Using GSS-API Mechanisms in SASL: The GS2 Mechanism Family
draft-ietf-sasl-gs2-13

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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. This is done by defining a new SASL mechanism family, called GS2. This mechanism family offers a number of improvements over the previous "SASL/GSSAPI" mechanism: it is more general, uses fewer messages for the authentication phase in some cases, and supports negotiable use of channel binding. Only GSS-API mechanisms that support channel binding are supported.

See <http://josefsson.org/sasl-gs2-*/> for more information.

Table of Contents

1.  Introduction
2.  Conventions used in this document
3.  Mechanism name
    3.1.  Generating SASL mechanism names from GSS-API OIDs
    3.2.  Computing mechanism names manually
    3.3.  Examples
    3.4.  Grandfathered mechanism names
4.  SASL Authentication Exchange Message Format
    4.1.  SASL Messages
5.  Channel Bindings
    5.1.  Channel Binding to TLS Channels
6.  Examples
7.  Authentication Conditions
8.  GSS-API Parameters
9.  Naming
10.  GSS_Inquire_SASLname_for_mech call
    10.1.  gss_inquire_saslname_for_mech
11.  GSS_Inquire_mech_for_SASLname call
    11.1.  gss_inquire_mech_for_saslname
12.  Security Layers
13.  Interoperability with the SASL GSSAPI mechanism
    13.1.  The interoperability problem
    13.2.  Resolving the problem
    13.3.  Additional Recommendations
14.  GSS-API Mechanisms that negotiate other mechanisms
    14.1.  The interoperability problem
    14.2.  Security problem
    14.3.  Resolving the problems
15.  IANA Considerations
16.  Security Considerations
17.  Acknowledgements
18.  References
    18.1.  Normative References
    18.2.  Informative References
§  Authors' Addresses

1.  Introduction

Generic Security Service Application Program Interface (GSS-API) [RFC2743] (Linn, J., “Generic Security Service Application Program Interface Version 2, Update 1,” January 2000.) is a framework that provides security services to applications using a variety of authentication "mechanisms". Simple Authentication and Security Layer (SASL) [RFC4422] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.) is a framework to provide authentication and "security layers" for connection based protocols, also using a variety of mechanisms. This document describes how to use a GSS-API mechanism as though it were a SASL mechanism. This facility is called GS2 -- a moniker that indicates that this is the second GSS-API->SASL mechanism bridge. The original GSS-API->SASL mechanism bridge was specified by [RFC2222] (Myers, J., “Simple Authentication and Security Layer (SASL),” October 1997.), now [RFC4752] (Melnikov, A., “The Kerberos V5 ("GSSAPI") Simple Authentication and Security Layer (SASL) Mechanism,” November 2006.); we shall sometimes refer to the original bridge as GS1 in this document.

All GSS-API mechanisms are implicitly registered for use within SASL by this specification. The SASL mechanisms defined in this document are known as the GS2 family of mechanisms.

The GS1 bridge failed to gain wide deployment for any GSS-API mechanism other than The "Kerberos V5 GSS-API mechanism" [RFC1964] (Linn, J., “The Kerberos Version 5 GSS-API Mechanism,” June 1996.) [RFC4121] (Zhu, L., Jaganathan, K., and S. Hartman, “The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2,” July 2005.), and has a number of problems that lead us to desire a new bridge. Specifically: a) GS1 was not round-trip optimized, b) GS1 did not support channel binding [RFC5056] (Williams, N., “On the Use of Channel Bindings to Secure Channels,” November 2007.). These problems and the opportunity to create the next SASL password-based mechanism, SCRAM (Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams, “Salted Challenge Response (SCRAM) SASL and GSS-API Mechanism,” February 2010.) [I‑D.ietf‑sasl‑scram], as a GSS-API mechanism used by SASL applications via GS2, provide the motivation for GS2.

In particular, the current consensus of the SASL community appears to be that SASL "security layers" (i.e., confidentiality and integrity protection of application data after authentication) are too complex and, since SASL applications tend to have an option to run over a Transport Layer Security (TLS) [RFC5246] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.) channel, redundant and best replaced with channel binding.

