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Neuman, et al. Standards Track [Page 1] RFC 4120 Kerberos V5 July 2005
Neuman, et al. Standards Track [Page 2] RFC 4120 Kerberos V5 July 2005
3.4. The KRB_SAFE Exchange .....................................42 3.4.1. Generation of a KRB_SAFE Message ...................42 3.4.2. Receipt of KRB_SAFE Message ........................43 3.5. The KRB_PRIV Exchange .....................................44 3.5.1. Generation of a KRB_PRIV Message ...................44 3.5.2. Receipt of KRB_PRIV Message ........................44 3.6. The KRB_CRED Exchange .....................................45 3.6.1. Generation of a KRB_CRED Message ...................45 3.6.2. Receipt of KRB_CRED Message ........................46 3.7. User-to-User Authentication Exchanges .....................47 4. Encryption and Checksum Specifications .........................48 5. Message Specifications .........................................50 5.1. Specific Compatibility Notes on ASN.1 .....................51 5.1.1. ASN.1 Distinguished Encoding Rules .................51 5.1.2. Optional Integer Fields ............................52 5.1.3. Empty SEQUENCE OF Types ............................52 5.1.4. Unrecognized Tag Numbers ...........................52 5.1.5. Tag Numbers Greater Than 30 ........................53 5.2. Basic Kerberos Types ......................................53 5.2.1. KerberosString .....................................53 5.2.2. Realm and PrincipalName ............................55 5.2.3. KerberosTime .......................................55 5.2.4. Constrained Integer Types ..........................55 5.2.5. HostAddress and HostAddresses ......................56 5.2.6. AuthorizationData ..................................57 5.2.7. PA-DATA ............................................60 5.2.8. KerberosFlags ......................................64 5.2.9. Cryptosystem-Related Types .........................65 5.3. Tickets ...................................................66 5.4. Specifications for the AS and TGS Exchanges ...............73 5.4.1. KRB_KDC_REQ Definition .............................73 5.4.2. KRB_KDC_REP Definition .............................81 5.5. Client/Server (CS) Message Specifications .................84 5.5.1. KRB_AP_REQ Definition ..............................84 5.5.2. KRB_AP_REP Definition ..............................88 5.5.3. Error Message Reply ................................89 5.6. KRB_SAFE Message Specification ............................89 5.6.1. KRB_SAFE definition ................................89 5.7. KRB_PRIV Message Specification ............................91 5.7.1. KRB_PRIV Definition ................................91 5.8. KRB_CRED Message Specification ............................92 5.8.1. KRB_CRED Definition ................................92 5.9. Error Message Specification ...............................94 5.9.1. KRB_ERROR Definition ...............................94 5.10. Application Tag Numbers ..................................96
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6. Naming Constraints .............................................97 6.1. Realm Names ...............................................97 6.2. Principal Names .......................................... 99 6.2.1. Name of Server Principals .........................100 7. Constants and Other Defined Values ............................101 7.1. Host Address Types .......................................101 7.2. KDC Messaging: IP Transports .............................102 7.2.1. UDP/IP transport ..................................102 7.2.2. TCP/IP Transport ..................................103 7.2.3. KDC Discovery on IP Networks ......................104 7.3. Name of the TGS ..........................................105 7.4. OID Arc for KerberosV5 ...................................106 7.5. Protocol Constants and Associated Values .................106 7.5.1. Key Usage Numbers .................................106 7.5.2. PreAuthentication Data Types ......................108 7.5.3. Address Types .....................................109 7.5.4. Authorization Data Types ..........................109 7.5.5. Transited Encoding Types ..........................109 7.5.6. Protocol Version Number ...........................109 7.5.7. Kerberos Message Types ............................110 7.5.8. Name Types ........................................110 7.5.9. Error Codes .......................................110 8. Interoperability Requirements .................................113 8.1. Specification 2 ..........................................113 8.2. Recommended KDC Values ...................................116 9. IANA Considerations ...........................................116 10. Security Considerations ......................................117 11. Acknowledgements .............................................121 A. ASN.1 Module ..................................................123 B. Changes since RFC 1510 ........................................131 Normative References .............................................134 Informative References ...........................................135
This document describes the concepts and model upon which the Kerberos network authentication system is based. It also specifies Version 5 of the Kerberos protocol. The motivations, goals, assumptions, and rationale behind most design decisions are treated cursorily; they are more fully described in a paper available in IEEE communications [NT94] and earlier in the Kerberos portion of the Athena Technical Plan [MNSS87].
The Kerberos model is based in part on Needham and Schroeder's trusted third-party authentication protocol [NS78] and on modifications suggested by Denning and Sacco [DS81]. The original design and implementation of Kerberos Versions 1 through 4 was the work of two former Project Athena staff members, Steve Miller of Digital Equipment Corporation and Clifford Neuman (now at the Information Sciences Institute of the University of Southern California), along with Jerome Saltzer, Technical Director of Project Athena, and Jeffrey Schiller, MIT Campus Network Manager. Many other members of Project Athena have also contributed to the work on Kerberos.
Reference implementations of both Version 4 and Version 5 of Kerberos are publicly available, and commercial implementations have been developed and are widely used. Details on the differences between Versions 4 and 5 can be found in [KNT94].
Neuman, et al. Standards Track [Page 5] RFC 4120 Kerberos V5 July 2005
1.1. The Kerberos Protocol
The basic Kerberos authentication process proceeds as follows: A client sends a request to the authentication server (AS) for "credentials" for a given server. The AS responds with these credentials, encrypted in the client's key. The credentials consist of a "ticket" for the server and a temporary encryption key (often called a "session key"). The client transmits the ticket (which contains the client's identity and a copy of the session key, all encrypted in the server's key) to the server. The session key (now shared by the client and server) is used to authenticate the client and may optionally be used to authenticate the server. It may also be used to encrypt further communication between the two parties or to exchange a separate sub-session key to be used to encrypt further communication. Note that many applications use Kerberos' functions only upon the initiation of a stream-based network connection. Unless an application performs encryption or integrity protection for the data stream, the identity verification applies only to the initiation of the connection, and it does not guarantee that subsequent messages on the connection originate from the same principal.
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The Kerberos protocol consists of several sub-protocols (or exchanges). There are two basic methods by which a client can ask a Kerberos server for credentials. In the first approach, the client sends a cleartext request for a ticket for the desired server to the AS. The reply is sent encrypted in the client's secret key. Usually this request is for a ticket-granting ticket (TGT), which can later be used with the ticket-granting server (TGS). In the second method, the client sends a request to the TGS. The client uses the TGT to authenticate itself to the TGS in the same manner as if it were contacting any other application server that requires Kerberos authentication. The reply is encrypted in the session key from the TGT. Though the protocol specification describes the AS and the TGS as separate servers, in practice they are implemented as different protocol entry points within a single Kerberos server.
To verify the identities of the principals in a transaction, the client transmits the ticket to the application server. Because the ticket is sent "in the clear" (parts of it are encrypted, but this encryption doesn't thwart replay) and might be intercepted and reused by an attacker, additional information is sent to prove that the message originated with the principal to whom the ticket was issued. This information (called the authenticator) is encrypted in the session key and includes a timestamp. The timestamp proves that the message was recently generated and is not a replay. Encrypting the authenticator in the session key proves that it was generated by a party possessing the session key. Since no one except the requesting principal and the server know the session key (it is never sent over the network in the clear), this guarantees the identity of the client.
1.2. Cross-Realm Operation
By establishing "inter-realm" keys, the administrators of two realms can allow a client authenticated in the local realm to prove its identity to servers in other realms. The exchange of inter-realm keys (a separate key may be used for each direction) registers the ticket-granting service of each realm as a principal in the other realm. A client is then able to obtain a TGT for the remote realm's ticket-granting service from its local realm. When that TGT is used, the remote ticket-granting service uses the inter-realm key (which usually differs from its own normal TGS key) to decrypt the TGT; thus it is certain that the ticket was issued by the client's own TGS. Tickets issued by the remote ticket-granting service will indicate to the end-service that the client was authenticated from another realm.
A realm is said to communicate with another realm if the two realms share an inter-realm key, or if the local realm shares an inter-realm key with an intermediate realm that communicates with the remote realm. An authentication path is the sequence of intermediate realms that are transited in communicating from one realm to another.
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If a hierarchical organization is not used, it may be necessary to consult a database in order to construct an authentication path between realms.
The application server is ultimately responsible for accepting or rejecting authentication and SHOULD check the transited field. The application server may choose to rely on the Key Distribution Center (KDC) for the application server's realm to check the transited field. The application server's KDC will set the TRANSITED-POLICY-CHECKED flag in this case. The KDCs for intermediate realms may also check the transited field as they issue TGTs for other realms, but they are encouraged not to do so. A client may request that the KDCs not check the transited field by setting the DISABLE-TRANSITED-CHECK flag. KDCs SHOULD honor this flag.
The Kerberos protocol provides the means for verifying (subject to the assumptions in Section 1.6) that the entity with which one communicates is the same entity that was registered with the KDC using the claimed identity (principal name). It is still necessary to determine whether that identity corresponds to the entity with which one intends to communicate.
One can also rely on trusted third parties to make this determination, but only when the data obtained from the third party is suitably integrity-protected while resident on the third-party
Implementations of Kerberos and protocols based on Kerberos MUST NOT use insecure DNS queries to canonicalize the hostname components of the service principal names (i.e., they MUST NOT use insecure DNS queries to map one name to another to determine the host part of the principal name with which one is to communicate). In an environment without secure name service, application authors MAY append a statically configured domain name to unqualified hostnames before passing the name to the security mechanisms, but they should do no more than that. Secure name service facilities, if available, might be trusted for hostname canonicalization, but such canonicalization by the client SHOULD NOT be required by KDC implementations.
1.4. Authorization
Separate authorization methods MAY be implemented as application- specific access control functions and may utilize files on the application server, on separately issued authorization credentials such as those based on proxies [Neu93], or on other authorization services. Separately authenticated authorization credentials MAY be embedded in a ticket's authorization data when encapsulated by the KDC-issued authorization data element.
1.5. Extending Kerberos without Breaking Interoperability
Kerberos provides a general mechanism for protocol extensibility. Some protocol messages contain typed holes -- sub-messages that contain an octet-string along with an integer that defines how to interpret the octet-string. The integer types are registered centrally, but they can be used both for vendor extensions and for extensions standardized through the IETF.
1.5.1. Compatibility with RFC 1510
In the meantime, all new or modified implementations of Kerberos that receive an unknown message extension SHOULD preserve the encoding of the extension but otherwise ignore its presence. Recipients MUST NOT decline a request simply because an extension is present.
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service cannot determine whether the restriction applies to that service, then a security weakness may result if the ticket can be used for that service. Authorization elements that are optional SHOULD be enclosed in the AD-IF-RELEVANT element.
Implementation note: Many RFC 1510 implementations ignore unknown authorization data elements. Depending on these implementations to honor authorization data restrictions may create a security weakness.
Care must be taken to ensure that old implementations can understand messages sent to them, even if they do not understand an extension that is used. Unless the sender knows that an extension is supported, the extension cannot change the semantics of the core message or previously defined extensions.
1.6. Environmental Assumptions
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* "Denial of service" attacks are not solved with Kerberos. There are places in the protocols where an intruder can prevent an application from participating in the proper authentication steps. Detection and solution of such attacks (some of which can appear to be not-uncommon "normal" failure modes for the system) are usually best left to the human administrators and users.
* "Password guessing" attacks are not solved by Kerberos. If a user chooses a poor password, it is possible for an attacker to successfully mount an offline dictionary attack by repeatedly attempting to decrypt, with successive entries from a dictionary, messages obtained which are encrypted under a key derived from the user's password.
* Principal identifiers are not recycled on a short-term basis. A typical mode of access control will use access control lists (ACLs) to grant permissions to particular principals. If a stale ACL entry remains for a deleted principal and the principal identifier is reused, the new principal will inherit rights specified in the stale ACL entry. By not re-using principal identifiers, the danger of inadvertent access is removed.
Below is a list of terms used throughout this document.
Authentication header A record containing a Ticket and an Authenticator to be presented to a server as part of the authentication process.
Authenticator A record containing information that can be shown to have been recently generated using the session key known only by the client and server.
Capability A token that grants the bearer permission to access an object or service. In Kerberos, this might be a ticket whose use is restricted by the contents of the authorization data field, but which lists no network addresses, together with the session key necessary to use the ticket.
Client A process that makes use of a network service on behalf of a user. Note that in some cases a Server may itself be a client of some other server (e.g., a print server may be a client of a file server).
Encryption Type (etype) When associated with encrypted data, an encryption type identifies the algorithm used to encrypt the data and is used to select the appropriate algorithm for decrypting the data. Encryption type tags are communicated in other messages to enumerate algorithms that are desired, supported, preferred, or allowed to be used for encryption of data between parties. This preference is combined with local information and policy to select an algorithm to be used.
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host on which it runs. The KDC services both initial ticket and ticket-granting ticket requests. The initial ticket portion is sometimes referred to as the Authentication Server (or service). The ticket-granting ticket portion is sometimes referred to as the ticket-granting server (or service).
Key Version Number (kvno) A tag associated with encrypted data identifies which key was used for encryption when a long-lived key associated with a principal changes over time. It is used during the transition to a new key so that the party decrypting a message can tell whether the data was encrypted with the old or the new key.
Principal A named client or server entity that participates in a network communication, with one name that is considered canonical.
Seal To encipher a record containing several fields in such a way that the fields cannot be individually replaced without knowledge of the encryption key or leaving evidence of tampering.
Server A particular Principal that provides a resource to network clients. The server is sometimes referred to as the Application Server.
Session key A temporary encryption key used between two principals, with a lifetime limited to the duration of a single login "session". In the Kerberos system, a session key is generated by the KDC. The session key is distinct from the sub-session key, described next.
Ticket A record that helps a client authenticate itself to a server; it contains the client's identity, a session key, a timestamp, and other information, all sealed using the server's secret key. It only serves to authenticate a client when presented along with a fresh Authenticator.
Each Kerberos ticket contains a set of flags that are used to indicate attributes of that ticket. Most flags may be requested by a client when the ticket is obtained; some are automatically turned on and off by a Kerberos server as required. The following sections explain what the various flags mean and give examples of reasons to use them. With the exception of the INVALID flag, clients MUST ignore ticket flags that are not recognized. KDCs MUST ignore KDC options that are not recognized. Some implementations of RFC 1510 are known to reject unknown KDC options, so clients may need to resend a request without new KDC options if the request was rejected when sent with options added since RFC 1510. Because new KDCs will ignore unknown options, clients MUST confirm that the ticket returned by the KDC meets their needs.
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2.1. Initial, Pre-authenticated, and Hardware-Authenticated Tickets
The PRE-AUTHENT and HW-AUTHENT flags provide additional information about the initial authentication, regardless of whether the current ticket was issued directly (in which case INITIAL will also be set) or issued on the basis of a TGT (in which case the INITIAL flag is clear, but the PRE-AUTHENT and HW-AUTHENT flags are carried forward from the TGT).
