Network Working Group J. Hutzelman Internet-Draft CMU Expires: January 17, 2005 J. Salowey Cisco Systems J. Galbraith Van Dyke Technologies, Inc. V. Welch U Chicago / ANL July 19, 2004 GSSAPI Authentication and Key Exchange for the Secure Shell Protocol draft-ietf-secsh-gsskeyex-08 Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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. This Internet-Draft will expire on January 17, 2005. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract The Secure Shell protocol (SSH) is a protocol for secure remote login and other secure network services over an insecure network. The Generic Security Service Application Program Interface (GSS-API) [GSSAPI] provides security services to callers in a mechanism-independent fashion. Hutzelman, et. al. Expires January 17, 2005 [Page 1] Internet-Draft SSH GSSAPI Methods July 2004 This memo describes methods for using the GSS-API for authentication and key exchange in SSH. It defines an SSH user authentication method which uses a specified GSSAPI mechanism to authenticate a user, and a family of SSH key exchange methods which use GSSAPI to authenticate the Diffie-Hellman exchange described in [SSH-TRANSPORT]. This memo also defines a new host public key algorithm which can be used when no operations are needed using a host's public key, and a new user authentication method which allows an authorization name to be used in conjunction with any authentication which has already occurred as a side-effect of GSSAPI-based key exchange. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [KEYWORDS]. Hutzelman, et. al. Expires January 17, 2005 [Page 2] Internet-Draft SSH GSSAPI Methods July 2004 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1 SSH terminology . . . . . . . . . . . . . . . . . . . . . . . 4 2. GSSAPI Authenticated Diffie-Hellman Key Exchange . . . . . . . 5 2.1 Generic GSSAPI Key Exchange . . . . . . . . . . . . . . . . . 5 2.2 Group Exchange . . . . . . . . . . . . . . . . . . . . . . . . 10 2.3 gss-group1-sha1-* . . . . . . . . . . . . . . . . . . . . . . 12 2.4 gss-gex-sha1-* . . . . . . . . . . . . . . . . . . . . . . . . 12 2.5 Other GSSAPI key exchange methods . . . . . . . . . . . . . . 13 3. GSSAPI User Authentication . . . . . . . . . . . . . . . . . . 14 3.1 GSSAPI Authentication Overview . . . . . . . . . . . . . . . . 14 3.2 Initiating GSSAPI authentication . . . . . . . . . . . . . . . 14 3.3 Initial server response . . . . . . . . . . . . . . . . . . . 15 3.4 GSSAPI session . . . . . . . . . . . . . . . . . . . . . . . . 15 3.5 Binding Encryption Keys . . . . . . . . . . . . . . . . . . . 16 3.6 Client acknowledgement . . . . . . . . . . . . . . . . . . . . 17 3.7 Completion . . . . . . . . . . . . . . . . . . . . . . . . . . 18 3.8 Error Status . . . . . . . . . . . . . . . . . . . . . . . . . 18 3.9 Error Token . . . . . . . . . . . . . . . . . . . . . . . . . 19 4. Authentication using GSSAPI Key Exchange . . . . . . . . . . . 20 5. Null Host Key Algorithm . . . . . . . . . . . . . . . . . . . 22 6. Summary of Message Numbers . . . . . . . . . . . . . . . . . . 23 7. GSSAPI Considerations . . . . . . . . . . . . . . . . . . . . 24 7.1 Naming Conventions . . . . . . . . . . . . . . . . . . . . . . 24 7.2 Channel Bindings . . . . . . . . . . . . . . . . . . . . . . . 24 7.3 SPNEGO . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26 9. Security Considerations . . . . . . . . . . . . . . . . . . . 27 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28 11. Changes the last version . . . . . . . . . . . . . . . . . . . 29 References . . . . . . . . . . . . . . . . . . . . . . . . . . 30 References . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 32 Full Copyright Statement . . . . . . . . . . . . . . . . . . . 34 Hutzelman, et. al. Expires January 17, 2005 [Page 3] Internet-Draft SSH GSSAPI Methods July 2004 1. Introduction This document describes the methods used to perform key exchange and user authentication in the Secure Shell protocol using the GSSAPI. To do this, it defines a family of key exchange methods, two user authentication methods, and a new host key algorithm. These definitions allow any GSSAPI mechanism to be used with the Secure Shell protocol. This document should be read only after reading the documents describing the SSH protocol architecture [SSH-ARCH], transport layer protocol [SSH-TRANSPORT], and user authentication protocol [SSH-USERAUTH]. This document freely uses terminology and notation from the architecture document without reference or further explanation. 1.1 SSH terminology The data types used in the packets are defined in the SSH architecture document [SSH-ARCH]. It is particularly important to note the definition of string allows binary content. The SSH_MSG_USERAUTH_REQUEST packet refers to a service; this service name is an SSH service name, and has no relationship to GSSAPI service names. Currently, the only defined service name is "ssh-connection", which refers to the SSH connection protocol [SSH-CONNECT]. Hutzelman, et. al. Expires January 17, 2005 [Page 4] Internet-Draft SSH GSSAPI Methods July 2004 2. GSSAPI Authenticated Diffie-Hellman Key Exchange This section defines a class of key exchange methods which combine the Diffie-Hellman key exchange from section 6 of [SSH-TRANSPORT] with mutual authentication using GSSAPI. Since the GSSAPI key exchange methods described in this section do not require the use of public key signature or encryption algorithms, they MAY be used with any host key algorithm, including the "null" algorithm described in Section 5. 