GS2 is designed to be as simple as possible. It adds to GSS-API security context token exchanges only the bare minimum to support SASL semantics and negotiation of use of channel binding. Specifically, GS2 adds a small header (2 bytes or 3 bytes plus the length of the client requested SASL authorization ID (authzid)) to the initial context token and to the application channel binding data, and it uses SASL mechanism negotiation to implement channel binding negotiation. All GS2 plaintext is protected via the use of GSS-API channel binding. Additionally, to simplify the implementation of GS2 mechanisms for implementors who will not implement a GSS-API framework, we compress the initial security context token header required by [RFC2743] (Linn, J., “Generic Security Service Application Program Interface Version 2, Update 1,” January 2000.) (see section 3.1).

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 [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

3.  Mechanism name

There are two SASL mechanism names for any GSS-API mechanism used through this facility. One denotes that the server supports channel binding. The other denotes that it does not.

The SASL mechanism name for a GSS-API mechanism is that which is provided by that mechanism when it was specified, if one was specified. This name denotes that the server does not support channel binding. Add the suffix "-PLUS" and the resulting name denotes that the server does support channel binding. SASL implementations can use the GSS_Inquire_SASLname_for_mech call (see below) to query for the SASL mechanism name of a GSS-API mechanism.

If the GSS_Inquire_SASLname_for_mech interface is not used, the GS2 implementation need some other mechanism to map mechanism OIDs to SASL name internally. In this case, the implementation can only support the mechanisms for which it knows the SASL name. If the GSS_Inquire_SASLname_for_mech call fails, and the GS2 implementation cannot map the OID to a SASL mechanism name using some other means, it cannot use the particular GSS-API mechanism since it does not know its SASL mechanism name.

3.1.  Generating SASL mechanism names from GSS-API OIDs

For GSS-API mechanisms whose SASL names are not defined together with the GSS-API mechanism or in this document, the SASL mechanism name is concatenation of the string "GS2-" and the Base32 encoding (Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” October 2006.) [RFC4648] (with an upper case alphabet) of the first 55 bits of the binary SHA-1 hash (National Institute of Standards and Technology, “Secure Hash Standard,” April 1995.) [FIPS.180‑1.1995] string computed over the ASN.1 DER encoding (International International Telephone and Telegraph Consultative Committee, “ASN.1 encoding rules: Specification of basic encoding Rules (BER), Canonical encoding rules (CER) and Distinguished encoding rules (DER),” July 2002.) [CCITT.X690.2002], including the tag and length octets, 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. This name denotes that the server does not support channel binding. Add the suffix "-PLUS" and the resulting name denotes that the server does support channel binding.

3.2.  Computing mechanism names manually

The hash-derived GS2 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.  Examples

The OID for the SPKM-1 mechanism (Adams, C., “The Simple Public-Key GSS-API Mechanism (SPKM),” October 1996.) [RFC2025] is 1.3.6.1.5.5.1.1. The ASN.1 DER encoding of the OID, including the tag and length, is (in hex) 06 07 2b 06 01 05 05 01 01. The SHA-1 hash of the ASN.1 DER encoding is (in hex) 1c f8 f4 2b 5a 9f 80 fa e9 f8 31 22 6d 5d 9d 56 27 86 61 ad. Convert the first 7 octets to binary, drop the last bit, and re-group them in groups of 5, and convert them back to decimal, which results in these computations:

hex:
1c f8 f4 2b 5a 9f 80

binary:
00011100 11111000 11110100 00101011 01011010
10011111 1000000

binary in groups of 5:
00011 10011 11100 01111 01000 01010 11010 11010
10011 11110 00000

decimal of each group:
3 19 28 15 8 10 26 26 19 30 0

base32 encoding:
D T 4 P I K 2 2 T 6 A

The last step translate each decimal value using table 3 in Base32 (Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” October 2006.) [RFC4648]. Thus the SASL mechanism name for the SPKM-1 GSSAPI mechanism is "GS2-DT4PIK22T6A".

The OID for the Kerberos V5 GSS-API mechanism (Linn, J., “The Kerberos Version 5 GSS-API Mechanism,” June 1996.) [RFC1964] is 1.2.840.113554.1.2.2 and its DER encoding is (in hex) 06 09 2A 86 48 86 F7 12 01 02 02. The SHA-1 hash is 82 d2 73 25 76 6b d6 c8 45 aa 93 25 51 6a fc ff 04 b0 43 60. Convert the 7 octets to binary, drop the last bit, and re-group them in groups of 5, and convert them back to decimal, which results in these computations:

hex:
82 d2 73 25 76 6b d6

binary:
10000010 11010010 01110011 00100101 01110110
01101011 1101011

binary in groups of 5:
10000 01011 01001 00111 00110 01001 01011 10110
01101 01111 01011

decimal of each group:
16 11 9 7 6 9 11 22 13 15 11

base32 encoding:
Q L J H G J L W N P L

The last step translate each decimal value using table 3 in Base32 (Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” October 2006.) [RFC4648]. Thus the SASL mechanism name for the Kerberos V5 GSSAPI mechanism would be "GS2-QLJHGJLWNPL" and (because this mechanism supports channel binding) "GS2-QLJHGJLWNPL-PLUS". Instead, the next section assigns the Kerberos V5 mechanism a non-hash-derived mechanism name.

3.4.  Grandfathered mechanism names

Some older GSS-API mechanisms were not specified with a SASL GS2 mechanism name. Using a shorter name can be useful nonetheless. We specify the names "GS2-KRB5" and "GS2-KRB5-PLUS" for the Kerberos V5 mechanism, to be used as if the original specification documented it. See Section 15 (IANA Considerations).

4.  SASL Authentication Exchange Message Format

4.1.  SASL Messages

During the SASL authentication exchange for GS2, a number of messages following the following format is sent between the client and server. This number is the same as the number of context tokens that the GSS-API mechanism would normally require in order to establish a security context (or to fail to do so).

Note that when using a GS2 mechanism the SASL client is always a GSS-API initiator and the SASL server is always a GSS-API acceptor. Thus the SASL client calls GSS_Init_sec_context and the server calls GSS_Accept_sec_context.

All the SASL authentication messages exchanged are exactly the same as the security context tokens of the GSS-API mechanism, except for the initial security context token.

The client and server MAY send GSS-API error tokens (tokens output by GSS_Init_sec_context() or GSS_Accept_sec_context() when the major status code is other than GSS_S_COMPLETE or GSS_S_CONTINUE_NEEDED). As this indicate an error condition, after sending the token, the sending side should fail the authentication.

The initial security context token is modified as follows:

• The [RFC2743] (Linn, J., “Generic Security Service Application Program Interface Version 2, Update 1,” January 2000.) section 3.1 initial context token header MUST be removed if present. If the header is not present, the client MUST send a "gs2-nonstd-flag" flag (see below). On the server side this header MUST be recomputed and restored prior to passing the token to GSS_Accept_sec_context, except when the "gs2-nonstd-flag" is sent.
• A GS2 header MUST be prefixed to the resulting initial context token. This header has the form "gs2-header" given below in ABNF [RFC5234] (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.).

   UTF8-1-safe    = %x01-2B / %x2D-3C / %x3E-7F
                    ;; As UTF8-1 in RFC 3629 except
                    ;; NUL, "=", and ",".
   UTF8-2         = <as defined in RFC 3629 (STD 63)>
   UTF8-3         = <as defined in RFC 3629 (STD 63)>
   UTF8-4         = <as defined in RFC 3629 (STD 63)>
   UTF8-char-safe = UTF8-1-safe / UTF8-2 / UTF8-3 / UTF8-4

   saslname     = 1*(UTF8-char-safe / "=2C" / "=3D")
   gs2-authzid  = "a=" saslname
                     ;; GS2 has to transport an authzid since
                     ;; the GSS-API has no equivalent
   gs2-nonstd-flag = "F"
                     ;; "F" means the mechanism is not a
                     ;; standard GSS-API mechanism in that the
                     ;; RFC2743 section 3.1 header was missing
   gs2-cb-flag  = "p" / "n" / "y"
                     ;; GS2 channel binding (CB) flag
                     ;; "p" -> client supports and used CB
                     ;; "n" -> client does not support CB
                     ;; "y" -> client supports CB, thinks the server
                     ;;           does not
   gs2-header   = [gs2-nonstd-flag] gs2-cb-flag [gs2-authzid] ","
                     ;; The GS2 header is gs2-header.

When the "gs2-nonstd-flag" flag is present, the client did not find/remove a [RFC2743] (Linn, J., “Generic Security Service Application Program Interface Version 2, Update 1,” January 2000.) section 3.1 token header from the initial token returned by GSS_Init_sec_context. This signals to the server that it MUST NOT re-add the data that is normally removed by the client.

The "gs2-cb-flag" signals the channel binding mode. One of "p", "n", or "y" is used. A "p" means the client supports and used a channel binding. A "n" means that the client does not support channel binding. A "y" means the client supports channel binding, but believes the server does not, so it did not use a channel binding. See the next section for more details.