The INVALID flag indicates that a ticket is invalid. Application servers MUST reject tickets that have this flag set. A postdated ticket will be issued in this form. Invalid tickets MUST be validated by the KDC before use, by being presented to the KDC in a TGS request with the VALIDATE option specified. The KDC will only validate tickets after their starttime has passed. The validation is required so that postdated tickets that have been stolen before their starttime can be rendered permanently invalid (through a hot-list mechanism) (see Section 3.3.3.1).
Applications may desire to hold tickets that can be valid for long periods of time. However, this can expose their credentials to potential theft for equally long periods, and those stolen credentials would be valid until the expiration time of the ticket(s). Simply using short-lived tickets and obtaining new ones periodically would require the client to have long-term access to its secret key, an even greater risk. Renewable tickets can be used to mitigate the consequences of theft. Renewable tickets have two "expiration times": the first is when the current instance of the ticket expires, and the second is the latest permissible value for an individual expiration time. An application client must periodically (i.e., before it expires) present a renewable ticket to the KDC, with the RENEW option set in the KDC request. The KDC will issue a new ticket with a new session key and a later expiration time. All other fields of the ticket are left unmodified by the renewal process. When the latest permissible expiration time arrives, the ticket expires permanently. At each renewal, the KDC MAY consult a hot-list to determine whether the ticket had been reported stolen since its
The RENEWABLE flag in a ticket is normally only interpreted by the ticket-granting service (discussed below in Section 3.3). It can usually be ignored by application servers. However, some particularly careful application servers MAY disallow renewable tickets.
2.4. Postdated Tickets
The MAY-POSTDATE flag in a ticket is normally only interpreted by the ticket-granting service. It can be ignored by application servers. This flag MUST be set in a TGT in order to issue a postdated ticket based on the presented ticket. It is reset by default; a client MAY request it by setting the ALLOW-POSTDATE option in the KRB_AS_REQ message. This flag does not allow a client to obtain a postdated TGT; postdated TGTs can only be obtained by requesting the postdating in the KRB_AS_REQ message. The life (endtime-starttime) of a postdated ticket will be the remaining life of the TGT at the time of the request, unless the RENEWABLE option is also set, in which case it can be the full life (endtime-starttime) of the TGT. The KDC MAY limit how far in the future a ticket may be postdated.
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the application client MUST present the ticket to the KDC to be validated before use.
At times it may be necessary for a principal to allow a service to perform an operation on its behalf. The service must be able to take on the identity of the client, but only for a particular purpose. A principal can allow a service to do this by granting it a proxy.
The PROXIABLE flag in a ticket is normally only interpreted by the ticket-granting service. It can be ignored by application servers. When set, this flag tells the ticket-granting server that it is OK to issue a new ticket (but not a TGT) with a different network address based on this ticket. This flag is set if requested by the client on initial authentication. By default, the client will request that it be set when requesting a TGT, and that it be reset when requesting any other ticket.
In order to complicate the use of stolen credentials, Kerberos tickets are often valid only from those network addresses specifically included in the ticket, but it is permissible as a policy option to allow requests and to issue tickets with no network addresses specified. When granting a proxy, the client MUST specify the new network address from which the proxy is to be used or indicate that the proxy is to be issued for use from any address.
2.6. Forwardable Tickets
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and wants authentication to work from that system as if the login were local.
This flag allows for authentication forwarding without requiring the user to enter a password again. If the flag is not set, then authentication forwarding is not permitted, but the same result can still be achieved if the user engages in the AS exchange, specifies the requested network addresses, and supplies a password.
If addressless tickets are forwarded from one system to another, clients SHOULD still use this option to obtain a new TGT in order to have different session keys on the different systems.
In Kerberos, the application server is ultimately responsible for accepting or rejecting authentication, and it SHOULD check that only suitably trusted KDCs are relied upon to authenticate a principal. The transited field in the ticket identifies which realms (and thus which KDCs) were involved in the authentication process, and an application server would normally check this field. If any of these are untrusted to authenticate the indicated client principal (probably determined by a realm-based policy), the authentication attempt MUST be rejected. The presence of trusted KDCs in this list does not provide any guarantee; an untrusted KDC may have fabricated the list.
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on the cross-realm TGT. A client MAY request that the KDCs not check the transited field by setting the DISABLE-TRANSITED-CHECK flag. KDCs are encouraged but not required to honor this flag.
2.8. OK as Delegate
The copy of the ticket flags in the encrypted part of the KDC reply may have the OK-AS-DELEGATE flag set to indicate to the client that the server specified in the ticket has been determined by the policy of the realm to be a suitable recipient of delegation. A client can use the presence of this flag to help it decide whether to delegate credentials (grant either a proxy or a forwarded TGT) to this server. It is acceptable to ignore the value of this flag. When setting this flag, an administrator should consider the security and placement of the server on which the service will run, as well as whether the service requires the use of delegated credentials.
There are three additional options that MAY be set in a client's request of the KDC.
The RENEWABLE-OK option indicates that the client will accept a renewable ticket if a ticket with the requested life cannot otherwise be provided. If a ticket with the requested life cannot be provided, then the KDC MAY issue a renewable ticket with a renew-till equal to the requested endtime. The value of the renew-till field MAY still be adjusted by site-determined limits or limits imposed by the individual principal or server.
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2.9.2. ENC-TKT-IN-SKEY
2.9.3. Passwordless Hardware Authentication
3. Message Exchanges
3.1. The Authentication Service Exchange
Message direction Message type Section 1. Client to Kerberos KRB_AS_REQ 5.4.1 2. Kerberos to client KRB_AS_REP or 5.4.2 KRB_ERROR 5.9.1
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without requiring further use of the client's secret key. This exchange is also used to request credentials for services that must not be mediated through the Ticket-Granting Service, but rather require knowledge of a principal's secret key, such as the password- changing service (the password-changing service denies requests unless the requester can demonstrate knowledge of the user's old password; requiring this knowledge prevents unauthorized password changes by someone walking up to an unattended session).
The AS exchange consists of two messages: KRB_AS_REQ from the client to Kerberos, and KRB_AS_REP or KRB_ERROR in reply. The formats for these messages are described in Sections 5.4.1, 5.4.2, and 5.9.1.
Without pre-authentication, the authentication server does not know whether the client is actually the principal named in the request. It simply sends a reply without knowing or caring whether they are the same. This is acceptable because nobody but the principal whose identity was given in the request will be able to use the reply. Its critical information is encrypted in that principal's key. However, an attacker can send a KRB_AS_REQ message to get known plaintext in order to attack the principal's key. Especially if the key is based on a password, this may create a security exposure. So the initial request supports an optional field that can be used to pass additional information that might be needed for the initial exchange. This field SHOULD be used for pre-authentication as described in sections 3.1.1 and 5.2.7.
3.1.1. Generation of KRB_AS_REQ Message
The client prepares the KRB_AS_REQ message and sends it to the KDC.
If all goes well, processing the KRB_AS_REQ message will result in the creation of a ticket for the client to present to the server. The format for the ticket is described in Section 5.3.
3.1.3. Generation of KRB_AS_REP Message
If required to do so, the server pre-authenticates the request, and if the pre-authentication check fails, an error message with the code KDC_ERR_PREAUTH_FAILED is returned. If pre-authentication is
In response to an AS request, if there are multiple encryption keys registered for a client in the Kerberos database, then the etype field from the AS request is used by the KDC to select the encryption method to be used to protect the encrypted part of the KRB_AS_REP message that is sent to the client. If there is more than one supported strong encryption type in the etype list, the KDC SHOULD use the first valid strong etype for which an encryption key is available.
When the AS server is to include pre-authentication data in a KRB-ERROR or in an AS-REP, it MUST use PA-ETYPE-INFO2, not PA-ETYPE- INFO, if the etype field of the client's AS-REQ lists at least one "newer" encryption type. Otherwise (when the etype field of the client's AS-REQ does not list any "newer" encryption types), it MUST send both PA-ETYPE-INFO2 and PA-ETYPE-INFO (both with an entry for each enctype). A "newer" enctype is any enctype first officially specified concurrently with or subsequent to the issue of this RFC. The enctypes DES, 3DES, or RC4 and any defined in [RFC1510] are not "newer" enctypes.
The KDC will attempt to assign the type of the random session key from the list of methods in the etype field. The KDC will select the appropriate type using the list of methods provided and information from the Kerberos database indicating acceptable encryption methods for the application server. The KDC will not issue tickets with a weak session key encryption type.
The expiration time of the ticket will be set to the earlier of the requested endtime and a time determined by local policy, possibly by using realm- or principal-specific factors. For example, the expiration time MAY be set to the earliest of the following:
* The ticket's starttime plus the maximum allowable lifetime associated with the client principal from the authentication server's database.
* The ticket's starttime plus the maximum lifetime set by the policy of the local realm.
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flag is set in the new ticket, and the renew-till value is set as if the 'RENEWABLE' option were requested (the field and option names are described fully in Section 5.4.1).
* Its requested value.
* The starttime of the ticket plus the maximum renewable lifetime set by the policy of the local realm.
If all of the above succeed, the server will encrypt the ciphertext part of the ticket using the encryption key extracted from the server principal's record in the Kerberos database using the encryption type associated with the server principal's key. (This choice is NOT affected by the etype field in the request.) It then formats a KRB_AS_REP message (see Section 5.4.2), copying the addresses in the request into the caddr of the response, placing any required pre- authentication data into the padata of the response, and encrypts the ciphertext part in the client's key using an acceptable encryption method requested in the etype field of the request, or in some key specified by pre-authentication mechanisms being used.
Several errors can occur, and the Authentication Server responds by returning an error message, KRB_ERROR, to the client, with the error-code and e-text fields set to appropriate values. The error message contents and details are described in Section 5.9.1.
If the reply message type is KRB_AS_REP, then the client verifies that the cname and crealm fields in the cleartext portion of the reply match what it requested. If any padata fields are present, they may be used to derive the proper secret key to decrypt the
Upon validation of the KRB_AS_REP message (by checking the returned nonce against that sent in the KRB_AS_REQ message), the client knows that the current time on the KDC is that read from the authtime field of the encrypted part of the reply. The client can optionally use this value for clock synchronization in subsequent messages by recording with the ticket the difference (offset) between the authtime value and the local clock. This offset can then be used by the same user to adjust the time read from the system clock when generating messages [DGT96].
Proper decryption of the KRB_AS_REP message is not sufficient for the host to verify the identity of the user; the user and an attacker could cooperate to generate a KRB_AS_REP format message that decrypts properly but is not from the proper KDC. If the host wishes to verify the identity of the user, it MUST require the user to present application credentials that can be verified using a securely-stored secret key for the host. If those credentials can be verified, then the identity of the user can be assured.
If the reply message type is KRB_ERROR, then the client interprets it as an error and performs whatever application-specific tasks are necessary for recovery.
Summary
The client/server authentication (CS) exchange is used by network applications to authenticate the client to the server and vice versa. The client MUST have already acquired credentials for the server using the AS or TGS exchange.
The KRB_AP_REQ contains authentication information that SHOULD be part of the first message in an authenticated transaction. It contains a ticket, an authenticator, and some additional bookkeeping information (see Section 5.5.1 for the exact format). The ticket by itself is insufficient to authenticate a client, since tickets are passed across the network in cleartext (tickets contain both an encrypted and unencrypted portion, so cleartext here refers to the entire unit, which can be copied from one message and replayed in another without any cryptographic skill). The authenticator is used to prevent invalid replay of tickets by proving to the server that the client knows the session key of the ticket and thus is entitled to use the ticket. The KRB_AP_REQ message is referred to elsewhere as the 'authentication header'.
When a client wishes to initiate authentication to a server, it obtains (either through a credentials cache, the AS exchange, or the TGS exchange) a ticket and session key for the desired service. The client MAY re-use any tickets it holds until they expire. To use a ticket, the client constructs a new Authenticator from the system time and its name, and optionally from an application-specific checksum, an initial sequence number to be used in KRB_SAFE or KRB_PRIV messages, and/or a session subkey to be used in negotiations for a session key unique to this particular session. Authenticators MUST NOT be re-used and SHOULD be rejected if replayed to a server. Note that this can make applications based on unreliable transports difficult to code correctly. If the transport might deliver duplicated messages, either a new authenticator MUST be generated for each retry, or the application server MUST match requests and replies and replay the first reply in response to a detected duplicate.
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If a sequence number is to be included, it SHOULD be randomly chosen so that even after many messages have been exchanged it is not likely to collide with other sequence numbers in use.
The Authenticator is encrypted in the session key and combined with the ticket to form the KRB_AP_REQ message, which is then sent to the end server along with any additional application-specific information.
Authentication is based on the server's current time of day (clocks MUST be loosely synchronized), the authenticator, and the ticket. Several errors are possible. If an error occurs, the server is expected to reply to the client with a KRB_ERROR message. This message MAY be encapsulated in the application protocol if its raw form is not acceptable to the protocol. The format of error messages is described in Section 5.9.1.
Because it is possible for the server to be registered in multiple realms, with different keys in each, the srealm field in the unencrypted portion of the ticket in the KRB_AP_REQ is used to specify which secret key the server should use to decrypt that ticket. The KRB_AP_ERR_NOKEY error code is returned if the server doesn't have the proper key to decipher the ticket.
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KRB_AP_ERR_BAD_INTEGRITY error is returned (chances are good that different keys were used to encrypt and decrypt).
Unless the application server provides its own suitable means to protect against replay (for example, a challenge-response sequence initiated by the server after authentication, or use of a server- generated encryption subkey), the server MUST utilize a replay cache to remember any authenticator presented within the allowable clock skew. Careful analysis of the application protocol and implementation is recommended before eliminating this cache. The replay cache will store at least the server name, along with the client name, time, and microsecond fields from the recently-seen authenticators, and if a matching tuple is found, the KRB_AP_ERR_REPEAT error is returned. Note that the rejection here is restricted to authenticators from the same principal to the same server. Other client principals communicating with the same server principal should not have their authenticators rejected if the time and microsecond fields happen to match some other client's authenticator.
Implementation note: If a client generates multiple requests to the KDC with the same timestamp, including the microsecond field, all but the first of the requests received will be rejected as replays. This
If multiple servers (for example, different services on one machine, or a single service implemented on multiple machines) share a service principal (a practice that we do not recommend in general, but that we acknowledge will be used in some cases), either they MUST share this replay cache, or the application protocol MUST be designed so as to eliminate the need for it. Note that this applies to all of the services. If any of the application protocols does not have replay protection built in, an authenticator used with such a service could later be replayed to a different service with the same service principal but no replay protection, if the former doesn't record the authenticator information in the common replay cache.
The server computes the age of the ticket: local (server) time minus the starttime inside the Ticket. If the starttime is later than the current time by more than the allowable clock skew, or if the INVALID flag is set in the ticket, the KRB_AP_ERR_TKT_NYV error is returned. Otherwise, if the current time is later than end time by more than the allowable clock skew, the KRB_AP_ERR_TKT_EXPIRED error is returned.
