INTERNET-DRAFT Clifford Neuman John Kohl Theodore Ts'o Tom Yu Sam Hartman Ken Raeburn February 22, 2002 Expires 22 August, 2002 The Kerberos Network Authentication Service (V5) draft-ietf-krb-wg-kerberos-clarifications-00.txt STATUS OF THIS MEMO This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. To learn the current status of any Internet-Draft, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.ietf.org (US East Coast), nic.nordu.net (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim). The distribution of this memo is unlimited. It is filed as draft-ietf-krb-wg-kerberos-clarifications-00.txt, and expires August 22, 2002. Please send comments to: ietf-krb-wg@anl.gov ABSTRACT This document provides an overview and specification of Version 5 of the Kerberos protocol, and updates RFC1510 to clarify aspects of the protocol and its intended use that require more detailed or clearer explanation than was provided in RFC1510. This document is intended to provide a detailed description of the protocol, suitable for implementation, together with descriptions of the appropriate use of protocol messages and fields within those messages. This document contains a subset of the changes considered and discussed in the Kerberos working group and is intended as an interim description of Kerberos. Additional changes to the Kerberos protocol have been proposed and will appear in a subsequent extensions document. This document is not intended to describe Kerberos to the end user, system administrator, or application developer. Higher level papers describing Version 5 of the Kerberos system [NT94] and documenting version 4 [SNS88], are available elsewhere. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 OVERVIEW This INTERNET-DRAFT describes the concepts and model upon which the Kerberos network authentication system is based. It also specifies Version 5 of the Kerberos protocol. The motivations, goals, assumptions, and rationale behind most design decisions are treated cursorily; they are more fully described in a paper available in IEEE communications [NT94] and earlier in the Kerberos portion of the Athena Technical Plan [MNSS87]. The protocols have been a proposed standard and are being considered for advancement for draft standard through the IETF standard process. Comments are encouraged on the presentation, but only minor refinements to the protocol as implemented or extensions that fit within current protocol framework will be considered at this time. 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 [NS78] and on modifications suggested by Denning and Sacco [DS81]. The original design and implementation of Kerberos Versions 1 through 4 was the work of two former Project Athena staff members, Steve Miller of Digital Equipment Corporation and Clifford Neuman (now at the Information Sciences Institute of the University of Southern California), along with Jerome Saltzer, Technical Director of Project Athena, and Jeffrey Schiller, MIT Campus Network Manager. Many other members of Project Athena have also contributed to the work on Kerberos. Version 5 of the Kerberos protocol (described in this document) has evolved from Version 4 based on new requirements and desires for features not available in Version 4. The design of Version 5 of the Kerberos protocol was led by Clifford Neuman and John Kohl with much input from the community. The development of the MIT reference implementation was led at MIT by John Kohl and Theodore T'so, with help and contributed code from many others. Since RFC1510 was issued, extensions and revisions to the protocol have been proposed by many individuals. Some of these proposals are reflected in this document. Where such changes involved significant effort, the document cites the contribution of the proposer. Reference implementations of both version 4 and version 5 of Kerberos are publicly available and commercial implementations have been developed and are widely used. Details on the differences between Kerberos Versions 4 and 5 can be found in [KNT92]. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 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 assertions 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[1.1]. Kerberos performs authentication under these conditions as a trusted third-party authentication service by using conventional (shared secret key [1.2]) cryptography. Kerberos extensions (outside the scope of this document) can provide for the use of public key cryptography during certain phases of the authentication protocol [@RFCE: if PKINIT advances concurrently include reference to the RFC here]. Such extensions support Kerberos authentication for users registered with public key certification authorities and provide certain benefits of public key cryptography in situations where they are needed. The basic Kerberos 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 a "ticket" for the server and a temporary encryption key (often called a "session key"). The client transmits the ticket (which contains the client's identity and a copy of the session key, all encrypted in the server's key) to the server. The session key (now shared by the client and server) is used to authenticate the client, and may optionally be used to authenticate the server. It may also be used to encrypt further communication between the two parties or to exchange a separate sub-session key to be used to encrypt further communication. Implementation of the basic protocol 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 directly or through the Generic Security Services Application Programming Interface, GSSAPI, described in separate document [ref to GSSAPI RFC]. These calls result in the transmission of the necessary messages to achieve authentication. The Kerberos protocol consists of several sub-protocols (or exchanges). There are two basic methods by which a client can ask a Kerberos server for credentials. In the first approach, the client sends a cleartext request for a ticket for the desired server to the AS. The reply is sent encrypted in the client's secret key. Usually this request is for a ticket-granting ticket (TGT) which can later be used with the ticket-granting server (TGS). In the second method, the client sends a request to the TGS. The client uses the TGT to authenticate itself to the TGS in the same manner as if it were contacting any other application server that requires Kerberos authentication. The reply is encrypted in the session key from the TGT. Though the protocol specification describes the AS and the TGS as separate servers, they are implemented in practice as different protocol entry points within a single Kerberos server. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 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 application 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 originated with the principal to whom the ticket was issued. This information (called the authenticator) is encrypted in the session key, and includes a timestamp. The timestamp proves that the message was recently generated and is not a replay. Encrypting the authenticator in the session key proves that it was generated by a party possessing the session key. Since no one except the requesting principal and the server know the session key (it is never sent over the network in the clear) this guarantees the identity of the client. 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 contained in the ticket or the sub-session key found in the authenticator. 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). There is also a protocol for maintaining multiple copies of the Kerberos database. Neither of these protocols are described in this document. 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 prove its identity to servers in other realms[1.3]. 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. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 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 a 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. The application server is ultimately responsible for accepting or rejecting authentication and should check the transited field. The application server may choose to rely on the KDC for the application server's realm to check the transited field. The application server's KDC will set the TRANSITED-POLICY-CHECKED flag in this case. The KDC's for intermediate realms may also check the transited field as they issue ticket-granting-tickets for other realms, but they are encouraged not to do so. A client may request that the KDC's not check the transited field by setting the DISABLE-TRANSITED-CHECK flag. KDC's are encouraged but not required to honor this flag. 1.2. Choosing a principal with which to communicate The Kerberos protocol provides the means for verifying (subject to the assumptions in 1.4) that the entity with which one communicates is the same entity that was registered with the KDC using the claimed identity (principal name). It is still necessary to determine whether that identity corresponds to the entity with which one intends to communicate. When appropriate data has been exchanged in advance, this determination may be performed syntactically by the application based on the application protocol specification, information provided by the user, and configuration files. For example, the server principal name (including realm) for a telnet server might be derived from the user specified host name (from the telnet command line), the "host/" prefix specified in the application protocol specification, and a mapping to a Kerberos realm derived syntactically from the domain part of the specified hostname and information from the local Kerberos realms database. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 One can also rely on trusted third parties to make this determination, but only when the data obtained from the third party is suitably integrity protected wile resident on the third party server and when transmitted. Thus, for example, one should not rely on an unprotected domain name system record to map a host alias to the primary name of a server, accepting the primary name as the party one intends to contact, since an attacker can modify the mapping and impersonate the party with which one intended to communicate. 