Network Working Group J. Hutzelman Internet-Draft CMU Expires:
July 14,August 31, 2001 J. Salowey Cisco Systems January 13,March 2, 2001 Using GSSAPI authentication for key exchange in Secure Shell draft-ietf-secsh-gsskeyex-00draft-ietf-secsh-gsskeyex-01 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 July 14,August 31, 2001. Copyright Notice Copyright (C) The Internet Society (2001). All Rights Reserved. Abstract This memo describes a method for using the Generic Security Service Application Program Interface  for key exchange in the Secure Shell protocol, by defining a class of SSH key exchange methods which use GSSAPI to authenticate the Diffie-Hellman exchange described in . 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 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 . 1. 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  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 heykey algorithm, including the "null" algorithm described in section 2 of this document. 1.1 Generic method description 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 or serverdoes not support the "external-keyx" user authentication method described in section 3 of this document, or if the clientdoes not intend to use that method, 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. * 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 the mutual_state flag is true, the resulting output token is sent to S along with "e".S. * If the resulting major_status code is GSS_S_CONTINUE_NEEDED, the the output_token (only)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 exactly one ofthe messagesfirst 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. The client SHOULD wait until the last message (that is, the one containing the token returned by GSS_Init_sec_context at the same time it returned GSS_S_COMPLETE) to send "e", so that it is not sent if there* It is an error inif the coursecall does not produce a token of setting upnon-zero length to be sent to the GSSAPI security context.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 mutual_state flag is 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 mod p. It computes K = e ^ y mod p, and H = hash(V_C || V_S || I_C || I_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 || e || f || K). It then calls GSS_VerifyMIC to verify that the MIC sent by S matches H. 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. 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_GSSAPI_INIT boolean TRUE string output_token (from GSS_Init_sec_context) mpint e Each time GSS_Init_sec_contextthe 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_GSSAPI_CONTINUE string output_token (from GSS_Accept_sec_context) If the client receives this message appears 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_GSSAPI_INIT boolean FALSE string output_token (from GSS_Init_sec_context) The server responds withand client continue to trade these two messages as long as the following: byte SSH_MSG_GSSAPI_CONTINUE string output_token (from GSS_Accept_sec_context)server's calls to GSS_Accept_sec_context result in major_status codes of GSS_S_CONTINUE_NEEDED. When GSS_Init_sec_context returnsa call results in a major_status code of GSS_S_COMPLETE, it sends one of two final messages. If the clientserver's final call to GSS_Accept_sec_contents (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_GSSAPI_INITSSH_MSG_GSSAPI_COMPLETE mpint f string per_msg_token (MIC of H) boolean TRUE string output_token (from GSS_Init_sec_context) mpint e The server responds withGSS_Accept_sec_context) If the client receives this message appears 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_contents (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_GSSAPI_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. 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 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. The GSS_GetMIC call MUST be applied over H, not the original data. 1.2 gss-group1-sha1-* Each of these methods specifies GSSAPI authenticated Diffie-Hellman key exchange as described in section 1.1 of this document, with SHA-1 as HASH, and the group defined in section 6.1 of . The method name for each method is the concatenation of the string "gss-group1-sha1-" with the Base64 encoding of the first ten bytes of theMD5 hash  of the ASN.1 DER encoding  of the underlying GSSAPI mechanism's OID. Base64 encoding is described in section 6.8 of . 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. 1.3 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 1.1 of this document, 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 1.2 of this document. 1.4 SPNEGO The use of the Simple and Protected GSS-API Negotiation Mechanism  in conjunction with the key exchange methods described in this document is both unnecessary and undesirable. As a result, key exchange mechanisms conforming to this document MUST NOT use SPNEGO as the underlying GSSAPI mechanism. Since SSH performs its own negotiation of key exchange methods, and there exists a separate method name corresponding to every possible underlying GSSAPI mechanism, 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. The key exchange methods described in this document 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. Thus, the extra level of protection offered by SPNEGO is unnecessary in this case. The use of SPNEGO combined with GSSAPI mechanisms used without SPNEGO can lead to interoperability problems. For example, a client which supports key exchange using the Kerberos V5 GSSAPI mechanism  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 . 1.5 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 . 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. 1.6 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. 2. 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 , 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 , 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. 3. External Key Exchange user authentication This section describes a user authentication method building on the framework described in . This method relies upon the key exchange to authenticate both the client and the server. If the key exchange did not successfully perform these functions then the server MUST always respond to this request with SSH_MSG_USERAUTH_FAILURE with partial success set to false. The new mechanism is defined as follows: byte SSH_MSG_USERAUTH_REQUEST string authorization-ID string service string "external-keyx" If the user authenticated in the key-exchange is allowed to assume the authorization identity, this method is successful, and the server responds with SSH_MSG_USERAUTH_SUCCESS if no more authentications are needed, or with SSH_MSG_USERAUTH_FAILURE with partial success set to true if more authentications are needed. If the user authenticated in the key-exchange is not allowed to assume the authorization identity, then SSH_MSG_USERAUTH_FAILURE is returned with partial success set to false. Any implementation supporting at least one key exchange method which conforms to section 1 of this document SHOULD also support the "external-keyx" user authentication method, in order to allow user authentication to be performed at the same time as key exchange, thereby reducing the number of round trips needed for connection setup. 4. Summary of Message Numbers The following message numbers have been defined in this document: #define SSH_MSG_GSSAPI_INIT 30 #define SSH_MSG_GSSAPI_CONTINUE 31 #define SSH_MSG_GSSAPI_COMPLETE 32 The numbers 30-49 are key exchange specific and may be redefined by other kex methods. 5. Security Considerations This document describes an authentication and key-exchange protocol. 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. 5. Trademark Issues SSH is a registered trademark and Secure Shell is a trademark of SSH Communications Security Corp (www.ssh.com), and are used herein to describe and refer6. Acknowledgements The authors would like to the SSH protocol, as permitted in section 9 of . These trademarks may not be used as part of a product name or in otherwise confusing manner without prior written permission of SSH Communications Security Corp.thank Sam Hartman and Simon Wilkinson for their invaluable assistance with this document. References  ISO/IEC, "Specification of Abstract Syntax Notation One (ASN.1)", ISO/IEC 8824, November 1998.  Linn, J., "Generic Security Service Application Program Interface Version 2, Update 1", RFC 2743, January 2000.  Kohl, J. and C. Neuman, "The Kerberos Network Authentication Service (V5)", RFC 1510, September 1993.  Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC 1964, June 1996.  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April 1992.  Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996.  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, BCP 14, March 1997.  Baize, E. and D. Pinkas, "The Simple and Protected GSS-API Negotiation Mechanism", RFC 2478, December 1998.  Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T. and S. Lehtinen, "SSH Protocol Architecture", November 2000.  Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T. and S. Lehtinen, "SSH Transport Layer Protocol", November 2000.  Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T. and S. Lehtinen, "SSH Authentication Protocol", November 2000. Authors' Addresses Jeffrey Hutzelman Carnegie Mellon University 5000 Forbes Ave Pittsburgh, PA 15213 US Phone: +1 412 268 7225 EMail: firstname.lastname@example.org URI: http://www.cs.cmu.edu/~jhutz/ Joseph Salowey Cisco Systems Bldg 20 725 Alder Drive Milpitas, CA 95035 US Phone: +1 408 525 6381 EMail: email@example.com Full Copyright Statement Copyright (C) The Internet Society (2001). 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. 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