Passing these checks provides only authentication of the named principal; it does not imply authorization to use the named service. Applications MUST make a separate authorization decision based upon the authenticated name of the user, the requested operation, local access control information such as that contained in a .k5login or .k5users file, and possibly a separate distributed authorization service.
Typically, a client's request will include both the authentication information and its initial request in the same message, and the server need not explicitly reply to the KRB_AP_REQ. However, if mutual authentication (authenticating not only the client to the server, but also the server to the client) is being performed, the KRB_AP_REQ message will have MUTUAL-REQUIRED set in its ap-options field, and a KRB_AP_REP message is required in response. As with the error message, this message MAY be encapsulated in the application protocol if its "raw" form is not acceptable to the application's protocol. The timestamp and microsecond field used in the reply MUST be the client's timestamp and microsecond field (as provided in the authenticator). If a sequence number is to be included, it SHOULD be randomly chosen as described above for the authenticator. A subkey MAY be included if the server desires to negotiate a different subkey. The KRB_AP_REP message is encrypted in the session key extracted from the ticket.
3.2.5. Receipt of KRB_AP_REP Message
3.2.6. Using the Encryption Key
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and random numbers and to generate a negotiated key to be returned in the KRB_AP_REP message.
One way that an application may choose to negotiate a key to be used for subsequent integrity and privacy protection is for the client to propose a key in the subkey field of the authenticator. The server can then choose a key using the key proposed by the client as input, returning the new subkey in the subkey field of the application reply. This key could then be used for subsequent communication.
3.3. The Ticket-Granting Service (TGS) Exchange
Message direction Message type Section 1. Client to Kerberos KRB_TGS_REQ 5.4.1 2. Kerberos to client KRB_TGS_REP or 5.4.2 KRB_ERROR 5.9.1
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key. Instead, the session key from the TGT or renewable ticket, or sub-session key from an Authenticator is used. As is the case for all application servers, expired tickets are not accepted by the TGS, so once a renewable or TGT expires, the client must use a separate exchange to obtain valid tickets.
3.3.1. Generation of KRB_TGS_REQ Message
If the client knows the service principal name and realm and it does not already possess a TGT for the appropriate realm, then one must be obtained. This is first attempted by requesting a TGT for the destination realm from a Kerberos server for which the client possesses a TGT (by using the KRB_TGS_REQ message recursively). The
If the Kerberos server returns a TGT for a realm 'closer' than the desired realm, the client MAY use local policy configuration to verify that the authentication path used is an acceptable one. Alternatively, a client MAY choose its own authentication path, rather than rely on the Kerberos server to select one. In either case, any policy or configuration information used to choose or validate authentication paths, whether by the Kerberos server or by the client, MUST be obtained from a trusted source.
Once the client obtains a TGT for the appropriate realm, it determines which Kerberos servers serve that realm and contacts one of them. The list might be obtained through a configuration file or network service, or it MAY be generated from the name of the realm. As long as the secret keys exchanged by realms are kept secret, only denial of service results from using a false Kerberos server.
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The client prepares the KRB_TGS_REQ message, providing an authentication header as an element of the padata field, and including the same fields as used in the KRB_AS_REQ message along with several optional fields: the enc-authorizatfion-data field for application server use and additional tickets required by some options.
If the sub-session key is not specified, the session key from the TGT will be used. If the enc-authorization-data is present, it MUST be encrypted in the sub-session key, if present, from the authenticator portion of the authentication header, or, if not present, by using the session key from the TGT.
3.3.2. Receipt of KRB_TGS_REQ Message
Once the accompanying ticket has been decrypted, the user-supplied checksum in the Authenticator MUST be verified against the contents of the request, and the message MUST be rejected if the checksums do not match (with an error code of KRB_AP_ERR_MODIFIED) or if the checksum is not collision-proof (with an error code of KRB_AP_ERR_INAPP_CKSUM). If the checksum type is not supported, the KDC_ERR_SUMTYPE_NOSUPP error is returned. If the authorization-data are present, they are decrypted using the sub-session key from the Authenticator.
As discussed in Section 3.1.2, the KDC MUST send a valid KRB_TGS_REP message if it receives a KRB_TGS_REQ message identical to one it has recently processed. However, if the authenticator is a replay, but the rest of the request is not identical, then the KDC SHOULD return KRB_AP_ERR_REPEAT.
The KRB_TGS_REP message shares its format with the KRB_AS_REP (KRB_KDC_REP), but with its type field set to KRB_TGS_REP. The detailed specification is in Section 5.4.2.
By default, the address field, the client's name and realm, the list of transited realms, the time of initial authentication, the expiration time, and the authorization data of the newly-issued ticket will be copied from the TGT or renewable ticket. If the transited field needs to be updated, but the transited type is not supported, the KDC_ERR_TRTYPE_NOSUPP error is returned.
If the FORWARDED option has been requested, then the resulting ticket will contain the addresses specified by the client. This option will only be honored if the FORWARDABLE flag is set in the TGT. The PROXY option is similar; the resulting ticket will contain the addresses
If the requested starttime is absent, indicates a time in the past, or is within the window of acceptable clock skew for the KDC and the POSTDATE option has not been specified, then the starttime of the ticket is set to the authentication server's current time. If it indicates a time in the future beyond the acceptable clock skew, but the POSTDATED option has not been specified or the MAY-POSTDATE flag is not set in the TGT, then the error KDC_ERR_CANNOT_POSTDATE is returned. Otherwise, if the TGT has the MAY-POSTDATE flag set, then the resulting ticket will be postdated, and the requested starttime is checked against the policy of the local realm. If acceptable, the ticket's starttime is set as requested, and the INVALID flag is set. The postdated ticket MUST be validated before use by presenting it to the KDC after the starttime has been reached. However, in no case may the starttime, endtime, or renew-till time of a newly-issued postdated ticket extend beyond the renew-till time of the TGT.
If (a) the name of the server in the ticket that is presented to the KDC as part of the authentication header is not that of the TGS itself, (b) the server is registered in the realm of the KDC, and (c) the RENEW option is requested, then the KDC will verify that the RENEWABLE flag is set in the ticket, that the INVALID flag is not set in the ticket, and that the renew_till time is still in the future. If the VALIDATE option is requested, the KDC will check that the starttime has passed and that the INVALID flag is set. If the PROXY option is requested, then the KDC will check that the PROXIABLE flag
The ciphertext part of the response in the KRB_TGS_REP message is encrypted in the sub-session key from the Authenticator, if present, or in the session key from the TGT. It is not encrypted using the client's secret key. Furthermore, the client's key's expiration date and the key version number fields are left out since these values are stored along with the client's database record, and that record is not needed to satisfy a request based on a TGT.
Whenever a request is made to the ticket-granting server, the presented ticket(s) is (are) checked against a hot-list of tickets that have been canceled. This hot-list might be implemented by storing a range of issue timestamps for 'suspect tickets'; if a presented ticket had an authtime in that range, it would be rejected. In this way, a stolen TGT or renewable ticket cannot be used to gain additional tickets (renewals or otherwise) once the theft has been reported to the KDC for the realm in which the server resides. Any normal ticket obtained before it was reported stolen will still be valid (because tickets require no interaction with the KDC), but only until its normal expiration time. If TGTs have been issued for cross-realm authentication, use of the cross-realm TGT will not be affected unless the hot-list is propagated to the KDCs for the realms for which such cross-realm tickets were issued.
If the identity of the server in the TGT that is presented to the KDC as part of the authentication header is that of the ticket-granting service, but the TGT was issued from another realm, the KDC will look up the inter-realm key shared with that realm and use that key to decrypt the ticket. If the ticket is valid, then the KDC will honor the request, subject to the constraints outlined above in the section describing the AS exchange. The realm part of the client's identity will be taken from the TGT. The name of the realm that issued the TGT, if it is not the realm of the client principal, will be added to the transited field of the ticket to be issued. This is accomplished by reading the transited field from the TGT (which is treated as an unordered set of realm names), adding the new realm to the set, and then constructing and writing out its encoded (shorthand) form (this may involve a rearrangement of the existing encoding).
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previous realm. This prevents a malicious Kerberos server from intentionally leaving out its own name (it could, however, omit other realms' names).
Because this field has the name of each transited realm added to it, it might potentially be very long. To decrease the length of this field, its contents are encoded. The initially supported encoding is optimized for the normal case of inter-realm communication: a hierarchical arrangement of realms using either domain or X.500 style realm names. This encoding (called DOMAIN-X500-COMPRESS) is now described.
A realm name ending with a "." is interpreted as being prepended to the previous realm. For example, we can encode traversal of EDU, MIT.EDU, ATHENA.MIT.EDU, WASHINGTON.EDU, and CS.WASHINGTON.EDU as:
Note that if either ATHENA.MIT.EDU, or CS.WASHINGTON.EDU were endpoints, they would not be included in this field, and we would have:
A realm name beginning with a "/" is interpreted as being appended to the previous realm. For the purpose of appending, the realm preceding the first listed realm is considered the null realm (""). If a realm name beginning with a "/" is to stand by itself, then it SHOULD be preceded by a space (" "). For example, we can encode traversal of /COM/HP/APOLLO, /COM/HP, /COM, and /COM/DEC as:
As in the example above, if /COM/HP/APOLLO and /COM/DEC were endpoints, they would not be included in this field, and we would have:
A null subfield preceding or following a "," indicates that all realms between the previous realm and the next realm have been traversed. For the purpose of interpreting null subfields, the client's realm is considered to precede those in the transited field, and the server's realm is considered to follow them. Thus, "," means that all realms along the path between the client and the server have been traversed. ",EDU, /COM," means that all realms from the client's realm up to EDU (in a domain style hierarchy) have been traversed, and that everything from /COM down to the server's realm in an X.500 style has also been traversed. This could occur if the EDU realm in one hierarchy shares an inter-realm key directly with the /COM realm in another hierarchy.
When the KRB_TGS_REP is received by the client, it is processed in the same manner as the KRB_AS_REP processing described above. The primary difference is that the ciphertext part of the response must be decrypted using the sub-session key from the Authenticator, if it was specified in the request, or the session key from the TGT, rather than the client's secret key. The server name returned in the reply is the true principal name of the service.
The KRB_SAFE message MAY be used by clients requiring the ability to detect modifications of messages they exchange. It achieves this by including a keyed collision-proof checksum of the user data and some control information. The checksum is keyed with an encryption key (usually the last key negotiated via subkeys, or the session key if no negotiation has occurred).
When an application wishes to send a KRB_SAFE message, it collects its data and the appropriate control information and computes a checksum over them. The checksum algorithm should be the keyed checksum mandated to be implemented along with the crypto system used for the sub-session or session key. The checksum is generated using the sub-session key, if present, or the session key. Some implementations use a different checksum algorithm for the KRB_SAFE messages, but doing so in an interoperable manner is not always possible.
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using the KRB_SAFE message MUST choose at least one of the two mechanisms. This choice SHOULD be based on the needs of the application protocol.
If the application protocol is expected to tolerate lost messages without their being resent, the use of the timestamp is the appropriate replay detection mechanism. Using timestamps is also the appropriate mechanism for multi-cast protocols in which all of one's peers share a common sub-session key, but some messages will be sent to a subset of one's peers.
3.4.2. Receipt of KRB_SAFE Message
The message is first checked by verifying that the protocol version and type fields match the current version and KRB_SAFE, respectively. A mismatch generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE error. The application verifies that the checksum used is a collision-proof keyed checksum that uses keys compatible with the sub-session or session key as appropriate (or with the application key derived from the session or sub-session keys). If it is not, a KRB_AP_ERR_INAPP_CKSUM error is generated. The sender's address MUST be included in the control information; the recipient verifies that the operating system's report of the sender's address matches the sender's address in the message, and (if a recipient address is specified or the recipient requires an address) that one of the recipient's addresses appears as the recipient's address in the message. To work with network address translation, senders MAY use the directional address type specified in Section 8.1 for the sender address and not include recipient addresses. A failed match for either case generates a KRB_AP_ERR_BADADDR error. Then the timestamp and usec and/or the sequence number fields are checked. If timestamp and usec are expected and not present, or if they are present but not current, the KRB_AP_ERR_SKEW error is generated. Timestamps are not required to be strictly ordered; they are only required to be in the skew window. If the server name, along with the client name, time,
If all the checks succeed, the application is assured that the message was generated by its peer and was not modified in transit.
3.5. The KRB_PRIV Exchange
3.5.1. Generation of a KRB_PRIV Message
3.5.2. Receipt of KRB_PRIV Message
The message is first checked by verifying that the protocol version and type fields match the current version and KRB_PRIV, respectively. A mismatch generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE error. The application then decrypts the ciphertext and processes
The sender's address MUST be included in the control information; the recipient verifies that the operating system's report of the sender's address matches the sender's address in the message. If a recipient address is specified or the recipient requires an address, then one of the recipient's addresses MUST also appear as the recipient's address in the message. Where a sender's or receiver's address might not otherwise match the address in a message because of network address translation, an application MAY be written to use addresses of the directional address type in place of the actual network address.
Next the timestamp and usec and/or the sequence number fields are checked. If timestamp and usec are expected and not present, or if they are present but not current, the KRB_AP_ERR_SKEW error is generated. If the server name, along with the client name, time, and microsecond fields from the Authenticator match any such recently- seen tuples, the KRB_AP_ERR_REPEAT error is generated. If an incorrect sequence number is included, or if a sequence number is expected but not present, the KRB_AP_ERR_BADORDER error is generated. If neither a time-stamp and usec nor a sequence number is present, a KRB_AP_ERR_MODIFIED error is generated.
3.6. The KRB_CRED Exchange
3.6.1. Generation of a KRB_CRED Message
Other information associated with each ticket and obtained during the KRB_TGS exchange is also placed in the corresponding KrbCredInfo sequence in the encrypted part of the KRB_CRED message. The current time and, if they are specifically required by the application, the nonce, s-address, and r-address fields are placed in the encrypted part of the KRB_CRED message, which is then encrypted under an encryption key previously exchanged in the KRB_AP exchange (usually the last key negotiated via subkeys, or the session key if no negotiation has occurred).
3.6.2. Receipt of KRB_CRED Message
If present or required, the recipient MAY verify that the operating system's report of the sender's address matches the sender's address in the message, and that one of the recipient's addresses appears as the recipient's address in the message. The address check does not provide any added security, since the address, if present, has already been checked in the KRB_AP_REQ message and there is not any benefit to be gained by an attacker in reflecting a KRB_CRED message back to its originator. Thus, the recipient MAY ignore the address even if it is present in order to work better in Network Address Translation (NAT) environments. A failed match for either case
If all the checks succeed, the application stores each of the new tickets in its credentials cache together with the session key and other information in the corresponding KrbCredInfo sequence from the encrypted part of the KRB_CRED message.
User-to-User authentication provides a method to perform authentication when the verifier does not have a access to long-term service key. This might be the case when running a server (for example, a window server) as a user on a workstation. In such cases, the server may have access to the TGT obtained when the user logged in to the workstation, but because the server is running as an unprivileged user, it might not have access to system keys. Similar situations may arise when running peer-to-peer applications.