The "gs2-authzid" holds the SASL authorization identity. It is encoded using UTF-8 (Yergeau, F., “UTF-8, a transformation format of ISO 10646,” November 2003.) [RFC3629] with three exceptions:

• The NUL characters is forbidden as required by section 3.4.1 of [RFC4422] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.).
• The server MUST replace any "," (comma) in the string with "=2C".
• The server MUST replace any "=" (equals) in the string with "=3D".

If a server sends a string that does not conform to this syntax, the client MUST reject authentication.

5.  Channel Bindings

If the server supports channel binding then it MUST list both forms of the SASL mechanism name for each GSS-API mechanism supported via GS2 (i.e., GSS-API mechanisms that support channel binding).

If the client supports channel binding and the server does not (i.e., the server did not advertise the -PLUS names) then the client MUST either fail authentication or it MUST set the channel binding flag in the GS2 initial security context token to "y" and MUST NOT include application channel binding data in the GSS-API channel binding input to GSS_Init_sec_context.

If the client supports channel binding and the server also does then the client MUST set the channel binding flag in the GS2 initial security context token to "p" and MUST include application channel binding data in the GSS-API channel binding input to GSS_Init_sec_context. This is done by pre-pending the gs2-header to the application's channel binding data. If the application did not provide channel binding data then the GS2 header is used as though it were application-provided channel binding data.

If the client does not support channel binding then it MUST set the channel binding flag in the GS2 initial security context token to "n" and MUST NOT include application channel binding data in the GSS-API channel binding input to GSS_Init_sec_context.

Upon receipt of the initial authentication message the server checks the channel binding flag in the GS2 header and constructs a channel binding data input for GSS_Accept_sec_context accordingly. If the client channel binding flag was "n" then the server MUST NOT include application channel binding data in the GSS-API channel binding input to GSS_Accept_sec_context. If the client channel binding flag was "y" and the server does support channel binding then the server MUST fail authentication. If the client channel binding flag was "p" the server MUST include application channel binding data in the GSS-API channel binding input to GSS_Accept_sec_context.

For more discussions of channel bindings, and the syntax of the channel binding data for various security protocols, see [RFC5056] (Williams, N., “On the Use of Channel Bindings to Secure Channels,” November 2007.).

5.1.  Channel Binding to TLS Channels

If an external TLS channel is to be bound into the GS2 authentication, and if the channel was established using a X.509 [RFC5280] (Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” May 2008.) server certificate to authenticate the server, then the GS2 client and server MUST use the 'tls-server-end-point' channel binding type. See the IANA Channel Binding Types registry.

If an external TLS channel is to be bound into the GS2 authentication, and if the channel was established either without the use of any X.509 server certificate to authenticate the server, or with a non X.509 server certificate, then the GS2 client and server MUST use the 'tls-unique' channel binding type.

6.  Examples

Example #1: a one round-trip GSS-API context token exchange, no channel binding, optional authzid given.

      C: Request authentication exchange
      S: Empty Challenge
      C: na=someuser,<initial context token with standard
                         header removed>
      S: Send reply context token as is
      C: Empty message
      S: Outcome of authentication exchange

Example #2: a one and one half round-trip GSS-API context token exchange.

      C: Request authentication exchange
      S: Empty Challenge
      C: na=someuser,<initial context token with standard
                         header removed>
      S: Send reply context token as is
      C: Send reply context token as is
      S: Outcome of authentication exchange

Example #3: a two round-trip GSS-API context token exchange, no standard token header.

      C: Request authentication exchange
      S: Empty Challenge
      C: Fna=someuser,<initial context token without
                          standard header>
      S: Send reply context token as is
      C: Send reply context token as is
      S: Send reply context token as is
      C: Empty message
      S: Outcome of authentication exchange

Example #4: using channel binding

      C: Request authentication exchange
      S: Empty Challenge
      C: pa=someuser,<initial context token with standard
                             header removed>
      S: Send reply context token as is
      ...

GSS-API authentication is always initiated by the client. The SASL framework allows either the client and server to initiate authentication. In GS2 the server will send an initial empty challenge (zero byte string) if it has not yet received a token from the client. See section 3 of [RFC4422] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.).

