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Versions: (draft-josefsson-sasl-gs2) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 RFC 5801

Network Working Group                                       S. Josefsson
Internet-Draft                                                    SJD AB
Intended status: Standards Track                             N. Williams
Expires: September 24, 2009                             Sun Microsystems
                                                          March 23, 2009


       Using GSS-API Mechanisms in SASL: The GS2 Mechanism Family
                         draft-ietf-sasl-gs2-11

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

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.
































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions used in this document  . . . . . . . . . . . . . .  5
   3.  Mechanism name . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Generating SASL mechanism names from GSS-API OIDs  . . . .  5
     3.2.  Computing mechanism names manually . . . . . . . . . . . .  5
     3.3.  Examples . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  SASL Authentication Exchange Message Format  . . . . . . . . .  7
     4.1.  SASL Messages  . . . . . . . . . . . . . . . . . . . . . .  7
   5.  Channel Bindings . . . . . . . . . . . . . . . . . . . . . . .  9
   6.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   7.  Authentication Conditions  . . . . . . . . . . . . . . . . . . 11
   8.  GSS-API Parameters . . . . . . . . . . . . . . . . . . . . . . 11
   9.  Naming . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   10. GSS_Mechanism_SASLname call  . . . . . . . . . . . . . . . . . 11
     10.1. gss_mechanism_saslname . . . . . . . . . . . . . . . . . . 13
   11. GSS_Inquire_mech_for_SASLname call . . . . . . . . . . . . . . 13
     11.1. gss_inquire_mech_for_saslname  . . . . . . . . . . . . . . 15
   12. Security Layers  . . . . . . . . . . . . . . . . . . . . . . . 15
   13. Interoperability with the SASL "GSSAPI" mechanism  . . . . . . 16
     13.1. The interoperability problem . . . . . . . . . . . . . . . 16
     13.2. Resolving the problem  . . . . . . . . . . . . . . . . . . 16
     13.3. Additional Recommendations . . . . . . . . . . . . . . . . 16
   14. Mechanisms that negotiate other mechanisms . . . . . . . . . . 17
     14.1. The interoperability problem . . . . . . . . . . . . . . . 17
     14.2. Security problem . . . . . . . . . . . . . . . . . . . . . 17
     14.3. Resolving the problems . . . . . . . . . . . . . . . . . . 17
   15. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   16. Security Considerations  . . . . . . . . . . . . . . . . . . . 18
   17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
   18. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     18.1. Normative References . . . . . . . . . . . . . . . . . . . 19
     18.2. Informative References . . . . . . . . . . . . . . . . . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
















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1.  Introduction

   Generic Security Service Application Program Interface (GSS-API)
   [RFC2743] is a framework that provides security services to
   applications using a variety of authentication "mechanisms".  Simple
   Authentication and Security Layer (SASL) [RFC4422] 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], now
   [RFC4752]; 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 V GSS-API mechanism" [RFC1964]
   [RFC4121], 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].  These problems and the
   opportunity to create the next SASL password-based mechanism, SCRAM
   [I-D.newman-auth-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] 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 4 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] (see section 3.1).




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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].


3.  Mechanism name

3.1.  Generating SASL mechanism names from GSS-API OIDs

   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_Mechanism_Name call (see below) to
   query for the SASL mechanism name of a GSS-API mechanism.

   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 [RFC4648]
   (with an upper case alphabet) of the first 55 bits of the binary
   SHA-1 hash [FIPS.180-1.1995] string computed over the ASN.1 DER
   encoding [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.






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3.3.  Examples

   The OID for the SPKM-1 mechanism [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
   [RFC4648].  Thus the SASL mechanism name for the SPKM-1 GSSAPI
   mechanism is "GS2-DT4PIK22T6A".

   The OID for the Kerberos V5 GSS-API mechanism [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 first ten octets to
   binary, and re-group them in groups of 5, and convert them back to
   decimal, which results in these computations:














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   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
   [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".  But instead, we
   assign the Kerberos V mechanism a non-hash-derived mechanism name:
   "KerberosV-GS2" and "KerberosV-GS2-PLUS" (see Section 15).