Message direction Message type Sections 0. Message from application server Not specified 1. Client to Kerberos KRB_TGS_REQ 3.3 & 5.4.1 2. Kerberos to client KRB_TGS_REP or 3.3 & 5.4.2 KRB_ERROR 5.9.1 3. Client to application server KRB_AP_REQ 3.2 & 5.5.1
Message 0 in the table above represents an application-specific negotiation between the client and server, at the end of which both have determined that they will use user-to-user authentication, and the client has obtained the server's TGT.
If validated according to the instructions in Section 3.3.3, the application ticket returned to the client (message 2 in the table above) will be encrypted using the session key from the additional ticket and the client will note this when it uses or stores the application ticket.
4. Encryption and Checksum Specifications
The KDC uses the principal's secret key (in the AS exchange) or a shared session key (in the TGS exchange) to encrypt responses to ticket requests; the ability to obtain the secret key or session key implies the knowledge of the appropriate keys and the identity of the KDC. The ability of a principal to decrypt the KDC response and to present a Ticket and a properly formed Authenticator (generated with the session key from the KDC response) to a service verifies the identity of the principal; likewise the ability of the service to extract the session key from the Ticket and to prove its knowledge thereof in a response verifies the identity of the service.
The string-to-key operation provided by [RFC3961] is used to produce a long-term key for a principal (generally for a user). The default salt string, if none is provided via pre-authentication data, is the concatenation of the principal's realm and name components, in order, with no separators. Unless it is indicated otherwise, the default string-to-key opaque parameter set as defined in [RFC3961] is used.
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Encrypted data, keys, and checksums are transmitted using the EncryptedData, EncryptionKey, and Checksum data objects defined in Section 5.2.9. The encryption, decryption, and checksum operations described in this document use the corresponding encryption, decryption, and get_mic operations described in [RFC3961], with implicit "specific key" generation using the "key usage" values specified in the description of each EncryptedData or Checksum object to vary the key for each operation. Note that in some cases, the value to be used is dependent on the method of choosing the key or the context of the message.
There might exist other documents that define protocols in terms of the RFC 1510 encryption types or checksum types. These documents would not know about key usages. In order that these specifications continue to be meaningful until they are updated, if no key usage values are specified, then key usages 1024 and 1025 must be used to derive keys for encryption and checksums, respectively. (This does not apply to protocols that do their own encryption independent of this framework, by directly using the key resulting from the Kerberos authentication exchange.) New protocols defined in terms of the Kerberos encryption and checksum types SHOULD use their own key usage values.
Implementation note: Although it is not recommended, some application protocols will continue to use the key data directly, even if only in currently existing protocol specifications. An implementation intended to support general Kerberos applications may therefore need to make key data available, as well as the attributes and operations described in [RFC3961]. One of the more common reasons for directly performing encryption is direct control over negotiation and selection of a "sufficiently strong" encryption algorithm (in the context of a given application). Although Kerberos does not directly provide a facility for negotiating encryption types between the
5. Message Specifications
The Kerberos protocol is defined here in terms of Abstract Syntax Notation One (ASN.1) [X680], which provides a syntax for specifying both the abstract layout of protocol messages as well as their encodings. Implementors not utilizing an existing ASN.1 compiler or support library are cautioned to understand the actual ASN.1 specification thoroughly in order to ensure correct implementation behavior. There is more complexity in the notation than is immediately obvious, and some tutorials and guides to ASN.1 are misleading or erroneous.
The type definitions in this section assume an ASN.1 module definition of the following form:
-- rest of definitions here
This specifies that the tagging context for the module will be explicit and non-automatic.
Note that elsewhere in this document, nomenclature for various message types is inconsistent, but it largely follows C language conventions, including use of underscore (_) characters and all-caps spelling of names intended to be numeric constants. Also, in some places, identifiers (especially those referring to constants) are written in all-caps in order to distinguish them from surrounding explanatory text.
5.1. Specific Compatibility Notes on ASN.1
5.1.1. ASN.1 Distinguished Encoding Rules
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Rules (BER); in particular, these implementations send indefinite encodings of lengths. Implementations MAY accept such encodings in the interest of backward compatibility, though implementors are warned that decoding fully-general BER is fraught with peril.
Some implementations do not internally distinguish between an omitted optional integer value and a transmitted value of zero. The places in the protocol where this is relevant include various microseconds fields, nonces, and sequence numbers. Implementations SHOULD treat omitted optional integer values as having been transmitted with a value of zero, if the application is expecting this.
There are places in the protocol where a message contains a SEQUENCE OF type as an optional member. This can result in an encoding that contains an empty SEQUENCE OF encoding. The Kerberos protocol does not semantically distinguish between an absent optional SEQUENCE OF type and a present optional but empty SEQUENCE OF type. Implementations SHOULD NOT send empty SEQUENCE OF encodings that are marked OPTIONAL, but SHOULD accept them as being equivalent to an omitted OPTIONAL type. In the ASN.1 syntax describing Kerberos messages, instances of these problematic optional SEQUENCE OF types are indicated with a comment.
Future revisions to this protocol may include new message types with different APPLICATION class tag numbers. Such revisions should protect older implementations by only sending the message types to parties that are known to understand them; e.g., by means of a flag bit set by the receiver in a preceding request. In the interest of robust error handling, implementations SHOULD gracefully handle receiving a message with an unrecognized tag anyway, and return an error message, if appropriate.
A naive implementation of a DER ASN.1 decoder may experience problems with ASN.1 tag numbers greater than 30, due to such tag numbers being encoded using more than one byte. Future revisions of this protocol may utilize tag numbers greater than 30, and implementations SHOULD be prepared to gracefully return an error, if appropriate, when they do not recognize the tag.
This section defines a number of basic types that are potentially used in multiple Kerberos protocol messages.
The original specification of the Kerberos protocol in RFC 1510 uses GeneralString in numerous places for human-readable string data. Historical implementations of Kerberos cannot utilize the full power of GeneralString. This ASN.1 type requires the use of designation and invocation escape sequences as specified in ISO-2022/ECMA-35 [ISO-2022/ECMA-35] to switch character sets, and the default character set that is designated as G0 is the ISO-646/ECMA-6 [ISO-646/ECMA-6] International Reference Version (IRV) (a.k.a. U.S. ASCII), which mostly works.
In practice, many implementations treat GeneralStrings as if they were 8-bit strings of whichever character set the implementation defaults to, without regard to correct usage of character-set designation escape sequences. The default character set is often determined by the current user's operating system-dependent locale. At least one major implementation places unescaped UTF-8 encoded Unicode characters in the GeneralString. This failure to adhere to the GeneralString specifications results in interoperability issues when conflicting character encodings are utilized by the Kerberos clients, services, and KDC.
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This unfortunate situation is the result of improper documentation of the restrictions of the ASN.1 GeneralString type in prior Kerberos specifications.
KerberosString ::= GeneralString (IA5String)
For compatibility, implementations MAY choose to accept GeneralString values that contain characters other than those permitted by IA5String, but they should be aware that character set designation codes will likely be absent, and that the encoding should probably be treated as locale-specific in almost every way. Implementations MAY also choose to emit GeneralString values that are beyond those permitted by IA5String, but they should be aware that doing so is extraordinarily risky from an interoperability perspective.
Other implementations have been known to use GeneralString to contain a UTF-8 encoding. This also violates the ASN.1 standard, since UTF-8 is a different encoding, not a 94 or 96 character "G" set as defined by ISO 2022. It is believed that these implementations do not even use the ISO 2022 escape sequence to change the character encoding. Even if implementations were to announce the encoding change by using that escape sequence, the ASN.1 standard prohibits the use of any escape sequences other than those used to designate/invoke "G" or "C" sets allowed by GeneralString.
Note that applying a new constraint to a previously unconstrained type constitutes creation of a new ASN.1 type. In this particular case, the change does not result in a changed encoding under DER.
Realm ::= KerberosString
Kerberos realm names are encoded as KerberosStrings. Realms shall not contain a character with the code 0 (the US-ASCII NUL). Most realms will usually consist of several components separated by periods (.), in the style of Internet Domain Names, or separated by slashes (/), in the style of X.500 names. Acceptable forms for realm names are specified in Section 6.1. A PrincipalName is a typed sequence of components consisting of the following subfields:
name-string This field encodes a sequence of components that form a name, each component encoded as a KerberosString. Taken together, a PrincipalName and a Realm form a principal identifier. Most PrincipalNames will have only a few components (typically one or two).
KerberosTime ::= GeneralizedTime -- with no fractional seconds
5.2.4. Constrained Integer Types
UInt32 ::= INTEGER (0..4294967295) -- unsigned 32 bit values
Although this results in changes to the abstract types from the RFC 1510 version, the encoding in DER should be unaltered. Historical implementations were typically limited to 32-bit integer values anyway, and assigned numbers SHOULD fall in the space of integer values representable in 32 bits in order to promote interoperability anyway.
pvno also TKT-VNO or AUTHENTICATOR-VNO, this recurring field is always the constant integer 5. There is no easy way to make this field into a useful protocol version number, so its value is fixed.
5.2.5. HostAddress and HostAddresses
-- NOTE: HostAddresses is always used as an OPTIONAL field and -- should not be empty. HostAddresses -- NOTE: subtly different from rfc1510, -- but has a value mapping and encodes the same ::= SEQUENCE OF HostAddress
addr-type This field specifies the type of address that follows. Pre- defined values for this field are specified in Section 7.5.3.
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5.2.6. AuthorizationData
ad-data This field contains authorization data to be interpreted according to the value of the corresponding ad-type field.
Each sequence of type and data is referred to as an authorization element. Elements MAY be application specific; however, there is a common set of recursive elements that should be understood by all implementations. These elements contain other elements embedded within them, and the interpretation of the encapsulating element determines which of the embedded elements must be interpreted, and which may be ignored.
Authorization data elements are considered critical if present in a ticket or authenticator. If an unknown authorization data element type is received by a server either in an AP-REQ or in a ticket contained in an AP-REQ, then, unless it is encapsulated in a known authorization data element amending the criticality of the elements it contains, authentication MUST fail. Authorization data is intended to restrict the use of a ticket. If the service cannot determine whether the restriction applies to that service, then a
In the definitions that follow, the value of the ad-type for the element will be specified as the least significant part of the subsection number, and the value of the ad-data will be as shown in the ASN.1 structure that follows the subsection heading.
DER encoding of AD-IF-RELEVANT 1
DER encoding of AD-AND-OR 5
5.2.6.1. IF-RELEVANT
AD elements encapsulated within the if-relevant element are intended for interpretation only by application servers that understand the particular ad-type of the embedded element. Application servers that do not understand the type of an element embedded within the if-relevant element MAY ignore the uninterpretable element. This element promotes interoperability across implementations that may have local extensions for authorization. The ad-type for AD-IF-RELEVANT is (1).
AD-KDCIssued ::= SEQUENCE { ad-checksum [0] Checksum, i-realm [1] Realm OPTIONAL, i-sname [2] PrincipalName OPTIONAL, elements [3] AuthorizationData }
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i-realm, i-sname The name of the issuing principal if different from that of the KDC itself. This field would be used when the KDC can verify the authenticity of elements signed by the issuing principal, and it allows this KDC to notify the application server of the validity of those elements.
The KDC-issued ad-data field is intended to provide a means for Kerberos principal credentials to embed within themselves privilege attributes and other mechanisms for positive authorization, amplifying the privileges of the principal beyond what can be done using credentials without such an a-data element.
For KDC-issued elements, this is prevented because the elements are signed by the KDC by including a checksum encrypted using the server's key (the same key used to encrypt the ticket or a key derived from that key). Elements encapsulated with in the KDC-issued element MUST be ignored by the application server if this "signature" is not present. Further, elements encapsulated within this element from a TGT MAY be interpreted by the KDC, and used as a basis according to policy for including new signed elements within derivative tickets, but they will not be copied to a derivative ticket directly. If they are copied directly to a derivative ticket by a KDC that is not aware of this element, the signature will not be correct for the application ticket elements, and the field will be ignored by the application server.
The ad-type for AD-KDC-ISSUED is (4).
AD-AND-OR ::= SEQUENCE { condition-count [0] Int32, elements [1] AuthorizationData }
The ad-type for AD-AND-OR is (5).
AD-MANDATORY-FOR-KDC ::= AuthorizationData
The ad-type for AD-MANDATORY-FOR-KDC is (8).
Historically, PA-DATA have been known as "pre-authentication data", meaning that they were used to augment the initial authentication with the KDC. Since that time, they have also been used as a typed hole with which to extend protocol exchanges with the KDC.
padata-type Indicates the way that the padata-value element is to be interpreted. Negative values of padata-type are reserved for unregistered use; non-negative values are used for a registered interpretation of the element type.
1 pa-tgs-req DER encoding of AP-REQ
3 pa-pw-salt salt (not ASN.1 encoded)
19 pa-etype-info2 DER encoding of ETYPE-INFO2
The padata field can also contain information needed to help the KDC or the client select the key needed for generating or decrypting the response. This form of the padata is useful for supporting the use of certain token cards with Kerberos. The details of such extensions are specified in separate documents. See [Pat92] for additional uses of this field.
In the case of requests for additional tickets (KRB_TGS_REQ), padata-value will contain an encoded AP-REQ. The checksum in the authenticator (which MUST be collision-proof) is to be computed over the KDC-REQ-BODY encoding.
There are pre-authentication types that may be used to pre- authenticate a client by means of an encrypted timestamp.
PA-ENC-TS-ENC ::= SEQUENCE { patimestamp [0] KerberosTime -- client's time --, pausec [1] Microseconds OPTIONAL }
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This pre-authentication type was not present in RFC 1510, but many implementations support it.
The padata-value for this pre-authentication type contains the salt for the string-to-key to be used by the client to obtain the key for decrypting the encrypted part of an AS-REP message. Unfortunately, for historical reasons, the character set to be used is unspecified and probably locale-specific.
In the trivial example, a zero-length salt string is very commonplace for realms that have converted their principal databases from Kerberos Version 4.
5.2.7.4. PA-ETYPE-INFO
ETYPE-INFO-ENTRY ::= SEQUENCE { etype [0] Int32, salt [1] OCTET STRING OPTIONAL }
The salt, like that of PA-PW-SALT, is also completely unspecified with respect to character set and is probably locale-specific.
This pre-authentication type was not present in RFC 1510, but many implementations that support encrypted timestamps for pre- authentication need to support ETYPE-INFO as well. As noted in Section 3.1.3, a KDC MUST NOT send PA-ETYPE-INFO when the client's AS-REQ includes at least one "newer" etype.
The ETYPE-INFO2 pre-authentication type is sent by the KDC in a KRB-ERROR indicating a requirement for additional pre-authentication. It is usually used to notify a client of which key to use for the encryption of an encrypted timestamp for the purposes of sending a PA-ENC-TIMESTAMP pre-authentication value. It MAY also be sent in an AS-REP to provide information to the client about which key salt to use for the string-to-key to be used by the client to obtain the key for decrypting the encrypted part the AS-REP.