2.1 Generic GSSAPI Key Exchange The following symbols are used in this description: o C is the client, and S is the server o p is a large safe prime, g is a generator for a subgroup of GF(p), and q is the order of the subgroup o V_S is S's version string, and V_C is C's version string o I_C is C's KEXINIT message, and I_S is S's KEXINIT message 1. C generates a random number x (1 < x < q) and computes e = g^x mod p. 2. C calls GSS_Init_sec_context, using the most recent reply token received from S during this exchange, if any. For this call, the client MUST set the mutual_req_flag to "true" to request that mutual authentication be performed. It also MUST set the integ_req_flag to "true" to request that per-message integrity protection be supported for this context. In addition, the deleg_req_flag MAY be set to "true" to request access delegation, if requested by the user. Since the key exchange process authenticates only the host, the setting of the anon_req_flag is immaterial to this process. If the client does not support the "gssapi-keyex" user authentication method described in Section 4, or does not intend to use that method in conjunction with the GSSAPI context established during key exchange, then the anon_req_flag SHOULD be set to "true". Otherwise, this flag MAY be set to true if the client wishes to hide its identity. Since the key exchange process will involve the exchange of only a single token once the context has been established, it is not necessary that the GSSAPI context support detection of replayed or out-of-sequence tokens. Thus, the setting of the replay_det_req_flag and sequence_req_flag are not needed for this process. These flags SHOULD be set to "false". Hutzelman, et. al. Expires January 17, 2005 [Page 5] Internet-Draft SSH GSSAPI Methods July 2004 * If the resulting major_status code is GSS_S_COMPLETE and the mutual_state flag is not true, then mutual authentication has not been established, and the key exchange MUST fail. * If the resulting major_status code is GSS_S_COMPLETE and the integ_avail flag is not true, then per-message integrity protection is not available, and the key exchange MUST fail. * If the resulting major_status code is GSS_S_COMPLETE and both the mutual_state and integ_avail flags are true, the resulting output token is sent to S. * If the resulting major_status code is GSS_S_CONTINUE_NEEDED, the the output_token is sent to S, which will reply with a new token to be provided to GSS_Init_sec_context. * The client MUST also include "e" with the first message it sends to the server during this process; if the server receives more than one "e" or none at all, the key exchange fails. * It is an error if the call does not produce a token of non-zero length to be sent to the server. In this case, the key exchange MUST fail. 3. S calls GSS_Accept_sec_context, using the token received from C. * If the resulting major_status code is GSS_S_COMPLETE and the mutual_state flag is not true, then mutual authentication has not been established, and the key exchange MUST fail. * If the resulting major_status code is GSS_S_COMPLETE and the integ_avail flag is not true, then per-message integrity protection is not available, and the key exchange MUST fail. * If the resulting major_status code is GSS_S_COMPLETE and both the mutual_state and integ_avail flags are true, then the security context has been established, and processing continues with step 4. * If the resulting major_status code is GSS_S_CONTINUE_NEEDED, then the output token is sent to C, and processing continues with step 2. * If the resulting major_status code is GSS_S_COMPLETE, but a non-zero-length reply token is returned, then that token is sent to the client. 4. S generates a random number y (0 < y < q) and computes f = g^y Hutzelman, et. al. Expires January 17, 2005 [Page 6] Internet-Draft SSH GSSAPI Methods July 2004 mod p. It computes K = e ^ y mod p, and H = hash(V_C || V_S || I_C || I_S || K_S || e || f || K). It then calls GSS_GetMIC to obtain a GSSAPI message integrity code for H. S then sends f and the MIC to C. 5. This step is performed only if the server's final call to GSS_Accept_sec_context produced a non-zero-length final reply token to be sent to the client _and_ no previous call by the client to GSS_Init_sec_context has resulted in a major_status of GSS_S_COMPLETE. Under these conditions, the client makes an additional call to GSS_Init_sec_context to process the final reply token. This call is made exactly as described above. However, if the resulting major_status is anything other than GSS_S_COMPLETE, or a non-zero-length token is returned, it is an error and the key exchange MUST fail. 6. C computes K = f^x mod p, and H = hash(V_C || V_S || I_C || I_S || K_S || e || f || K). It then calls GSS_VerifyMIC to verify that the MIC sent by S matches H. If the MIC is not successfully verified, the key exchange MUST fail. Either side MUST NOT send or accept e or f values that are not in the range [1, p-1]. If this condition is violated, the key exchange fails. If any call to GSS_Init_sec_context or GSS_Accept_sec_context returns a major_status other than GSS_S_COMPLETE or GSS_S_CONTINUE_NEEDED, or any other GSSAPI call returns a major_status other than GSS_S_COMPLETE, the key exchange fails. In this case, several mechanisms are available for communicating error information to the peer before terminating the connection as required by [SSH-TRANSPORT]: o If the key exchange fails due to any GSSAPI error on the server (including errors returned by GSS_Accept_sec_context), the server MAY send a message informing the client of the details of the error. In this case, if an error token is also sent (see below), then this message MUST be sent before the error token. o If the key exchange fails due to a GSSAPI error returned from the server's call to GSS_Accept_sec_context, and an "error token" is also returned, then the server SHOULD send the error token to the client to allow completion of the GSS security exchange. o If the key exchange fails due to a GSSAPI error returned from the client's call to GSS_Init_sec_context, and an "error token" is also returned, then the client SHOULD send the error token to the server to allow completion of the GSS security exchange. Hutzelman, et. al. Expires January 17, 2005 [Page 7] Internet-Draft SSH GSSAPI Methods July 2004 As noted in Section 9, it may be desirable under site security policy to obscure information about the precise nature of the error; thus, it is RECOMMENDED that implementations provide a method to suppress these messages as a matter of policy. This is implemented with the following messages. The hash algorithm for computing the exchange hash is defined by the method name, and is called HASH. The group used for Diffie-Hellman key exchange and the underlying GSSAPI mechanism are also defined by the method name. After the client's first call to GSS_Init_sec_context, it sends the following: byte SSH_MSG_KEXGSS_INIT string output_token (from GSS_Init_sec_context) mpint e Upon