7.  Authentication Conditions

Authentication MUST NOT succeed if any one of the following conditions are true:

• GSS_Init/Accept_sec_context return anything other than GSS_S_CONTINUE_NEEDED or GSS_S_COMPLETE.
• If the client's GS2 channel binding flag was "y" and the server supports channel binding.
• If the client requires use of channel binding and the server did not advertise support for channel binding.
• Authorization of client principal (i.e., src_name in GSS_Accept_sec_context) to requested authzid failed.
• If the client is not authorized to the requested authzid or an authzid could not be derived from the client's initiator principal name.

8.  GSS-API Parameters

GS2 does not use any GSS-API per-message tokens. Therefore the setting of req_flags related to per-message tokens is irrelevant.

9.  Naming

There's no requirement that any particular GSS-API name-types be used. However, typically SASL servers will have host-based acceptor principal names (see [RFC2743] (Linn, J., “Generic Security Service Application Program Interface Version 2, Update 1,” January 2000.) section 4.1) and clients will typically have username initiator principal names (see [RFC2743] (Linn, J., “Generic Security Service Application Program Interface Version 2, Update 1,” January 2000.) section 4.2).

10.  GSS_Inquire_SASLname_for_mech call

To allow SASL implementations to query for the SASL mechanism name of a GSS-API mechanism, we specify a new GSS-API function for this purpose.

   Inputs:

   o desired_mech OBJECT IDENTIFIER

   Outputs:

   o sasl_mech_name UTF-8 STRING -- SASL name for this mechanism

   o mech_name UTF-8 STRING -- name of this mechanism, possibly
     localized

   o mech_description UTF-8 STRING -- possibly localized
     description of this mechanism.

   Return major_status codes:

   o  GSS_S_COMPLETE indicates successful completion, and that output
      parameters holds correct information.

   o  GSS_S_BAD_MECH indicates that a desired_mech was unsupported by
      the GSS-API implementation.

   The GSS_Inquire_SASLname_for_mech call is used to get the SASL
   mechanism name for a GSS-API mechanism.  It also returns a name
   and description of the mechanism in a human readable form.

   The output variable sasl_mech_name will hold the IANA registered
   mechanism name for the GSS-API mechanism, or if none is
   registered, a mechanism named computed from the OID as
   described in section 3.1 of this document.

10.1.  gss_inquire_saslname_for_mech

The C binding for the GSS_Inquire_SASLname_for_mech call is as follows.

   OM_uint32 gss_inquire_saslname_for_mech(
     OM_uint32     *minor_status,
     const gss_OID  desired_mech,
     gss_buffer_t   sasl_mech_name,
     gss_buffer_t   mech_name,
     gss_buffer_t   mech_description,
   );

   Purpose:

   Output the SASL mechanism name of a GSS-API mechanism.
   It also returns a name and description of the mechanism in a
   human readable form.

   Parameters:

   minor_status      Integer, modify
                     Mechanism specific status code.

   Function value:   GSS status code

   GSS_S_COMPLETE    Successful completion

   GSS_S_BAD_MECH    The desired_mech OID is unsupported

11.  GSS_Inquire_mech_for_SASLname call

To allow SASL clients to more efficiently identify which GSS-API mechanism a particular SASL mechanism name refers to we specify a new GSS-API utility function for this purpose.

   Inputs:

   o sasl_mech_name UTF-8 STRING -- SASL name of mechanism

   Outputs:

   o  mech_type OBJECT IDENTIFIER -- must be explicit mechanism,
      and not "default" specifier

   Return major_status codes:

   o  GSS_S_COMPLETE indicates successful completion, and that output
      parameters holds correct information.

   o  GSS_S_BAD_MECH indicates that no supported GSS-API mechanism
      had the indicated sasl_mech_name.

   The GSS_Inquire_mech_for_SASLname call is used to get the GSS-API
   mechanism OID associated with a SASL mechanism name.

11.1.  gss_inquire_mech_for_saslname

The C binding for the GSS_Inquire_mech_for_SASLname call is as follows.

   OM_uint32 gss_inquire_mech_for_saslname(
     OM_uint32           *minor_status,
     const gss_buffer_t   sasl_mech_name,
     gss_OID             *mech_type
   );

   Purpose:

   Output GSS-API mechanism OID of mechanism associated with given
   sasl_mech_name.

   Parameters:

   minor_status      Integer, modify
                     Mechanism specific status code.

