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RFC 1510

            The Kerberos Network Authentication Service (V5)

Overview

   Project Athena, Athena, Athena MUSE, Discuss, Hesiod, Kerberos,
   Moira, and Zephyr are trademarks of the Massachusetts Institute of
   Technology (MIT).  No commercial use of these trademarks may be made
   without prior written permission of MIT.

   This RFC 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; for Version 4 they are fully
   described in the Kerberos portion of the Athena Technical Plan [1].
   The protocols are under review, and are not being submitted for
   consideration as an Internet standard at this time.  Comments are
   encouraged.  Requests for addition to an electronic mailing list for
   discussion of Kerberos, kerberos@MIT.EDU, may be addressed to
   kerberos-request@MIT.EDU.  This mailing list is gatewayed onto the
   Usenet as the group comp.protocols.kerberos.  Requests for further
   information, including documents and code availability, may be sent
   to info-kerberos@MIT.EDU.





Kohl & Neuman                                                   [Page 1]

RFC 1510 Kerberos September 1993 Background The Kerberos model is based in part on Needham and Schroeder's trusted third-party authentication protocol [3] and on modifications suggested by Denning and Sacco [4]. 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. Version 4 is publicly available, and has seen wide use across the Internet. Version 5 (described in this document) has evolved from Version 4 based on new requirements and desires for features not available in Version 4. Details on the differences between Kerberos Versions 4 and 5 can be found in [5]. Table of Contents 1. Introduction ....................................... 5 1.1. Cross-Realm Operation ............................ 7 1.2. Environmental assumptions ........................ 8 1.3. Glossary of terms ................................ 9 2. Ticket flag uses and requests ...................... 12 2.1. Initial and pre-authenticated tickets ............ 12 2.2. Invalid tickets .................................. 12 2.3. Renewable tickets ................................ 12 2.4. Postdated tickets ................................ 13 2.5. Proxiable and proxy tickets ...................... 14 2.6. Forwardable tickets .............................. 15 2.7. Other KDC options ................................ 15 3. Message Exchanges .................................. 16 3.1. The Authentication Service Exchange .............. 16 3.1.1. Generation of KRB_AS_REQ message ............... 17 3.1.2. Receipt of KRB_AS_REQ message .................. 17 3.1.3. Generation of KRB_AS_REP message ............... 17 3.1.4. Generation of KRB_ERROR message ................ 19 3.1.5. Receipt of KRB_AS_REP message .................. 19 3.1.6. Receipt of KRB_ERROR message ................... 20 3.2. The Client/Server Authentication Exchange ........ 20 3.2.1. The KRB_AP_REQ message ......................... 20 3.2.2. Generation of a KRB_AP_REQ message ............. 20 3.2.3. Receipt of KRB_AP_REQ message .................. 21 3.2.4. Generation of a KRB_AP_REP message ............. 23 3.2.5. Receipt of KRB_AP_REP message .................. 23 Kohl & Neuman [Page 2]
RFC 1510 Kerberos September 1993 3.2.6. Using the encryption key ....................... 24 3.3. The Ticket-Granting Service (TGS) Exchange ....... 24 3.3.1. Generation of KRB_TGS_REQ message .............. 25 3.3.2. Receipt of KRB_TGS_REQ message ................. 26 3.3.3. Generation of KRB_TGS_REP message .............. 27 3.3.3.1. Encoding the transited field ................. 29 3.3.4. Receipt of KRB_TGS_REP message ................. 31 3.4. The KRB_SAFE Exchange ............................ 31 3.4.1. Generation of a KRB_SAFE message ............... 31 3.4.2. Receipt of KRB_SAFE message .................... 32 3.5. The KRB_PRIV Exchange ............................ 33 3.5.1. Generation of a KRB_PRIV message ............... 33 3.5.2. Receipt of KRB_PRIV message .................... 33 3.6. The KRB_CRED Exchange ............................ 34 3.6.1. Generation of a KRB_CRED message ............... 34 3.6.2. Receipt of KRB_CRED message .................... 34 4. The Kerberos Database .............................. 35 4.1. Database contents ................................ 35 4.2. Additional fields ................................ 36 4.3. Frequently Changing Fields ....................... 37 4.4. Site Constants ................................... 37 5. Message Specifications ............................. 38 5.1. ASN.1 Distinguished Encoding Representation ...... 38 5.2. ASN.1 Base Definitions ........................... 38 5.3. Tickets and Authenticators ....................... 42 5.3.1. Tickets ........................................ 42 5.3.2. Authenticators ................................. 47 5.4. Specifications for the AS and TGS exchanges ...... 49 5.4.1. KRB_KDC_REQ definition ......................... 49 5.4.2. KRB_KDC_REP definition ......................... 56 5.5. Client/Server (CS) message specifications ........ 58 5.5.1. KRB_AP_REQ definition .......................... 58 5.5.2. KRB_AP_REP definition .......................... 60 5.5.3. Error message reply ............................ 61 5.6. KRB_SAFE message specification ................... 61 5.6.1. KRB_SAFE definition ............................ 61 5.7. KRB_PRIV message specification ................... 62 5.7.1. KRB_PRIV definition ............................ 62 5.8. KRB_CRED message specification ................... 63 5.8.1. KRB_CRED definition ............................ 63 5.9. Error message specification ...................... 65 5.9.1. KRB_ERROR definition ........................... 66 6. Encryption and Checksum Specifications ............. 67 6.1. Encryption Specifications ........................ 68 6.2. Encryption Keys .................................. 71 6.3. Encryption Systems ............................... 71 6.3.1. The NULL Encryption System (null) .............. 71 6.3.2. DES in CBC mode with a CRC-32 checksum (descbc-crc)71 Kohl & Neuman [Page 3]
RFC 1510 Kerberos September 1993 6.3.3. DES in CBC mode with an MD4 checksum (descbc-md4) 72 6.3.4. DES in CBC mode with an MD5 checksum (descbc-md5) 72 6.4. Checksums ........................................ 74 6.4.1. The CRC-32 Checksum (crc32) .................... 74 6.4.2. The RSA MD4 Checksum (rsa-md4) ................. 75 6.4.3. RSA MD4 Cryptographic Checksum Using DES (rsa-md4-des) ......................................... 75 6.4.4. The RSA MD5 Checksum (rsa-md5) ................. 76 6.4.5. RSA MD5 Cryptographic Checksum Using DES (rsa-md5-des) ......................................... 76 6.4.6. DES cipher-block chained checksum (des-mac) 6.4.7. RSA MD4 Cryptographic Checksum Using DES alternative (rsa-md4-des-k) ........................... 77 6.4.8. DES cipher-block chained checksum alternative (des-mac-k) ........................................... 77 7. Naming Constraints ................................. 78 7.1. Realm Names ...................................... 77 7.2. Principal Names .................................. 79 7.2.1. Name of server principals ...................... 80 8. Constants and other defined values ................. 80 8.1. Host address types ............................... 80 8.2. KDC messages ..................................... 81 8.2.1. IP transport ................................... 81 8.2.2. OSI transport .................................. 82 8.2.3. Name of the TGS ................................ 82 8.3. Protocol constants and associated values ......... 82 9. Interoperability requirements ...................... 86 9.1. Specification 1 .................................. 86 9.2. Recommended KDC values ........................... 88 10. Acknowledgments ................................... 88 11. References ........................................ 89 12. Security Considerations ........................... 90 13. Authors' Addresses ................................ 90 A. Pseudo-code for protocol processing ................ 91 A.1. KRB_AS_REQ generation ............................ 91 A.2. KRB_AS_REQ verification and KRB_AS_REP generation 92 A.3. KRB_AS_REP verification .......................... 95 A.4. KRB_AS_REP and KRB_TGS_REP common checks ......... 96 A.5. KRB_TGS_REQ generation ........................... 97 A.6. KRB_TGS_REQ verification and KRB_TGS_REP generation 98 A.7. KRB_TGS_REP verification ......................... 104 A.8. Authenticator generation ......................... 104 A.9. KRB_AP_REQ generation ............................ 105 A.10. KRB_AP_REQ verification ......................... 105 A.11. KRB_AP_REP generation ........................... 106 A.12. KRB_AP_REP verification ......................... 107 A.13. KRB_SAFE generation ............................. 107 A.14. KRB_SAFE verification ........................... 108 Kohl & Neuman [Page 4]