ETYPE-INFO2 ::= SEQUENCE SIZE (1..MAX) OF ETYPE-INFO2-ENTRY
The interpretation of s2kparams is specified in the cryptosystem description associated with the etype. Each cryptosystem has a default interpretation of s2kparams that will hold if that element is omitted from the encoding of ETYPE-INFO2-ENTRY.
The preferred ordering of the "hint" pre-authentication data that affect client key selection is: ETYPE-INFO2, followed by ETYPE-INFO, followed by PW-SALT. As noted in Section 3.1.3, a KDC MUST NOT send ETYPE-INFO or PW-SALT when the client's AS-REQ includes at least one "newer" etype.
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The ETYPE-INFO2 pre-authentication type was not present in RFC 1510.
For several message types, a specific constrained bit string type, KerberosFlags, is used.
Compatibility note: The following paragraphs describe a change from the RFC 1510 description of bit strings that would result in incompatility in the case of an implementation that strictly conformed to ASN.1 DER and RFC 1510.
For example, if the notation for KDCOptions were to include the "NamedBit" notation, as in RFC 1510, and a KDCOptions value to be encoded had only the "forwardable" (bit number one) bit set, the DER encoding MUST include only two bits: the first reserved bit ("reserved", bit number zero, value zero) and the one-valued bit (bit number one) for "forwardable".
Consequently, this document removes the "NamedBit" notations for individual bits, relegating them to comments. The size constraint on the KerberosFlags type requires that at least 32 bits be encoded at all times, though a lenient implementation MAY choose to accept fewer than 32 bits and to treat the missing bits as set to zero.
5.2.9. Cryptosystem-Related Types
EncryptedData ::= SEQUENCE { etype [0] Int32 -- EncryptionType --, kvno [1] UInt32 OPTIONAL, cipher [2] OCTET STRING -- ciphertext }
kvno This field contains the version number of the key under which data is encrypted. It is only present in messages encrypted under long lasting keys, such as principals' secret keys.
The EncryptionKey type is the means by which cryptographic keys used for encryption are transferred.
keyvalue This field contains the key itself, encoded as an octet string.
Checksum ::= SEQUENCE { cksumtype [0] Int32, checksum [1] OCTET STRING }
checksum This field contains the checksum itself, encoded as an octet string.
5.3. Tickets
Ticket ::= [APPLICATION 1] SEQUENCE { tkt-vno [0] INTEGER (5), realm [1] Realm, sname [2] PrincipalName, enc-part [3] EncryptedData -- EncTicketPart }
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EncTicketPart ::= [APPLICATION 3] SEQUENCE { flags [0] TicketFlags, key [1] EncryptionKey, crealm [2] Realm, cname [3] PrincipalName, transited [4] TransitedEncoding, authtime [5] KerberosTime, starttime [6] KerberosTime OPTIONAL, endtime [7] KerberosTime, renew-till [8] KerberosTime OPTIONAL, caddr [9] HostAddresses OPTIONAL, authorization-data [10] AuthorizationData OPTIONAL }
TicketFlags ::= KerberosFlags -- reserved(0), -- forwardable(1), -- forwarded(2), -- proxiable(3), -- proxy(4), -- may-postdate(5), -- postdated(6), -- invalid(7), -- renewable(8), -- initial(9), -- pre-authent(10), -- hw-authent(11), -- the following are new since 1510 -- transited-policy-checked(12), -- ok-as-delegate(13)
realm This field specifies the realm that issued a ticket. It also serves to identify the realm part of the server's principal identifier. Since a Kerberos server can only issue tickets for servers within its realm, the two will always be identical.
enc-part This field holds the encrypted encoding of the EncTicketPart sequence. It is encrypted in the key shared by Kerberos and the end server (the server's secret key), using a key usage value of 2.
Bit(s) Name Description
1 forwardable The FORWARDABLE flag is normally only interpreted by the TGS, and can be ignored by end servers. When set, this flag tells the ticket-granting server that it is OK to issue a new TGT with a different network address based on the presented ticket.
3 proxiable The PROXIABLE flag is normally only interpreted by the TGS, and can be ignored by end servers. The PROXIABLE flag has an interpretation identical to that of the FORWARDABLE flag, except that the PROXIABLE flag tells the ticket-granting server that only non-TGTs may be issued with different network addresses.
5 may-postdate The MAY-POSTDATE flag is normally only interpreted by the TGS, and can be ignored by end servers. This flag tells the
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ticket-granting server that a post-dated ticket MAY be issued based on this TGT.
7 invalid This flag indicates that a ticket is invalid, and it must be validated by the KDC before use. Application servers must reject tickets which have this flag set.
9 initial This flag indicates that this ticket was issued using the AS protocol, and not issued based on a TGT.
11 hw-authent This flag indicates that the protocol employed for initial authentication required the use of hardware expected to be possessed solely by the named client. The hardware authentication method is selected by the KDC and the strength of the method is not indicated.
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transited field, relying on the validation performed by the KDC. At its option the application server MAY still apply its own validation based on a separate policy for acceptance.
13 ok-as-delegate This flag indicates that the server (not the client) specified in the ticket has been determined by policy of the realm to be a suitable recipient of delegation. A client can use the presence of this flag to help it decide whether to delegate credentials (either grant a proxy or a forwarded TGT) to this server. The client is free to ignore the value of this flag. When setting this flag, an administrator should consider the security and placement of the server on which the service will run, as well as whether the service requires the use of delegated credentials.
14-31 reserved Reserved for future use.
crealm This field contains the name of the realm in which the client is registered and in which initial authentication took place.
transited This field lists the names of the Kerberos realms that took part in authenticating the user to whom this ticket was issued. It does not specify the order in which the realms were transited. See Section 3.3.3.2 for details on how this field encodes the traversed realms. When the names of CAs are to be embedded in the transited field (as specified for some extensions to the
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protocol), the X.500 names of the CAs SHOULD be mapped into items in the transited field using the mapping defined by RFC 2253.
starttime This field in the ticket specifies the time after which the ticket is valid. Together with endtime, this field specifies the life of the ticket. If the starttime field is absent from the ticket, then the authtime field SHOULD be used in its place to determine the life of the ticket.
renew-till This field is only present in tickets that have the RENEWABLE flag set in the flags field. It indicates the maximum endtime that may be included in a renewal. It can be thought of as the absolute expiration time for the ticket, including all renewals.
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deployment of network address translation, it is recommended that policy allow the issue and acceptance of such tickets.
Note that the network address from which a connection is received cannot be reliably determined. Even if it could be, an attacker who has compromised the client's workstation could use the credentials from there. Including the network addresses only makes it more difficult, not impossible, for an attacker to walk off with stolen credentials and then to use them from a "safe" location.
This field contains restrictions on any authority obtained on the basis of authentication using the ticket. It is possible for any principal in possession of credentials to add entries to the authorization data field since these entries further restrict what can be done with the ticket. Such additions can be made by specifying the additional entries when a new ticket is obtained during the TGS exchange, or they MAY be added during chained delegation using the authorization data field of the authenticator.
The data in this field may be specific to the end service; the field will contain the names of service specific objects, and the rights to those objects. The format for this field is described
By using the authorization_data field, a principal is able to issue a proxy that is valid for a specific purpose. For example, a client wishing to print a file can obtain a file server proxy to be passed to the print server. By specifying the name of the file in the authorization_data field, the file server knows that the print server can only use the client's rights when accessing the particular file to be printed.
Similarly, if one specifies the authorization-data field of a proxy and leaves the host addresses blank, the resulting ticket and session key can be treated as a capability. See [Neu93] for some suggested uses of this field.
5.4. Specifications for the AS and TGS Exchanges
5.4.1. KRB_KDC_REQ Definition
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The message fields are as follows:
TGS-REQ ::= [APPLICATION 12] KDC-REQ
KDC-REQ-BODY ::= SEQUENCE { kdc-options [0] KDCOptions, cname [1] PrincipalName OPTIONAL -- Used only in AS-REQ --, realm [2] Realm -- Server's realm -- Also client's in AS-REQ --, sname [3] PrincipalName OPTIONAL, from [4] KerberosTime OPTIONAL, till [5] KerberosTime, rtime [6] KerberosTime OPTIONAL, nonce [7] UInt32, etype [8] SEQUENCE OF Int32 -- EncryptionType -- in preference order --, addresses [9] HostAddresses OPTIONAL, enc-authorization-data [10] EncryptedData OPTIONAL -- AuthorizationData --, additional-tickets [11] SEQUENCE OF Ticket OPTIONAL -- NOTE: not empty }
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-- opt-hardware-auth(11), -- unused12(12), -- unused13(13), -- 15 is reserved for canonicalize -- unused15(15), -- 26 was unused in 1510 -- disable-transited-check(26), -- -- renewable-ok(27), -- enc-tkt-in-skey(28), -- renew(30), -- validate(31)
pvno This field is included in each message, and specifies the protocol version number. This document specifies protocol version 5.
padata Contains pre-authentication data. Requests for additional tickets (KRB_TGS_REQ) MUST contain a padata of PA-TGS-REQ.
req-body This field is a placeholder delimiting the extent of the remaining fields. If a checksum is to be calculated over the request, it is calculated over an encoding of the KDC-REQ-BODY sequence which is enclosed within the req-body field.
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acceptable simply to copy the options field to the flags field. There are various checks that must be made before an option is honored anyway.
Bits Name Description
1 FORWARDABLE The FORWARDABLE option indicates that the ticket to be issued is to have its forwardable flag set. It may only be set on the initial request, or in a subsequent request if the TGT on which it is based is also forwardable.
3 PROXIABLE The PROXIABLE option indicates that the ticket to be issued is to have its proxiable flag set. It may only be set on the initial request, or a subsequent request if the TGT on which it is based is also proxiable.
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host from which the resulting ticket is to be valid are included in the addresses field of the request.
6 POSTDATED The POSTDATED option indicates that this is a request for a postdated ticket. This option will only be honored if the TGT on which it is based has its MAY-POSTDATE flag set. The resulting ticket will also have its INVALID flag set, and that flag may be reset by a subsequent request to the KDC after the starttime in the ticket has been reached.
8 RENEWABLE The RENEWABLE option indicates that the ticket to be issued is to have its RENEWABLE flag set. It may only be set on the initial request, or when the TGT on which the request is based is also renewable. If this option is requested, then the rtime field in the request contains the desired absolute expiration time for the ticket.
10 RESERVED Reserved for future use.
12-25 RESERVED Reserved for future use.
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on the TGT. If this flag is set in the request, checking of the transited field is disabled. Tickets issued without the performance of this check will be noted by the reset (0) value of the TRANSITED-POLICY-CHECKED flag, indicating to the application server that the transited field must be checked locally. KDCs are encouraged but not required to honor the DISABLE-TRANSITED-CHECK option.
27 RENEWABLE-OK The RENEWABLE-OK option indicates that a renewable ticket will be acceptable if a ticket with the requested life cannot otherwise be provided, in which case a renewable ticket may be issued with a renew- till equal to the requested endtime. The value of the renew-till field may still be limited by local limits, or limits selected by the individual principal or server.
29 RESERVED Reserved for future use.
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field as part of the authentication header.
cname and sname These fields are the same as those described for the ticket in section 5.3. The sname may only be absent when the ENC-TKT-IN- SKEY option is specified. If the sname is absent, the name of the server is taken from the name of the client in the ticket passed as additional-tickets.
realm This field specifies the realm part of the server's principal identifier. In the AS exchange, this is also the realm part of the client's principal identifier.
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till This field contains the expiration date requested by the client in a ticket request. It is not optional, but if the requested endtime is "19700101000000Z", the requested ticket is to have the maximum endtime permitted according to KDC policy. Implementation note: This special timestamp corresponds to a UNIX time_t value of zero on most systems.
nonce This field is part of the KDC request and response. It is intended to hold a random number generated by the client. If the same number is included in the encrypted response from the KDC, it provides evidence that the response is fresh and has not been replayed by an attacker. Nonces MUST NEVER be reused.
addresses This field is included in the initial request for tickets, and it is optionally included in requests for additional tickets from the ticket-granting server. It specifies the addresses from which the requested ticket is to be valid. Normally it includes the addresses for the client's host. If a proxy is requested, this field will contain other addresses. The contents of this field are usually copied by the KDC into the caddr field of the resulting ticket.
The optional fields (addresses, authorization-data, and additional- tickets) are only included if necessary to perform the operation specified in the kdc-options field.
5.4.2. KRB_KDC_REP Definition
The KRB_KDC_REP message contains the following fields:
TGS-REP ::= [APPLICATION 13] KDC-REP
EncASRepPart ::= [APPLICATION 25] EncKDCRepPart
EncKDCRepPart ::= SEQUENCE { key [0] EncryptionKey, last-req [1] LastReq, nonce [2] UInt32, key-expiration [3] KerberosTime OPTIONAL, flags [4] TicketFlags, authtime [5] KerberosTime, starttime [6] KerberosTime OPTIONAL, endtime [7] KerberosTime, renew-till [8] KerberosTime OPTIONAL, srealm [9] Realm, sname [10] PrincipalName, caddr [11] HostAddresses OPTIONAL }
pvno and msg-type These fields are described above in Section 5.4.1. msg-type is either KRB_AS_REP or KRB_TGS_REP.
crealm, cname, srealm, and sname These fields are the same as those described for the ticket in section 5.3.
enc-part This field is a place holder for the ciphertext and related information that forms the encrypted part of a message. The description of the encrypted part of the message follows each appearance of this field.
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The key usage value for encrypting this field is 3 in an AS-REP message, using the client's long-term key or another key selected via pre-authentication mechanisms. In a TGS-REP message, the key usage value is 8 if the TGS session key is used, or 9 if a TGS authenticator subkey is used.
key This field is the same as described for the ticket in Section 5.3.
lr-type This field indicates how the following lr-value field is to be interpreted. Negative values indicate that the information pertains only to the responding server. Non-negative values pertain to all servers for the realm.
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lr-value This field contains the time of the last request. The time MUST be interpreted according to the contents of the accompanying lr- type subfield.
key-expiration The key-expiration field is part of the response from the KDC and specifies the time that the client's secret key is due to expire. The expiration might be the result of password aging or an account expiration. If present, it SHOULD be set to the earlier of the user's key expiration and account expiration. The use of this field is deprecated, and the last-req field SHOULD be used to convey this information instead. This field will usually be left out of the TGS reply since the response to the TGS request is encrypted in a session key and no client information has to be retrieved from the KDC database. It is up to the application client (usually the login program) to take appropriate action (such as notifying the user) if the expiration time is imminent.
5.5. Client/Server (CS) Message Specifications
5.5.1. KRB_AP_REQ Definition
APOptions ::= KerberosFlags -- reserved(0), -- use-session-key(1), -- mutual-required(2)
ap-options This field appears in the application request (KRB_AP_REQ) and affects the way the request is processed. It is a bit-field, where the selected options are indicated by the bit being set (1), and the unselected options and reserved fields by being reset (0). The encoding of the bits is specified in Section 5.2. The meanings of the options are as follows:
0 reserved Reserved for future expansion of this field.
