Network Working Group S. Josefsson Internet-Draft February 13, 2006 Expires: August 17, 2006 Using GSS-API Mechanisms in SASL: The GS2 Mechanism Family draft-ietf-sasl-gs2-00 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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 August 17, 2006. Copyright Notice Copyright (C) The Internet Society (2006). Abstract This document describes how to use a Generic Security Service Application Program Interface (GSS-API) mechanism in the the Simple Authentication and Security Layer (SASL) framework. See for more information. Josefsson Expires August 17, 2006 [Page 1] Internet-Draft SASL GS2-* February 2006 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions Used in this Document . . . . . . . . . . . . . . 3 3. Mechanism Name . . . . . . . . . . . . . . . . . . . . . . . . 3 3.1. Generating SASL mechanism names from GSS-API OIDs . . . . 3 3.2. Computing mechanism names manually . . . . . . . . . . . . 4 3.3. Example . . . . . . . . . . . . . . . . . . . . . . . . . 4 4. Packet format . . . . . . . . . . . . . . . . . . . . . . . . 4 4.1. SASL tokens . . . . . . . . . . . . . . . . . . . . . . . 4 4.2. Context token . . . . . . . . . . . . . . . . . . . . . . 5 4.3. Wrap token . . . . . . . . . . . . . . . . . . . . . . . . 6 4.3.1. Wrap token input for client requests . . . . . . . . . 7 4.3.2. Wrap token input for server responses . . . . . . . . 8 4.3.3. Wrap token input for server requests . . . . . . . . . 8 4.3.4. Wrap token input for client responses . . . . . . . . 9 5. Protocol overview . . . . . . . . . . . . . . . . . . . . . . 9 6. Authentication Conditions . . . . . . . . . . . . . . . . . . 12 7. GSS-API parameters . . . . . . . . . . . . . . . . . . . . . . 12 8. Security layer bits . . . . . . . . . . . . . . . . . . . . . 13 9. Interoperability with the GSSAPI mechanism . . . . . . . . . . 13 9.1. Interoperability problem . . . . . . . . . . . . . . . . . 13 9.2. Resolving the problem . . . . . . . . . . . . . . . . . . 13 9.3. Additional recommendations . . . . . . . . . . . . . . . . 13 10. Mechanisms that negotiate other mechanisms . . . . . . . . . . 14 10.1. Interoperability problem . . . . . . . . . . . . . . . . . 14 10.2. Security problem . . . . . . . . . . . . . . . . . . . . . 14 10.3. Resolving the problems . . . . . . . . . . . . . . . . . . 14 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 12. Security Considerations . . . . . . . . . . . . . . . . . . . 15 13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16 14. Copying conditions . . . . . . . . . . . . . . . . . . . . . . 16 15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16 15.1. Normative References . . . . . . . . . . . . . . . . . . . 16 15.2. Informative References . . . . . . . . . . . . . . . . . . 17 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18 Intellectual Property and Copyright Statements . . . . . . . . . . 19 Josefsson Expires August 17, 2006 [Page 2] Internet-Draft SASL GS2-* February 2006 1. Introduction Generic Security Service Application Program Interface (GSS-API) [3] is a framework that provide security services to applications. Simple Authentication and Security Layer (SASL) [2] is a framework to provide authentication and security layers for connection based protocols. This document describe how to use a GSS-API mechanism in a connection-based protocol using the SASL framework. All GSSAPI mechanisms are implicitly registered for use within SASL by this specification. The SASL mechanism defined in this document is known as the GS2 family. The "Kerberos V5 GSS-API mechanism" [9] is also supported in SASL through "SASL GSSAPI mechanisms" [11]. The difference between that protocol and the one described here, is that this protocol offer more features (i.e., channel bindings and round-trip optimizations) while the other protocol is more widely deployed. There are interoperability concerns by having the same GSS-API mechanism available under more than one SASL mechanism name, see the section "Interoperability with the GSSAPI mechanism" below. There are interoperability and security concerns if this SASL mechanism is used together with a GSS-API mechanism that negotiate other GSS-API mechanisms (such as SPNEGO [10]), see the section "Mechanisms that negotiate other mechanisms" below. SASL mechanism names starting with "GS2-" are reserved for SASL mechanisms which conform to this document. The IESG is considered to be the owner of all SASL mechanisms which conform to this document. This does not necessarily imply that the IESG is considered to be the owner of the underlying GSSAPI mechanism. 