RFC 1510 Kerberos September 1993 A.15. KRB_SAFE and KRB_PRIV common checks ............. 108 A.16. KRB_PRIV generation ............................. 109 A.17. KRB_PRIV verification ........................... 110 A.18. KRB_CRED generation ............................. 110 A.19. KRB_CRED verification ........................... 111 A.20. KRB_ERROR generation ............................ 112 1. Introduction Kerberos provides a means of verifying the identities of principals, (e.g., a workstation user or a network server) on an open (unprotected) network. This is accomplished without relying on authentication by the host operating system, without basing trust on host addresses, without requiring physical security of all the hosts on the network, and under the assumption that packets traveling along the network can be read, modified, and inserted at will. (Note, however, that many applications use Kerberos' functions only upon the initiation of a stream-based network connection, and assume the absence of any "hijackers" who might subvert such a connection. Such use implicitly trusts the host addresses involved.) Kerberos performs authentication under these conditions as a trusted third- party authentication service by using conventional cryptography, i.e., shared secret key. (shared secret key - Secret and private are often used interchangeably in the literature. In our usage, it takes two (or more) to share a secret, thus a shared DES key is a secret key. Something is only private when no one but its owner knows it. Thus, in public key cryptosystems, one has a public and a private key.) The authentication process proceeds as follows: A client sends a request to the authentication server (AS) requesting "credentials" for a given server. The AS responds with these credentials, encrypted in the client's key. The credentials consist of 1) a "ticket" for the server and 2) 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. The implementation consists of one or more authentication servers running on physically secure hosts. The authentication servers maintain a database of principals (i.e., users and servers) and their secret keys. Code libraries provide encryption and implement the Kerberos protocol. In order to add authentication to its Kohl & Neuman [Page 5]
RFC 1510 Kerberos September 1993 transactions, a typical network application adds one or two calls to the Kerberos library, which results in the transmission of the necessary messages to achieve authentication. The Kerberos protocol consists of several sub-protocols (or exchanges). There are two 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 sends the TGT to the TGS in the same manner as if it were contacting any other application server which requires Kerberos credentials. The reply is encrypted in the session key from the TGT. Once obtained, credentials may be used to verify the identity of the principals in a transaction, to ensure the integrity of messages exchanged between them, or to preserve privacy of the messages. The application is free to choose whatever protection may be necessary. To verify the identities of the principals in a transaction, the client transmits the ticket to the server. Since 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 was originated by 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. The integrity of the messages exchanged between principals can also be guaranteed using the session key (passed in the ticket and contained in the credentials). This approach provides detection of both replay attacks and message stream modification attacks. It is accomplished by generating and transmitting a collision-proof checksum (elsewhere called a hash or digest function) of the client's message, keyed with the session key. Privacy and integrity of the messages exchanged between principals can be secured by encrypting the data to be passed using the session key passed in the ticket, and contained in the credentials. The authentication exchanges mentioned above require read-only access to the Kerberos database. Sometimes, however, the entries in the Kohl & Neuman [Page 6]
RFC 1510 Kerberos September 1993 database must be modified, such as when adding new principals or changing a principal's key. This is done using a protocol between a client and a third Kerberos server, the Kerberos Administration Server (KADM). The administration protocol is not described in this document. There is also a protocol for maintaining multiple copies of the Kerberos database, but this can be considered an implementation detail and may vary to support different database technologies. 1.1. Cross-Realm Operation The Kerberos protocol is designed to operate across organizational boundaries. A client in one organization can be authenticated to a server in another. Each organization wishing to run a Kerberos server establishes its own "realm". The name of the realm in which a client is registered is part of the client's name, and can be used by the end-service to decide whether to honor a request. By establishing "inter-realm" keys, the administrators of two realms can allow a client authenticated in the local realm to use its authentication remotely (Of course, with appropriate permission the client could arrange registration of a separately-named principal in a remote realm, and engage in normal exchanges with that realm's services. However, for even small numbers of clients this becomes cumbersome, and more automatic methods as described here are necessary). 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 ticket-granting ticket for the remote realm's ticket- granting service from its local realm. When that ticket-granting ticket is used, the remote ticket-granting service uses the inter- realm key (which usually differs from its own normal TGS key) to decrypt the ticket-granting ticket, and is thus certain that it 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. Realms are typically organized hierarchically. Each realm shares a key with its parent and a different key with each child. If an inter-realm key is not directly shared by two realms, the hierarchical organization allows an authentication path to be easily constructed. If a hierarchical organization is not used, it may be necessary to consult some database in order to construct an Kohl & Neuman [Page 7]
RFC 1510 Kerberos September 1993 authentication path between realms. Although realms are typically hierarchical, intermediate realms may be bypassed to achieve cross-realm authentication through alternate authentication paths (these might be established to make communication between two realms more efficient). It is important for the end-service to know which realms were transited when deciding how much faith to place in the authentication process. To facilitate this decision, a field in each ticket contains the names of the realms that were involved in authenticating the client. 1.2. Environmental assumptions Kerberos imposes a few assumptions on the environment in which it can properly function: + "Denial of service" attacks are not solved with Kerberos. There are places in these protocols where an intruder 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) is usually best left to the human administrators and users. + 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. + "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. + Each host on the network must have a clock which is "loosely synchronized" to the time of the other hosts; this synchronization is used to reduce the bookkeeping needs of application servers when they do replay detection. The degree of "looseness" can be configured on a per-server basis. If the clocks are synchronized over the network, the clock synchronization protocol must itself be secured from network attackers. + 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 Kohl & Neuman [Page 8]
RFC 1510 Kerberos September 1993 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. 1.3. Glossary of terms Below is a list of terms used throughout this document. Authentication Verifying the claimed identity of a principal. Authentication header A record containing a Ticket and an Authenticator to be presented to a server as part of the authentication process. Authentication path A sequence of intermediate realms transited in the authentication process when communicating from one realm to another. 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. Authorization The process of determining whether a client may use a service, which objects the client is allowed to access, and the type of access allowed for each. 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. Kohl & Neuman [Page 9]
RFC 1510 Kerberos September 1993 Ciphertext The output of an encryption function. Encryption transforms plaintext into ciphertext. 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). Credentials A ticket plus the secret session key necessary to successfully use that ticket in an authentication exchange. KDC Key Distribution Center, a network service that supplies tickets and temporary session keys; or an instance of that service or the 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). Kerberos Aside from the 3-headed dog guarding Hades, the name given to Project Athena's authentication service, the protocol used by that service, or the code used to implement the authentication service. Plaintext The input to an encryption function or the output of a decryption function. Decryption transforms ciphertext into plaintext. Principal A uniquely named client or server instance that participates in a network communication. Kohl & Neuman [Page 10]