2 mutual-required The MUTUAL-REQUIRED option tells the server that the client requires mutual authentication, and that it must respond with a KRB_AP_REP message.
ticket This field is a ticket authenticating the client to the server.
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authenticator This contains the encrypted authenticator, which includes the client's choice of a subkey.
-- Unencrypted authenticator Authenticator ::= [APPLICATION 2] SEQUENCE { authenticator-vno [0] INTEGER (5), crealm [1] Realm, cname [2] PrincipalName, cksum [3] Checksum OPTIONAL, cusec [4] Microseconds, ctime [5] KerberosTime, subkey [6] EncryptionKey OPTIONAL, seq-number [7] UInt32 OPTIONAL, authorization-data [8] AuthorizationData OPTIONAL }
crealm and cname These fields are the same as those described for the ticket in section 5.3.
cusec This field contains the microsecond part of the client's timestamp. Its value (before encryption) ranges from 0 to 999999. It often appears along with ctime. The two fields are used together to specify a reasonably accurate timestamp.
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subkey This field contains the client's choice for an encryption key to be used to protect this specific application session. Unless an application specifies otherwise, if this field is left out, the session key from the ticket will be used.
For sequence numbers to support the detection of replays adequately, they SHOULD be non-repeating, even across connection boundaries. The initial sequence number SHOULD be random and uniformly distributed across the full space of possible sequence numbers, so that it cannot be guessed by an attacker and so that it and the successive sequence numbers do not repeat other sequences. In the event that more than 2^32 messages are to be generated in a series of KRB_PRIV or KRB_SAFE messages, rekeying SHOULD be performed before sequence numbers are reused with the same encryption key.
Implementation note: As noted before, some implementations omit the optional sequence number when its value would be zero. Implementations MAY accept an omitted sequence number when expecting a value of zero, and SHOULD NOT transmit an Authenticator with a initial sequence number of zero.
The KRB_AP_REP message contains the Kerberos protocol version number, the message type, and an encrypted time-stamp. The message is sent in response to an application request (KRB_AP_REQ) for which the mutual authentication option has been selected in the ap-options field.
EncAPRepPart ::= [APPLICATION 27] SEQUENCE { ctime [0] KerberosTime, cusec [1] Microseconds, subkey [2] EncryptionKey OPTIONAL, seq-number [3] UInt32 OPTIONAL }
pvno and msg-type These fields are described above in Section 5.4.1. msg-type is KRB_AP_REP.
ctime This field contains the current time on the client's host.
subkey This field contains an encryption key that is to be used to protect this specific application session. See Section 3.2.6 for specifics on how this field is used to negotiate a key. Unless an application specifies otherwise, if this field is left out, the sub-session key from the authenticator or if the latter is also left out, the session key from the ticket will be used.
5.5.3. Error Message Reply
5.6. KRB_SAFE Message Specification
5.6.1. KRB_SAFE definition
KRB-SAFE ::= [APPLICATION 20] SEQUENCE { pvno [0] INTEGER (5), msg-type [1] INTEGER (20), safe-body [2] KRB-SAFE-BODY, cksum [3] Checksum }
pvno and msg-type These fields are described above in Section 5.4.1. msg-type is KRB_SAFE.
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safe-body This field is a placeholder for the body of the KRB-SAFE message.
The checksum is computed over the encoding of the KRB-SAFE sequence. First, the cksum is set to a type zero, zero-length value, and the checksum is computed over the encoding of the KRB- SAFE sequence. Then the checksum is set to the result of that computation. Finally, the KRB-SAFE sequence is encoded again. This method, although different than the one specified in RFC 1510, corresponds to existing practice.
timestamp This field is part of the KRB_SAFE and KRB_PRIV messages. Its contents are the current time as known by the sender of the message. By checking the timestamp, the recipient of the message is able to make sure that it was recently generated, and is not a replay.
seq-number This field is described above in Section 5.3.2.
This field specifies the address in use by the sender of the message.
This section specifies the format of a message that can be used by either side (client or server) of an application to send a message to its peer securely and privately. It presumes that a session key has previously been exchanged (for example, by using the KRB_AP_REQ/KRB_AP_REP messages).
The KRB_PRIV message contains user data encrypted in the Session Key. The message fields are as follows:
EncKrbPrivPart ::= [APPLICATION 28] SEQUENCE { user-data [0] OCTET STRING, timestamp [1] KerberosTime OPTIONAL, usec [2] Microseconds OPTIONAL, seq-number [3] UInt32 OPTIONAL, s-address [4] HostAddress -- sender's addr --, r-address [5] HostAddress OPTIONAL -- recip's addr }
enc-part This field holds an encoding of the EncKrbPrivPart sequence encrypted under the session key, with a key usage value of 13. This encrypted encoding is used for the enc-part field of the KRB-PRIV message.
seq-number This field is described above in Section 5.3.2.
This section specifies the format of a message that can be used to send Kerberos credentials from one principal to another. It is presented here to encourage a common mechanism to be used by applications when forwarding tickets or providing proxies to subordinate servers. It presumes that a session key has already been exchanged, perhaps by using the KRB_AP_REQ/KRB_AP_REP messages.
The KRB_CRED message contains a sequence of tickets to be sent and information needed to use the tickets, including the session key from each. The information needed to use the tickets is encrypted under an encryption key previously exchanged or transferred alongside the KRB_CRED message. The message fields are as follows:
EncKrbCredPart ::= [APPLICATION 29] SEQUENCE { ticket-info [0] SEQUENCE OF KrbCredInfo, nonce [1] UInt32 OPTIONAL, timestamp [2] KerberosTime OPTIONAL, usec [3] Microseconds OPTIONAL, s-address [4] HostAddress OPTIONAL, r-address [5] HostAddress OPTIONAL }
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pvno and msg-type These fields are described above in Section 5.4.1. msg-type is KRB_CRED.
enc-part This field holds an encoding of the EncKrbCredPart sequence encrypted under the session key shared by the sender and the intended recipient, with a key usage value of 14. This encrypted encoding is used for the enc-part field of the KRB-CRED message.
nonce If practical, an application MAY require the inclusion of a nonce generated by the recipient of the message. If the same value is included as the nonce in the message, it provides evidence that the message is fresh and has not been replayed by an attacker. A nonce MUST NEVER be reused.
s-address and r-address These fields are described above in Section 5.6.1. They are used optionally to provide additional assurance of the integrity of the KRB-CRED message.
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The following fields are optional. If present, they can be associated with the credentials in the remote ticket file. If left out, then it is assumed that the recipient of the credentials already knows their values.
flags, authtime, starttime, endtime, renew-till, srealm, sname, and caddr These fields contain the values of the corresponding fields from the ticket found in the ticket field. Descriptions of the fields are identical to the descriptions in the KDC-REP message.
This section specifies the format for the KRB_ERROR message. The fields included in the message are intended to return as much information as possible about an error. It is not expected that all the information required by the fields will be available for all types of errors. If the appropriate information is not available when the message is composed, the corresponding field will be left out of the message.
5.9.1. KRB_ERROR Definition
KRB-ERROR ::= [APPLICATION 30] SEQUENCE { pvno [0] INTEGER (5), msg-type [1] INTEGER (30), ctime [2] KerberosTime OPTIONAL, cusec [3] Microseconds OPTIONAL, stime [4] KerberosTime, susec [5] Microseconds, error-code [6] Int32, crealm [7] Realm OPTIONAL, cname [8] PrincipalName OPTIONAL, realm [9] Realm -- service realm --, sname [10] PrincipalName -- service name --, e-text [11] KerberosString OPTIONAL,
pvno and msg-type These fields are described above in Section 5.4.1. msg-type is KRB_ERROR.
stime This field contains the current time on the server. It is of type KerberosTime.
error-code This field contains the error code returned by Kerberos or the server when a request fails. To interpret the value of this field see the list of error codes in Section 7.5.9. Implementations are encouraged to provide for national language support in the display of error messages.
realm and sname These fields are described above in Section 5.3.
METHOD-DATA ::= SEQUENCE OF PA-DATA
TYPED-DATA ::= SEQUENCE SIZE (1..MAX) OF SEQUENCE { data-type [0] Int32, data-value [1] OCTET STRING OPTIONAL }
The following table lists the application class tag numbers used by various data types defined in this section.
0 unused
2 Authenticator non-PDU
4-9 unused
11 AS-REP PDU
13 TGS-REP PDU
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14 AP-REQ PDU
16 RESERVED16 TGT-REQ (for user-to-user)
18-19 unused
21 KRB-PRIV PDU
23-24 unused
26 EncTGSRepPart non-PDU
28 EncKrbPrivPart non-PDU
30 KRB-ERROR PDU
6. Naming Constraints
Although realm names are encoded as GeneralStrings and technically a realm can select any name it chooses, interoperability across realm boundaries requires agreement on how realm names are to be assigned, and what information they imply.
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Kerberos realm names are case sensitive. Realm names that differ only in the case of the characters are not equivalent. There are presently three styles of realm names: domain, X500, and other. Examples of each style follow:
Domain style realm names MUST look like domain names: they consist of components separated by periods (.) and they contain neither colons (:) nor slashes (/). Though domain names themselves are case insensitive, in order for realms to match, the case must match as well. When establishing a new realm name based on an internet domain name it is recommended by convention that the characters be converted to uppercase.
Names that fall into the other category MUST begin with a prefix that contains no equals sign (=) or period (.), and the prefix MUST be followed by a colon (:) and the rest of the name. All prefixes expect those beginning with used. Presently none are assigned.
These rules guarantee that there will be no conflicts between the various name styles. The following additional constraints apply to the assignment of realm names in the domain and X.500 categories: either the name of a realm for the domain or X.500 formats must be used by the organization owning (to whom it was assigned) an Internet domain name or X.500 name, or, in the case that no such names are registered, authority to use a realm name MAY be derived from the authority of the parent realm. For example, if there is no domain name for E40.MIT.EDU, then the administrator of the MIT.EDU realm can authorize the creation of a realm with that name.
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its children in the X.500 and domain name systems as well. If the parent assigns a realm name without also registering it in the domain name or X.500 hierarchy, it is the parent's responsibility to make sure that in the future there will not exist a name identical to the realm name of the child unless it is assigned to the same entity as the realm name.
As was the case for realm names, conventions are needed to ensure that all agree on what information is implied by a principal name. The name-type field that is part of the principal name indicates the kind of information implied by the name. The name-type SHOULD be treated only as a hint to interpreting the meaning of a name. It is not significant when checking for equivalence. Principal names that differ only in the name-type identify the same principal. The name type does not partition the name space. Ignoring the name type, no two names can be the same (i.e., at least one of the components, or the realm, MUST be different). The following name types are defined:
NT-UNKNOWN 0 Name type not known NT-PRINCIPAL 1 Just the name of the principal as in DCE, or for users NT-SRV-INST 2 Service and other unique instance (krbtgt) NT-SRV-HST 3 Service with host name as instance (telnet, rcommands) NT-SRV-XHST 4 Service with host as remaining components NT-UID 5 Unique ID NT-X500-PRINCIPAL 6 Encoded X.509 Distinguished name [RFC2253] NT-SMTP-NAME 7 Name in form of SMTP email name (e.g., user@example.com) NT-ENTERPRISE 10 Enterprise name - may be mapped to principal name
If the first component of a name identifies a service and the remaining components identify an instance of the service in a server-specified manner, then the name type of SRV-INST SHOULD be
If the first component of a name identifies a service and there is a single component following the service name identifying the instance as the host on which the server is running, then the name type SRV-HST SHOULD be used. This type is typically used for Internet services such as telnet and the Berkeley R commands. If the separate components of the host name appear as successive components following the name of the service, then the name type SRV-XHST SHOULD be used. This type might be used to identify servers on hosts with X.500 names, where the slash (/) might otherwise be ambiguous.
A name type of SMTP allows a name to be of a form that resembles an SMTP email name. This name, including an "@" and a domain name, is used as the one component of the principal name.
Names of any type with an initial component of 'krbtgt' are reserved for the Kerberos ticket-granting service. See Section 7.3 for the form of such names.
The principal identifier for a server on a host will generally be composed of two parts: (1) the realm of the KDC with which the server is registered, and (2) a two-component name of type NT-SRV-HST, if the host name is an Internet domain name, or a multi-component name of type NT-SRV-XHST, if the name of the host is of a form (such as X.500) that allows slash (/) separators. The first component of the two- or multi-component name will identify the service, and the latter components will identify the host. Where the name of the host is not case sensitive (for example, with Internet domain names) the name of the host MUST be lowercase. If specified by the application protocol for services such as telnet and the Berkeley R commands that run with system privileges, the first component MAY be the string 'host' instead of a service-specific identifier.
7.1. Host Address Types
Internet (IPv4) Addresses
Internet (IPv6) Addresses
* the Unspecified Address * the Loopback Address * Link-Local addresses
IPv4-mapped IPv6 addresses MUST be represented as addresses of type 2.
DECnet Phase IV addresses are 16-bit addresses, encoded in LSB order. The type of DECnet Phase IV addresses is twelve (12).
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Netbios Addresses
Directional Addresses
Directional addresses MUST only be used for the sender address field in the KRB_SAFE or KRB_PRIV messages. They MUST NOT be used as a ticket address or in a KRB_AP_REQ message. This address type SHOULD only be used in situations where the sending party knows that the receiving party supports the address type. This generally means that directional addresses may only be used when the application protocol requires their support. Directional addresses are type (3).
Kerberos defines two IP transport mechanisms for communication between clients and servers: UDP/IP and TCP/IP.
Kerberos servers (KDCs) supporting IP transports MUST accept UDP requests and SHOULD listen for them on port 88 (decimal) unless specifically configured to listen on an alternative UDP port. Alternate ports MAY be used when running multiple KDCs for multiple realms on the same host.
When contacting a KDC for a KRB_KDC_REQ request using UDP/IP transport, the client shall send a UDP datagram containing only an encoding of the request to the KDC. The KDC will respond with a reply datagram containing only an encoding of the reply message (either a KRB_ERROR or a KRB_KDC_REP) to the sending port at the sender's IP address. The response to a request made through UDP/IP transport MUST also use UDP/IP transport. If the response cannot be handled using UDP (for example, because it is too large), the KDC MUST return KRB_ERR_RESPONSE_TOO_BIG, forcing the client to retry the request using the TCP transport.
Kerberos servers (KDCs) supporting IP transports MUST accept TCP requests and SHOULD listen for them on port 88 (decimal) unless specifically configured to listen on an alternate TCP port. Alternate ports MAY be used when running multiple KDCs for multiple realms on the same host.
Implementation note: Some extensions to the Kerberos protocol will not succeed if any client or KDC not supporting the TCP transport is involved. Implementations of RFC 1510 were not required to support TCP/IP transports.