2. Conventions Used in this Document 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]. 3. Mechanism Name 3.1. Generating SASL mechanism names from GSS-API OIDs The SASL mechanism name for a GSS-API mechanism is the concatenation Josefsson Expires August 17, 2006 [Page 3] Internet-Draft SASL GS2-* February 2006 of the string "GS2-" and the Base32 encoding [5] (with an upper case alphabet) of the first ten bytes of the binary SHA-1 hash [4] string computed over the ASN.1 DER encoding [7] of the GSS-API mechanism's Object Identifier. The Base32 rules on padding characters and characters outside of the base32 alphabet are not relevant to this use of Base32. If any padding or non-alphabet characters are encountered, the name is not a GS2 family mechanism name. 3.2. Computing mechanism names manually The SASL mechanism name may be computed manually. This is useful when the set of supported GSS-API mechanisms is known in advance. It also obliterate the need to implement Base32, SHA-1 and DER in the SASL mechanism. The computed mechanism name can be used directly in the implementation, and the implementation need not concern itself with that the mechanism is part of a mechanism family. 3.3. Example For example, the OID for the SPKM-1 mechanism [12] is 1.3.6.1.5.5.1.1. The ASN.1 DER encoding of the OID is 06 07 2b 06 01 05 05 01 01. The SHA-1 hash of the ASN.1 DER encoding is 1cf8f42b5a9f80fae9f831226d5d9d56278661ad. The Base32 encoding of the first ten bytes of this is "dt4pik22t6epv2py". Thus the SASL mechanism name for the SPKM-1 GSSAPI mechanism is "GS2- DT4PIK22T6EPV2PY". 4. Packet format 4.1. SASL tokens All top-level SASL packets for the GS2 mechanism family follow the following format: 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Context length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | / / Context token / / --------------------/ / ---------------------/ / /--------------------/ / / [Wrap token] / / / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Josefsson Expires August 17, 2006 [Page 4] Internet-Draft SASL GS2-* February 2006 The "Context length" field is a 4 octet (32 bit) integer encoded in network byte order, it indicate the length of the "Context token" field. The "Context token" field contain a GSS-API context establishment token generated by GSS_Init_sec_context or GSS_Accept_sec_context. The "Wrap token" field is optional, and if present will contain the output generated by GSS_Wrap. The length field does not include the length of the length field itself. Whether the "Wrap token" field is included or not can be infered from the length field; if the length field is shorter than the entire packet size minus 4 octets, the "Wrap token" field is present and begins after length+4 octets into the packet. The tokens need not be aligned to 32-bit a boundary. There is no padding between the tokens. Packets shorter than 4 octets are invalid. If the length field is longer than the entire packet size, minus 4 octets, the packet is invalid. 4.2. Context token The format of the "Context token" field inside the SASL token are defined by the GSS-API specifications, see GSS_Init_sec_context and GSS_Accept_sec_context. The client calls GSS_Init_sec_context, passing in input_context_handle of 0 (initially), mech_type of the GSSAPI mechanism for which this SASL mechanism is registered, any chan_binding if requested by the application (XXX?), and targ_name equal to output_name from GSS_Import_Name called with input_name_type of GSS_C_NT_HOSTBASED_SERVICE and input_name_string of "service@hostname" where "service" is the service name specified in the protocol's profile, and "hostname" is the fully qualified host name of the server. If the client will be requesting a security layer, it MUST also supply to the GSS_Init_sec_context a mutual_req_flag of TRUE, a sequence_req_flag of TRUE, and an integ_req_flag of TRUE. If the client will be requesting a security layer providing confidentiality protection, it MUST also supply to the GSS_Init_sec_context a conf_req_flag of TRUE. If GSS_Init_sec_context returns GSS_S_CONTINUE_NEEDED, then the client should expect the server to issue a token in a subsequent challenge or as additional