RFC 1510 Kerberos September 1993 Principal identifier The name used to uniquely identify each different principal. Seal To encipher a record containing several fields in such a way that the fields cannot be individually replaced without either knowledge of the encryption key or leaving evidence of tampering. Secret key An encryption key shared by a principal and the KDC, distributed outside the bounds of the system, with a long lifetime. In the case of a human user's principal, the secret key is derived from a password. Server A particular Principal which provides a resource to network clients. Service A resource provided to network clients; often provided by more than one server (for example, remote file service). Session key A temporary encryption key used between two principals, with a lifetime limited to the duration of a single login "session". Sub-session key A temporary encryption key used between two principals, selected and exchanged by the principals using the session key, and with a lifetime limited to the duration of a single association. 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. Kohl & Neuman [Page 11]
RFC 1510 Kerberos September 1993 2. Ticket flag uses and requests Each Kerberos ticket contains a set of flags which are used to indicate various 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 gives examples of reasons to use such a flag. 2.1. Initial and pre-authenticated tickets The INITIAL flag indicates that a ticket was issued using the AS protocol and not issued based on a ticket-granting ticket. Application servers that want to require the knowledge of a client's secret key (e.g., a passwordchanging program) can insist that this flag be set in any tickets they accept, and thus be assured that the client's key was recently presented to the application client. The PRE-AUTHENT and HW-AUTHENT flags provide addition 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 ticket-granting ticket (in which case the INITIAL flag is clear, but the PRE-AUTHENT and HW-AUTHENT flags are carried forward from the ticket-granting ticket). 2.2. Invalid tickets The INVALID flag indicates that a ticket is invalid. Application servers must reject tickets which have this flag set. A postdated ticket will usually be issued in this form. Invalid tickets must be validated by the KDC before use, by presenting them 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 which have been stolen before their starttime can be rendered permanently invalid (through a hot- list mechanism). 2.3. Renewable tickets Applications may desire to hold tickets which 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 shortlived 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 Kohl & Neuman [Page 12]
RFC 1510 Kerberos September 1993 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 if the ticket had been reported stolen since its last renewal; it will refuse to renew such stolen tickets, and thus the usable lifetime of stolen tickets is reduced. 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 wish to disallow renewable tickets. If a renewable ticket is not renewed by its expiration time, the KDC will not renew the ticket. The RENEWABLE flag is reset by default, but a client may request it be set by setting the RENEWABLE option in the KRB_AS_REQ message. If it is set, then the renew-till field in the ticket contains the time after which the ticket may not be renewed. 2.4. Postdated tickets Applications may occasionally need to obtain tickets for use much later, e.g., a batch submission system would need tickets to be valid at the time the batch job is serviced. However, it is dangerous to hold valid tickets in a batch queue, since they will be on-line longer and more prone to theft. Postdated tickets provide a way to obtain these tickets from the KDC at job submission time, but to leave them "dormant" until they are activated and validated by a further request of the KDC. If a ticket theft were reported in the interim, the KDC would refuse to validate the ticket, and the thief would be foiled. 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 ticket-granting ticket in order to issue a postdated ticket based on the presented ticket. It is reset by default; it may be requested by a client by setting the ALLOW- POSTDATE option in the KRB_AS_REQ message. This flag does not allow a client to obtain a postdated ticket-granting ticket; postdated ticket-granting tickets can only by 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 ticket- Kohl & Neuman [Page 13]
RFC 1510 Kerberos September 1993 granting ticket 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 ticket-granting ticket. The KDC may limit how far in the future a ticket may be postdated. The POSTDATED flag indicates that a ticket has been postdated. The application server can check the authtime field in the ticket to see when the original authentication occurred. Some services may choose to reject postdated tickets, or they may only accept them within a certain period after the original authentication. When the KDC issues a POSTDATED ticket, it will also be marked as INVALID, so that the application client must present the ticket to the KDC to be validated before use. 2.5. Proxiable and proxy tickets 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 take on the principal's identity for a particular purpose 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 ticket-granting ticket) with a different network address based on this ticket. This flag is set by default. This flag allows a client to pass a proxy to a server to perform a remote request on its behalf, e.g., a print service client can give the print server a proxy to access the client's files on a particular file server in order to satisfy a print request. In order to complicate the use of stolen credentials, Kerberos tickets are usually valid from only those network addresses specifically included in the ticket (It is permissible to request or issue tickets with no network addresses specified, but we do not recommend it). For this reason, a client wishing to grant a proxy must request a new ticket valid for the network address of the service to be granted the proxy. The PROXY flag is set in a ticket by the TGS when it issues a proxy ticket. Application servers may check this flag and require additional authentication from the agent presenting the proxy in order to provide an audit trail. Kohl & Neuman [Page 14]
RFC 1510 Kerberos September 1993 2.6. Forwardable tickets Authentication forwarding is an instance of the proxy case where the service is granted complete use of the client's identity. An example where it might be used is when a user logs in to a remote system and wants authentication to work from that system as if the login were local. The FORWARDABLE flag in a ticket is normally only interpreted by the ticket-granting service. It can be ignored by application servers. The FORWARDABLE flag has an interpretation similar to that of the PROXIABLE flag, except ticket-granting tickets may also be issued with different network addresses. This flag is reset by default, but users may request that it be set by setting the FORWARDABLE option in the AS request when they request their initial ticket-granting ticket. 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 end result can still be achieved if the user engages in the AS exchange with the requested network addresses and supplies a password. The FORWARDED flag is set by the TGS when a client presents a ticket with the FORWARDABLE flag set and requests it be set by specifying the FORWARDED KDC option and supplying a set of addresses for the new ticket. It is also set in all tickets issued based on tickets with the FORWARDED flag set. Application servers may wish to process FORWARDED tickets differently than non-FORWARDED tickets. 2.7. Other KDC options There are two additional options which 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 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. The ENC-TKT-IN-SKEY option is honored only by the ticket-granting service. It indicates that the to-be-issued ticket for the end server is to be encrypted in the session key from the additional ticket-granting ticket provided with the request. See section 3.3.3 for specific details. Kohl & Neuman [Page 15]