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.

   Also, the server SHOULD refrain from sending GSS-API error tokens
   (tokens output by GSS_Init_sec_context() or GSS_Accept_sec_context()
   along with a major status code other than GSS_S_COMPLETE or
   GSS_S_CONTINUE_NEEDED) as SASL applications handle error conditions.



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   The initial security context token is modified as follows:
   o  The [RFC2743] section 3.1 initial context token header MUST be
      removed if present, and its presence is noted (see below).  On the
      server side this header MUST be recomputed and restored prior to
      passing the token to GSS_Accept_sec_context().
   o  A GS2 header MUST be prefixed to the resulting initial context
      token.  This header has the form given below in ABNF [RFC5234].

      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 / 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-std-mech = "F"
                        ;; "F" means the mechanism is NOT is a
                        ;; standard GSS-API mechanism in that the
                        ;; RFC2743 section 3.1 header was missing
      gs2-cb-flag  = "n" / "y" / "p"
                        ;; GS2 channel binding (CB) flag
                        ;; "n" -> client does not support CB
                        ;; "y" -> client supports CB, thinks the server
                        ;;           does not
                        ;; "p" -> client supports and used CB
      gs2-header   = [gs2-std-mech] gs2-cb-flag [gs2-authzid] ","
                        ;; The GS2 header is gs2-header.
                        ;; gs2-std-mech is present if the GSS-API
                        ;; mechanism's initial context token did not
                        ;; have the standard header defined in
                        ;; [RFC2743] section 3.1.

   The GS2 header is also prepended 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.

   The "gs2-authzid" holds the SASL authorization identity.  It is
   encoded using UTF-8 [RFC3629] with three exceptions:
   o  The NUL characters is forbidden as required by section 3.4.1 of
      [RFC4422].





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   o  The server MUST replace any occurance of "," (comma) in the string
      with "=2C".
   o  The server MUST replace any occurance of "=" (comma) 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().

   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].





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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: nauthzid=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: nauthzid=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.

         C: Request authentication exchange
         S: Empty Challenge
         C: nauthzid=someuser, <initial context token with standard
                                header removed>
         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: yauthzid=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



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   challenge (zero byte string) if it has not yet received a token from
   the client.  See section 3 of [RFC4422].


7.  Authentication Conditions

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

   o  GSS_Init/Accept_sec_context() return anything other than
      GSS_S_CONTINUE_NEEDED or GSS_S_COMPLETE.
   o  If the client's GS2 channel binding flag was "y" and the server
      supports channel binding.
   o  If the client requires use of channel binding and the server did
      not advertise support for channel binding.
   o  Authorization of client principal (i.e., src_name in
      GSS_Accept_sec_context()) to requested authzid failed.
   o  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] section 4.1) and clients will
   typically have username initiator principal names (see [RFC2743]
   section 4.2).


10.  GSS_Mechanism_SASLname 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.









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      Inputs:

      o desired_mech OBJECT IDENTIFIER

      Outputs:

      o sasl_mech_name OCTET STRING -- SASL name for this mechanism
        (really, ASCII)

      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 disred_mech was unsupported by
         the GSS-API implementation.

      The GSS_Mechanism_SASLname 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.

























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10.1.  gss_mechanism_saslname

   The C binding for the GSS_Mechanism_SASLname call is as follows.

     OM_uint32 gss_mechanism_saslname(
       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.  Also output
     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.

















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      Inputs:

      o sasl_mech_name OCTET STRING -- SASL name of mechanism
        (really, ASCII)

      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.































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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", "n", "y" or "p", 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



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   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 [RFC1964] mechanism is currently used in SASL
   under the name "GSSAPI", see GSSAPI mechanism [RFC4752].  The
   Kerberos V5 mechanism may also be used with the GS2 family.  This
   causes an interopability 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], [mitm]).