1.3. Authorization As an authentication service, Kerberos provides a means of verifying the identity of principals on a network. Authentication is usually useful primarily as a first step in the process of authorization, determining whether a client may use a service, which objects the client is allowed to access, and the type of access allowed for each. Kerberos does not, by itself, provide authorization. Possession of a client ticket for a service provides only for authentication of the client to that service, and in the absence of a separate authorization procedure, it should not be considered by an application as authorizing the use of that service. Such separate authorization methods may be implemented as application specific access control functions and may utilize files on the application server, or on separately issued authorization credentials such as those based on proxies [Neu93], or on other authorization services. Separately authenticated authorization credentials may be embedded in a tickets authorization data when encapsulated by the kdc-issued authorization data element. Applications should not accept the mere issuance of a service ticket by the Kerberos server (even by a modified Kerberos server) as granting authority to use the service, since such applications may become vulnerable to the bypass of this authorization check in an environment if they interoperate with other KDCs or where other options for application authentication (e.g. the PKTAPP proposal) are provided. 1.4. Extending Kerberos Without Breaking Interoperability As the deployed base of Kerberos implementations grows, extending Kerberos becomes more important. Unfortunately some extensions to the existing Kerberos protocol create interoperability issues because of uncertainty regarding the treatment of certain extensibility options by some implementations. This section includes guidelines that will enable future implementations to maintain interoperability. Kerberos provides a general mechanism for protocol extensibility. Some protocol messages contain typed holes -- sub-messages that contain an octet-string along with an integer that defines how to interpret the octet-string. The integer types are registered centrally, but can be used both for vendor extensions and for extensions standardized through the IETF. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 1.4.1. Compatibility with RFC 1510 It is important to note that existing Kerberos message formats can not be readily extended by adding fields to the ASN.1 types. Sending additional fields often results in the entire message being discarded without an error indication. Future versions of this specification will provide guidelines to ensure that ASN.1 fields can be added without creating an interoperability problem. In the meantime, all new or modified implementations of Kerberos that receive an unknown message extension should preserve the encoding of the extension but otherwise ignore the presence of the extension. Recipients MUST NOT decline a request simply because an extension is present. There is one exception to this rule. If an unknown authorization data element type is received by a server either in an AP-REQ or in a ticket contained in an AP-REQ, then authentication SHOULD fail. Authorization data is intended to restrict the use of the ticket. If the service cannot determine whether the restriction applies to that service then a security weakness may result if the ticket can be used for that service. Authorization elements that are optional can be enclosed in AD-IF-RELEVANT element. Implementation note: Many RFC 1510 implementations ignore unknown authorization data elements. Depending on these implementations to honor authorization data restrictions may create a security weakness. 1.4.2. Sending Extensible Messages Care must be taken to ensure that old implementations can understand messages sent to them even if they do not understand an extension that is used. Unless the sender knows an extension is supported, the extension cannot change the semantics of the core message or previously defined extensions. For example, an extension including key information necessary to decrypt the encrypted part of a KDC-REP could only be used in situations where the recipient was known to support the extension. Thus when designing such extensions it is important to provide a way for the recipient to notify the sender of support for the extension. For example in the case of an extension that changes the KDC-REP reply key, the client could indicate support for the extension by including a padata element in the AS-REQ sequence. The KDC should only use the extension if this padata element is present in the AS-REQ. Even if policy requires the use of the extension, it is better to return an error indicating that the extension is required than to use the extension when the recipient may not support it; debugging why implementations do not interoperate is easier when errors are returned. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 1.6. 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 the protocols where an intruder can prevent an application from participating in the proper authentication steps. Detection and solution of such attacks (some of which can appear to be not-uncommon "normal" failure modes for the system) 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, but is typically on the order of 5 minutes. 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. 1.7. 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. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 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 named client or server entity that participates in a network communication, with one name that is considered canonical. Principal identifier The canonical 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 may be derived from a password. Server A particular Principal which provides a resource to network clients. The server is sometimes referred to as the Application Server. 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". draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 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 attributes of that ticket. Most flags may be requested by a client when the ticket is obtained; some are automatically turned on and off by a Kerberos server as required. The following sections explain what the various flags mean and give examples of reasons to use them. With the exception of the ANONYMOUS and INVALID flags clients MUST ignore ticket flags that are not recognized. KDCs MUST ignore KDC options that are not recognized. Some implementations of RFC 1510 are known to reject unknown KDC options, so clients may need to resend a request without KDC new options absent if the request was rejected when sent with option added since RFC 1510. Since new KDCs will ignore unknown options, clients MUST confirm that the ticket returned by the KDC meets their needs. For example, as discussed in section 2.8, a client requiring anonymous communication needs to make sure that the ticket is actually anonymous. A KDC that prohibits issuing of anonymous tickets or that does not understand the anonymous option would not return an anonymous ticket. Note that it is not in general possible to determine whether an option was not honored because it was not understood or because it was rejected either through configuration or policy. When adding a new option to the Kerberos protocol, designers should consider whether the distinction is important for their option. In cases where it is, a mechanism for the KDC to return an indication that the option was understood but rejected needs to be provided in the specification of the option. Often in such cases, the mechanism needs to be broad enough to permit an error or reason to be returned. 2.1. Initial, pre-authenticated, and hardware 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 demonstrated knowledge of a client's secret key (e.g. a password-changing 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 additional information about the initial authentication, regardless of whether the current ticket was issued directly (in which case INITIAL will also be set) or issued on the basis of a 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). draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 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) (see section 3.3.3.1). 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 short-lived tickets and obtaining new ones periodically would require the client to have long-term access to its secret key, an even greater risk. Renewable tickets can be used to mitigate the consequences of theft. Renewable tickets have two "expiration times": the first is when the current instance of the ticket expires, and the second is the latest permissible value for an individual expiration time. An application client must periodically (i.e. before it expires) present a renewable ticket to the KDC, with the RENEW option set in the KDC request. The KDC will issue a new ticket with a new session key and a later expiration time. All other fields of the ticket are left unmodified by the renewal process. When the latest permissible expiration time arrives, the ticket expires permanently. At each renewal, the KDC may consult a hot-list to determine 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. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 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 process of granting a proxy using the proxy and proxiable flags is used to provide credentials for use with specific services. Though conceptually also a proxy, user's wishing to delegate their identity for ANY purpose must use the ticket forwarding mechanism described in the next section to forward a ticket granting ticket. 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 if requested by the client on initial authentication. By default, the client will request that it be set when requesting a ticket granting ticket, and reset when requesting any other ticket. 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[2.1]. When granting a proxy, the client must specify the new network address from which the proxy is to be used, or indicate that the proxy is to be issued for use from any address. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 The PROXY flag is set in a ticket by the TGS when it issues a proxy ticket. Application servers may check this flag and at their option they may 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 a proxy where the service granted is 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 result can still be achieved if the user engages in the AS exchange specifying 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 a forwarded ticket 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 choose to process FORWARDED tickets differently than non-FORWARDED tickets. 2.7 Transited Policy Checking In Kerberos, the application server is ultimately responsible for accepting or rejecting authentication and should check that only suitably trusted KDC's are relied upon to authenticate a principal. The transited field in the ticket identifies which KDC's were involved in the authentication process and an application server would normally check this field. While the end server ultimately decides whether authentication is valid, the KDC for the end server's realm may apply a realm specific policy for validating the transited field and accepting credentials for cross-realm authentication. When the KDC applies such checks and accepts such cross-realm authentication it will set the TRANSITED-POLICY-CHECKED flag in the service tickets it issues based on the cross-realm TGT. A client may request that the KDC's not check the transited field by setting the DISABLE-TRANSITED-CHECK flag. KDC's are encouraged but not required to honor this flag. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 2.8 Anonymous Tickets When policy allows, a KDC may issue anonymous tickets for the purpose of enabling encrypted communication between a client and server without identifying the client to the server. Such anonymous tickets are issued with a generic principal name configured on the KDC (e.g. "anonymous@") and will have the ANONYMOUS flag set. A server accepting such a ticket may assume that subsequent requests using the same ticket and session key originate from the same user. Requests with the same username but different tickets are likely to originate from different users. Users request anonymous ticket by setting the REQUEST-ANONYMOUS option in an AS or TGS request. If a client requires anonymous communication then the client should check to make sure that the resulting ticket is actually anonymous. A KDC that does not understand the anonymous-requested flag will not return an error, but will instead return a normal ticket. 2.9. Other KDC options There are three additional options which may be set in a client's request of the KDC. 2.9.1 Renewable-OK 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. 2.9.2 ENC-TKT-IN-SKEY In its basic form the Kerberos protocol supports authentication in a client server setting and is not well suited to authentication in a peer-to-peer environment because the long term key of the user does not remain on the workstation after initial login. Authentication of such peers may be supported by Kerberos in its user-to-user variant. The ENC-TKT-IN-SKEY option supports user-to-user authentication by allowing the KDC to issue a service ticket encrypted using the session key from another ticket granting ticket issued to another user. The ENC-TKT-IN-SKEY option is honored only by the ticket-granting service. It indicates that the ticket to be issued for the end server is to be encrypted in the session key from the additional second ticket-granting ticket provided with the request. See section 3.3.3 for specific details. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 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 initiated by a client when it wishes to obtain authentication credentials for a given server but currently holds no credentials. In its basic form, 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[3.1]. This exchange does not by itself provide any assurance of the the identity of the user[3.2]. 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. Without pre-authentication, the authentication server does not know whether the client is actually the principal named in the request. It simply sends a reply without knowing or caring whether they are the same. This is acceptable because nobody but the principal whose identity was given in the request will be able to use the reply. Its critical information is encrypted in that principal's key. However, an attacker can send a KRB_AS_REQ message to get known plaintext in order to attack the principal's key. Especially if the key is based on a password, this may create a security exposure. So, the initial request supports an optional field that can be used to pass additional information that might be needed for the initial exchange. This field should be used for pre-authentication as described in section 3.1.1. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 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 contains information which can be used to associate it with the message to which it replies. The contents of the KRB_ERROR message are not integrity-protected. As such, the client cannot detect replays, fabrications or modifications. A solution to this problem will be included in a future version of the protocol. 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 pre-authentication 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; whether the client requests an anonymous ticket; 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 non-renewable ticket (due to configuration constraints; see section 4). 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 the requested client principal named in the request is not known because it doesn't exist in the KDC's principal database, then an error message with a KDC_ERR_C_PRINCIPAL_UNKNOWN is returned. 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 pre-authentication is required, but was not present in the request, an error message with the code KDC_ERR_PREAUTH_REQUIRED is returned and the PA-ETYPE-INFO pre-authentication field will be included in the KRB-ERROR message. If the server cannot accommodate an encryption type requested by the client, an error message with code KDC_ERR_ETYPE_NOSUPP is returned. Otherwise the KDC generates a 'random' session key[3.3]. When responding to an AS request, if there are multiple encryption keys registered for a client in the Kerberos database , then the etype field from the AS request is used by the KDC to select the encryption method to be used to protect the encrypted part of the KRB_AS_REP message which is sent to the client. If there is more than one supported strong encryption type in the etype list, the first valid etype for which an encryption key is available is used. The encryption method used to protect the encrypted part of the KRB_TGS_REP message is the keytype of the session key found in the ticket granting ticket presented in the KRB_TGS_REQ. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 When the user's key is generated from a password or pass phrase, the string-to-key function for the particular encryption key type is used, as specified in [KCRYPTO]. The salt value and additional parameters for the string-to-key function have default values (specified by section 6 and by the encryption mechanism specification, respectively) that may be overridden by preauthentication data (PA-PW-SALT, PA-AFS3-SALT, PA-ETYPE-INFO, PA-S2K-PARAMS, etc). Since the KDC is presumed to store a copy of the resulting key only, these values should not be changed for password-based keys except when changing the principal's key. It is not possible to reliably generate a user's key given a pass phrase without contacting the KDC, since it will not be known whether alternate salt or parameter values are required. When the etype field is present in a KDC request, whether an AS or TGS request, the KDC will attempt to assign the type of the random session key from the list of methods in the etype field. The KDC will select the appropriate type using the list of methods provided together with information from the Kerberos database indicating acceptable encryption methods for the application server. The KDC will not issue tickets with a weak session key encryption type. If the requested start time is absent, indicates a time in the past, or is within the window of acceptable clock skew for the KDC and the POSTDATE option has not been specified, then the start time of the ticket is set to the authentication server's current time. If it indicates a time in the future beyond the acceptable clock skew, but the POSTDATED option has not been specified 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 earlier of the requested endtime and a time determined by local policy, possibly determined using realm or principal specific factors. For example, the expiration time may 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 from the authentication server's database (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). draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 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, ANONYMOUS. If the new ticket is post-dated (the start time is in the future), its INVALID flag will also be set. If all of the above succeed, the server will encrypt ciphertext part of the ticket using the encryption key extracted from the server principal's record in the Kerberos database using the encryption type associated with the server principal's key (this choice is NOT affected by the etype field in the request). It then formats a KRB_AS_REP message (see section 5.4.2), copying the addresses in the request into the caddr of the response, placing any required pre-authentication data into the padata of the response, and encrypts the ciphertext part in the client's key using an acceptable encryption method requested in the etype field of the request, or in some key specified by pre-authentication mechanisms being used. 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, e-text, and optional e-cksum 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 (or are otherwise expected values), 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). Proper decryption of the KRB_AS_REP message is not sufficient for the host to verify the identity of the user; the user and an attacker could cooperate to generate a KRB_AS_REP format message 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 for the host. If those credentials can be verified, then the identity of the user can be assured. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 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[3.4], 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 the ticket. 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 TGS exchange) a ticket and session key for the desired service. The client may re-use any tickets it holds until they expire. To use a ticket the client constructs a new Authenticator from the 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[3.5]. 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. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 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 [3.6]. 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 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; this normally is caused by a client error or attempted attack. 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. Unless the application server provides its own suitable means to protect against replay (for example, a challenge-response sequence initiated by the server after authentication, or use of a server-generated encryption subkey), the server must utilize a replay cache to remember any authenticator presented within the allowable clock skew. Careful analysis of the application protocol and implementation is recommended before eliminating this cache. The replay cache will store the server name, along with the client name, time and microsecond fields from the recently-seen authenticators and if a matching tuple is found, the KRB_AP_ERR_REPEAT error is returned [3.7]. If a server loses track of authenticators presented within the allowable clock skew, it must reject all requests until the clock skew interval has passed, providing assurance that any lost or re-played authenticators will fall outside the allowable clock skew and can no longer be successfully replayed[3.8]. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 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. If multiple servers (for example, different services on one machine, or a single service implemented on multiple machines) share a service principal (a practice we do not recommend in general, but acknowledge will be used in some cases), they should also share this replay cache, or the application protocol should be designed so as to eliminate the need for it. Note that this applies to all of the services, if any of the application protocols does not have replay protection built in; an authenticator used with such a service could later be replayed to a different service with the same service principal but no replay protection, if the former doesn't record the authenticator information in the common replay cache. The server computes the age of the ticket: local (server) time minus the 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. Passing these checks provides only authentication of the named principal; it does not imply authorization to use the named service. Applications must make a separate authorization decisions based upon the authenticated name of the user, the requested operation, local access control information such as that contained in a .k5login or .k5users file, and possibly a separate distributed authorization service. 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)[3.9]. 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. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 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[3.10] 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. 3.2.6. Using the encryption key [This seems inconsistent with crypto-architecture; we should look at before publication.] 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. 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 several alternatives. 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 session key from the ticket and subkeys in the KRB_AP_REP message and the authenticator[3.11]. To mitigate the effect of failures in random number generation on the client it is strongly encouraged that any key derived by an application for subsequent use include the full key entropy derived from the KDC generated session key carried in the ticket. 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, but an example of how this might be done follows. One way that an application may choose to negotiate a key to be used for subsequent integrity and privacy protection is for the client to propose a key in the subkey field of the authenticator. The server can then choose a key using the proposed key from the client as input, returning the new subkey in the subkey field of the application reply. This key could then be used for subsequent communication. To make this example more concrete, if the communication patterns of an application dictates the use of encryption modes of operation incompatible with the encryption system used for the authenticator, then a key compatible with the required encryption system may be generated by either the client, the server, or collaboratively by both and exchanged using the subkey field. This generation might involve the use of a random number as a pre-key, initially generated by either party, which could then be encrypted using the session key from the ticket, and the result exchanged and used for subsequent encryption. By encrypting the pre-key with the session key from the ticket, randomness from the KDC generated key is assured of being present in the negotiated key. Application developers must be careful however, to use a means of introducing this entropy that does not allow an attacker to learn the session key from the ticket if it learns the key generated and used for subsequent communication. The reader should note that this is only an example, and that an analysis of the particular cryptosystem to be used, must be made before deciding how to generate values for the subkey fields, and the key to be used for subsequent communication. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 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 response 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 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 sub-session 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. The same comments about integrity protection of KRB_ERROR messages mentioned in section 3.1 apply to the TGS exchange. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 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 believed to be registered[3.12]. If the client knows the service principal name and realm and it 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 a Kerberos server for which the client possesses a ticket-granting ticket (using the 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 between the client's realm and the requested realm server's realm). 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 or it may be generated from the name of the realm; as long as the secret keys exchanged by realms are kept secret, only denial of service results from using a false Kerberos server. 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[3.13]. 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, using the session key from the ticket-granting ticket. Once prepared, the message is sent to a Kerberos server for the destination realm. 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 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. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 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 or for a ticket granting server of an intermediate KDC to be contacted to obtain the requested ticket. The Kerberos database is queried to retrieve the record for the appropriate 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. If the requested server cannot be found in the TGS database, then a TGT for another trusted realm may be returned instead of a ticket for the service. This TGT is a referral mechanism to cause the client to retry the request to the realm of the TGT. These are the only cases where the response for 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. 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 (endtime-starttime) 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. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 If the requested start time is absent, indicates a time in the past, or is within the window of acceptable clock skew for the KDC and the POSTDATE option has not been specified, then the start time of the ticket is set to the authentication server's current time. If it indicates a time in the future beyond the acceptable clock skew, but the POSTDATED option has not been specified or the MAY-POSTDATE flag is not set in the TGT, then the error KDC_ERR_CANNOT_POSTDATE is returned. Otherwise, if the ticket-granting ticket has the MAY-POSTDATE flag set, then the resulting ticket will be postdated and the requested starttime is checked against the policy of the local realm. If acceptable, the ticket's 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. 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, the server is registered in the realm of the KDC, and the RENEW option is requested, then the KDC will verify that the RENEWABLE flag is set in the ticket, that the INVALID flag is not set in the ticket, and that the renew_till time is still in the future. If the VALIDATE option is requested, the KDC will check that the starttime has passed and 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, and the ticket passes the hotlist check described in the next section, the KDC will issue the appropriate new ticket. 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. 3.3.3.1. Checking for revoked tickets 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 timestamps for 'suspect tickets'; if a presented ticket had an authtime in that range, it would be rejected. In this way, a stolen ticket-granting draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 ticket or renewable ticket cannot be used to gain additional tickets (renewals or otherwise) once the theft has been reported to the KDC for the realm in which the server resides. Any normal ticket obtained before it was reported stolen will still be valid (because they require no interaction with the KDC), but only until their normal expiration time. If TGT's have been issued for cross-realm authentication, use of the cross-realm TGT will not be affected unless the hot-list is propagated to the KDC's for the realms for which such cross-realm tickets were issued. 