RFC 1510 Kerberos September 1993 3. Message Exchanges The following sections describe the interactions between network clients and servers and the messages involved in those exchanges. 3.1. The Authentication Service Exchange Summary 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 The Authentication Service (AS) Exchange between the client and the Kerberos Authentication Server is usually initiated by a client when it wishes to obtain authentication credentials for a given server but currently holds no credentials. The client's secret key is used for encryption and decryption. This exchange is typically used at the initiation of a login session, to obtain credentials for a Ticket- Granting Server, which will subsequently be used to obtain credentials for other servers (see section 3.3) without requiring further use of the client's secret key. This exchange is also used to request credentials for services which must not be mediated through the Ticket-Granting Service, but rather require a principal's secret key, such as the password-changing service. (The password- changing request must not be honored unless the requester can provide the old password (the user's current secret key). Otherwise, it would be possible for someone to walk up to an unattended session and change another user's password.) This exchange does not by itself provide any assurance of the the identity of the user. (To authenticate a user logging on to a local system, the credentials obtained in the AS exchange may first be used in a TGS exchange to obtain credentials for a local server. Those credentials must then be verified by the local server through successful completion of the Client/Server exchange.) The 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. In the request, the client sends (in cleartext) its own identity and the identity of the server for which it is requesting credentials. The response, KRB_AS_REP, contains a ticket for the client to present to the server, and a session key that will be shared by the client and the server. The session key and additional information are encrypted in the client's secret key. The KRB_AS_REP message contains information which can be used to detect replays, and to Kohl & Neuman [Page 16]
RFC 1510 Kerberos September 1993 associate it with the message to which it replies. Various errors can occur; these are indicated by an error response (KRB_ERROR) instead of the KRB_AS_REP response. The error message is not encrypted. The KRB_ERROR message also contains information which can be used to associate it with the message to which it replies. The lack of encryption in the KRB_ERROR message precludes the ability to detect replays or fabrications of such messages. In the normal case 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. 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 may be used for preauthentication if desired, but the mechanism is not currently specified. 3.1.1. Generation of KRB_AS_REQ message The client may specify a number of options in the initial request. Among these options are whether preauthentication is to be performed; whether the requested ticket is to be renewable, proxiable, or forwardable; whether it should be postdated or allow postdating of derivative tickets; and whether a renewable ticket will be accepted in lieu of a non-renewable ticket if the requested ticket expiration date cannot be satisfied by a nonrenewable ticket (due to configuration constraints; see section 4). See section A.1 for pseudocode. The client prepares the KRB_AS_REQ message and sends it to the KDC. 3.1.2. Receipt of KRB_AS_REQ message 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 5.3.1. The contents of the ticket are determined as follows. 3.1.3. Generation of KRB_AS_REP message The authentication server looks up the client and server principals named in the KRB_AS_REQ in its database, extracting their respective keys. If required, 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 the server cannot accommodate the requested encryption type, an error message with code Kohl & Neuman [Page 17]
RFC 1510 Kerberos September 1993 KDC_ERR_ETYPE_NOSUPP is returned. Otherwise it generates a "random" session key ("Random" means that, among other things, it should be impossible to guess the next session key based on knowledge of past session keys. This can only be achieved in a pseudo-random number generator if it is based on cryptographic principles. It would be more desirable to use a truly random number generator, such as one based on measurements of random physical phenomena.). If the requested start time is absent or indicates a time in the past, then the start time of the ticket is set to the authentication server's current time. If it indicates a time in the future, but the POSTDATED option has not been specified, then the error KDC_ERR_CANNOT_POSTDATE is returned. Otherwise the requested start time is checked against the policy of the local realm (the administrator might decide to prohibit certain types or ranges of postdated tickets), and if acceptable, the ticket's start time is set as requested and the INVALID flag is set in the new ticket. The postdated ticket must be validated before use by presenting it to the KDC after the start time has been reached. The expiration time of the ticket will be set to the minimum of the following: +The expiration time (endtime) requested in the KRB_AS_REQ message. +The ticket's start time plus the maximum allowable lifetime associated with the client principal (the authentication server's database includes a maximum ticket lifetime field in each principal's record; see section 4). +The ticket's start time plus the maximum allowable lifetime associated with the server principal. +The ticket's start time plus the maximum lifetime set by the policy of the local realm. If the requested expiration time minus the start time (as determined above) is less than a site-determined minimum lifetime, an error message with code KDC_ERR_NEVER_VALID is returned. If the requested expiration time for the ticket exceeds what was determined as above, and if the "RENEWABLE-OK" option was requested, then the "RENEWABLE" 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). If the RENEWABLE option has been requested or if the RENEWABLE-OK option has been set and a renewable ticket is to be issued, then the renew-till field is set to the minimum of: Kohl & Neuman [Page 18]
RFC 1510 Kerberos September 1993 +Its requested value. +The start time of the ticket plus the minimum of the two maximum renewable lifetimes associated with the principals' database entries. +The start time of the ticket plus the maximum renewable lifetime set by the policy of the local realm. The flags field of the new ticket will have the following options set if they have been requested and if the policy of the local realm allows: FORWARDABLE, MAY-POSTDATE, POSTDATED, PROXIABLE, RENEWABLE. If the new ticket is postdated (the start time is in the future), its INVALID flag will also be set. If all of the above succeed, the server 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 the requested encryption method, and sends it to the client. See section A.2 for pseudocode. 3.1.4. Generation of KRB_ERROR message 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. 3.1.5. Receipt of KRB_AS_REP message 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 message. The client decrypts the encrypted part of the response using its secret key, verifies that the nonce in the encrypted part matches the nonce it supplied in its request (to detect replays). It also verifies that the sname and srealm in the response match those in the request, and that the host address field is also correct. It then stores the ticket, session key, start and expiration times, and other information for later use. The key-expiration field from the encrypted part of the response may be checked to notify the user of impending key expiration (the client program could then suggest remedial action, such as a password change). See section A.3 for pseudocode. Proper decryption of the KRB_AS_REP message is not sufficient to Kohl & Neuman [Page 19]
RFC 1510 Kerberos September 1993 verify the identity of the user; the user and an attacker could cooperate to generate a KRB_AS_REP format message which 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 which can be verified using a securely-stored secret key. If those credentials can be verified, then the identity of the user can be assured. 3.1.6. Receipt of KRB_ERROR message If the reply message type is KRB_ERROR, then the client interprets it as an error and performs whatever application-specific tasks are necessary to recover. 3.2. The Client/Server Authentication Exchange Summary Message direction Message type Section Client to Application server KRB_AP_REQ 5.5.1 [optional] Application server to client KRB_AP_REP or 5.5.2 KRB_ERROR 5.9.1 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. 3.2.1. The KRB_AP_REQ message The KRB_AP_REQ contains authentication information which 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.), so 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 it. The KRB_AP_REQ message is referred to elsewhere as the "authentication header." 3.2.2. Generation of a KRB_AP_REQ message When a client wishes to initiate authentication to a server, it obtains (either through a credentials cache, the AS exchange, or the Kohl & Neuman [Page 20]