information to the outcome of the authentication. The client must pass the context token to another call to GSS_Init_sec_context, repeating the actions in this paragraph, until GSS_S_COMPLETE is returned or authentication is aborted. If the Josefsson Expires August 17, 2006 [Page 5] Internet-Draft SASL GS2-* February 2006 server supply data beyond the context token, the context token should be processed first, and then the overflow data should be passed to GSS_Unwrap and the unwrapped data should be interpreted. During the authentication exchange, the client will generate one Wrap token using GSS_Wrap. The server passes the first client response to GSS_Accept_sec_context as input_token, setting input_context_handle to 0 (initially), mech_type of the GSSAPI mechanism for which this SASL mechanism is registered, any chan_binding if requested by the application (XXX?), and acceptor_cred_handle equal to output_cred_handle from GSS_Acquire_cred called with desired_name equal to output_name from GSS_Import_name with input_name_type of GSS_C_NT_HOSTBASED_SERVICE and input_name_string of "service@hostname" where "service" is the service name specified in the protocol's profile, and "hostname" is the fully qualified host name of the server. The server must pass any resulting challenge from the client to another call to GSS_Accept_sec_context, repeating the actions in this paragraph, until GSS_S_COMPLETE is returned or authentication is aborted. If the client supply data beyond the context token, the context token should be processed first, and then the overflow data should be passed to GSS_Unwrap and the unwrapped data should be interpreted. During the authentication exchange, the server will generate one Wrap token using GSS_Wrap. 4.3. Wrap token The Wrap token MUST NOT be sent before the PROT_READY flag has been set locally (by GSS_Init_sec_context or Gss_Accept_sec_context), or if the PROT_READY flag is never set, before the context has been fully established. The GSS_Wrap token does not have to be sent directly when the PROT_READY flag is set. During any exchange, exactly one GSS_Wrap token is sent in each direction. The input to the GSS_Wrap function MUST follow the format described below. If not exactly one GSS_Wrap token is received by the client and by the server, the authentication MUST fail. If PROT_READY is never set by GSS_Init_sec_context or GSS_Accept_sec_context, the flag is implied by successful context negotiation. This is for GSS-API v1 mechanisms that do not support PROT_READY. This may result in a SASL token consisting of a context token length of 0 and a Wrap token. The entity that sends its first Wrap token will have to specify a bitmap of supported and preferred quality of protection schemes. The entity that reply to the Wrap tokens will pick a scheme, based on the bitmask and local policy. Josefsson Expires August 17, 2006 [Page 6] Internet-Draft SASL GS2-* February 2006 Four input formats to the GSS_Wrap function are defined. The first two input formats are used when the client sends the GSS_Wrap token first and the server reponds. The other two input formats are used when the server sends the GSS_Wrap token first and the client responds. The input formats below are passed to GSS_Wrap with conf_flag set to FALSE, and the Wrap token output will be the generated output_message. Some fields in the input formats are optional, indicated by brackets ("[" and "]") and explained by the text below. 4.3.1. Wrap token input for client requests The input to GSS_Wrap when the client sends the GSS_Wrap token first is as follows. 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | client_qops | client_maxbuf | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | channel_binding_length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |[client_cbqops]| [channel_binding_data] / / / / / [authzid] / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The "client_qops" integer indicate the client's preferred quality of protection if channel bindings are absent or the negotiation of the channel binding fails. The "client_maxbuf" field indicate the maximum protected buffer size the client can receive. The "channel_binding_length" is a network byte order integer that indicate the length of the "channel_binding_data" field. The optional field "client_cbqops" is present only if "channel_binding_length" is non-zero, and indicate the client's preferred quality of protection if channel binding negotiation succeeds. The optional field "channel_binding_data" is present only if "channel_binding_length" is non-zero, and contain the actual channel binding data. Josefsson Expires August 17, 2006 [Page 7] Internet-Draft SASL GS2-* February 2006 The optional field "authzid" contain the authorization identity. The authorization identity is encoded using UTF-8 [6]. The authorization identity is not terminated with the NUL (U+0000) character. Servers MUST validate that the authorization identity is valid UTF-8. 