3.3.3.2. 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, 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.". draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 Note that if ATHENA.MIT.EDU, or CS.WASHINGTON.EDU were end-points, 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[18]. 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". Like the example above, if /COM/HP/APOLLO and /COM/DEC are endpoints, they they would not be included in this field, and we would have: "/COM,/HP" A null subfield preceding or following a "," indicates that all realms between the previous realm and the next realm have been traversed[19]. Thus, "," means that all realms along the path between the client and the server have been traversed. ",EDU, /COM," means that that all realms from the client's realm up to EDU (in a domain style hierarchy) have been traversed, and that everything from /COM down to the server's realm in an X.500 style has also been traversed. This could occur if the EDU realm in one hierarchy shares an inter-realm key directly with the /COM realm in another hierarchy. 3.3.4. Receipt of KRB_TGS_REP message When the KRB_TGS_REP is received by the client, it is processed in the same manner as the KRB_AS_REP processing described above. The primary difference is that the ciphertext part of the response must be decrypted using the session key from the ticket-granting ticket rather than the client's secret key. The server name returned in the reply is the true principal name of the service. 3.4. The KRB_SAFE Exchange The KRB_SAFE message may be used by clients requiring the ability to detect modifications of messages they exchange. It achieves this by including a keyed collision-proof checksum of the user data and some control information. The checksum is keyed with an encryption key (usually the last key negotiated via subkeys, or the session key if no negotiation has occurred). 3.4.1. Generation of a KRB_SAFE message When an application wishes to send a KRB_SAFE message, it collects its data and the appropriate control information and computes a checksum over them. The checksum algorithm should be the keyed checksum mandated to be implemented along with the crypto system used for the sub-session or session key. The checksum is generated using the sub-session key if present, or the session key. Some implementations use a different checksum algorithm for KRB_SAFE messages but doing so in a interoperable manner is impossible. Implementations should accept any checksum algorithm they implement that both has adequate security and that has keys compatible with the sub-session or session key. Unkeyed or non-collision-proof checksums are not suitable for this use. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 The control information for the KRB_SAFE message includes both a timestamp and a sequence number. The designer of an application using the KRB_SAFE message must choose at least one of the two mechanisms. This choice should be based on the needs of the application protocol. Sequence numbers are useful when all messages sent will be received by one's peer. Connection state is presently required to maintain the session key, so maintaining the next sequence number should not present an additional problem. If the application protocol is expected to tolerate lost messages without them being resent, the use of the timestamp is the appropriate replay detection mechanism. Using timestamps is also the appropriate mechanism for multi-cast protocols where all of one's peers share a common sub-session key, but some messages will be sent to a subset of one's peers. After computing the checksum, the client then transmits the information and checksum to the recipient in the message format specified in section 5.6.1. 3.4.2. Receipt of KRB_SAFE message When an application receives a KRB_SAFE message, it verifies it as follows. If any error occurs, an error code is reported for use by the application. The message is first checked by verifying that the protocol version and type fields match the current version and KRB_SAFE, respectively. A mismatch generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE error. The application verifies that the checksum used is a collision-proof keyed checksum that uses keys compatible with the sub-session or session key as appropriate, and if it is not, a KRB_AP_ERR_INAPP_CKSUM error is generated. If the sender's address was included in the control information, the recipient verifies that the operating system's report of the sender's address matches the sender's address in the message, and (if a recipient address is specified or the recipient requires an address) that one of the recipient's addresses appears as the recipient's address in the message. A failed match for either case generates a KRB_AP_ERR_BADADDR error. Then the timestamp and usec and/or the sequence number fields are checked. If timestamp and usec are expected and not present, or they are present but not current, the KRB_AP_ERR_SKEW error is generated. If the server name, along with the client name, time and microsecond fields from the Authenticator match any recently-seen (sent or received[20] ) such tuples, the KRB_AP_ERR_REPEAT error is generated. If an incorrect sequence number is included, or a sequence number is expected but not present, the KRB_AP_ERR_BADORDER error is generated. If neither a time-stamp and usec or a sequence number is present, a KRB_AP_ERR_MODIFIED error is generated. Finally, the checksum is computed over the data and control information, and if it doesn't match the received checksum, a KRB_AP_ERR_MODIFIED error is generated. If all the checks succeed, the application is assured that the message was generated by its peer and was not modified in transit. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 3.5. The KRB_PRIV Exchange The KRB_PRIV message may be used by clients requiring confidentiality and the ability to detect modifications of exchanged messages. It achieves this by encrypting the messages and adding control information. 3.5.1. Generation of a KRB_PRIV message When an application wishes to send a KRB_PRIV message, it collects its data and the appropriate control information (specified in section 5.7.1) and encrypts them under an encryption key (usually the last key negotiated via subkeys, or the session key if no negotiation has occurred). As part of the control information, the client must choose to use either a timestamp or a sequence number (or both); see the discussion in section 3.4.1 for guidelines on which to use. After the user data and control information are encrypted, the client transmits the ciphertext and some 'envelope' information to the recipient. 3.5.2. Receipt of KRB_PRIV message When an application receives a KRB_PRIV message, it verifies it as follows. If any error occurs, an error code is reported for use by the application. The message is first checked by verifying that the protocol version and type fields match the current version and KRB_PRIV, respectively. A mismatch generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE error. The application then decrypts the ciphertext and processes the resultant plaintext. If decryption shows the data to have been modified, a KRB_AP_ERR_BAD_INTEGRITY error is generated. If the sender's address was included in the control information, the recipient verifies that the operating system's report of the sender's address matches the sender's address in the message, and (if a recipient address is specified or the recipient requires an address) that one of the recipient's addresses appears as the recipient's address in the message. A failed match for either case generates a KRB_AP_ERR_BADADDR error. Then the timestamp and usec and/or the sequence number fields are checked. If timestamp and usec are expected and not present, or they are present but not current, the KRB_AP_ERR_SKEW error is generated. If the server name, along with the client name, time and microsecond fields from the Authenticator match any recently-seen such tuples, the KRB_AP_ERR_REPEAT error is generated. If an incorrect sequence number is included, or a sequence number is expected but not present, the KRB_AP_ERR_BADORDER error is generated. If neither a time-stamp and usec or a sequence number is present, a KRB_AP_ERR_MODIFIED error is generated. If all the checks succeed, the application can assume the message was generated by its peer, and was securely transmitted (without intruders able to see the unencrypted contents). 3.6. The KRB_CRED Exchange The KRB_CRED message may be used by clients requiring the ability to send Kerberos credentials from one host to another. It achieves this by sending the tickets together with encrypted data containing the session keys and other information associated with the tickets. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 3.6.1. Generation of a KRB_CRED message When an application wishes to send a KRB_CRED message it first (using the KRB_TGS exchange) obtains credentials to be sent to the remote host. It then constructs a KRB_CRED message using the ticket or tickets so obtained, placing the session key needed to use each ticket in the key field of the corresponding KrbCredInfo sequence of the encrypted part of the the KRB_CRED message. Other information associated with each ticket and obtained during the KRB_TGS exchange is also placed in the corresponding KrbCredInfo sequence in the encrypted part of the KRB_CRED message. The current time and, if specifically required by the application the nonce, s-address, and r-address fields, are placed in the encrypted part of the KRB_CRED message which is then encrypted under an encryption key previously exchanged in the KRB_AP exchange (usually the last key negotiated via subkeys, or the session key if no negotiation has occurred). Implementation note: [ Regarding unencrypted KRB_CRED messages go here. We need to make sure we understand what MIT does and reconcile with Microsoft.] 