RFC 1510 Kerberos September 1993 TGS exchange) a ticket and session key for the desired service. The client may re-use any tickets it holds until they expire. The client then constructs a new Authenticator from the the system time, its name, and optionally 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 may not be re-used and will 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. In such cases, a new authenticator must be generated for each retry.). 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 client may indicate a requirement of mutual authentication or the use of a session-key based ticket by setting the appropriate flag(s) in the ap-options field of the message. 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. See section A.9 for pseudocode. 3.2.3. Receipt of KRB_AP_REQ message 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. The algorithm for verifying authentication information is as follows. If the message type is not KRB_AP_REQ, the server returns the KRB_AP_ERR_MSG_TYPE error. If the key version indicated by the Ticket in the KRB_AP_REQ is not one the server can use (e.g., it indicates an old key, and the server no longer possesses a copy of the old key), the KRB_AP_ERR_BADKEYVER error is returned. If the USE- SESSION-KEY flag is set in the ap-options field, it indicates to the server that the ticket is encrypted in the session key from the server's ticket-granting ticket rather than its secret key (This is used for user-to-user authentication as described in [6]). Since 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 Kohl & Neuman [Page 21]
RFC 1510 Kerberos September 1993 error code is returned if the server doesn't have the proper key to decipher the ticket. The ticket is decrypted using the version of the server's key specified by the ticket. If the decryption routines detect a modification of the ticket (each encryption system must provide safeguards to detect modified ciphertext; see section 6), the KRB_AP_ERR_BAD_INTEGRITY error is returned (chances are good that different keys were used to encrypt and decrypt). The authenticator is decrypted using the session key extracted from the decrypted ticket. If decryption shows it to have been modified, the KRB_AP_ERR_BAD_INTEGRITY error is returned. The name and realm of the client from the ticket are compared against the same fields in the authenticator. If they don't match, the KRB_AP_ERR_BADMATCH error is returned (they might not match, for example, if the wrong session key was used to encrypt the authenticator). The addresses in the ticket (if any) are then searched for an address matching the operating-system reported address of the client. If no match is found or the server insists on ticket addresses but none are present in the ticket, the KRB_AP_ERR_BADADDR error is returned. If the local (server) time and the client time in the authenticator differ by more than the allowable clock skew (e.g., 5 minutes), the KRB_AP_ERR_SKEW error is returned. If the server name, along with the client name, time and microsecond fields from the Authenticator match any recently-seen such tuples, 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 be have their authenticators rejected if the time and microsecond fields happen to match some other client's authenticator.). The server must remember any authenticator presented within the allowable clock skew, so that a replay attempt is guaranteed to fail. If a server loses track of any authenticator presented within the allowable clock skew, it must reject all requests until the clock skew interval has passed. This assures that any lost or re-played authenticators will fall outside the allowable clock skew and can no longer be successfully replayed (If this is not done, an attacker could conceivably record the ticket and authenticator sent over the network to a server, then disable the client's host, pose as the disabled host, and replay the ticket and authenticator to subvert the authentication.). If a sequence number is provided in the authenticator, the server saves it for later use in processing KRB_SAFE and/or KRB_PRIV messages. If a subkey is present, the server either saves it for later use or uses it to help generate its own choice for a subkey to be returned in a KRB_AP_REP message. Kohl & Neuman [Page 22]
RFC 1510 Kerberos September 1993 The server computes the age of the ticket: local (server) time minus the start time inside the Ticket. If the start time 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. If all these checks succeed without an error, the server is assured that the client possesses the credentials of the principal named in the ticket and thus, the client has been authenticated to the server. See section A.10 for pseudocode. 3.2.4. Generation of a KRB_AP_REP message 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 (not only authenticating 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). [Note: In the Kerberos version 4 protocol, the timestamp in the reply was the client's timestamp plus one. This is not necessary in version 5 because version 5 messages are formatted in such a way that it is not possible to create the reply by judicious message surgery (even in encrypted form) without knowledge of the appropriate encryption keys.] 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. See section A.11 for pseudocode. 3.2.5. Receipt of KRB_AP_REP message If a KRB_AP_REP message is returned, the client uses the session key from the credentials obtained for the server (Note that for encrypting the KRB_AP_REP message, the sub-session key is not used, even if present in the Authenticator.) to decrypt the message, and verifies that the timestamp and microsecond fields match those in the Authenticator it sent to the server. If they match, then the client is assured that the server is genuine. The sequence number and subkey (if present) are retained for later use. See section A.12 for Kohl & Neuman [Page 23]
RFC 1510 Kerberos September 1993 pseudocode. 3.2.6. Using the encryption key After the KRB_AP_REQ/KRB_AP_REP exchange has occurred, the client and server share an encryption key which can be used by the application. The "true session key" to be used for KRB_PRIV, KRB_SAFE, or other application-specific uses may be chosen by the application based on the subkeys in the KRB_AP_REP message and the authenticator (Implementations of the protocol may wish to provide routines to choose subkeys based on session keys and random numbers and to orchestrate a negotiated key to be returned in the KRB_AP_REP message.). In some cases, the use of this session key will be implicit in the protocol; in others the method of use must be chosen from a several alternatives. We leave the protocol negotiations of how to use the key (e.g., selecting an encryption or checksum type) to the application programmer; the Kerberos protocol does not constrain the implementation options. With both the one-way and mutual authentication exchanges, the peers should take care not to send sensitive information to each other without proper assurances. In particular, applications that require privacy or integrity should use the KRB_AP_REP or KRB_ERROR responses from the server to client to assure both client and server of their peer's identity. If an application protocol requires privacy of its messages, it can use the KRB_PRIV message (section 3.5). The KRB_SAFE message (section 3.4) can be used to assure integrity. 3.3. The Ticket-Granting Service (TGS) Exchange Summary 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 The TGS exchange between a client and the Kerberos Ticket-Granting Server is initiated by a client when it wishes to obtain authentication credentials for a given server (which might be registered in a remote realm), when it wishes to renew or validate an existing ticket, or when it wishes to obtain a proxy ticket. In the first case, the client must already have acquired a ticket for the Ticket-Granting Service using the AS exchange (the ticket-granting ticket is usually obtained when a client initially authenticates to the system, such as when a user logs in). The message format for the TGS exchange is almost identical to that for the AS exchange. The primary difference is that encryption and decryption in the TGS Kohl & Neuman [Page 24]
RFC 1510 Kerberos September 1993 exchange does not take place under the client's key. Instead, the session key from the ticket-granting ticket 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 ticket-granting ticket expires, the client must use a separate exchange to obtain valid tickets. The TGS exchange consists of two messages: A request (KRB_TGS_REQ) from the client to the Kerberos Ticket-Granting Server, and a reply (KRB_TGS_REP or KRB_ERROR). The KRB_TGS_REQ message includes information authenticating the client plus a request for credentials. The authentication information consists of the authentication header (KRB_AP_REQ) which includes the client's previously obtained ticket- granting, renewable, or invalid ticket. In the ticket-granting ticket and proxy cases, the request may include one or more of: a list of network addresses, a collection of typed authorization data to be sealed in the ticket for authorization use by the application server, or additional tickets (the use of which are described later). The TGS reply (KRB_TGS_REP) contains the requested credentials, encrypted in the session key from the ticket-granting ticket or renewable ticket, or if present, in the subsession key from the Authenticator (part of the authentication header). The KRB_ERROR message contains an error code and text explaining what went wrong. The KRB_ERROR message is not encrypted. The KRB_TGS_REP message contains information which can be used to detect replays, and to associate it with the message to which it replies. The KRB_ERROR message also contains information which can be used to associate it with the message to which it replies, but the lack of encryption in the KRB_ERROR message precludes the ability to detect replays or fabrications of such messages. 3.3.1. Generation of KRB_TGS_REQ message Before sending a request to the ticket-granting service, the client must determine in which realm the application server is registered [Note: This can be accomplished in several ways. It might be known beforehand (since the realm is part of the principal identifier), or it might be stored in a nameserver. Presently, however, this information is obtained from a configuration file. If the realm to be used is obtained from a nameserver, there is a danger of being spoofed if the nameservice providing the realm name is not authenticated. This might result in the use of a realm which has been compromised, and would result in an attacker's ability to compromise the authentication of the application server to the client.]. If the client does not already possess a ticket-granting ticket for the appropriate realm, then one must be obtained. This is first attempted by requesting a ticket-granting ticket for the destination realm from the local Kerberos server (using the Kohl & Neuman [Page 25]