4.3.2. Wrap token input for server responses The data format for input to GSS_Wrap when the server responds to the previous GSS_Wrap token data is as follows. 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | server_qop | server_maxbuf | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The "server_qop" field integer indicate the selected quality of protection. The "server_maxbuf" field indicate the maximum data buffer size the server can receive. It MUST be 0 if the server doesn't advertise support for any security layer, the client MUST verify this. 4.3.3. Wrap token input for server requests The data format for input to GSS_Wrap when the server sends the GSS_Wrap token first is as follows. 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | server_qops | server_maxbuf | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | channel_binding_length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |[server_cbqops]| [channel_binding_data] / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The "server_qops" field is an integer indicating the server's preferred quality of protection if channel bindings are absent or the negotiation of the channel binding fails. The "server_maxbuf" is the same as above. The "channel_binding_length" is a network byte order integer that indicate the length of the "channel_binding_data" field. The optional field "server_cbqops" is present only if Josefsson Expires August 17, 2006 [Page 8] Internet-Draft SASL GS2-* February 2006 "channel_binding_length" is non-zero, and indicate the server's preferred quality of protection if channel binding negotiation succeeds. The optional field "channel_binding_data" is present only if "channel_binding_length" is non-zero, and contain the actual channel binding data. 4.3.4. Wrap token input for client responses The data format for input to GSS_Wrap when the client responds to the previous token is as follows. 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | client_qop | client_maxbuf | / [authzid] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The "client_qop" field is the selected quality of protection. The "client_maxbuf" and "authzid" fields are as above. 5. Protocol overview This section describe several high-level protocol exchanges. The descriptions do not assume any properties of the actual GSS-API mechanism. Protocol profiles, GSS-API mechanism specific behaviour, and to some extent implementation and policy choices, will dictate which packets are sent in what order. The protocol exchanges are examples and other exchanges are permitted and will occur. An authentication exchange using GS2 may look like: Josefsson Expires August 17, 2006 [Page 9] Internet-Draft SASL GS2-* February 2006 C: Request authentication exchange S: Send [length=0] token C: Send [length, GSS_Init_sec_context] token ... S: After PROT_READY is set, send [length, GSS_Accept_sec_context, GSS_Wrap(server_qops, server_maxbuf, channel_binding_length=0)] ... C: After PROT_READY is set, send [length, GSS_Init_sec_context, GSS_Wrap (client_qop, client_maxbuf, authzid)] S: Send [length, GSS_Accept_sec_context] token C: Send [length, GSS_Init_sec_context] token ... S: Outcome of authentication exchange Because GSS-API authentication is initiated by the client, the length field will be 0 in the initial token from the server to the client when the protocol profile do not support additional information to be sent together with the authentication request. The next example illustrate when the client sends its Wrap token first. C: Request authentication exchange S: Send [length=0] token C: Send [length, GSS_Init_sec_context] token ... C: After PROT_READY is set, send [length, GSS_Init_sec_context, GSS_Wrap(client_qops, client_maxbuf, channel_binding_length=0, authzid)] ... S: After PROT_READY is set, send [length, GSS_Accept_sec_context, GSS_Wrap (server_qop, server_maxbuf)] C: Send [length, GSS_Init_sec_context] token S: Send [length, GSS_Accept_sec_context] token ... S: Outcome of authentication exchange If the protocol profile support the optional initial client response, the first empty message can be optimized away, and then the protocol exchange will look like: Josefsson Expires August 17, 2006 [Page 10] Internet-Draft SASL GS2-* February 2006 C: Request authentication exchange and send [length, GSS_Init_sec_context] token S: Send [length, GSS_Accept_sec_context] token ... S: Outcome of authentication exchange If the protocol profile can also send additional information when indicating the outcome of the authentication, then the protocol exchange will look like: C: Request authentication exchange and send [length, GSS_Init_sec_context] token S: Send [length, GSS_Accept_sec_context] token ... C: Send [length, GSS_Init_sec_context] token S: Indicate successful authentication and send [length, GSS_Accept_sec_context] token as additional information. If the PROT_READY flag is never set by the GSS-API mechanism, the GSS_Wrap message will be sent after the context has been established. The protocol may look like: C: Request authentication exchange ... S: GSS_Accept_sec_context() returns GSS_S_COMPLETE and outputs a token, send [length, context token, GSS_Wrap(server_qops, server_maxbuf, channel_binding_length=0)] C: GSS_Init_sec_context() returns GSS_S_COMPLETE and does not output a token, send [length=0, context token, GSS_Wrap(client_qop, client_maxbuf, channel_binding_length=0, authzid)] S: Outcome of authentication exchange Alternatively, if the client finishes first, it may look like: C: Request authentication exchange ... C: GSS_Init_sec_context() returns GSS_S_COMPLETE and outputs a token, send [length, context token, GSS_Wrap(client_qops, client_maxbuf, channel_binding_length=0, authzid)] S: GSS_Accept_sec_context() returns GSS_S_COMPLETE and does not output a token, send [length, context token, GSS_Wrap(server_qop, server_maxbuf, channel_binding_length=0)] Josefsson Expires August 17, 2006 [Page 11] Internet-Draft SASL GS2-* February 2006 S: Outcome of authentication exchange If the protocol support initial data from the client, and the PROT_READY flag is set in the client after the first call to GSS_Init_sec_context, and the server can send additional data to the client when indicating successful authentication, the following protocol exchange will occur. C: Request authentication exchange and send [length, GSS_Init_sec_context, GSS_Wrap (client_qops, client_maxbuf, channel_binding_length=0, authzid)] token S: Indicate successful authentication and send [length, GSS_Accept_sec_context, GSS_Wrap(server_qop, server_maxbuf)] token as additional information. The last example illustrate the optimal (round-trip wise) authentication possible using this protocol. 6. Authentication Conditions Authentication MUST NOT succeed if any one of the following conditions are true: o An invalid SASL token is received (i.e., length shorter than 4 octets). o GSS_Init/Accept_sec_context() return anything other than GSS_S_CONTINUE_NEEDED or GSS_S_COMPLETE. o GSS_Wrap() returns anything other than GSS_S_COMPLETE. o GSS_Unwrap() returns anything other than GSS_S_COMPLETE. (There can't be supplementary status codes in GS2 at this point, so any indications of out of order processing or replays should be fatal.) o The token returned from GSS_Unwrap fail to parse correctly (e.g., too short, invalid maximum buffer size) as the expected Wrap token. o Local policy reject the attempt. For example, client and server can't agree on qop proposal. o (server-side) Authorization of client principal (i.e., src_name in GSS_Acecpt_sec_context) to requested authzid failed. 7. GSS-API parameters The implementation MAY set any GSSAPI flags or arguments not mentioned in this specification as is necessary for the implementation to enforce its security policy. Josefsson Expires August 17, 2006 [Page 12] Internet-Draft SASL GS2-* February 2006 8. Security layer bits The security layers and their corresponding bit-masks are as follows: 1 No security layer 2 Integrity protection. Sender calls GSS_Wrap with conf_flag set to FALSE 4 Confidentiality protection. Sender calls GSS_Wrap with conf_flag set to TRUE Other bit-masks may be defined in the future; bits which are not understood must be negotiated off. Note that SASL negotiates the maximum size of the output_message to send. Implementations can use the GSS_Wrap_size_limit call to determine the corresponding maximum size input_message. 