The client MUST be prepared to have the stream closed by the KDC at any time after the receipt of a response. A stream closure SHOULD NOT be treated as a fatal error. Instead, if multiple exchanges are required (e.g., certain forms of pre-authentication), the client may need to establish a new connection when it is ready to send
A client MAY send multiple requests before receiving responses, though it must be prepared to handle the connection being closed after the first response.
If multiple requests are sent over a single TCP connection and the KDC sends multiple responses, the KDC is not required to send the responses in the order of the corresponding requests. This may permit some implementations to send each response as soon as it is ready, even if earlier requests are still being processed (for example, waiting for a response from an external device or database).
Kerberos client implementations MUST provide a means for the client to determine the location of the Kerberos Key Distribution Centers (KDCs). Traditionally, Kerberos implementations have stored such configuration information in a file on each client machine. Experience has shown that this method of storing configuration information presents problems with out-of-date information and scaling, especially when using cross-realm authentication. This section describes a method for using the Domain Name System [RFC1035] for storing KDC location information.
In Kerberos, realm names are case sensitive. Although it is strongly encouraged that all realm names be all uppercase, this recommendation has not been adopted by all sites. Some sites use all lowercase names and other use mixed case. DNS, on the other hand, is case insensitive for queries. Because the realm names "MYREALM", "myrealm", and "MyRealm" are all different, but resolve the same in the domain name system, it is necessary that only one of the possible combinations of upper- and lowercase characters be used in realm names.
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7.2.3.2. Specifying KDC Location Information with DNS SRV records
_Service._Proto.Realm TTL Class SRV Priority Weight Port Target
The Proto can be either "udp" or "tcp". If these SRV records are to be used, both "udp" and "tcp" records MUST be specified for all KDC deployments.
TTL, Class, SRV, Priority, Weight, and Target have the standard meaning as defined in RFC 2782.
Implementation note: Many existing client implementations do not support KDC Discovery and are configured to send requests to the IANA assigned port (88 decimal), so it is strongly recommended that KDCs be configured to listen on that port.
These are DNS records for a Kerberos realm EXAMPLE.COM. It has two Kerberos servers, kdc1.example.com and kdc2.example.com. Queries should be directed to kdc1.example.com first as per the specified priority. Weights are not used in these sample records.
7.3. Name of the TGS
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get tickets from the ATHENA.MIT.EDU KDC has a principal identifier of "ATHENA.MIT.EDU" (realm), ("krbtgt", "ATHENA.MIT.EDU") (name). A TGT issued by the ATHENA.MIT.EDU realm to be used to get tickets from the MIT.EDU realm has a principal identifier of "ATHENA.MIT.EDU" (realm), ("krbtgt", "MIT.EDU") (name).
This OID MAY be used to identify Kerberos protocol messages encapsulated in other protocols. It also designates the OID arc for KerberosV5-related OIDs assigned by future IETF action. Implementation note: RFC 1510 had an incorrect value (5) for "dod" in its OID.
Assignment of OIDs beneath the id-krb5 arc must be obtained by contacting the registrar for the id-krb5 arc, or its designee. At the time of the issuance of this RFC, such registrations can be obtained by contacting krb5-oid-registrar@mit.edu.
The following tables list constants used in the protocol and define their meanings. In the "specification" section, ranges are specified that limit the values of constants for which values are defined here. This allows implementations to make assumptions about the maximum values that will be received for these constants. Implementations receiving values outside the range specified in the "specification" section MAY reject the request, but they MUST recover cleanly.
The encryption and checksum specifications in [RFC3961] require as input a "key usage number", to alter the encryption key used in any specific message in order to make certain types of cryptographic attack more difficult. These are the key usage values assigned in this document:
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2. AS-REP Ticket and TGS-REP Ticket (includes TGS session key or application session key), encrypted with the service key (Section 5.3) 3. AS-REP encrypted part (includes TGS session key or application session key), encrypted with the client key (Section 5.4.2) 4. TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with the TGS session key (Section 5.4.1) 5. TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with the TGS authenticator subkey (Section 5.4.1) 6. TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator cksum, keyed with the TGS session key (Section 5.5.1) 7. TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator (includes TGS authenticator subkey), encrypted with the TGS session key (Section 5.5.1) 8. TGS-REP encrypted part (includes application session key), encrypted with the TGS session key (Section 5.4.2) 9. TGS-REP encrypted part (includes application session key), encrypted with the TGS authenticator subkey (Section 5.4.2) 10. AP-REQ Authenticator cksum, keyed with the application session key (Section 5.5.1) 11. AP-REQ Authenticator (includes application authenticator subkey), encrypted with the application session key (Section 5.5.1) 12. AP-REP encrypted part (includes application session subkey), encrypted with the application session key (Section 5.5.2) 13. KRB-PRIV encrypted part, encrypted with a key chosen by the application (Section 5.7.1) 14. KRB-CRED encrypted part, encrypted with a key chosen by the application (Section 5.8.1) 15. KRB-SAFE cksum, keyed with a key chosen by the application (Section 5.6.1) 16-18. Reserved for future use in Kerberos and related protocols. 19. AD-KDC-ISSUED checksum (ad-checksum in 5.2.6.4) 20-21. Reserved for future use in Kerberos and related protocols. 22-25. Reserved for use in the Kerberos Version 5 GSS-API mechanisms [RFC4121]. 26-511. Reserved for future use in Kerberos and related protocols. 512-1023. Reserved for uses internal to a Kerberos implementation. 1024. Encryption for application use in protocols that do not specify key usage values
7.5.2. PreAuthentication Data Types
PA-TGS-REQ 1 PA-ENC-TIMESTAMP 2 PA-PW-SALT 3 [reserved] 4 PA-ENC-UNIX-TIME 5 (deprecated) PA-SANDIA-SECUREID 6 PA-SESAME 7 PA-OSF-DCE 8 PA-CYBERSAFE-SECUREID 9 PA-AFS3-SALT 10 PA-ETYPE-INFO 11 PA-SAM-CHALLENGE 12 (sam/otp) PA-SAM-RESPONSE 13 (sam/otp) PA-PK-AS-REQ_OLD 14 (pkinit) PA-PK-AS-REP_OLD 15 (pkinit) PA-PK-AS-REQ 16 (pkinit) PA-PK-AS-REP 17 (pkinit) PA-ETYPE-INFO2 19 (replaces pa-etype-info) PA-USE-SPECIFIED-KVNO 20 PA-SAM-REDIRECT 21 (sam/otp) PA-GET-FROM-TYPED-DATA 22 (embedded in typed data) TD-PADATA 22 (embeds padata) PA-SAM-ETYPE-INFO 23 (sam/otp) PA-ALT-PRINC 24 (crawdad@fnal.gov) PA-SAM-CHALLENGE2 30 (kenh@pobox.com) PA-SAM-RESPONSE2 31 (kenh@pobox.com) PA-EXTRA-TGT 41 Reserved extra TGT TD-PKINIT-CMS-CERTIFICATES 101 CertificateSet from CMS TD-KRB-PRINCIPAL 102 PrincipalName TD-KRB-REALM 103 Realm TD-TRUSTED-CERTIFIERS 104 from PKINIT TD-CERTIFICATE-INDEX 105 from PKINIT TD-APP-DEFINED-ERROR 106 application specific TD-REQ-NONCE 107 INTEGER TD-REQ-SEQ 108 INTEGER PA-PAC-REQUEST 128 (jbrezak@exchange.microsoft.com)
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7.5.3. Address Types
IPv4 2 Directional 3 ChaosNet 5 XNS 6 ISO 7 DECNET Phase IV 12 AppleTalk DDP 16 NetBios 20 IPv6 24
Authorization Data Type Ad-type Value
7.5.5. Transited Encoding Types
DOMAIN-X500-COMPRESS 1 Reserved values All others
Label Value Meaning or MIT Code
Message Type Value Meaning
7.5.8. Name Types
KRB_NT_UNKNOWN 0 Name type not known KRB_NT_PRINCIPAL 1 Just the name of the principal as in DCE, or for users KRB_NT_SRV_INST 2 Service and other unique instance (krbtgt) KRB_NT_SRV_HST 3 Service with host name as instance (telnet, rcommands) KRB_NT_SRV_XHST 4 Service with host as remaining components KRB_NT_UID 5 Unique ID KRB_NT_X500_PRINCIPAL 6 Encoded X.509 Distinguished name [RFC2253] KRB_NT_SMTP_NAME 7 Name in form of SMTP email name (e.g., user@example.com) KRB_NT_ENTERPRISE 10 Enterprise name; may be mapped to principal name
Error Code Value Meaning
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KDC_ERR_SVC_UNAVAILABLE 29 A service is not available KRB_AP_ERR_BAD_INTEGRITY 31 Integrity check on decrypted field failed KRB_AP_ERR_TKT_EXPIRED 32 Ticket expired KRB_AP_ERR_TKT_NYV 33 Ticket not yet valid KRB_AP_ERR_REPEAT 34 Request is a replay KRB_AP_ERR_NOT_US 35 The ticket isn't for us KRB_AP_ERR_BADMATCH 36 Ticket and authenticator don't match KRB_AP_ERR_SKEW 37 Clock skew too great KRB_AP_ERR_BADADDR 38 Incorrect net address KRB_AP_ERR_BADVERSION 39 Protocol version mismatch KRB_AP_ERR_MSG_TYPE 40 Invalid msg type KRB_AP_ERR_MODIFIED 41 Message stream modified KRB_AP_ERR_BADORDER 42 Message out of order KRB_AP_ERR_BADKEYVER 44 Specified version of key is not available KRB_AP_ERR_NOKEY 45 Service key not available KRB_AP_ERR_MUT_FAIL 46 Mutual authentication failed KRB_AP_ERR_BADDIRECTION 47 Incorrect message direction KRB_AP_ERR_METHOD 48 Alternative authentication method required KRB_AP_ERR_BADSEQ 49 Incorrect sequence number in message KRB_AP_ERR_INAPP_CKSUM 50 Inappropriate type of checksum in message KRB_AP_PATH_NOT_ACCEPTED 51 Policy rejects transited path KRB_ERR_RESPONSE_TOO_BIG 52 Response too big for UDP; retry with TCP KRB_ERR_GENERIC 60 Generic error (description in e-text) KRB_ERR_FIELD_TOOLONG 61 Field is too long for this implementation KDC_ERROR_CLIENT_NOT_TRUSTED 62 Reserved for PKINIT KDC_ERROR_KDC_NOT_TRUSTED 63 Reserved for PKINIT KDC_ERROR_INVALID_SIG 64 Reserved for PKINIT KDC_ERR_KEY_TOO_WEAK 65 Reserved for PKINIT KDC_ERR_CERTIFICATE_MISMATCH 66 Reserved for PKINIT KRB_AP_ERR_NO_TGT 67 No TGT available to validate USER-TO-USER KDC_ERR_WRONG_REALM 68 Reserved for future use KRB_AP_ERR_USER_TO_USER_REQUIRED 69 Ticket must be for USER-TO-USER KDC_ERR_CANT_VERIFY_CERTIFICATE 70 Reserved for PKINIT KDC_ERR_INVALID_CERTIFICATE 71 Reserved for PKINIT KDC_ERR_REVOKED_CERTIFICATE 72 Reserved for PKINIT
8. Interoperability Requirements
In order to ensure the interoperability of realms, it is necessary to define a minimal configuration that must be supported by all implementations. This minimal configuration is subject to change as technology does. For example, if at some later date it is discovered that one of the required encryption or checksum algorithms is not secure, it will be replaced.
This section defines the second specification of these options. Implementations which are configured in this way can be said to support Kerberos Version 5 Specification 2 (5.2). Specification 1 (deprecated) may be found in RFC 1510.
TCP/IP and UDP/IP transport MUST be supported by clients and KDCs claiming conformance to specification 2.
The following encryption and checksum mechanisms MUST be supported:
Implementations SHOULD support other mechanisms as well, but the additional mechanisms may only be used when communicating with principals known to also support them. The following mechanisms from [RFC3961] and [RFC3962] SHOULD be supported:
Implementations MAY support other mechanisms as well, but the additional mechanisms may only be used when communicating with principals known to support them also.
Realm Names
Transited Field Encoding
Pre-authentication Methods
The ETYPE-INFO2 method MUST be supported; this method is used to communicate the set of supported encryption types, and corresponding salt and string to key parameters. The ETYPE-INFO method SHOULD be supported for interoperability with older implementation.
Mutual authentication (via the KRB_AP_REP message) MUST be supported.
All KDCs MUST pass through tickets that carry no addresses (i.e., if a TGT contains no addresses, the KDC will return derivative tickets). Implementations SHOULD default to requesting addressless tickets, as this significantly increases interoperability with network address translation. In some cases, realms or application servers MAY require that tickets have an address.
Proxies and forwarded tickets MUST be supported. Individual realms and application servers can set their own policy on when such tickets will be accepted.
User-to-User Authentication
Authorization Data
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Implementations MUST make the contents of any authorization data subfields available to the server when a ticket is used. Implementations are not required to allow clients to specify the contents of the authorization data fields.
All protocol constants are constrained to 32-bit (signed) values unless further constrained by the protocol definition. This limit is provided to allow implementations to make assumptions about the maximum values that will be received for these constants. Implementations receiving values outside this range MAY reject the request, but they MUST recover cleanly.
Following is a list of recommended values for a KDC configuration.
9. IANA Considerations
"reserved" realm name types in Section 6.1 and "other" realm types except those beginning with "X-" or "x-" will not be registered without IETF standards action, at which point guidelines for further assignment will be specified. Realm name types beginning with "X-" or "x-" are for private use.
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Additional key usage numbers, as defined in Section 7.5.1, will be assigned subject to review by the Kerberos working group or other expert review.
Additional authorization data types as defined in Section 7.5.4, will be assigned subject to review by the Kerberos working group or other expert review. Although it is anticipated that there may be significant demand for private use types, provision is intentionally not made for a private use portion of the namespace because conflicts between privately assigned values could have detrimental security implications.
Additional Kerberos message types, as described in Section 7.5.7, will be assigned subject to review by the Kerberos working group or other expert review.
Additional error codes described in Section 7.5.9 will be assigned subject to review by the Kerberos working group or other expert review.
As an authentication service, Kerberos provides a means of verifying the identity of principals on a network. By itself, Kerberos does not provide authorization. Applications should not accept the issuance of a service ticket by the Kerberos server as granting authority to use the service, since such applications may become vulnerable to the bypass of this authorization check in an environment where they inter-operate with other KDCs or where other options for application authentication are provided.
Interoperability conflicts caused by incompatible character-set usage (see 5.2.1) can result in denial of service for clients that utilize character-sets in Kerberos strings other than those stored in the KDC database.
Principals must keep their secret keys secret. If an intruder somehow steals a principal's key, it will be able to masquerade as that principal or impersonate any server to the legitimate principal.