receiving the SSH_MSG_KEXGSS_INIT message, the server MAY send the following message, prior to any other messages, to inform the client of its host key. byte SSH_MSG_KEXGSS_HOSTKEY string server public host key and certificates (K_S) Since this key exchange method does not require the host key to be used for any encryption operations, this message is OPTIONAL. If the "null" host key algorithm described in Section 5 is used, this message MUST NOT be sent. If this message is sent, the server public host key(s) and/or certificate(s) in this message are encoded as a single string, in the format specified by the public key type in use (see [SSH-TRANSPORT], section 4.6). Each time the server's call to GSS_Accept_sec_context returns a major_status code of GSS_S_CONTINUE_NEEDED, it sends the following reply to the client: byte SSH_MSG_KEXGSS_CONTINUE string output_token (from GSS_Accept_sec_context) If the client receives this message after a call to GSS_Init_sec_context has returned a major_status code of GSS_S_COMPLETE, a protocol error has occurred and the key exchange MUST fail. Each time the client receives the message described above, it makes another call to GSS_Init_sec_context. It then sends the following: byte SSH_MSG_KEXGSS_CONTINUE string output_token (from GSS_Init_sec_context) Hutzelman, et. al. Expires January 17, 2005 [Page 8] Internet-Draft SSH GSSAPI Methods July 2004 The server and client continue to trade these two messages as long as the server's calls to GSS_Accept_sec_context result in major_status codes of GSS_S_CONTINUE_NEEDED. When a call results in a major_status code of GSS_S_COMPLETE, it sends one of two final messages. If the server's final call to GSS_Accept_sec_context (resulting in a major_status code of GSS_S_COMPLETE) returns a non-zero-length token to be sent to the client, it sends the following: byte SSH_MSG_KEXGSS_COMPLETE mpint f string per_msg_token (MIC of H) boolean TRUE string output_token (from GSS_Accept_sec_context) If the client receives this message after a call to GSS_Init_sec_context has returned a major_status code of GSS_S_COMPLETE, a protocol error has occurred and the key exchange MUST fail. If the server's final call to GSS_Accept_sec_context (resulting in a major_status code of GSS_S_COMPLETE) returns a zero-length token or no token at all, it sends the following: byte SSH_MSG_KEXGSS_COMPLETE mpint f string per_msg_token (MIC of H) boolean FALSE If the client receives this message when no call to GSS_Init_sec_context has yet resulted in a major_status code of GSS_S_COMPLETE, a protocol error has occurred and the key exchange MUST fail. If either the client's call to GSS_Init_sec_context or the server's call to GSS_Accept_sec_context returns an error status and produces an output token (called an "error token"), then the following SHOULD be sent to convey the error information to the peer: byte SSH_MSG_KEXGSS_CONTINUE string error_token If a server sends both this message and an SSH_MSG_KEXGSS_ERROR message, the SSH_MSG_KEXGSS_ERROR message MUST be sent first, to allow clients to record and/or display the error information before processing the error token. This is important because a client processing an error token will likely disconnect without reading any further messages. Hutzelman, et. al. Expires January 17, 2005 [Page 9] Internet-Draft SSH GSSAPI Methods July 2004 In the event of a GSSAPI error on the server, the server MAY send the following message before terminating the connection: byte SSH_MSG_KEXGSS_ERROR uint32 major_status uint32 minor_status string message string language tag The message text MUST be encoded in the UTF-8 encoding described in [UTF8]. Language tags are those described in [LANGTAG]. Note that the message text may contain multiple lines separated by carriage return-line feed (CRLF) sequences. Application developers should take this into account when displaying these messages. The hash H is computed as the HASH hash of the concatenation of the following: string V_C, the client's version string (CR and NL excluded) string V_S, the server's version string (CR and NL excluded) string I_C, the payload of the client's SSH_MSG_KEXINIT string I_S, the payload of the server's SSH_MSG_KEXINIT string K_S, the host key mpint e, exchange value sent by the client mpint f, exchange value sent by the server mpint K, the shared secret This value is called the exchange hash, and it is used to authenticate the key exchange. The exchange hash SHOULD be kept secret. If no SSH_MSG_KEXGSS_HOSTKEY message has been sent by the server or received by the client, then the empty string is used in place of K_S when computing the exchange hash. The GSS_GetMIC call MUST be applied over H, not the original data. 2.2 Group Exchange This section describes a modification to the generic GSSAPI authenticated Diffie-Hellman key exchange to allow the negotiation of the group to be used, using a method based on that described in [GROUP-EXCHANGE]. The server keeps a list of safe primes and corresponding generators that it can select from. These are chosen as described in section 5 of [GROUP-EXCHANGE]. The client requests a modulus from the server, indicating the minimum, maximum, and preferred sizes; the server responds with a suitable modulus and generator. The exchange then proceeds as described in Section 2.1 above. Hutzelman, et. al. Expires January 17, 2005 [Page 10] Internet-Draft SSH GSSAPI Methods July 2004 This description uses the following symbols, in addition to those defined above: o n is the size of the modulus p in bits that the client would like to receive from the server o min and max are the minimal and maximal sizes of p in bits that are acceptable to the client 1. C sends "min || n || max" to S, indicating the minimal acceptable group size, the preferred size of the group, and the maximal group size in bits the client will accept. 