RFC 1510 Kerberos September 1993 KRB_TGS_REQ message recursively). The Kerberos server may return a TGT for the desired realm in which case one can proceed. Alternatively, the Kerberos server may return a TGT for a realm which is "closer" to the desired realm (further along the standard hierarchical path), in which case this step must be repeated with a Kerberos server in the realm specified in the returned TGT. If neither are returned, then the request must be retried with a Kerberos server for a realm higher in the hierarchy. This request will itself require a ticket-granting ticket for the higher realm which must be obtained by recursively applying these directions. Once the client obtains a ticket-granting ticket for the appropriate realm, it determines which Kerberos servers serve that realm, and contacts one. The list might be obtained through a configuration file or network service; as long as the secret keys exchanged by realms are kept secret, only denial of service results from a false Kerberos server. As in the AS exchange, the client may specify a number of options in the KRB_TGS_REQ message. 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-authorization- data field for application server use and additional tickets required by some options. In preparing the authentication header, the client can select a sub- session key under which the response from the Kerberos server will be encrypted (If the client selects a sub-session key, care must be taken to ensure the randomness of the selected subsession key. One approach would be to generate a random number and XOR it with the session key from the ticket-granting ticket.). If the sub-session key is not specified, the session key from the ticket-granting ticket 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 in the session key from the ticket-granting ticket. Once prepared, the message is sent to a Kerberos server for the destination realm. See section A.5 for pseudocode. 3.3.2. Receipt of KRB_TGS_REQ message The KRB_TGS_REQ message is processed in a manner similar to the KRB_AS_REQ message, but there are many additional checks to be performed. First, the Kerberos server must determine which server the accompanying ticket is for and it must select the appropriate key to decrypt it. For a normal KRB_TGS_REQ message, it will be for the Kohl & Neuman [Page 26]
RFC 1510 Kerberos September 1993 ticket granting service, and the TGS's key will be used. If the TGT was issued by another realm, then the appropriate inter-realm key must be used. If the accompanying ticket is not a ticket granting ticket for the current realm, but is for an application server in the current realm, the RENEW, VALIDATE, or PROXY options are specified in the request, and the server for which a ticket is requested is the server named in the accompanying ticket, then the KDC will decrypt the ticket in the authentication header using the key of the server for which it was issued. If no ticket can be found in the padata field, the KDC_ERR_PADATA_TYPE_NOSUPP error is returned. 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 rejected if the checksums do not match (with an error code of KRB_AP_ERR_MODIFIED) or if the checksum is not keyed or 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. If any of the decryptions indicate failed integrity checks, the KRB_AP_ERR_BAD_INTEGRITY error is returned. 3.3.3. Generation of KRB_TGS_REP message 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. The response will include a ticket for the requested server. The Kerberos database is queried to retrieve the record for the requested server (including the key with which the ticket will be encrypted). If the request is for a ticket granting ticket for a remote realm, and if no key is shared with the requested realm, then the Kerberos server will select the realm "closest" to the requested realm with which it does share a key, and use that realm instead. This is the only case where the response from the KDC will be for a different server than that requested by the client. 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 ticket-granting ticket (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. Kohl & Neuman [Page 27]
RFC 1510 Kerberos September 1993 If the request specifies an endtime, then the endtime of the new ticket is set to the minimum of (a) that request, (b) the endtime from the TGT, and (c) the starttime of the TGT plus the minimum of the maximum life for the application server and the maximum life for the local realm (the maximum life for the requesting principal was already applied when the TGT was issued). If the new ticket is to be a renewal, then the endtime above is replaced by the minimum of (a) the value of the renew_till field of the ticket and (b) the starttime for the new ticket plus the life (endtimestarttime) of the old ticket. 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 specified by the client. It will be honored only if the PROXIABLE flag in the TGT is set. The PROXY option will not be honored on requests for additional ticket-granting tickets. If the requested start time is absent or indicates a time in the past, then the start time of the ticket is set to the authentication server's current time. If it indicates a time in the future, 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 ticket-granting ticket has the MAYPOSTDATE 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 start time 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 ticket-granting ticket. If the ENC-TKT-IN-SKEY option has been specified and an additional ticket has been included in the request, the KDC will decrypt the additional ticket using the key for the server to which the additional ticket was issued and verify that it is a ticket-granting ticket. If the name of the requested server is missing from the request, the name of the client in the additional ticket will be used. Otherwise the name of the requested server will be compared to the name of the client in the additional ticket and if different, the request will be rejected. If the request succeeds, the session key from the additional ticket will be used to encrypt the new ticket that is issued instead of using the key of the server for which the new ticket will be used (This allows easy implementation of user-to- user authentication [6], which uses ticket-granting ticket session keys in lieu of secret server keys in situations where such secret Kohl & Neuman [Page 28]
RFC 1510 Kerberos September 1993 keys could be easily compromised.). If 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 ticket- granting server itself, and the server is registered in the realm of the KDC, If the RENEW option is requested, then the KDC will verify that the RENEWABLE flag is set in the ticket and that the renew_till time is still in the future. If the VALIDATE option is rqeuested, the KDC will check that the starttime has passed and the INVALID flag is set. If the PROXY option is requested, then the KDC will check that the PROXIABLE flag is set in the ticket. If the tests succeed, the KDC will issue the appropriate new ticket. Whenever a request is made to the ticket-granting server, the presented ticket(s) is(are) checked against a hot-list of tickets which have been canceled. This hot-list might be implemented by storing a range of issue dates for "suspect tickets"; if a presented ticket had an authtime in that range, it would be rejected. In this way, a stolen ticket-granting ticket or renewable ticket cannot be used to gain additional tickets (renewals or otherwise) once the theft has been reported. Any normal ticket obtained before it was reported stolen will still be valid (because they require no interaction with the KDC), but only until their normal expiration time. 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 the session key key from the ticket-granting ticket. 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 ticket-granting ticket. See section A.6 for pseudocode. 3.3.3.1. Encoding the transited field 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 ticket-granting ticket. The name of the realm that issued the ticket-granting ticket will be added to the transited field of the ticket to be issued. This is accomplished by reading the transited field from the ticket-granting ticket (which is treated as an unordered set of realm names), adding the new realm to the set, Kohl & Neuman [Page 29]