9. Interoperability with the GSSAPI mechanism The GSSAPI mechanism [11] describe how the Kerberos V5 GSS-API mechanism [9] is used in SASL under the mechanism name "GSSAPI". The same mechanism may also be used with the GS2 family. This causes an interopability problem, which is discussed and resolved below. 9.1. Interoperability problem If a client (or server) only support Kerberos V5 under the "GSSAPI" name and the server (or client) only support Kerberos V5 under the GS2 family, the authentication negotiation will fail. 9.2. Resolving the problem If the Kerberos V5 mechanism is supported under GS2 in a server, the server SHOULD also support Kerberos V5 through the "GSSAPI" mechanism, to avoid interoperability problems with older clients. Reasons for violating this recommendation may include security considerations regarding the absent features in the GS2 mechanism. The Kerberos V5 "GSSAPI" SASL mechanism lack channel bindings, which could enable certain tunnel attacks [16]. 9.3. Additional recommendations It is RECOMMENDED to negotiate Kerberos V5 through the GS2 mechanism rather than through the "GSSAPI" mechanism, if both are available, because of the additional features in the GS2 mechanism. Josefsson Expires August 17, 2006 [Page 13] Internet-Draft SASL GS2-* February 2006 10. Mechanisms that negotiate other mechanisms A GSS-API mechanism that negotiate other mechanisms interact badly with the SASL mechanism negotiation. There are two problems. The first is an interoperability problem and the second is a security concern. The problems are described and resolved below. 10.1. Interoperability problem If a client implement GSS-API mechanism X, potentially negotiated through a GSS-API mechanism Y, and the server also implement GSS-API mechanism X negotiated through a GSS-API mechanism Z, the authentication negotiation will fail. 10.2. Security problem If a client's policy is to first prefer GSSAPI mechanism X, then non- GSSAPI mechanism Y, then GSSAPI mechanism Z, and if a server supports mechanisms Y and Z but not X, then if the client attempts to negotiate mechanism X by using a GSS-API mechanism that negotiate other mechanisms (such as SPNEGO), it may end up using mechanism Z when it should have used mechanism Y. For this reason, the use of GSS-API mechanisms that negotiate other mechanisms are disallowed under GS2. 10.3. Resolving the problems GSS-API mechanisms that negotiate other mechanisms MUST NOT be used with the GS2 SASL mechanism. This specifically exclude negotiating SPNEGO [10] under GS2. The GSS_C_MA_MECH_NEGO attribute of GSS_Inquire_attrs_for_mech() [15] can be used to identify such mechanisms. 11. IANA Considerations The IANA is advised that SASL mechanism names starting with "GS2-" are reserved for SASL mechanisms which conform to this document. The IANA is directed to place a statement to that effect in the sasl- mechanisms registry. Josefsson Expires August 17, 2006 [Page 14] Internet-Draft SASL GS2-* February 2006 Subject: Registration of SASL mechanism GS2-* Family of SASL mechanisms: YES SASL mechanism prefix: GS2- Security considerations: RFC [THIS-DOC] Published specification: RFC [THIS-DOC] Person & email address to contact for further information: Simon Josefsson Intended usage: COMMON Owner/Change controller: iesg@ietf.org Note: Compare with the GSSAPI and GSS-SPNEGO mechanisms. 12. Security Considerations Security issues are discussed throughout this memo. When a server or client supports multiple authentication mechanisms, each of which has a different security strength, it is possible for an active attacker to cause a party to use the least secure mechanism supported. There are several ways to mitigate this problem: 1. Integrity protected transports can be used, e.g., TLS [13]. To protect against certain tunnel attacks [16] with that solution, a mechanism that support channel bindings that can bind the security layer (e.g., the TLS session id) to the authentication is required. 