2. S finds a group that best matches the client's request, and sends "p || g" to C. 3. The exchange proceeds as described in Section 2.1 above, beginning with step 1, except that the exchange hash is computed as described below. Servers and clients SHOULD support gorups with a modulus length of k bits, where 1024 <= k <= 8192. The recommended values for min and max are 1024 and 8192, respectively. This is implemented using the following messages, in addition to those described above: First, the client sends: byte SSH_MSG_KEXGSS_GROUPREQ uint32 min, minimal size in bits of an acceptable group uint32 n, preferred size in bits of the group the server should send uint32 max, maximal size in bits of an acceptable group The server responds with: byte SSH_MSG_KEXGSS_GROUP mpint p, safe prime mpint g, generator for subgroup in GF(p) Hutzelman, et. al. Expires January 17, 2005 [Page 11] Internet-Draft SSH GSSAPI Methods July 2004 This is followed by the message exchange described above in Section 2.1, except that the exchange hash H is computed as the HASH hash of the concatenation of the following: string V_C, the client's version string (CR and NL excluded) string V_S, the server's version string (CR and NL excluded) string I_C, the payload of the client's SSH_MSG_KEXINIT string I_S, the payload of the server's SSH_MSG_KEXINIT string K_S, the host key uint32 min, minimal size in bits of an acceptable group uint32 n, preferred size in bits of the group the server should send uint32 max, maximal size in bits of an acceptable group mpint p, safe prime mpint g, generator for subgroup in GF(p) mpint e, exchange value sent by the client mpint f, exchange value sent by the server mpint K, the shared secret 2.3 gss-group1-sha1-* Each of these methods specifies GSSAPI authenticated Diffie-Hellman key exchange as described in Section 2.1 with SHA-1 as HASH, and the group defined in section 6.1 of [SSH-TRANSPORT]. The method name for each method is the concatenation of the string "gss-group1-sha1-" with the Base64 encoding of the MD5 hash [MD5] of the ASN.1 DER encoding [ASN1] of the underlying GSSAPI mechanism's OID. Base64 encoding is described in section 6.8 of [MIME]. Each and every such key exchange method is implicitly registered by this specification. The IESG is considered to be the owner of all such key exchange methods; this does NOT imply that the IESG is considered to be the owner of the underlying GSSAPI mechanism. 2.4 gss-gex-sha1-* Each of these methods specifies GSSAPI authenticated Diffie-Hellman key exchange as described in Section 2.2 with SHA-1 as HASH. The method name for each method is the concatenation of the string "gss-gex-sha1-" with the Base64 encoding of the MD5 hash [MD5] of the ASN.1 DER encoding [ASN1] of the underlying GSSAPI mechanism's OID. Base64 encoding is described in section 6.8 of [MIME]. Each and every such key exchange method is implicitly registered by this specification. The IESG is considered to be the owner of all such key exchange methods; this does NOT imply that the IESG is considered to be the owner of the underlying GSSAPI mechanism. Hutzelman, et. al. Expires January 17, 2005 [Page 12] Internet-Draft SSH GSSAPI Methods July 2004 2.5 Other GSSAPI key exchange methods Key exchange method names starting with "gss-" are reserved for key exchange methods which conform to this document; in particular, for those methods which use the GSSAPI authenticated Diffie-Hellman key exchange algorithm described in Section 2.1, including any future methods which use different groups and/or hash functions. The intent is that the names for any such future methods methods be defined in a similar manner to that used in Section 2.3. Hutzelman, et. al. Expires January 17, 2005 [Page 13] Internet-Draft SSH GSSAPI Methods July 2004 3. GSSAPI User Authentication This section describes a general-purpose user authentication method based on [GSSAPI]. It is intended to be run over the SSH user authentication protocol [SSH-USERAUTH]. The authentication method name for this protocol is "gssapi-with-mic". 3.1 GSSAPI Authentication Overview GSSAPI authentication must maintain a context. Authentication begins when the client sends a SSH_MSG_USERAUTH_REQUEST, which specifies the mechanism OIDs the client supports. If the server supports any of the requested mechanism OIDs, the server sends a SSH_MSG_USERAUTH_GSSAPI_RESPONSE message containing the mechanism OID. After the client receives SSH_MSG_USERAUTH_GSSAPI_RESPONSE, the client and server exchange SSH_MSG_USERAUTH_GSSAPI_TOKEN packets until the authentication mechanism either succeeds or fails. If at any time during the exchange, the client sends a new SSH_MSG_USERAUTH_REQUEST packet, the GSSAPI context is completely discarded and destroyed, and any further GSSAPI authentication MUST restart from the beginning. 3.2 Initiating GSSAPI authentication The GSSAPI authentication method is initiated when the client sends a SSH_MSG_USERAUTH_REQUEST: byte SSH_MSG_USERAUTH_REQUEST string user name (in ISO-10646 UTF-8 encoding) string service name (in US-ASCII) string "gssapi-with-mic" (US-ASCII method name) uint32 n, the number of mechanism OIDs client supports string[n] mechanism OIDs Mechanism OIDs are encoded according to the ASN.1 distinguished encoding rules (DER), as described in [ASN1] and in section 3.1 of [GSSAPI]. The mechanism OIDs MUST be listed in order of preference, and the server must choose the first mechanism OID on the list that it supports. The client SHOULD send GSSAPI mechanism OID's only for mechanisms which are of the same priority, compared to non-GSSAPI authentication methods. Otherwise, authentication methods may be Hutzelman, et. al. Expires January 17, 2005 [Page 14] Internet-Draft SSH GSSAPI Methods July 2004 executed out of order. Thus, the client could first send a SSH_MSG_USERAUTH_REQUEST for one GSSAPI mechanism, then try public key authentication, and then try another GSSAPI mechanism. If the server does not support any of the specified OIDs, the server MUST fail the request by sending a SSH_MSG_USERAUTH_FAILURE packet. The user name may be an empty string if it can be deduced from the results of the GSSAPI authentication. If the user name is not empty, and the requested user does not exist, the server MAY disconnect, or MAY send a bogus list of acceptable authentications but never accept any. This makes it possible for the server to avoid disclosing information about which accounts exist. In any case, if the user does not exist, the authentication request MUST NOT be accepted. The client MAY at any time continue with a new SSH_MSG_USERAUTH_REQUEST message, in which case the server MUST abandon the previous authentication attempt and continue with the new one. 3.3 Initial server response The server responds to the SSH_MSG_USERAUTH_REQUEST with either a SSH_MSG_USERAUTH_FAILURE if none of the mechanisms are supported, or with SSH_MSG_USERAUTH_GSSAPI_RESPONSE as follows: byte SSH_MSG_USERAUTH_GSSAPI_RESPONSE string selected mechanism OID The mechanism OID must be one of the OIDs sent by the client in the SSH_MSG_USERAUTH_REQUEST packet. 