RFC 1510 Kerberos September 1993 then constructing and writing out its encoded (shorthand) form (this may involve a rearrangement of the existing encoding). Note that the ticket-granting service does not add the name of its own realm. Instead, its responsibility is to add the name of the previous realm. This prevents a malicious Kerberos server from intentionally leaving out its own name (it could, however, omit other realms' names). The names of neither the local realm nor the principal's realm are to be included in the transited field. They appear elsewhere in the ticket and both are known to have taken part in authenticating the principal. Since the endpoints are not included, both local and single-hop inter-realm authentication result in a transited field that is empty. Because the name of each realm transited is added to this field, 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. Realm names in the transited field are separated by a ",". The ",", "\", trailing "."s, and leading spaces (" ") are special characters, and if they are part of a realm name, they must be quoted in the transited field by preceding them with a "\". 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: "EDU,MIT.,ATHENA.,WASHINGTON.EDU,CS.". Note that if ATHENA.MIT.EDU, or CS.WASHINGTON.EDU were endpoints, that they would not be included in this field, and we would have: "EDU,MIT.,WASHINGTON.EDU" 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 to be the null realm ("")). If it 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: "/COM,/HP,/APOLLO, /COM/DEC". Kohl & Neuman [Page 30]
RFC 1510 Kerberos September 1993
RFC 1510 Kerberos September 1993 this checksum type is the old method for encoding the DESMAC checksum and it is no longer recommended. The DES specifications identify some "weak keys"; those keys shall not be used for generating DES-MAC checksums for use in Kerberos. 7. Naming Constraints 7.1. Realm Names Although realm names are encoded as GeneralStrings and although a realm can technically select any name it chooses, interoperability across realm boundaries requires agreement on how realm names are to be assigned, and what information they imply. To enforce these conventions, each realm must conform to the conventions itself, and it must require that any realms with which inter-realm keys are shared also conform to the conventions and require the same from its neighbors. There are presently four styles of realm names: domain, X500, other, and reserved. Examples of each style follow: domain: host.subdomain.domain (example) X500: C=US/O=OSF (example) other: NAMETYPE:rest/of.name=without-restrictions (example) reserved: reserved, but will not conflict with above Domain names must look like domain names: they consist of components separated by periods (.) and they contain neither colons (:) nor slashes (/). X.500 names contain an equal (=) and cannot contain a colon (:) before the equal. The realm names for X.500 names will be string representations of the names with components separated by slashes. Leading and trailing slashes will not be included. Names that fall into the other category must begin with a prefix that contains no equal (=) or period (.) and the prefix must be followed by a colon (:) and the rest of the name. All prefixes must be assigned before they may be used. Presently none are assigned. The reserved category includes strings which do not fall into the first three categories. All names in this category are reserved. It is unlikely that names will be assigned to this category unless there is a very strong argument for not using the "other" category. These rules guarantee that there will be no conflicts between the Kohl & Neuman [Page 78]
RFC 1510 Kerberos September 1993 various name styles. The following additional constraints apply to the assignment of realm names in the domain and X.500 categories: the name of a realm for the domain or X.500 formats must either 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. This is acceptable because the organization to which the parent is assigned is presumably the organization authorized to assign names to 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 there will not in the future exists a name identical to the realm name of the child unless it is assigned to the same entity as the realm name. 7.2. Principal Names 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 as a hint. 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). This constraint may be eliminated in the future. The following name types are defined: name-type value meaning 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 When a name implies no information other than its uniqueness at a particular time the name type PRINCIPAL should be used. The principal name type should be used for users, and it might also be used for a unique server. If the name is a unique machine generated ID that is guaranteed never to be reassigned then the name type of UID should be used (note that it is generally a bad idea to reassign names of any type since stale entries might remain in access control lists). Kohl & Neuman [Page 79]
RFC 1510 Kerberos September 1993 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 used. An example of this name type is the Kerberos ticket-granting ticket which has a first component of krbtgt and a second component identifying the realm for which the ticket is valid. If instance is a single component following the service name and the instance identifies 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 SRVXHST 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 UNKNOWN should be used when the form of the name is not known. When comparing names, a name of type UNKNOWN will match principals authenticated with names of any type. A principal authenticated with a name of type UNKNOWN, however, will only match other names of type UNKNOWN. Names of any type with an initial component of "krbtgt" are reserved for the Kerberos ticket granting service. See section 8.2.3 for the form of such names. 7.2.1. Name of server principals 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 lower case. For services such as telnet and the Berkeley R commands which run with system privileges, the first component will be the string "host" instead of a service specific identifier. 8. Constants and other defined values 8.1. Host address types All negative values for the host address type are reserved for local use. All non-negative values are reserved for officially assigned Kohl & Neuman [Page 80]
RFC 1510 Kerberos September 1993 type fields and interpretations. The values of the types for the following addresses are chosen to match the defined address family constants in the Berkeley Standard Distributions of Unix. They can be found in <sys/socket.h> with symbolic names AF_xxx (where xxx is an abbreviation of the address family name). Internet addresses Internet addresses are 32-bit (4-octet) quantities, encoded in MSB order. The type of internet addresses is two (2). CHAOSnet addresses CHAOSnet addresses are 16-bit (2-octet) quantities, encoded in MSB order. The type of CHAOSnet addresses is five (5). ISO addresses ISO addresses are variable-length. The type of ISO addresses is seven (7). Xerox Network Services (XNS) addresses XNS addresses are 48-bit (6-octet) quantities, encoded in MSB order. The type of XNS addresses is six (6). AppleTalk Datagram Delivery Protocol (DDP) addresses AppleTalk DDP addresses consist of an 8-bit node number and a 16- bit network number. The first octet of the address is the node number; the remaining two octets encode the network number in MSB order. The type of AppleTalk DDP addresses is sixteen (16). DECnet Phase IV addresses DECnet Phase IV addresses are 16-bit addresses, encoded in LSB order. The type of DECnet Phase IV addresses is twelve (12). 8.2. KDC messages 8.2.1. IP transport When contacting a Kerberos server (KDC) for a KRB_KDC_REQ request using IP transport, the client shall send a UDP datagram containing only an encoding of the request to port 88 (decimal) at the KDC's IP Kohl & Neuman [Page 81]