2. A client or server which supports mechanisms of different strengths should have a configurable minimum strength that it will use. It is not sufficient for this minimum strength check to only be on the server, since an active attacker can change which mechanisms the client sees as being supported, causing the client to send authentication credentials for its weakest supported mechanism. Because the negotiation of a GSS-API mechanism may be done in the clear, it is important for the GSS-API mechanisms to be designed such that an active attacker cannot obtain an authentication with weaker security properties by modifying the challenges and responses. The integrity protection provided by the security layer is useless to the client unless the client also requests mutual authentication. Therefore, a client wishing to benefit from the integrity protection of a security layer MUST pass to the GSS_Init_sec_context call a mutual_req_flag of TRUE. When constructing the input_name_string, the client should not canonicalize the server's fully qualified domain name using an insecure or untrusted directory service, e.g., the Domain Name System Josefsson Expires August 17, 2006 [Page 15] Internet-Draft SASL GS2-* February 2006 [8] without DNSSEC [14]. Additional security considerations are in the SASL and GSSAPI specifications. Additional security considerations for the Kerberos V5 GSSAPI mechanism can be found in [9]. We stress that service names should not be canonicalized using an unsecured directory service such as the DNS without DNSSEC. 13. Acknowledgements This document is a revision of RFC 2222. This version was derived from draft-ietf-sasl-gssapi-02 which was prepared by Alexey Melnikov with significant contributions from John G. Myers, although this document has been rewritten by the current author. Contributions of many members of the SASL mailing list are gratefully acknowledged. In particular, ideas from Sam Hartman, Jeffrey Hutzelman, and Nicolas Williams influenced the design of this protocol. 14. Copying conditions Regarding the portion of this document that was written by Simon Josefsson ("the author", for the remainder of this section), the author makes no guarantees and is not responsible for any damage resulting from its use. The author grants irrevocable permission to anyone to use, modify, and distribute it in any way that does not diminish the rights of anyone else to use, modify, and distribute it, provided that redistributed derivative works do not contain misleading author or version information. Derivative works need not be licensed under similar terms. Contact the author to confirm which sections are available under this license. 15. References 15.1. Normative References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [2] Myers, J., "Simple Authentication and Security Layer (SASL)", RFC 2222, October 1997. [3] Linn, J., "Generic Security Service Application Program Interface Version 2, Update 1", RFC 2743, January 2000. Josefsson Expires August 17, 2006 [Page 16] Internet-Draft SASL GS2-* February 2006 [4] Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1 (SHA1)", RFC 3174, September 2001. [5] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", draft-josefsson-rfc3548bis-00 (work in progress), November 2005. [6] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003. [7] "Information Processing Systems - Open Systems Interconnection - Specification of Abstract Syntax Notation One (ASN.1)", ISO Standard 8824. 15.2. Informative References [8] Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987. [9] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC 1964, June 1996. [10] Baize, E. and D. Pinkas, "The Simple and Protected GSS-API Negotiation Mechanism", RFC 2478, December 1998. [11] Melnikov, A., "SASL GSSAPI mechanisms", draft-ietf-sasl-gssapi-03 (work in progress), September 2005. [12] Adams, C., "The Simple Public-Key GSS-API Mechanism (SPKM)", RFC 2025, October 1996. [13] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999. [14] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, March 2005. [15] Williams, N., "Extended Generic Security Service Mechanism Inquiry APIs", draft-ietf-kitten-extended-mech-inquiry-01 (work in progress), October 2005. [16] Asokan, N., Niemi, V., and K. Nyberg, "Man-in-the-Middle in Tunneled Authentication", WWW http://www.saunalahti.fi/~asokan/research/mitm.html. Josefsson Expires August 17, 2006 [Page 17] Internet-Draft SASL GS2-* February 2006 Author's Address Simon Josefsson Email: simon@josefsson.org Josefsson Expires August 17, 2006 [Page 18] Internet-Draft SASL GS2-* February 2006 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Josefsson Expires August 17, 2006 [Page 19]