3.4 GSSAPI session Once the mechanism OID has been selected, the client will then initiate an exchange of one or more pairs of SSH_MSG_USERAUTH_GSSAPI_TOKEN packets. These packets contain the tokens produced from the 'GSS_Init_sec_context()' and 'GSS_Accept_sec_context()' calls. The actual number of packets exchanged is determined by the underlying GSSAPI mechanism. byte SSH_MSG_USERAUTH_GSSAPI_TOKEN string data returned from either GSS_Init_sec_context() or GSS_Accept_sec_context() If an error occurs during this exchange on server side, the server can terminate the method by sending a SSH_MSG_USERAUTH_FAILURE packet. If an error occurs on client side, the client can terminate Hutzelman, et. al. Expires January 17, 2005 [Page 15] Internet-Draft SSH GSSAPI Methods July 2004 the method by sending a new SSH_MSG_USERAUTH_REQUEST packet. When calling GSS_Init_sec_context(), the client MUST set the the integ_req_flag to "true" to request that per-message integrity protection be supported for this context. In addition, the deleg_req_flag MAY be set to "true" to request access delegation, if requested by the user. Since the user authentication process by its nature authenticates only the client, the setting of the mutual_req_flag is not needed for this process. This flag SHOULD be set to "false". Since the user authentication process will involve the exchange of only a single token once the context has been established, it is not necessary that the context support detection of replayed or out-of-sequence tokens. Thus, the setting of the replay_det_req_flag and sequence_req_flag are not needed for this process. These flags SHOULD be set to "false". Additional SSH_MSG_USERAUTH_GSSAPI_TOKEN messages are sent if and only if the calls to the GSSAPI routines produce send tokens of non-zero length. Any major status code other than GSS_S_COMPLETE or GSS_S_CONTINUE_NEEDED SHOULD be a failure. 3.5 Binding Encryption Keys In some cases, it is possible to obtain improved security by allowing access only if the client sends a valid message integrity code (MIC) binding the GSSAPI context to the keys used for encryption and integrity protection of the SSH session. With this extra level of protection, a "man-in-the-middle" attacker who has convinced a client of his authenticity cannot then relay user authentication messages between the real client and server, thus gaining access to the real server. This additional protection is available when the negotiated GSSAPI context supports per-message integrity protection, as indicated by the setting of the integ_avail flag on successful return from GSS_Init_sec_context() or GSS_Accept_sec_context(). When the client's call to GSS_Init_sec_context() returns GSS_S_COMPLETE with the integ_avail flag set, the client MUST conclude the user authentication exchange by sending the following message: byte SSH_MSG_USERAUTH_GSSAPI_MIC string MIC Hutzelman, et. al. Expires January 17, 2005 [Page 16] Internet-Draft SSH GSSAPI Methods July 2004 This message MUST be sent only if GSS_Init_sec_context() returned GSS_S_COMPLETE. If a token is also returned then the SSH_MSG_USERAUTH_GSSAPI_TOKEN message MUST be sent before this one. The contents of the MIC field are obtained by calling GSS_GetMIC over the following, using the GSSAPI context which was just established: string session identifier byte SSH_MSG_USERAUTH_REQUEST string user name string service string "gssapi-with-mic" If this message is received by the server before the GSSAPI context is fully established, the server MUST fail the authentication. If this message is received by the server when the negotiated GSSAPI context does not support per-message integrity protection, the server MUST fail the authentication. 3.6 Client acknowledgement Some servers may wish to permit user authentication to proceed even when the negotitated GSSAPI context does not support per-message integrity protection. In such cases, it is possible for the server to successfully complete the GSSAPI method, while the client's last call to GSS_Init_sec_context fails. If the server simply assumed success on the part of the client and completed the authentication service, it is possible that the client would fail to complete the authentication method, but not be able to retry other methods because the server had already moved on. To protect against this, a final message is sent by the client to indicate it has completed authentication. When the client's call to GSS_Init_sec_context() returns GSS_S_COMPLETE with the integ_avail flag not set, the client MUST conclude the user authentication exchange by sending the following message: byte SSH_MSG_USERAUTH_GSSAPI_EXCHANGE_COMPLETE This message MUST be sent only if GSS_Init_sec_context() returned GSS_S_COMPLETE. If a token is also returned then the SSH_MSG_USERAUTH_GSSAPI_TOKEN message MUST be sent before this one. If this message is received by the server before the GSSAPI context is fully established, the server MUST fail the authentication. Hutzelman, et. al. Expires January 17, 2005 [Page 17] Internet-Draft SSH GSSAPI Methods July 2004 If this message is received by the server when the negotiated GSSAPI context supports per-message integrity protection, the server MUST fail the authentication. It is a site policy descision for the server whether or not to permit authentication using GSSAPI mechanisms and/or contexts which do not support per-message integrity protection. The server MAY fail the otherwise valid gssapi-with-mic authentication if per-message integrity protection is not supported. 3.7 Completion As with all SSH authentication methods, successful completion is indicated by a SSH_MSG_USERAUTH_SUCCESS if no other authentication is required, or a SSH_MSG_USERAUTH_FAILURE with the partial success flag set if the server requires further authentication. This packet should be sent immediately following receipt of the the SSH_MSG_USERAUTH_GSSAPI_EXCHANGE_COMPLETE packet. 