13.3.  Additional Recommendations

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

   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.





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14.  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] 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()
   [I-D.ietf-kitten-extended-mech-inquiry] can be used to identify such
   mechanisms.


15.  IANA Considerations

   The SASL names for the Kerberos V GSS-API mechanism [RFC4121]
   [RFC1964] used via GS2 SHALL be "KerberosV-GS2" and "KerberosV-GS2-
   PLUS".




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   The SASL names for the SPNEGO GSS-API mechanism used via GS2 SHALL be
   "SPNEGO" and "SPNEGO-PLUS".  As described in Section 14 the SASL
   "SPNEGO" and "SPNEGO-PLUS" MUST NOT be used.  These names are
   provided as a convienience 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



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   will have protected the SASL mechanism negotiation.

   The channel binding data MAY be sent (byt 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].

   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 [RFC1034] without DNSSEC [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 apoplications via GS2.

   The security considerations of SASL [RFC4422], the GSS-API [RFC2743],
   channel binding [RFC5056], any external channels (such as TLS,
   [RFC5246], channel binding types (see the IANA channel binding type
   registry), and GSS-API mechanisms (such as the Kerberos V mechanism
   [RFC4121] [RFC1964]), 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

   [FIPS.180-1.1995]
              National Institute of Standards and Technology, "Secure



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              Hash Standard", FIPS PUB 180-1, April 1995,
              <http://www.itl.nist.gov/fipspubs/fip180-1.htm>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2743]  Linn, J., "Generic Security Service Application Program
              Interface Version 2, Update 1", RFC 2743, January 2000.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC4422]  Melnikov, A. and K. Zeilenga, "Simple Authentication and
              Security Layer (SASL)", RFC 4422, June 2006.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

   [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure
              Channels", RFC 5056, November 2007.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [CCITT.X690.2002]
              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)", CCITT Recommendation X.690, July 2002.

18.2.  Informative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1964]  Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
              RFC 1964, June 1996.

   [RFC2025]  Adams, C., "The Simple Public-Key GSS-API Mechanism
              (SPKM)", RFC 2025, October 1996.

   [RFC2222]  Myers, J., "Simple Authentication and Security Layer
              (SASL)", RFC 2222, October 1997.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.



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   [RFC4121]  Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
              Version 5 Generic Security Service Application Program
              Interface (GSS-API) Mechanism: Version 2", RFC 4121,
              July 2005.

   [RFC4178]  Zhu, L., Leach, P., Jaganathan, K., and W. Ingersoll, "The
              Simple and Protected Generic Security Service Application
              Program Interface (GSS-API) Negotiation Mechanism",
              RFC 4178, October 2005.

   [RFC4752]  Melnikov, A., "The Kerberos V5 ("GSSAPI") Simple
              Authentication and Security Layer (SASL) Mechanism",
              RFC 4752, November 2006.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [I-D.newman-auth-scram]
              Menon-Sen, A., Melnikov, A., and C. Newman, "Salted
              Challenge Response (SCRAM) SASL Mechanism",
              draft-newman-auth-scram-10 (work in progress),
              February 2009.

   [I-D.ietf-kitten-extended-mech-inquiry]
              Williams, N., "Extended Generic Security Service Mechanism
              Inquiry APIs", draft-ietf-kitten-extended-mech-inquiry-04
              (work in progress), March 2008.

   [mitm]     Asokan, N., Niemi, V., and K. Nyberg, "Man-in-the-Middle
              in Tunneled Authentication",
              WWW http://www.saunalahti.fi/~asokan/research/mitm.html.


Authors' Addresses

   Simon Josefsson
   SJD AB
   Hagagatan 24
   Stockholm  113 47
   SE

   Email: simon@josefsson.org
   URI:   http://josefsson.org/








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   Nicolas Williams
   Sun Microsystems
   5300 Riata Trace Ct
   Austin, TX  78727
   USA

   Email: Nicolas.Williams@sun.com












































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