   Function value:   GSS status code

   GSS_S_COMPLETE    Successful completion

   GSS_S_BAD_MECH    The desired_mech OID is unsupported

12.  Security Layers

GS2 does not currently support SASL security layers. Applications that need integrity protection or confidentiality and integrity protection MUST use either channel binding to a secure external channel or a SASL mechanism that does provide security layers.

NOTE WELL: the GS2 client's first authentication message MUST always start with "F", "p", "n" or "y", otherwise the server MUST fail authentication. This will allow us to add support for security layers in the future if it were to become necessary. Note that adding security layer support to GS2 must not break existing SASL/GS2 applications, which can be accomplished by making security layers optional.

[A sketch of how to add sec layer support... Add a way for the client to: a) make an offer of sec layers and max buffer, b) make an opportunistic selection of sec layer and buffer size, both in the first client authentication message, and starting with a character other than "F", "n", "y" or "p". The server could accept the opportunistic proposal (reply token prefixed with a byte indicating acceptance) or reject it along with an indication of the server's acceptable sec layers and max buffer size. In the latter case the GSS-API security context token exchange must be abandoned and recommenced, although this would be a detail of the GS2 bridge not exposed to the SASL application. The negotiation would be protected via GSS channel binding, as with the rest of GS2.]

13.  Interoperability with the SASL GSSAPI mechanism

The Kerberos V5 GSS-API (Linn, J., “The Kerberos Version 5 GSS-API Mechanism,” June 1996.) [RFC1964] mechanism is currently used in SASL under the name GSSAPI, see GSSAPI mechanism (Melnikov, A., “The Kerberos V5 ("GSSAPI") Simple Authentication and Security Layer (SASL) Mechanism,” November 2006.) [RFC4752]. The Kerberos V5 mechanism may also be used with the GS2 family. This causes an interoperability problem, which is discussed and resolved below.

13.1.  The interoperability problem

The SASL "GSSAPI" mechanism is not wire-compatible with the Kerberos V GSS-API mechanism used as a SASL GS2 mechanism.

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 mechanism negotiation will fail.

13.2.  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 SASL "GSSAPI" mechanism lacks support for channel bindings, which means that using an external secure channel may not be sufficient protection against active attackers (see [RFC5056] (Williams, N., “On the Use of Channel Bindings to Secure Channels,” November 2007.), [mitm] (Asokan, N., Niemi, V., and K. Nyberg, “Man-in-the-Middle in Tunneled Authentication,” .)).

13.3.  Additional Recommendations

If the application requires security layers then it MUST prefer the SASL "GSSAPI" mechanism over "GS2-KRB5" or "GS2-KRB5-PLUS".

If the application can use channel binding to an external channel then it is RECOMMENDED that it select Kerberos V5 through the GS2 mechanism rather than the "GSSAPI" mechanism.

14.  GSS-API 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.

14.1.  The 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.

14.2.  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 ideally should have used mechanism Y. For this reason, the use of GSS-API mechanisms that negotiate other mechanisms are disallowed under GS2.

14.3.  Resolving the problems

GSS-API mechanisms that negotiate other mechanisms MUST NOT be used with the GS2 SASL mechanism. Specifically SPNEGO [RFC4178] (Zhu, L., Leach, P., Jaganathan, K., and W. Ingersoll, “The Simple and Protected Generic Security Service Application Program Interface (GSS-API) Negotiation Mechanism,” October 2005.) MUST NOT be used as a GS2 mechanism. To make this easier for SASL implementations we assign a symbolic SASL mechanism name to the SPNEGO GSS-API mechanism: "SPNEGO". SASL client implementations MUST NOT choose the SPNEGO mechanism under any circumstances. [What about SASL apps that don't do mechanism negotiation? Probably none exist. But if any did then presumably it would OK to use the SPNEGO mechanism, no? -Nico]

The GSS_C_MA_MECH_NEGO attribute of GSS_Inquire_attrs_for_mech (Williams, N., “Extended Generic Security Service Mechanism Inquiry APIs,” April 2009.) [I‑D.ietf‑kitten‑extended‑mech‑inquiry] can be used to identify such mechanisms.

15.  IANA Considerations

The SASL names for the Kerberos V5 GSS-API mechanism [RFC4121] (Zhu, L., Jaganathan, K., and S. Hartman, “The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2,” July 2005.) [RFC1964] (Linn, J., “The Kerberos Version 5 GSS-API Mechanism,” June 1996.) used via GS2 SHALL be "GS2-KRB5" and "GS2-KRB5-PLUS".

