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<Network Working Group>                                       Larry Zhu
Internet Draft                                       Karthik Jaganathan
Updates: 1964                                                 Microsoft
Category: Standards Track                                   Sam Hartman
draft-ietf-krb-wg-gssapi-cfx-06.txt                                 MIT
                                                      February 16, 2004
                                               Expires: August 16, 2004

          The Kerberos Version 5 GSS-API Mechanism: Version 2

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of [RFC-2026].

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Drafts.  Internet-Drafts are draft documents valid for a maximum of
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   To learn the current status of any Internet-Draft, please check the
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   The distribution of this memo is unlimited.  It is filed as
   draft-ietf-krb-wg-gssapi-cfx-06.txt, and expires on August 10
   2004.  Please send comments to: ietf-krb-wg@anl.gov.


   This document defines protocols, procedures, and conventions to be
   employed by peers implementing the Generic Security Service
   Application Program Interface (GSS-API) when using the Kerberos
   Version 5 mechanism.

   RFC-1964 is updated and incremental changes are proposed in response
   to recent developments such as the introduction of Kerberos
   cryptosystem framework.  These changes support the inclusion of new
   cryptosystems, by defining new per-message tokens along with their
   encryption and checksum algorithms based on the cryptosystem

Conventions used in this document

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   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC-2119].

   The term "little endian order" is used for brevity to refer to the
   least-significant-octet-first encoding, while the term "big endian
   order" is for the most-significant-octet-first encoding.

Table of Contents

   1. Introduction ...............................................  2
   2. Key Derivation for Per-Message Tokens ......................  3
   3. Quality of Protection ......................................  4
   4. Definitions and Token Formats ..............................  4
   4.1. Context Establishment Tokens .............................  4
   4.1.1. Authenticator Checksum .................................  5
   4.2. Per-Message Tokens .......................................  8
   4.2.1. Sequence Number ........................................  8
   4.2.2. Flags Field ............................................  8
   4.2.3. EC Field ...............................................  9
   4.2.4. Encryption and Checksum Operations .....................  9
   4.2.5. RRC Field .............................................. 10
   4.2.6. Message Layouts ........................................ 10
   4.3. Context Deletion Tokens .................................. 11
   4.4. Token Identifier Assignment Considerations ............... 11
   5. Parameter Definitions ...................................... 12
   5.1. Minor Status Codes ....................................... 12
   5.1.1. Non-Kerberos-specific codes ............................ 12
   5.1.2. Kerberos-specific-codes ................................ 12
   5.2. Buffer Sizes ............................................. 13
   6. Backwards Compatibility Considerations ..................... 13
   7. Security Considerations .................................... 13
   8. Acknowledgments ............................................ 14
   9. Intellectual Property Statement ............................ 15
   10. References ................................................ 15
   10.1. Normative References .................................... 15
   10.2. Informative References .................................. 15
   11. Author's Address .......................................... 15
   Full Copyright Statement ...................................... 17

1. Introduction

   [KCRYPTO] defines a generic framework for describing encryption and
   checksum types to be used with the Kerberos protocol and associated

   [RFC-1964] describes the GSS-API mechanism for Kerberos Version 5.
   It defines the format of context establishment, per-message and
   context deletion tokens and uses algorithm identifiers for each
   cryptosystem in per message and context deletion tokens.

   The approach taken in this document obviates the need for algorithm
   identifiers.  This is accomplished by using the same encryption
   algorithm, specified by the crypto profile [KCRYPTO] for the session
   key or subkey that is created during context negotiation, and its
   required checksum algorithm.  Message layouts of the per-message
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   tokens are therefore revised to remove algorithm indicators and also
   to add extra information to support the generic crypto framework

   Tokens transferred between GSS-API peers for security context
   establishment are also described in this document.  The data
   elements exchanged between a GSS-API endpoint implementation and the
   Kerberos Key Distribution Center (KDC) [KRBCLAR] are not specific to
   GSS-API usage and are therefore defined within [KRBCLAR] rather than
   within this specification.

   The new token formats specified in this document MUST be used with
   all "newer" encryption types [KRBCLAR] and MAY be used with "older"
   encryption types, provided that the initiator and acceptor know,
   from the context establishment, that they can both process these new
   token formats.