3.8 Error Status In the event a GSSAPI error occurs on the server during context establishment, the server MAY send the following message to inform the client of the details of the error before sending a SSH_MSG_USERAUTH_FAILURE message: byte SSH_MSG_USERAUTH_GSSAPI_ERROR uint32 major_status uint32 minor_status string message string language tag The message text MUST be encoded in the UTF-8 encoding described in [UTF8]. Language tags are those described in [LANGTAG]. Note that the message text may contain multiple lines separated by carriage return-line feed (CRLF) sequences. Application developers should take this into account when displaying these messages. Clients receiving this message MAY log the error details and/or report them to the user. Any server sending this message MUST ignore any SSH_MSG_UNIMPLEMENTED sent by the client in response. Hutzelman, et. al. Expires January 17, 2005 [Page 18] Internet-Draft SSH GSSAPI Methods July 2004 3.9 Error Token In the event that, during context establishment, a client's call to GSS_Init_sec_context or a server's call to GSS_Accept_sec_context returns a token along with an error status, the resulting "error token" SHOULD be sent to the peer using the following message: byte SSH_MSG_USERAUTH_GSSAPI_ERRTOK string error token This message implies that the authentication is about to fail, and is defined to allow the error token to be communicated without losing synchronization. When a server sends this message, it MUST be followed by a SSH_MSG_USERAUTH_FAILURE message, which is to be interpreted as applying to the same authentication request. A client receiving this message SHOULD wait for the following SSH_MSG_USERAUTH_FAILURE message before beginning another authentication attempt. When a client sends this message, it MUST be followed by a new authentication request or by terminating the connection. A server receiving this message MUST NOT send a SSH_MSG_USERAUTH_FAILURE in reply, since such a message might otherwise be interpreted by a client as a response to the following authentication sequence. Any server sending this message MUST ignore any SSH_MSG_UNIMPLEMENTED sent by the client in response. If a server sends both this message and an SSH_MSG_USERAUTH_GSSAPI_ERROR message, the SSH_MSG_USERAUTH_GSSAPI_ERROR message MUST be sent first, to allow the client to store and/or display the error status before processing the error token. Hutzelman, et. al. Expires January 17, 2005 [Page 19] Internet-Draft SSH GSSAPI Methods July 2004 4. Authentication using GSSAPI Key Exchange This section describes a user authentication method building on the framework described in [SSH-USERAUTH]. This method performs user authentication by making use of an existing GSSAPI context established during key exchange. The authentication method name for this protocol is "gssapi-keyex". This method may be used only if the initial key exchange was performed using a GSSAPI-based key exchange method defined in accordance with Section 2. The GSSAPI context used with this method is always that established during an initial GSSAPI-based key exchange. Any context established during key exchange for the purpose of rekeying MUST NOT be used with this method. The server SHOULD include this user authentication method in the list of methods that can continue (in a SSH_MSG_USERAUTH_FAILURE) if the initial key exchange was performed using a GSSAPI-based key exchange method and provides information about the user's identity which is useful to the server. It MUST NOT include this method if the initial key exchange was not performed using a GSSAPI-based key exchange method defined in accordance with Section 2. The client SHOULD attempt to use this method if it is advertised by the server, initial key exchange was performed using a GSSAPI-based key exchange method, and this method has already been tried. The client SHOULD NOT try this method more than once per session. It MUST NOT try this method if initial key exchange was not performed using a GSSAPI-based key exchange method defined in accordance with Section 2. If a server receives a request for this method when initial key exchange was not performed using a GSSAPI-based key exchange method defined in accordance with Section 2, it MUST return SSH_MSG_USERAUTH_FAILURE. This method is defined as a single message: byte SSH_MSG_USERAUTH_REQUEST string user name string service string "gssapi-keyex" string MIC Hutzelman, et. al. Expires January 17, 2005 [Page 20] Internet-Draft SSH GSSAPI Methods July 2004 The contents of the MIC field are obtained by calling GSS_GetMIC over the following, using the GSSAPI context which was established during initial key exchange: string session identifier byte SSH_MSG_USERAUTH_REQUEST string user name string service string "gssapi-keyex" Upon receiving this message when initial key exchange was performed using a GSSAPI-based key exchange method, the server uses GSS_VerifyMIC() to verify that the MIC received is valid. If the MIC is not valid, the user authentication fails, and the server MUST return SSH_MSG_USERAUTH_FAILURE. If the MIC is valid and the server is satisfied as to the user's credentials, it MAY return either SSH_MSG_USERAUTH_SUCCESS, or SSH_MSG_USERAUTH_FAILURE with the partial success flag set, depending on whether additional authentications are needed. Hutzelman, et. al. Expires January 17, 2005 [Page 21] Internet-Draft SSH GSSAPI Methods July 2004 5. Null Host Key Algorithm The "null" host key algorithm has no associated host key material, and provides neither signature nor encryption algorithms. Thus, it can be used only with key exchange methods that do not require any public-key operations and do not require the use of host public key material. The key exchange methods described in section 1 of this document are examples of such methods. This algorithm is used when, as a matter of configuration, the host does not have or does not wish to use a public key. For example, it can be used when the administrator has decided as a matter of policy to require that all key exchanges be authenticated using Kerberos [KRB5], and thus the only permitted key exchange method is the GSSAPI-authenticated Diffie-Hellman exchange described above, with Kerberos V5 as the underlying GSSAPI mechanism. In such a configuration, the server implementation