RFC 1510
The Kerberos Network Authentication Service (V5)
Network Working Group J. Kohl Request for Comments: 1510 Digital Equipment Corporation C. Neuman ISI September 1993 The Kerberos Network Authentication Service (V5) Status of this Memo This RFC specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" for the standardization state and status of this protocol. Distribution of this memo is unlimited. Abstract This document gives an overview and specification of Version 5 of the protocol for the Kerberos network authentication system. Version 4, described elsewhere [1,2], is presently in production use at MIT's Project Athena, and at other Internet sites. 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.
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
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
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
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
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
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
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
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.
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.
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.
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
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-
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.
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.
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
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
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:
+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
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.
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