   "Newer" encryption types are those which have been specified along
   with or since the new Kerberos cryptosystem specification [KCRYPTO],
   as defined in section 3.1.3 of [KRBCLAR].  The list of not-newer
   encryption types is as follows [KCRYPTO]:

             Encryption Type             Assigned Number
            des-cbc-crc                        1
            des-cbc-md4                        2
            des-cbc-md5                        3
            des3-cbc-md5                       5
            des3-cbc-sha1                      7
            dsaWithSHA1-CmsOID                 9
            md5WithRSAEncryption-CmsOID       10
            sha1WithRSAEncryption-CmsOID      11
            rc2CBC-EnvOID                     12
            rsaEncryption-EnvOID              13
            rsaES-OAEP-ENV-OID                14
            des-ede3-cbc-Env-OID              15
            des3-cbc-sha1-kd                  16
            rc4-hmac                          23

2. Key Derivation for Per-Message Tokens

   To limit the exposure of a given key, [KCRYPTO] adopted "one-way"
   "entropy-preserving" derived keys, for different purposes or key
   usages, from a base key or protocol key.

   This document defines four key usage values below that are used to
   derive a specific key for signing and sealing messages, from the
   session key or subkey [KRBCLAR] created during the context

        Name                         Value
       KG-USAGE-ACCEPTOR-SEAL         22
       KG-USAGE-ACCEPTOR-SIGN         23
       KG-USAGE-INITIATOR-SEAL        24

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       KG-USAGE-INITIATOR-SIGN        25

   When the sender is the context acceptor, KG-USAGE-ACCEPTOR-SIGN is
   used as the usage number in the key derivation function for deriving
   keys to be used in MIC tokens (as defined in section, and
   KG-USAGE-ACCEPTOR-SEAL is used for Wrap tokens(as defined in section; similarly when the sender is the context initiator, KG-
   USAGE-INITIATOR-SIGN is used as the usage number in the key
   derivation function for MIC tokens, KG-USAGE-INITIATOR-SEAL is used
   for Wrap Tokens.  Even if the Wrap token does not provide for
   confidentiality the same usage values specified above are used.

   During the context initiation and acceptance sequence, the acceptor
   MAY assert a subkey, and if so, subsequent messages MUST use this
   subkey as the protocol key and these messages MUST be flagged as
   "AcceptorSubkey" as described in section 4.2.2.

3. Quality of Protection

   The GSS-API specification [RFC-2743] provides for Quality of
   Protection (QOP) values that can be used by applications to request
   a certain type of encryption or signing.  A zero QOP value is used
   to indicate the "default" protection; applications which do not use
   the default QOP are not guaranteed to be portable across
   implementations or even inter-operate with different deployment
   configurations of the same implementation.  Using an algorithm that
   is different from the one for which the key is defined may not be
   appropriate.  Therefore, when the new method in this document is
   used, the QOP value is ignored.

   The encryption and checksum algorithms in per-message tokens are now
   implicitly defined by the algorithms associated with the session key
   or subkey.  Algorithms identifiers as described in [RFC-1964] are
   therefore no longer needed and removed from the new token headers.

4. Definitions and Token Formats

   This section provides terms and definitions, as well as descriptions
   for tokens specific to the Kerberos Version 5 GSS-API mechanism.

4.1. Context Establishment Tokens

   All context establishment tokens emitted by the Kerberos Version 5
   GSS-API mechanism SHALL have the framing described in section 3.1 of
   [RFC-2743], as illustrated by the following pseudo-ASN.1 structures:



         MechType ::= OBJECT IDENTIFIER
         -- representing Kerberos V5 mechanism

         GSSAPI-Token ::=
         -- option indication (delegation, etc.) indicated within
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         -- mechanism-specific token
                 thisMech MechType,
                 innerToken ANY DEFINED BY thisMech
                    -- contents mechanism-specific
                    -- ASN.1 structure not required


   Where the innerToken field starts with a two-octet token-identifier
   (TOK_ID) expressed in big endian order, followed by a Kerberos

   Here are the TOK_ID values used in the context establishment tokens:

         Token               TOK_ID Value in Hex
         KRB_AP_REQ            01 00
         KRB_AP_REP            02 00
         KRB_ERROR             03 00

   Where Kerberos message KRB_AP_REQUEST, KRB_AP_REPLY, and KRB_ERROR
   are defined in [KRBCLAR].

   If an unknown token identifier (TOK_ID) is received in the initial
   context establishment token, the receiver MUST return
   GSS_S_CONTINUE_NEEDED major status, and the returned output token
   MUST contain a KRB_ERROR message with the error code

4.1.1. Authenticator Checksum

   The authenticator in the KRB_AP_REQ message MUST include the
   optional sequence number and the checksum field.  The checksum field
   is used to convey service flags, channel bindings, and optional
   delegation information.

   The checksum type MUST be 0x8003. When delegation is used, a ticket-
   granting ticket will be transferred in a KRB_CRED message.  This
   ticket SHOULD have its forwardable flag set.  The EncryptedData
   field of the KRB_CRED message [KRBCLAR] MUST be encrypted in the
   session key of the ticket used to authenticate the context.