Unless pre-authentication options are required by the policy of a realm, the KDC will not know whether a request for authentication succeeds. An attacker can request a reply with credentials for any principal. These credentials will likely not be of much use to the attacker unless it knows the client's secret key, but the availability of the response encrypted in the client's secret key provides the attacker with ciphertext that may be used to mount brute force or dictionary attacks to decrypt the credentials, by guessing the user's password. For this reason it is strongly encouraged that Kerberos realms require the use of pre-authentication. Even with
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pre-authentication, attackers may try brute force or dictionary attacks against credentials that are observed by eavesdropping on the network.
Although the DES-CBC-MD5 encryption method and DES-MD5 checksum methods are listed as SHOULD be implemented for backward compatibility, the single DES encryption algorithm on which these are based is weak, and stronger algorithms should be used whenever possible.
Principal identifiers must not recycled on a short-term basis. A typical mode of access control will use access control lists (ACLs) to grant permissions to particular principals. If a stale ACL entry remains for a deleted principal and the principal identifier is reused, the new principal will inherit rights specified in the stale ACL entry. By not reusing principal identifiers, the danger of inadvertent access is removed.
Many RFC 1510-compliant implementations ignore unknown authorization data elements. Depending on these implementations to honor authorization data restrictions may create a security weakness.
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Kerberos credentials contain clear-text information identifying the principals to which they apply. If privacy of this information is needed, this exchange should itself be encapsulated in a protocol providing for confidentiality on the exchange of these credentials.
Unless the application server provides its own suitable means to protect against replay (for example, a challenge-response sequence initiated by the server after authentication, or use of a server- generated encryption subkey), the server must utilize a replay cache to remember any authenticator presented within the allowable clock skew. All services sharing a key need to use the same replay cache. If separate replay caches are used, then an authenticator used with one such service could later be replayed to a different service with the same service principal.
Implementations of Kerberos should not use untrusted directory servers to determine the realm of a host. To allow this would allow the compromise of the directory server to enable an attacker to direct the client to accept authentication with the wrong principal (i.e., one with a similar name, but in a realm with which the legitimate host was not registered).
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wrong principal. Though the client will know who it is communicating with, it will not be the principal with which it intended to communicate.
The Kerberos protocol in its basic form does not provide perfect forward secrecy for communications. If traffic has been recorded by an eavesdropper, then messages encrypted using the KRB_PRIV message, or messages encrypted using application-specific encryption under keys exchanged using Kerberos can be decrypted if the user's, application server's, or KDC's key is subsequently discovered. This is because the session key used to encrypt such messages, when transmitted over the network, is encrypted in the key of the application server. It is also encrypted under the session key from the user's TGT when it is returned to the user in the KRB_TGS_REP message. The session key from the TGT is sent to the user in the KRB_AS_REP message encrypted in the user's secret key and embedded in the TGT, which was encrypted in the key of the KDC. Applications requiring perfect forward secrecy must exchange keys through mechanisms that provide such assurance, but may use Kerberos for authentication of the encrypted channel established through such other means.
This document is a revision to RFC 1510 which was co-authored with John Kohl. The specification of the Kerberos protocol described in this document is the result of many years of effort. Over this period, many individuals have contributed to the definition of the protocol and to the writing of the specification. Unfortunately, it is not possible to list all contributors as authors of this document, though there are many not listed who are authors in spirit, including those who contributed text for parts of some sections, who contributed to the design of parts of the protocol, and who contributed significantly to the discussion of the protocol in the IETF common authentication technology (CAT) and Kerberos working groups.
Neuman, et al. Standards Track [Page 121] RFC 4120 Kerberos V5 July 2005
Among those contributing to the development and specification of Kerberos were Jeffrey Altman, John Brezak, Marc Colan, Johan Danielsson, Don Davis, Doug Engert, Dan Geer, Paul Hill, John Kohl, Marc Horowitz, Matt Hur, Jeffrey Hutzelman, Paul Leach, John Linn, Ari Medvinsky, Sasha Medvinsky, Steve Miller, Jon Rochlis, Jerome Saltzer, Jeffrey Schiller, Jennifer Steiner, Ralph Swick, Mike Swift, Jonathan Trostle, Theodore Ts'o, Brian Tung, Jacques Vidrine, Assar Westerlund, and Nicolas Williams. Many other members of MIT Project Athena, the MIT networking group, and the Kerberos and CAT working groups of the IETF contributed but are not listed.
KerberosV5Spec2 { iso(1) identified-organization(3) dod(6) internet(1) security(5) kerberosV5(2) modules(4) krb5spec2(2) } DEFINITIONS EXPLICIT TAGS ::= BEGIN
Int32 ::= INTEGER (-2147483648..2147483647) -- signed values representable in 32 bits
Microseconds ::= INTEGER (0..999999) -- microseconds
Realm ::= KerberosString
KerberosTime ::= GeneralizedTime -- with no fractional seconds
-- NOTE: HostAddresses is always used as an OPTIONAL field and -- should not be empty. HostAddresses -- NOTE: subtly different from rfc1510,
-- NOTE: AuthorizationData is always used as an OPTIONAL field and -- should not be empty. AuthorizationData ::= SEQUENCE OF SEQUENCE { ad-type [0] Int32, ad-data [1] OCTET STRING }
KerberosFlags ::= BIT STRING (SIZE (32..MAX)) -- minimum number of bits shall be sent, -- but no fewer than 32
EncryptionKey ::= SEQUENCE { keytype [0] Int32 -- actually encryption type --, keyvalue [1] OCTET STRING }
Ticket ::= [APPLICATION 1] SEQUENCE { tkt-vno [0] INTEGER (5), realm [1] Realm, sname [2] PrincipalName, enc-part [3] EncryptedData -- EncTicketPart }
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cname [3] PrincipalName, transited [4] TransitedEncoding, authtime [5] KerberosTime, starttime [6] KerberosTime OPTIONAL, endtime [7] KerberosTime, renew-till [8] KerberosTime OPTIONAL, caddr [9] HostAddresses OPTIONAL, authorization-data [10] AuthorizationData OPTIONAL }
TicketFlags ::= KerberosFlags -- reserved(0), -- forwardable(1), -- forwarded(2), -- proxiable(3), -- proxy(4), -- may-postdate(5), -- postdated(6), -- invalid(7), -- renewable(8), -- initial(9), -- pre-authent(10), -- hw-authent(11), -- the following are new since 1510 -- transited-policy-checked(12), -- ok-as-delegate(13)
TGS-REQ ::= [APPLICATION 12] KDC-REQ
KDC-REQ-BODY ::= SEQUENCE { kdc-options [0] KDCOptions,
KDCOptions ::= KerberosFlags -- reserved(0), -- forwardable(1), -- forwarded(2), -- proxiable(3), -- proxy(4), -- allow-postdate(5), -- postdated(6), -- unused7(7), -- renewable(8), -- unused9(9), -- unused10(10), -- opt-hardware-auth(11), -- unused12(12), -- unused13(13), -- 15 is reserved for canonicalize -- unused15(15), -- 26 was unused in 1510 -- disable-transited-check(26), -- -- renewable-ok(27), -- enc-tkt-in-skey(28), -- renew(30), -- validate(31)
TGS-REP ::= [APPLICATION 13] KDC-REP
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KDC-REP ::= SEQUENCE { pvno [0] INTEGER (5), padata [2] SEQUENCE OF PA-DATA OPTIONAL -- NOTE: not empty --, crealm [3] Realm, cname [4] PrincipalName, ticket [5] Ticket, enc-part [6] EncryptedData -- EncASRepPart or EncTGSRepPart, -- as appropriate }
EncTGSRepPart ::= [APPLICATION 26] EncKDCRepPart
LastReq ::= SEQUENCE OF SEQUENCE { lr-type [0] Int32, lr-value [1] KerberosTime }
APOptions ::= KerberosFlags -- reserved(0), -- use-session-key(1),
-- Unencrypted authenticator Authenticator ::= [APPLICATION 2] SEQUENCE { authenticator-vno [0] INTEGER (5), crealm [1] Realm, cname [2] PrincipalName, cksum [3] Checksum OPTIONAL, cusec [4] Microseconds, ctime [5] KerberosTime, subkey [6] EncryptionKey OPTIONAL, seq-number [7] UInt32 OPTIONAL, authorization-data [8] AuthorizationData OPTIONAL }
EncAPRepPart ::= [APPLICATION 27] SEQUENCE { ctime [0] KerberosTime, cusec [1] Microseconds, subkey [2] EncryptionKey OPTIONAL, seq-number [3] UInt32 OPTIONAL }
KRB-SAFE-BODY ::= SEQUENCE { user-data [0] OCTET STRING, timestamp [1] KerberosTime OPTIONAL, usec [2] Microseconds OPTIONAL, seq-number [3] UInt32 OPTIONAL, s-address [4] HostAddress, r-address [5] HostAddress OPTIONAL }
Neuman, et al. Standards Track [Page 128] RFC 4120 Kerberos V5 July 2005
enc-part [3] EncryptedData -- EncKrbPrivPart }
KRB-CRED ::= [APPLICATION 22] SEQUENCE { pvno [0] INTEGER (5), msg-type [1] INTEGER (22), tickets [2] SEQUENCE OF Ticket, enc-part [3] EncryptedData -- EncKrbCredPart }
KrbCredInfo ::= SEQUENCE { key [0] EncryptionKey, prealm [1] Realm OPTIONAL, pname [2] PrincipalName OPTIONAL, flags [3] TicketFlags OPTIONAL, authtime [4] KerberosTime OPTIONAL, starttime [5] KerberosTime OPTIONAL, endtime [6] KerberosTime OPTIONAL, renew-till [7] KerberosTime OPTIONAL, srealm [8] Realm OPTIONAL, sname [9] PrincipalName OPTIONAL, caddr [10] HostAddresses OPTIONAL }
Neuman, et al. Standards Track [Page 129] RFC 4120 Kerberos V5 July 2005
susec [5] Microseconds, error-code [6] Int32, crealm [7] Realm OPTIONAL, cname [8] PrincipalName OPTIONAL, realm [9] Realm -- service realm --, sname [10] PrincipalName -- service name --, e-text [11] KerberosString OPTIONAL, e-data [12] OCTET STRING OPTIONAL }
TYPED-DATA ::= SEQUENCE SIZE (1..MAX) OF SEQUENCE { data-type [0] Int32, data-value [1] OCTET STRING OPTIONAL }
PA-ENC-TIMESTAMP ::= EncryptedData -- PA-ENC-TS-ENC
ETYPE-INFO-ENTRY ::= SEQUENCE { etype [0] Int32, salt [1] OCTET STRING OPTIONAL }
ETYPE-INFO2-ENTRY ::= SEQUENCE { etype [0] Int32, salt [1] KerberosString OPTIONAL, s2kparams [2] OCTET STRING OPTIONAL }
AD-IF-RELEVANT ::= AuthorizationData
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}
AD-MANDATORY-FOR-KDC ::= AuthorizationData
B. Changes since RFC 1510
Discussion was added to Section 1 regarding the ability to rely on the KDC to check the transited field, and on the inclusion of a flag in a ticket indicating that this check has occurred. This is a new capability not present in RFC 1510. Pre-existing implementations may ignore or not set this flag without negative security implications.
The introduction mentions the use of public key cryptography for initial authentication in Kerberos by reference. RFC 1510 did not include such a reference.
Discussion of extensibility has been added to the introduction.
Neuman, et al. Standards Track [Page 131] RFC 4120 Kerberos V5 July 2005
sections in the specification were renumbered, and text was revised to make the description and intent of existing options clearer, especially with respect to the ENC-TKT-IN-SKEY option (now section 2.9.2) which is used for user-to-user authentication. The new option and ticket flag transited policy checking (Section 2.7) was added.
In RFC 1510, Section 4 described the database in the KDC. This discussion was not necessary for interoperability and unnecessarily constrained implementation. The old Section 4 was removed.
Significant changes were made to the layout of Section 5 to clarify the correct behavior for optional fields. Many of these changes were made necessary because of improper ASN.1 description in the original Kerberos specification which left the correct behavior underspecified. Additionally, the wording in this section was tightened wherever possible to ensure that implementations conforming to this specification will be extensible with the addition of new fields in future specifications.
Neuman, et al. Standards Track [Page 132] RFC 4120 Kerberos V5 July 2005
definition in RFC 1510. The transited-policy-checked(12) and the ok-as-delegate(13) ticket flags were added. The disable-transited- check(26) KDC option was added.
There were two definitions of METHOD-DATA in RFC 1510. The second one, intended for use with KRB_AP_ERR_METHOD was removed leaving the SEQUENCE OF PA-DATA definition.
Words were added describing the convention that domain-based realm names for newly-created realms should be specified as uppercase. This recommendation does not make lowercase realm names illegal. Words were added highlighting that the slash-separated components in the X.500 style of realm names is consistent with existing RFC 1510 based implementations, but that it conflicts with the general recommendation of X.500 name representation specified in RFC 2253.
"Specification 2" in Section 8 of the current document changes the required encryption and checksum methods to bring them in line with the best current practices and to deprecate methods that are no longer considered sufficiently strong.
The pseudo-code has been removed from the appendix. The pseudo-code was sometimes misinterpreted to limit implementation choices and in RFC 1510, it was not always consistent with the words in the specification. Effort was made to clear up any ambiguities in the specification, rather than to rely on the pseudo-code.
END NOTES
Neuman, et al. Standards Track [Page 133] RFC 4120 Kerberos V5 July 2005
Normative References
[RFC3962] Raeburn, K., "Advanced Encryption Standard (AES) Encryption for Kerberos 5", RFC 3962, February 2005.
[ISO-2022/ECMA-35] International Organization for Standardization, "Character code structure and extension techniques", ISO/IEC 2022:1994.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000.
[RFC3513] Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6) Addressing Architecture", RFC 3513, April 2003.
Informative References
[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC 1964, June 1996.
[DS81] Dorothy E. Denning and Giovanni Maria Sacco, "Time-stamps in Key Distribution Protocols," Communications of the ACM, Vol. 24 (8), p. 533- 536, August 1981.
[MNSS87] S. P. Miller, B. C. Neuman, J. I. Schiller, and J. H. Saltzer, Section E.2.1: Kerberos Authentication and Authorization System, M.I.T. Project Athena, Cambridge, Massachusetts, December 21, 1987.
[Neu93] B. Clifford Neuman, "Proxy-Based Authorization and Accounting for Distributed Systems," in Proceedings of the 13th International Conference on Distributed Computing Systems, Pittsburgh, PA, May 1993.
[Pat92] J. Pato, Using Pre-Authentication to Avoid Password Guessing Attacks, Open Software Foundation DCE Request for Comments 26 (December 1992.
[RFC4086] Eastlake, D., 3rd, Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2", RFC 4121, July 2005.
Neuman, et al. Standards Track [Page 136] RFC 4120 Kerberos V5 July 2005
Authors' Addresses
EMail: bcn@isi.edu
Tom Yu Massachusetts Institute of Technology 77 Massachusetts Avenue Cambridge, MA 02139, USA
EMail: hartmans-ietf@mit.edu
Kenneth Raeburn Massachusetts Institute of Technology 77 Massachusetts Avenue Cambridge, MA 02139, USA
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