RFC 1510 Kerberos September 1993 address; 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. 8.2.2. OSI transport During authentication of an OSI client to and OSI server, the mutual authentication of an OSI server to an OSI client, the transfer of credentials from an OSI client to an OSI server, or during exchange of private or integrity checked messages, Kerberos protocol messages may be treated as opaque objects and the type of the authentication mechanism will be: OBJECT IDENTIFIER ::= {iso (1), org(3), dod(5),internet(1), security(5), kerberosv5(2)} Depending on the situation, the opaque object will be an authentication header (KRB_AP_REQ), an authentication reply (KRB_AP_REP), a safe message (KRB_SAFE), a private message (KRB_PRIV), or a credentials message (KRB_CRED). The opaque data contains an application code as specified in the ASN.1 description for each message. The application code may be used by Kerberos to determine the message type. 8.2.3. Name of the TGS The principal identifier of the ticket-granting service shall be composed of three parts: (1) the realm of the KDC issuing the TGS ticket (2) a two-part name of type NT-SRVINST, with the first part "krbtgt" and the second part the name of the realm which will accept the ticket-granting ticket. For example, a ticket-granting ticket issued by the ATHENA.MIT.EDU realm to be used to get tickets from the ATHENA.MIT.EDU KDC has a principal identifier of "ATHENA.MIT.EDU" (realm), ("krbtgt", "ATHENA.MIT.EDU") (name). A ticket-granting ticket 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). 8.3. Protocol constants and associated values The following tables list constants used in the protocol and defines their meanings. Kohl & Neuman [Page 82]
RFC 1510 Kerberos September 1993 ---------------+-----------+----------+----------------+--------------- Encryption type|etype value|block size|minimum pad size|confounder size ---------------+-----------+----------+----------------+--------------- NULL 0 1 0 0 des-cbc-crc 1 8 4 8 des-cbc-md4 2 8 0 8 des-cbc-md5 3 8 0 8 -------------------------------+-------------------+------------- Checksum type |sumtype value |checksum size -------------------------------+-------------------+------------- CRC32 1 4 rsa-md4 2 16 rsa-md4-des 3 24 des-mac 4 16 des-mac-k 5 8 rsa-md4-des-k 6 16 rsa-md5 7 16 rsa-md5-des 8 24 -------------------------------+----------------- padata type |padata-type value -------------------------------+----------------- PA-TGS-REQ 1 PA-ENC-TIMESTAMP 2 PA-PW-SALT 3 -------------------------------+------------- authorization data type |ad-type value -------------------------------+------------- reserved values 0-63 OSF-DCE 64 SESAME 65 -------------------------------+----------------- alternate authentication type |method-type value -------------------------------+----------------- reserved values 0-63 ATT-CHALLENGE-RESPONSE 64 -------------------------------+------------- transited encoding type |tr-type value -------------------------------+------------- DOMAIN-X500-COMPRESS 1 reserved values all others Kohl & Neuman [Page 83]
RFC 1510 Kerberos September 1993 --------------+-------+----------------------------------------- Label |Value |Meaning or MIT code --------------+-------+----------------------------------------- pvno 5 current Kerberos protocol version number message types KRB_AS_REQ 10 Request for initial authentication KRB_AS_REP 11 Response to KRB_AS_REQ request KRB_TGS_REQ 12 Request for authentication based on TGT KRB_TGS_REP 13 Response to KRB_TGS_REQ request KRB_AP_REQ 14 application request to server KRB_AP_REP 15 Response to KRB_AP_REQ_MUTUAL KRB_SAFE 20 Safe (checksummed) application message KRB_PRIV 21 Private (encrypted) application message KRB_CRED 22 Private (encrypted) message to forward credentials KRB_ERROR 30 Error response 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 error codes KDC_ERR_NONE 0 No error KDC_ERR_NAME_EXP 1 Client's entry in database has expired KDC_ERR_SERVICE_EXP 2 Server's entry in database has expired KDC_ERR_BAD_PVNO 3 Requested protocol version number not supported KDC_ERR_C_OLD_MAST_KVNO 4 Client's key encrypted in old master key KDC_ERR_S_OLD_MAST_KVNO 5 Server's key encrypted in old master key KDC_ERR_C_PRINCIPAL_UNKNOWN 6 Client not found in Kerberos database KDC_ERR_S_PRINCIPAL_UNKNOWN 7 Server not found in Kerberos database KDC_ERR_PRINCIPAL_NOT_UNIQUE 8 Multiple principal entries in database Kohl & Neuman [Page 84]
RFC 1510 Kerberos September 1993 KDC_ERR_NULL_KEY 9 The client or server has a null key KDC_ERR_CANNOT_POSTDATE 10 Ticket not eligible for postdating KDC_ERR_NEVER_VALID 11 Requested start time is later than end time KDC_ERR_POLICY 12 KDC policy rejects request KDC_ERR_BADOPTION 13 KDC cannot accommodate requested option KDC_ERR_ETYPE_NOSUPP 14 KDC has no support for encryption type KDC_ERR_SUMTYPE_NOSUPP 15 KDC has no support for checksum type KDC_ERR_PADATA_TYPE_NOSUPP 16 KDC has no support for padata type KDC_ERR_TRTYPE_NOSUPP 17 KDC has no support for transited type KDC_ERR_CLIENT_REVOKED 18 Clients credentials have been revoked KDC_ERR_SERVICE_REVOKED 19 Credentials for server have been revoked KDC_ERR_TGT_REVOKED 20 TGT has been revoked KDC_ERR_CLIENT_NOTYET 21 Client not yet valid - try again later KDC_ERR_SERVICE_NOTYET 22 Server not yet valid - try again later KDC_ERR_KEY_EXPIRED 23 Password has expired - change password to reset KDC_ERR_PREAUTH_FAILED 24 Pre-authentication information was invalid KDC_ERR_PREAUTH_REQUIRED 25 Additional pre-authentication required* 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 Kohl & Neuman [Page 85]
RFC 1510 Kerberos September 1993 message KRB_ERR_GENERIC 60 Generic error (description in e-text) KRB_ERR_FIELD_TOOLONG 61 Field is too long for this implementation *This error carries additional information in the e-data field. The contents of the e-data field for this message is described in section 5.9.1. 9. Interoperability requirements Version 5 of the Kerberos protocol supports a myriad of options. Among these are multiple encryption and checksum types, alternative encoding schemes for the transited field, optional mechanisms for pre-authentication, the handling of tickets with no addresses, options for mutual authentication, user to user authentication, support for proxies, forwarding, postdating, and renewing tickets, the format of realm names, and the handling of authorization data. In order to ensure the interoperability of realms, it is necessary to define a minimal configuration which 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. 9.1. Specification 1 This section defines the first specification of these options. Implementations which are configured in this way can be said to support Kerberos Version 5 Specification 1 (5.1). Encryption and checksum methods The following encryption and checksum mechanisms must be supported. Implementations may support other mechanisms as well, but the additional mechanisms may only be used when communicating with principals known to also support them: Encryption: DES-CBC-MD5 Checksums: CRC-32, DES-MAC, DES-MAC-K, and DES-MD5 Realm Names All implementations must understand hierarchical realms in both the Internet Domain and the X.500 style. When a ticket granting ticket for an unknown realm is requested, the KDC must be able to determine the names of the intermediate realms between the KDCs realm and the requested realm. Kohl & Neuman [Page 86]
RFC 1510 Kerberos September 1993
RFC 1510 Kerberos September 1993 body.ticket-info[n].srealm = tickets[n].srealm; body.ticket-info[n].sname = tickets[n].sname; body.ticket-info[n].caddr = tickets[n].caddr; done get system_time; body.timestamp, body.usec := system_time; if (using nonce) then body.nonce := nonce; endif if (using s-address) then body.s-address := sender host addresses; endif if (limited recipients) then body.r-address := recipient host address; endif encode body into OCTET STRING; select encryption type; encrypt OCTET STRING into packet.enc-part.cipher using negotiated encryption key; A.19. KRB_CRED verification receive packet; if (packet.pvno != 5) then either process using other protocol spec or error_out(KRB_AP_ERR_BADVERSION); endif if (packet.msg-type != KRB_CRED) then error_out(KRB_AP_ERR_MSG_TYPE); endif cleartext := decrypt(packet.enc-part) using negotiated key; if (decryption_error()) then error_out(KRB_AP_ERR_BAD_INTEGRITY); endif if ((packet.r-address is present or required) and (packet.s-address != O/S_sender(packet)) then /* O/S report of sender not who claims to have sent it */ error_out(KRB_AP_ERR_BADADDR); endif if ((packet.r-address is present) and (packet.r-address != local_host_address)) then /* was not sent to proper place */ error_out(KRB_AP_ERR_BADADDR); Kohl & Neuman [Page 111]
RFC 1510 Kerberos September 1993 endif if (not in_clock_skew(packet.timestamp,packet.usec)) then error_out(KRB_AP_ERR_SKEW); endif if (repeated(packet.timestamp,packet.usec,packet.s-address)) then error_out(KRB_AP_ERR_REPEAT); endif if (packet.nonce is required or present) and (packet.nonce != expected-nonce) then error_out(KRB_AP_ERR_MODIFIED); endif for (ticket[n] in tickets that were forwarded) do save_for_later(ticket[n],key[n],principal[n], server[n],times[n],flags[n]); return A.20. KRB_ERROR generation /* assemble packet: */ packet.pvno := protocol version; /* 5 */ packet.msg-type := message type; /* KRB_ERROR */ get system_time; packet.stime, packet.susec := system_time; packet.realm, packet.sname := server name; if (client time available) then packet.ctime, packet.cusec := client_time; endif packet.error-code := error code; if (client name available) then packet.cname, packet.crealm := client name; endif if (error text available) then packet.e-text := error text; endif if (error data available) then packet.e-data := error data; endif



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