supports the "ssh-dss" key algorithm (as required by [SSH-TRANSPORT]), but could be prohibited by configuration from using it. In this situation, the server needs some key exchange algorithm to advertise; the "null" algorithm fills this purpose. Note that the use of the "null" algorithm in this way means that the server will not be able to interoperate with clients which do not support this algorithm. This is not a significant problem, since in the configuration described, it will also be unable to interoperate with implementations that do not support the GSSAPI-authenticated key exchange and Kerberos. Any implementation supporting at least one key exchange method which conforms to section 1 of this document MUST also support the "null" host key algorithm. Servers MUST NOT advertise the "null" host key algorithm unless it is the only algorithm advertised. Hutzelman, et. al. Expires January 17, 2005 [Page 22] Internet-Draft SSH GSSAPI Methods July 2004 6. Summary of Message Numbers The following message numbers have been defined for use with GSSAPI-based key exchange methods: #define SSH_MSG_KEXGSS_INIT 30 #define SSH_MSG_KEXGSS_CONTINUE 31 #define SSH_MSG_KEXGSS_COMPLETE 32 #define SSH_MSG_KEXGSS_HOSTKEY 33 #define SSH_MSG_KEXGSS_ERROR 34 #define SSH_MSG_KEXGSS_GROUPREQ 40 #define SSH_MSG_KEXGSS_GROUP 41 The numbers 30-49 are specific to key exchange and may be redefined by other kex methods. The following message numbers have been defined for use with the 'gssapi' user authentication method: #define SSH_MSG_USERAUTH_GSSAPI_RESPONSE 60 #define SSH_MSG_USERAUTH_GSSAPI_TOKEN 61 #define SSH_MSG_USERAUTH_GSSAPI_EXCHANGE_COMPLETE 63 #define SSH_MSG_USERAUTH_GSSAPI_ERROR 64 #define SSH_MSG_USERAUTH_GSSAPI_ERRTOK 65 #define SSH_MSG_USERAUTH_GSSAPI_MIC 66 The numbers 60-79 are specific to user authentication and may be redefined by other user auth methods. Note that in the method described in this document, message number 62 is unused. Hutzelman, et. al. Expires January 17, 2005 [Page 23] Internet-Draft SSH GSSAPI Methods July 2004 7. GSSAPI Considerations 7.1 Naming Conventions In order to establish a GSSAPI security context, the SSH client needs to determine the appropriate targ_name to use in identifying the server when calling GSS_Init_sec_context. For this purpose, the GSSAPI mechanism-independent name form for host-based services is used, as described in section 4.1 of [GSSAPI]. In particular, the targ_name to pass to GSS_Init_sec_context is obtained by calling GSS_Import_name with an input_name_type of GSS_C_NT_HOSTBASED_SERVICE, and an input_name_string consisting of the string "host@" concatenated with the hostname of the SSH server. 7.2 Channel Bindings This document recommends that channel bindings SHOULD NOT be specified in the calls during context establishment. This document does not specify any standard data to be used as channel bindings and the use of network addresses as channel bindings may break SSH in environments where it is most useful. 7.3 SPNEGO The use of the Simple and Protected GSS-API Negotiation Mechanism [SPNEGO] in conjunction with the authentication and key exchange methods described in this document is both unnecessary and undesirable. As a result, mechanisms conforming to this document MUST NOT use SPNEGO as the underlying GSSAPI mechanism. Since SSH performs its own negotiation of authentication and key exchange methods, the negotiation capability of SPNEGO alone does not provide any added benefit. In fact, as described below, it has the potential to result in the use of a weaker method than desired. Normally, SPNEGO provides the added benefit of protecting the GSSAPI mechanism negotiation. It does this by having the server compute a MIC of the list of mechanisms proposed by the client, and then checking that value at the client. In the case of key exchange, this protection is not needed because the key exchange methods described here already perform an equivalent operation; namely, they generate a MIC of the SSH exchange hash, which is a hash of several items including the lists of key exchange mechanisms supported by both sides. In the case of user authentication, the protection is not needed because the negotiation occurs over a secure channel, and the host's identity has already been proved to the user. The use of SPNEGO combined with GSSAPI mechanisms used without Hutzelman, et. al. Expires January 17, 2005 [Page 24] Internet-Draft SSH GSSAPI Methods July 2004 SPNEGO can lead to interoperability problems. For example, a client which supports key exchange using the Kerberos V5 GSSAPI mechanism [KRB5-GSS] only underneath SPNEGO will not interoperate with a server which supports key exchange only using the Kerberos V5 GSSAPI mechanism directly. As a result, allowing GSSAPI mechanisms to be used both with and without SPNEGO is undesirable. If a client's policy is to first prefer GSSAPI-based key exchange method X, then non-GSSAPI method Y, then GSSAPI-based method Z, and if a server supports mechanisms Y and Z but not X, then an attempt to use SPNEGO to negotiate a GSSAPI mechanism might result in the use of method Z when method Y would have been preferable. As a result, the use of SPNEGO could result in the subversion of the negotiation algorithm for key exchange methods as described in section 5.1 of [SSH-TRANSPORT] and/or the negotiation algorithm for user authentication methods as described in [SSH-USERAUTH]. Hutzelman, et. al. Expires January 17, 2005 [Page 25] Internet-Draft SSH GSSAPI Methods July 2004 8. IANA Considerations Consistent with section 7 of [SSH-ARCH], this document makes the following registrations: The family of SSH key exchange method names beginning with "gss-group1-sha1-" and not containing the at-sign ('@'), to name the key exchange methods defined in Section 2.3. The family of SSH key exchange method names beginning with "gss-gex-sha1-" and not containing the at-sign ('@'), to name the key exchange methods defined in Section 2.4. All other SSH key exchange method names beginning with "gss-" and not containing the at-sign ('@'), to be reserved for future key exchange methods defined in conformance with this document, as noted in Section 2.5. The SSH host public key algorithm name "null", to name the NULL host key algorithm defined in Section 5. The SSH user