The SASL names for the SPNEGO GSS-API mechanism used via GS2 SHALL be "SPNEGO" and "SPNEGO-PLUS". As described in Section 14 (GSS-API Mechanisms that negotiate other mechanisms) the SASL "SPNEGO" and "SPNEGO-PLUS" MUST NOT be used. These names are provided as a convenience for SASL library implementors.

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.

The IANA is further advised that SASL mechanisms MUST NOT end in "-PLUS" except as a version of another mechanism name simply suffixed with "-PLUS".

  Subject: Registration of SASL mechanism GS2-*
  SASL mechanism prefix: GS2-
  Security considerations: RFC [THIS-DOC]
  Published specification: RFC [THIS-DOC]
  Person & email address to contact for further information:
    Simon Josefsson <simon@josefsson.org>
  Intended usage: COMMON
  Owner/Change controller: iesg@ietf.org
  Note: Compare with the GSSAPI and GSS-SPNEGO mechanisms.

16.  Security Considerations

Security issues are also discussed throughout this memo.

The security provided by a GS2 mechanism depends on the security of the GSS-API mechanism. The GS2 mechanism family depends on channel binding support, so GSS-API mechanisms that do not support channel binding cannot be successfully used as SASL mechanisms via the GS2 bridge.

Because GS2 does not support security layers it is strongly RECOMMENDED that channel binding to a secure external channel be used. Successful channel binding eliminates the possibility of man-in-the-middle (MITM) attacks, provided that the external channel and its channel binding data are secure and provided that the GSS-API mechanism used is secure. Authentication failure because of channel binding failure may indicate that an MITM attack was attempted, but note that a real MITM attacker would likely attempt to close the connection to the client or simulate network partition , thus MITM attack detection is heuristic.

Use of channel binding will also protect the SASL mechanism negotiation -- if there is no MITM then the external secure channel will have protected the SASL mechanism negotiation.

The channel binding data MAY be sent (but the actual GSS-API mechanism used) without confidentiality protection and knowledge of it is assumed to provide no advantage to an MITM (who can, in any case, compute the channel binding data independently). If the external channel does not provide confidentiality protection and the GSS-API mechanism does not provide confidentiality protection for the channel binding data, then passive attackers (eavesdroppers) can recover the channel binding data. See [RFC5056] (Williams, N., “On the Use of Channel Bindings to Secure Channels,” November 2007.).

When constructing the input_name_string for GSS_Import_name with the GSS_C_NT_HOSTBASED_SERVICE name type, the client SHOULD NOT canonicalize the server's fully qualified domain name using an insecure or untrusted directory service, such as the Domain Name System (Mockapetris, P., “Domain names - concepts and facilities,” November 1987.) [RFC1034] without DNSSEC (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “DNS Security Introduction and Requirements,” March 2005.) [RFC4033].

GS2 does not directly use any cryptographic algorithms, therefore it is automatically "algorithm agile", or, as agile as the GSS-API mechanisms that are available for use in SASL applications via GS2.

The security considerations of SASL [RFC4422] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.), the GSS-API [RFC2743] (Linn, J., “Generic Security Service Application Program Interface Version 2, Update 1,” January 2000.), channel binding [RFC5056] (Williams, N., “On the Use of Channel Bindings to Secure Channels,” November 2007.), any external channels (such as TLS, [RFC5246] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.), channel binding types (see the IANA channel binding type registry), and GSS-API mechanisms (such as the Kerberos V5 mechanism [RFC4121] (Zhu, L., Jaganathan, K., and S. Hartman, “The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2,” July 2005.) [RFC1964] (Linn, J., “The Kerberos Version 5 GSS-API Mechanism,” June 1996.)), may also apply.

17.  Acknowledgements

The history of GS2 can be traced to the "GSSAPI" mechanism originally specified by RFC2222. This document was derived from draft-ietf-sasl-gssapi-02 which was prepared by Alexey Melnikov with significant contributions from John G. Myers, although the majority of this document has been rewritten by the current authors.

Contributions of many members of the SASL mailing list are gratefully acknowledged. In particular, ideas and feedback from Sam Hartman, Jeffrey Hutzelman, Alexey Melnikov, and Tom Yu improved the document and the protocol.

18.  References

18.1. Normative References

18.2. Informative References

Authors' Addresses