3.6.2. Receipt of KRB_CRED message When an application receives a KRB_CRED message, it verifies it. If any error occurs, an error code is reported for use by the application. The message is verified by checking that the protocol version and type fields match the current version and KRB_CRED, respectively. A mismatch generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE error. The application then decrypts the ciphertext and processes the resultant plaintext. If decryption shows the data to have been modified, a KRB_AP_ERR_BAD_INTEGRITY error is generated. If present or required, the recipient verifies that the operating system's report of the sender's address matches the sender's address in the message, and that one of the recipient's addresses appears as the recipient's address in the message. A failed match for either case generates a KRB_AP_ERR_BADADDR error. The timestamp and usec fields (and the nonce field if required) are checked next. If the timestamp and usec are not present, or they are present but not current, the KRB_AP_ERR_SKEW error is generated. If all the checks succeed, the application stores each of the new tickets in its ticket cache together with the session key and other information in the corresponding KrbCredInfo sequence from the encrypted part of the KRB_CRED message. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 4. SECTION HAS BEEN DELETED 5. Message Specifications NOTE: This section is awaiting update. It is the old text from the Salt Lake version. It has not been updated and made uniform with the single ASN.1 Module in Appenix A. Appendix A should be considered the authoritative component of the current state of the draft until this section is update. NOTE: We are continuing to work on changes to message format extensibility as discussed at the London meeting. We believe the general form discussed in London will continue to be a useful strategy for pursuing this goal. We expect to have additional information by the Salt Lake City meeting. TODO: TypedData needs to be looked at carefully, particularly with regard to TD-APP-DEFINED-ERROR, etc. Some significant changes from 1510 to here have been written up; more proofreading is needed. - tlyu The Kerberos protocol is defined here in terms of Abstract Syntax Notation One (ASN.1), which provides a syntax for specifying both the abstract layout of protocol messages as well as their encodings. Implementors not utilizing an existing ASN.1 compiler or support library are cautioned to thoroughly understand the actual ASN.1 specification to ensure correct implementation behavior, as there is more complexity in the notation than is immediately obvious, and some tutorials and guides to ASN.1 are misleading or erroneous. Note that in several places, there have been changes here from RFC 1510 that change the abstract types. This is in part to address widespread assumptions that various implementations have made, in some cases unintentionally violating the ASN.1 standard in various ways. These will be clearly flagged when they occur. The changes to the abstract types can cause incompatible encodings to be emitted when certain encoding rules, e.g. the Packed Encoding Rules (PER) are used. This should not be relevant for Kerberos, since Kerberos explicitly specifies the use of the Distinguished Encoding Rules (DER). This might be an issue for protocols wishing to use Kerberos types with other encoding rules. (This practice is not recommended.) With very few exceptions (most notably the usages of BIT STRING), the encodings emitted by the DER, which are the only encodings permitted by this document and by RFC 1510, remain identical. The type definitions in this section assume an ASN.1 module definition of the following form: Kerberos5 { iso (1), org(3), dod(6), internet(1), security(5), kerberosV5(2) } DEFINITIONS ::= BEGIN -- rest of definitions here END This specifies an explicit non-automatic tagging for the ASN.1 type definitions. Note that in some other publications [RFC1510] [RFC1964], the "dod" portion of the object identifier is erroneously specified as having the value "5". draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 Note that elsewhere in this document, nomenclature for various message types is inconsistent, but seems to largely follow C language conventions, including use of underscore (_) characters and all-caps spelling of names intended to be numeric constants. Also, in some places, identifiers (especially ones refering to constants) are written in all-caps in order to distinguish them from surrounding explanatory text. The ASN.1 notation does not permit underscores in identifiers, so in actual ASN.1 definitions, underscores are replaced with hyphens (-). Additionally, structure member names and defined values in ASN.1 must begin with a lowercase letter, while type names must begin with an uppercase letter. 5.1. Specific Compatibility Notes on ASN.1 For compatibility purposes, implementors should heed the following specific notes regarding the use of ASN.1 in Kerberos. These notes do not describe a non-standard usage of ASN.1, but rather some historical quirks and non-compliance of various implementations, as well as historical ambiguities, which, while being valid ASN.1, can lead to confusion during implementation. 5.1.1. ASN.1 Distinguished Encoding Rules The encoding of Kerberos protocol messages shall obey the Distinguished Encoding Rules (DER) of ASN.1 as described in X.690 (1997). Some implementations (believed to be primarly ones derived from DCE 1.1 and earlier) are known to use the more general Basic Encoding Rules (BER); in particular, these implementations send indefinite encodings of lengths. Implementations may accept such encodings in the interests of backwards compatibility, though implementors are warned that decoding fully-general BER is fraught with peril. 5.1.2. Optional Fields in ASN.1 Sequences Some implementations behave as if certain default values are equivalent to omission of an optional value. Implementations should handle this case gracefully. For example, the seq-number field in an Authenticator is optional, but some implementations use an internal value of zero to indicate that the field is to be omitted upon encoding. [While it is possible to use the DEFAULT qualifier for the ASN.1 notation of a SEQUENCE member in order to mandate this behavior, the result would be that the member would be mandatory to omit if the value intended is that specified by the DEFAULT keyword. This limits the possible semantics of the protocol.] 5.1.3. Zero-length SEQUENCE Types There are places in the protocol where a message contains a SEQUENCE OF type as an optional member, or a SEQUENCE type where all members are optional. This can result in an encoding that contains an zero-length SEQUENCE or SEQUENCE OF encoding. In general, implementations should not send zero-length SEQUENCE OF or SEQUENCE encodings that are marked OPTIONAL, but should accept them. [XXX there may be cases where an empty SEQUENCE type has useful semantics, though] draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 5.1.4. Unrecognized Tag Numbers Future revisions to this protocol may include new message types with different APPLICATION class tag numbers. Such revisions should protect older implementations by only sending the message types to parties that are known to understand them, e.g. by means of a flag bit set by the receiver in a preceding request. In the interest of robust error handling, implementations should gracefully handle receiving a message with an unrecognized tag anyway, and return an error message if appropriate. 5.1.5. Tag Numbers Greater Than 30 A naive implementation of a DER ASN.1 decoder may experience problems with ASN.1 tag numbers greater than 30, due such tag numbers being encoded using more than one byte. Future revisions of this protocol may utilize tag numbers greater than 30, and implementations should be prepared to gracefully return an error, if appropriate, if they do not recognize the tag. 5.2. Basic Kerberos Types This section defines a number of basic types that are potentially used in multiple Kerberos protocol messages. 5.2.1. KerberosString [XXX The following paragraphs may need some editing, or maybe they want to live in a footnote] The original specification of the Kerberos protocol in RFC 1510 uses GeneralString in numerous places for human-readable string data. Historical implementations of Kerberos cannot utilize the full power of GeneralString. This ASN.1 type requires the use of designation and invocation escape sequences as specified in ISO 2022 to switch character sets, and the default character set that is designated for G0 is basically US ASCII, which mostly works. In practice, many implementations end up treating GeneralStrings as if they were strings of whatever character set the implementation defaults to, without regard for correct usage of character set designation escape sequences. Also, DER prohibits the invocation of character sets into any but the G0 and C0 sets, which seems to outright prohibit the encoding of characters with the high bit set. Unfortunately, this seems to have the side effect of prohibiting the transmission of Latin-1 characters or any other characters that belong to a 96-character set, since it is prohibited to invoke them into G0. Some inconclusive discussion has taken place within the ASN.1 community on this subject. For now, we must assume that the ASN.1 specification of GeneralString as currently published is fundamentally flawed in several ways. One method of resolving these myriad difficulties is to constrain the use of GeneralString to only include IA5String, which is essentially the US-ASCII. US-ASCII control characters should in general not be used in KerberosString, except for cases such as newlines in lengthy error messages. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 The new (since RFC 1510) type KerberosString, defined below, is a CHOICE containing a GeneralString that is constrained to only contain characters in IA5String (which are US-ASCII). Note that the ASN.1 standard does not permit the use of escape sequences to change the character sets while encoding an IA5String. KerberosString ::= CHOICE { general GeneralString (IA5String), ... } This CHOICE is extensible, so that when an interoperable solution for internationalization is chosen, it will be easier to specify the changed types. In the future, changes to this protocol that allow for extensions to this CHOICE will be specified so that the transmitting party has some way of knowing whether the receiving party can accept the chosen alternative of the CHOICE. Implementations may choose to accept GeneralString values that contain characters other than those permitted by IA5String, but they should be aware that character set designation codes will likely be absent, and that the encoding should probably be treated as locale-specific in almost every way. Implementations may also choose to emit GeneralString values that are beyond those permitted by IA5String, but should be aware that doing so is extraordinarily risky from an interoperability perspective. Some existing implementations use GeneralString to encode unescaped locale-specific characters. This is in violation of the ASN.1 standard. Most of these implementations encode US-ASCII in the left-hand half, so as long the implementation transmits only US-ASCII, the ASN.1 standard is not violated in this regard. As soon as such an implementation encodes unescaped locale-specific characters with the high bit set, it violates the ASN.1 standard. Other implementations have been known to use GeneralString to contain a UTF-8 encoding. This also violates the ASN.1 standard, since UTF-8 is a different encoding, not a 94 or 96 character "G" set as defined by ISO 2022. It is believed that these implementations do not even use the ISO 2022 escape sequence to change the character encoding. Even if implementations were to announce the change of encoding by using that escape sequence, the ASN.1 standard prohibits the use of any escape sequences other than those used to designate/invoke "G" or "C" sets allowed by GeneralString. Future revisions to this protocol will almost certainly allow for a more interoperable representation of principal names, probably including UTF8String. Note that both applying a new constraint to a previously unconstrained type and replacing a type with a CHOICE containing that type constitute creations of new ASN.1 types. In the case here, the change here does not result in a changed encoding under DER. Also, note that various text in the ASN.1 standard actually suggests the strategy of replacing a type with a CHOICE containing that type for certain deprecated types, even though this creates a new type. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 5.2.2. Realm and PrincipalName Realm ::= KerberosString PrincipalName ::= SEQUENCE { name-type[0] Int32, name-string[1] SEQUENCE OF KerberosString } Kerberos realm names are encoded as KerberosStrings. Realms shall not contain a character with the code 0 (the ASCII NUL). Most realms will usually consist of several components separated by periods (.), in the style of Internet Domain Names, or separated by slashes (/) in the style of X.500 names. Acceptable forms for realm names are specified in section 7. A PrincipalName is a typed sequence of components consisting of the following sub-fields: name-type This field specifies the type of name that follows. Pre-defined values for this field are specified in section 7.2. 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. name-string This field encodes a sequence of components that form a name, each component encoded as a KerberosString. Taken together, a PrincipalName and a Realm form a principal identifier. Most PrincipalNames will have only a few components (typically one or two). 5.2.3. KerberosTime KerberosTime ::= GeneralizedTime -- with no fractional seconds The timestamps used in Kerberos are encoded as GeneralizedTimes. A KerberosTime value shall not include any fractional portions of the seconds. As required by the DER, it further shall not include any separators, and it shall specify the UTC time zone (Z). Example: The only valid format for UTC time 6 minutes, 27 seconds after 9 pm on 6 November 1985 is 19851106210627Z. 5.2.4. Constrained Integer types Some integer members of types should be constrained to values representable in 32 bits, for compatibility with reasonable implementation limits. Int32 ::= INTEGER (-2147483648..2147483647) -- signed values representable in 32 bits UInt32 :: = INTEGER (0..4294967295) -- unsigned 32 bit values Microseconds ::= INTEGER (0..99999) -- microseconds draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 While this results in changes to the abstract types from the RFC 1510 version, the encoding in DER should be unaltered. Historical implementations were typically limited to 32-bit integer values anyway, and assigned numbers should fall in the space of integer values representable in 32 bits in order to promote interoperability anyway. There are some members of messages types that are still defined as unconstrained INTEGER types, but many of these have a (non-ASN.1) constraint applied in the descriptive text. There are specific cases where more discussion needs to occur regarding possible constraints, such as for the nonce fields in various messages. 5.2.5. HostAddress and HostAddresses HostAddress ::= SEQUENCE { addr-type[0] Int32, address[1] OCTET STRING } HostAddresses ::= SEQUENCE OF HostAddress The host address encodings consists of two fields: addr-type This field specifies the type of address that follows. Pre-defined values for this field are specified in section 8.1. address This field encodes a single address of type addr-type. The two forms differ slightly. HostAddress contains exactly one address; HostAddresses contains a sequence of possibly many addresses. 5.2.6. AuthorizationData AuthorizationData ::= SEQUENCE OF SEQUENCE { ad-type[0] Int32, ad-data[1] OCTET STRING } ad-data This field contains authorization data to be interpreted according to the value of the corresponding ad-type field. ad-type This field specifies the format for the ad-data subfield. All negative values are reserved for local use. Non-negative values are reserved for registered use. Each sequence of type and data is referred to as an authorization element. Elements may be application specific, however, there is a common set of recursive elements that should be understood by all implementations. These elements contain other elements embedded within them, and the interpretation of the encapsulating element determines which of the embedded elements must be interpreted, and which may be ignored. Definitions for these common elements may be found in Appendix B. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 5.2.7. PA-DATA Historically, PA-DATA have been known as "pre-authentication data", meaning that they were used to augment the initial authentication with the KDC. Since that time, they have also been used as a typed hole with which to extend protocol exchanges with the KDC. PA-DATA ::= SEQUENCE { padata-type[1] Int32, padata-value[2] OCTET STRING -- might be encoded AP-REQ } padata-type indicates the way that the padata-value element is to be interpreted. Negative values of padata-type are reserved for unregistered use; non-negative values are used for a registered interpretation of the element type. padata-value Usually contains the DER encoding of another type; the padata-type field identifies which type is encoded here. padata-type name contents of padata-value 1 pa-tgs-req DER encoding of AP-REQ 2 pa-enc-timestamp DER encoding of PA-ENC-TIMESTAMP 3 pa-pw-salt salt (not ASN.1 encoded) 10 pa-etype-info DER encoding of PA-ETYPE-INFO 20 pa-use-specified-kvno DER encoding of INTEGER [XXX -- the following paragraph needs discussion, as does the general concept of authenticating the cleartext pieces of the protocol] This field may also contain information needed by certain extensions to the Kerberos protocol. For example, it might be used to initially verify the identity of a client before any response is returned. When this field is used to authenticate or pre-authenticate a request, it should contain a keyed checksum over the KDC-REQ-BODY to bind the pre-authentication data to rest of the request. The KDC, as a matter of policy, may decide whether to honor a KDC-REQ which includes any pre-authentication data that does not contain the checksum field. It may also be used by the client to specify the version of a key that is being used for accompanying preauthentication, and/or which should be used to encrypt the reply from the KDC. [XXX the following paragraph should apply perhaps to PA-DATA in general] The padata field can also contain information needed to help the KDC or the client select the key needed for generating or decrypting the response. This form of the padata is useful for supporting the use of certain token cards with Kerberos. The details of such extensions are specified in separate documents. See [Pat92] for additional uses of this field. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 5.2.7.1. PA-TGS-REQ In the case of requests for additional tickets (KRB_TGS_REQ), padata-value will contain an encoded AP-REQ. The checksum in the authenticator (which must be collision-proof) is to be computed over the KDC-REQ-BODY encoding. 5.2.7.2. Encrypted Timestamp Pre-authentication There are pre-authentication types that may be used to pre-authenticate a client by means of an encrypted timestamp. The original PA-ENC-TIMESTAMP does not contain a checksum of the KDC-REQ-BODY, while the PA-ENC-TIMESTAMP2 does. PA-ENC-TIMESTAMP ::= EncryptedData -- encrypted PA-ENC-TS-ENC PA-ENC-TS-ENC ::= SEQUENCE { patimestamp[0] KerberosTime, -- client's time pausec[1] Microseconds OPTIONAL } -- XXX maybe remove ENC-TIMESTAMP2 for now? PA-ENC-TIMESTAMP2 ::= EncryptedData -- encrypted PA-ENC-TS2-ENC PA-ENC-TS2-ENC ::= SEQUENCE { patimestamp[0] KerberosTime, -- client's time pausec[1] Microseconds OPTIONAL, pachecksum[2] Checksum OPTIONAL -- keyed checksum of KDC-REQ-BODY } Patimestamp contains the client's time, and pausec contains the microseconds, which may be omitted if a client will not generate more than one request per second. The ciphertext (padata-value) consists of the PA-ENC-TS-ENC or PA-ENC-TS2-ENC encoding, encrypted using the client's secret key. This preauthentication type was not present in RFC 1510, but many implementations support it. 5.2.7.3. PA-PW-SALT The padata-value for this preauthentication type contains the salt for the string-to-key to be used by the client to obtain the key for decrypting the encrypted part of an AS-REP message. Unfortunately, for historical reasons, the character set to be used is unspecified and probably locale-specific. This preauthentication type was not present in RFC 1510, but many implementations support it. It is necessary in any case where the salt for the string-to-key algorithm is not the default. In the trivial example, a zero-length salt string is very commonplace for realms that have converted their principal databases from Kerberos 4. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 5.2.7.4. PA-ETYPE-INFO The ETYPE-INFO preauthentication type is sent by the KDC in a KRB-ERROR indicating a requirement for additional preauthentication. It is usually used to notify a client of which key to use for the encryption of an encrypted timestamp for the purposes of sending a PA-ENC-TIMESTAMP preauthentication value. ETYPE-INFO-ENTRY ::= SEQUENCE { etype[0] INTEGER, salt[1] OCTET STRING OPTIONAL } ETYPE-INFO ::= SEQUENCE OF ETYPE-INFO-ENTRY The salt, like that of PA-PW-SALT, is also completely unspecified with respect to character set and is probably locale-specific. [XXX -- not clear whether ETYPE-INFO or PW-SALT should take precedence if they conflict] This preauthentication type was not present in RFC 1510, but many implementations that support encrypted timestamps for preauthentication need to support ETYPE-INFO as well. 5.2.7.5. PA-USE-SPECIFIED-KVNO The KDC should only accept and abide by the value of the use-specified-kvno preauthentication data field when the specified key is still valid and until use of a new key is confirmed. This situation is likely to occur primarily during the period during which an updated key is propagating to other KDC's in a realm. 5.2.8. KerberosFlags For several message types, a specific constrained bit string type, KerberosFlags, is used. KerberosFlags ::= BIT STRING (SIZE (32..MAX)) Compatibility note: the following paragraphs describe a change from the RFC1510 description of bit strings that would result in incompatility in the case of an implementation that strictly conformed to ASN.1 DER and RFC1510. ASN.1 bit strings have multiple uses. The simplest use of a bit string is to contain a vector of bits, with no particular meaning attached to individual bits. This vector of bits is not necessarily a multiple of eight bits long. The use in Kerberos of a bit string as a compact boolean vector wherein each element has a distinct meaning poses some problems. The natural notation for a compact boolean vector is the ASN.1 "NamedBit" notation, and the DER require that encodings of a bit string using "NamedBit" notation exclude any trailing zero bits. This truncation is easy to neglect, especially given C language implementations that may naturally choose to store boolean vectors as 32 bit integers. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 For example, if the notation for KDCOptions were to include the "NamedBit" notation, as in RFC 1510, and a KDCOptions value to be encoded had only the "forwardable" (bit number one) bit set, the DER encoding must only include two bits: the first reserved bit ("reserved", bit number zero, value zero) and the one-valued bit (bit number one) for "forwardable". Most existing implementations of Kerberos unconditionally send 32 bits on the wire when encoding bit strings used as boolean vectors. This behavior violates the ASN.1 syntax used for flag values in RFC 1510, but occurs on such a widely installed base that the protocol description is being modified to accomodate it. Consequently, this document removes the "NamedBit" notations for individual bits, relegating them to comments. The size constraint on the KerberosFlags type requires that at least 32 bits be encoded at all times, though a lenient implementation may choose to accept fewer than 32 bits and to treat the missing bits as set to zero. Currently, no uses of KerberosFlags specify more than 32 bits worth of flags, although future revisions of this document may do so. When more than 32 bits are to be transmitted in a KerberosFlags value, future revisions to this document will likely specify that the smallest number of bits needed to encode the highest-numbered one-valued bit should be sent. This is somewhat similar to the DER encoding of a bit string that is declared with the "NamedBit" notation. 5.2.9. Cryptosystem-related Types Many Kerberos protocol messages contain an EncryptedData as a container for arbitrary encrypted data, which is often the encrypted encoding of another data type. Fields within EncryptedData assist the recipient in selecting a key with which to decrypt the enclosed data. EncryptedData ::= SEQUENCE { etype[0] Int32, -- EncryptionType kvno[1] INTEGER OPTIONAL, cipher[2] OCTET STRING -- ciphertext } etype This field identifies which encryption algorithm was used to encipher the cipher. Detailed specifications for selected encryption types appear in section 6. kvno This field contains the version number of the key under which data is encrypted. It is only present in messages encrypted under long lasting keys, such as principals' secret keys. cipher This field contains the enciphered text, encoded as an OCTET STRING. The EncryptionKey type is the means by which cryptographic keys used for encryption are transfered. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 EncryptionKey ::= SEQUENCE { keytype[0] Int32, -- actually encryption type keyvalue[1] OCTET STRING } keytype This field specifies the encryption type of the encryption key that follows in the keyvalue field. While its name is "keytype", it actually specifies an encryption type. Previously, multiple cryptosystems that performed encryption differently but were capable of using keys with the same characteristics were permitted to share an assigned number to designate the type of key; this usage is now deprecated. keyvalue This field contains the key itself, encoded as an octet string. All negative values for the encryption key type are reserved for local use. All non-negative values are reserved for officially assigned type fields and interpretations. Messages containing cleartext data to be authenticated will usually do so by using a member of type Checksum. Most instances of Checksum use a keyed hash, though exceptions will be noted. Checksum ::= SEQUENCE { cksumtype[0] Int32, checksum[1] OCTET STRING } cksumtype This field indicates the algorithm used to generate the accompanying checksum. checksum This field contains the checksum itself, encoded as an octet string. Detailed specification of selected checksum types appear in section 6. Negative values for the checksum type are reserved for local use. All non-negative values are reserved for officially assigned type fields and interpretations. 5.3. Tickets and Authenticators This section describes the format and encryption parameters for tickets and authenticators. When a ticket or authenticator is included in a protocol message it is treated as an opaque object. 5.3.1. Tickets A ticket is a record that helps a client authenticate to a service. A Ticket contains the following information: Ticket ::= [APPLICATION 1] SEQUENCE { tkt-vno[0] INTEGER, realm[1] Realm, sname[2] PrincipalName, enc-part[3] EncryptedData --EncTicketPart } draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 -- Encrypted part of ticket EncTicketPart ::= [APPLICATION 3] SEQUENCE { flags[0] TicketFlags, key[1] EncryptionKey, crealm[2] Realm, cname[3] PrincipalName, transited[4] TransitedEncoding, authtime[5] KerberosTime, starttime[6] KerberosTime OPTIONAL, endtime[7] KerberosTime, renew-till[8] KerberosTime OPTIONAL, caddr[9] HostAddresses OPTIONAL, authorization-data[10] AuthorizationData OPTIONAL } -- encoded Transited field TransitedEncoding ::= SEQUENCE { tr-type[0] Int32, -- must be registered contents[1] OCTET STRING } TicketFlags ::= KerberosFlags -- reserved(0), -- forwardable(1), -- forwarded(2), -- proxiable(3), -- proxy(4), -- may-postdate(5), -- postdated(6), -- invalid(7), -- renewable(8), -- initial(9), -- pre-authent(10), -- hw-authent(11), -- transited-policy-checked(12), -- ok-as-delegate(13) -- anonymous(14) The encoding of EncTicketPart is encrypted in the key shared by Kerberos and the end server (the server's secret key). See section 6 for the format of the ciphertext. tkt-vno This field specifies the version number for the ticket format. This document describes version number 5. realm This field specifies the realm that issued a ticket. It also serves to identify the realm part of the server's principal identifier. Since a Kerberos server can only issue tickets for servers within its realm, the two will always be identical. sname This field specifies all components of the name part of the server's identity, including those parts that identify a specific instance of a service. draft-ietf-krb-wg-kerberos-clarifications-00 Expires 22 August 2002 enc-part This field holds the encrypted encoding of the EncTicketPart sequence. flags This field indicates which of various options were used or requested when the ticket was issued. It is a bit-field, where the selected options are indicated by the bit being set (1), and the unselected options and reserved fields being reset (0). [XXX X.690 ref and notes on pitfalls?] The meanings of the flags are: Bit(s) Name Description 0 reserved Reserved for future expansion of this field. The FORWARDABLE flag is normally only interpreted by the TGS, and can be ignored by end servers. When set, this 1 forwardable flag tells the ticket-granting server that it is OK to issue a new ticket-granting ticket with a different network address based on the presented ticket. When set, this flag indicates that the ticket has either been forwarded or 2 forwarded was issued based on authentication involving a forwarded ticket-granting ticket. The PROXIABLE flag is normally only interpreted by the TGS, and can be ignored by end servers. The PROXIABLE flag has an interpretation identical 3 proxiable to that of the FORWARDABLE flag, except that the PROXIABLE flag tells the ticket-granting server that only non-ticket-granting tickets may be issued with different network addresses. 4 proxy When set, this flag indicates that a ticket is a proxy. The MAY-POSTDATE flag is normally only interpreted by the TGS, and can be 5 may-postdate ignored by end servers. This flag tells the ticke