authentication method name "gssapi-with-mic", to name the GSSAPI user authentication method defined in Section 3. The SSH user authentication method name "gssapi-keyex", to name the GSSAPI user authentication method defined in Section 4. The SSH user authentication method name "gssapi" is to be reserved, in order to avoid conflicts with implementations supporting an earlier version of this specification. The SSH user authentication method name "external-keyx" is to be reserved, in order to avoid conflicts with implementations supporting an earlier version of this specification. This document creates no new registries. Hutzelman, et. al. Expires January 17, 2005 [Page 26] Internet-Draft SSH GSSAPI Methods July 2004 9. Security Considerations This document describes authentication and key-exchange protocols. As such, security considerations are discussed throughout. This protocol depends on the SSH protocol itself, the GSSAPI, any underlying GSSAPI mechanisms which are used, and any protocols on which such mechanisms might depend. Each of these components plays a part in the security of the resulting connection, and each will have its own security considerations. The key exchange method described in section 1 of this document depends on the underlying GSSAPI mechanism to provide both mutual authentication and per-message integrity services. If either of these features is not supported by a particular GSSAPI mechanism, or by a particular implementation of a GSSAPI mechanism, then the key exchange is not secure and MUST fail. In order for the "external-keyx" user authentication method to be used, it MUST have access to user authentication information obtained as a side-effect of the key exchange. If this information is unavailable, the authentication MUST fail. Revealing information about the reason for an authentication failure may be considered by some sites to be an unacceptable security risk for a production environment. However, having that information available can be invaluable for debugging purposes. Thus, it is RECOMMENDED that implementations provide a means for controlling, as a matter of policy, whether to send SSH_MSG_USERAUTH_GSSAPI_ERROR, SSH_MSG_USERAUTH_GSSAPI_ERRTOK, and SSH_MSG_KEXGSS_ERROR messages, and SSH_MSG_KEXGEE_CONTINUE messages containing a GSSAPI error token. Hutzelman, et. al. Expires January 17, 2005 [Page 27] Internet-Draft SSH GSSAPI Methods July 2004 10. Acknowledgements The authors would like to thank the following individuals for their invaluable assistance and contributions to this document: o Sam Hartman o Love Hornquist-Astrand o Joel N. Weber II o Simon Wilkinson o Nicolas Williams Much of the text describing DH group exchnage was borrowed from [GROUP-EXCHANGE], by Markus Friedl, Niels Provos, and William A. Simpson. Hutzelman, et. al. Expires January 17, 2005 [Page 28] Internet-Draft SSH GSSAPI Methods July 2004 11. Changes the last version This section lists important changes since the previous version of this internet-draft. This section should be removed at the time of publication of this document as an RFC. o Added the description of the generic GSSAPI key exchange with group exchange, and the definition of the gss-gex-sha1-* methods. o Updated references to the ssh core documents. o Converted from numbered to symbolic references. Hutzelman, et. al. Expires January 17, 2005 [Page 29] Internet-Draft SSH GSSAPI Methods July 2004 Normative References [ASN1] ISO/IEC, "ASN.1 Encoding Rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)", ITU-T Recommendation X.690 (1997), ISO/IEC 8825-1:1998, November 1998. [GROUP-EXCHANGE] Friedl, M., Provos, N. and W. A. Simpson, "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol", draft-ietf-secsh-dh-group-exchange-04.txt (work in progress), July 2003. [GSSAPI] Linn, J., "Generic Security Service Application Program Interface Version 2, Update 1", RFC 2743, January 2000. [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, BCP 14, March 1997. [LANGTAG] Alvestrand, H., "Tags for the Identification of Languages", RFC 1766, March 1995. [MD5] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April 1992. [MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. [SSH-ARCH] Ylonen, T. and D. Moffat, "SSH Protocol Architecture", draft-ietf-secsh-architecture-15.txt (work in progress), October 2003. [SSH-CONNECT] Ylonen, T. and D. Moffat, "SSH Connection Protocol", draft-ietf-secsh-connect-18.txt (work in progress), October 2003. [SSH-TRANSPORT] Ylonen, T. and D. Moffat, "SSH Transport Layer Protocol", draft-ietf-secsh-transport-17.txt (work in progress), October 2003. [SSH-USERAUTH] Ylonen, T. and D. Moffat, "SSH Authentication Protocol", draft-ietf-secsh-userauth-18.txt (work in progress), October 2003. Hutzelman, et. al. Expires January 17, 2005 [Page 30] Internet-Draft SSH GSSAPI Methods July 2004 [UTF8] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC 2279, January 1998. Hutzelman, et. al. Expires January 17, 2005 [Page 31] Internet-Draft SSH GSSAPI Methods July 2004 Non-Normative References [KRB5] Kohl, J. and C. Neuman, "The Kerberos Network Authentication Service (V5)", RFC 1510, September 1993. [KRB5-GSS] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC 1964, June 1996. [SPNEGO] Baize, E. and D. Pinkas, "The Simple and Protected GSS-API Negotiation Mechanism", RFC 2478, December 1998. Authors' Addresses Jeffrey Hutzelman Carnegie Mellon University 5000 Forbes Ave Pittsburgh, PA 15213 US Phone: +1 412 268 7225 EMail: jhutz+@cmu.edu URI: http://www.cs.cmu.edu/~jhutz/ Joseph Salowey Cisco Systems 2901 Third Avenue Seattle, WA 98121 US Phone: +1 206 256 3380 EMail: jsalowey@cisco.com Joseph Galbraith Van Dyke Technologies, Inc. 4848 Tramway Ridge Dr. NE Suite 101 Albuquerque, NM 87111 US EMail: galb@vandyke.com Hutzelman, et. al. Expires January 17, 2005 [Page 32] Internet-Draft SSH GSSAPI Methods July 2004 Von Welch University of Chicago & Argonne National Laboratory Distributed Systems Laboratory 701 E. Washington Urbana, IL 61801 US EMail: welch@mcs.anl.gov Hutzelman, et. al. Expires January 17, 2005 [Page 33] Internet-Draft SSH GSSAPI Methods July 2004 Full Copyright Statement Copyright (C) The Internet Society (2004). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. 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