   The authenticator checksum field SHALL have the following format:

      Octet        Name      Description
      0..3         Lgth    Number of octets in Bnd field;  Represented
                           in little-endian order;  Currently contains
                           hex value 10 00 00 00 (16).
      4..19        Bnd     Channel binding information, as described in
      20..23       Flags   Four-octet context-establishment flags in
                           little-endian order as described in section
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      24..25       DlgOpt  The delegation option identifier (=1) in
                           little-endian order [optional].  This field
                           and the next two fields are present if and
                           only if GSS_C_DELEG_FLAG is set as described
                           in section
      26..27       Dlgth   The length of the Deleg field in little-
                           endian order [optional].
      28..(n-1)    Deleg   A KRB_CRED message (n = Dlgth + 28)
      n..last      Exts    Extensions [optional].

   The length of the checksum field MUST be at least 24 octets when
   GSS_C_DELEG_FLAG is not set (as described in section, and
   at least 28 octets plus Dlgth octets when GSS_C_DELEG_FLAG is set.
   When GSS_C_DELEG_FLAG is set, the DlgOpt, Dlgth and Deleg fields
   of the checksum data MUST immediately follow the Flags field.  The
   optional trailing octets (namely the "Exts" field) facilitate
   future extensions to this mechanism.  When delegation is not used
   but the Exts field is present, the Exts field starts at octet 24
   (DlgOpt, Dlgth and Deleg are absent).

   Initiators that do not support the extensions MUST NOT include more
   than 24 octets in the checksum field, when GSS_C_DELEG_FLAG is not
   set, or more than 28 octets plus the KRB_CRED in the Deleg field,
   when GSS_C_DELEG_FLAG is set.  Acceptors that do not understand the
   extensions MUST ignore any octets past the Deleg field of the
   checksum data, when GSS_C_DELEG_FLAG is set, or past the Flags field
   of the checksum data, when GSS_C_DELEG_FLAG is not set. Checksum Flags Field

   The checksum "Flags" field is used to convey service options or
   extension negotiation information.

   The following context establishment flags are defined in [RFC-2744].

        Flag Name              Value
       GSS_C_DELEG_FLAG           1
       GSS_C_MUTUAL_FLAG          2
       GSS_C_REPLAY_FLAG          4
       GSS_C_SEQUENCE_FLAG        8
       GSS_C_CONF_FLAG           16
       GSS_C_INTEG_FLAG          32

   Context establishment flags are exposed to the calling application.
   If the calling application desires a particular service option then
   it requests that option via GSS_Init_sec_context() [RFC-2743].  If
   the corresponding return state values [RFC-2743] indicate that any
   of above optional context level services will be active on the
   context, the corresponding flag values in the table above MUST be
   set in the checksum Flags field.

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   Flag values 4096..524288 (2^12, 2^13, ..., 2^19) are reserved for
   use with legacy vendor-specific extensions to this mechanism.

   All other flag values not specified herein are reserved for future
   use.  Future revisions of this mechanism may use these reserved
   flags and may rely on implementations of this version to not use
   such flags in order to properly negotiate mechanism versions.
   Undefined flag values MUST be cleared by the sender, and unknown
   flags MUST be ignored by the receiver. Channel Binding Information

   These tags are intended to be used to identify the particular
   communications channel for which the GSS-API security context
   establishment tokens are intended, thus limiting the scope within
   which an intercepted context establishment token can be reused by an
   attacker (see [RFC-2743], section 1.1.6).

   When using C language bindings, channel bindings are communicated
   to the GSS-API using the following structure [RFC-2744]:

      typedef struct gss_channel_bindings_struct {
         OM_uint32       initiator_addrtype;
         gss_buffer_desc initiator_address;
         OM_uint32       acceptor_addrtype;
         gss_buffer_desc acceptor_address;
         gss_buffer_desc application_data;
      } *gss_channel_bindings_t;

   The member fields and constants used for different address types
   are defined in [RFC-2744].

   The "Bnd" field contains the MD5 hash of channel bindings, taken
   over all non-null components of bindings, in order of declaration.
   Integer fields within channel bindings are represented in little-
   endian order for the purposes of the MD5 calculation.

   In computing the contents of the Bnd field, the following detailed
   points apply:

   (1) For purposes of MD5 hash computation, each integer field and
   input length field SHALL be formatted into four octets, using
   little endian octet ordering.

   (2) All input length fields within gss_buffer_desc elements of a
   gss_channel_bindings_struct even those which are zero-valued, SHALL
   be included in the hash calculation; the value elements of
   gss_buffer_desc elements SHALL be dereferenced, and the resulting
   data SHALL be included within the hash computation, only for the
   case of gss_buffer_desc elements having non-zero length specifiers.

   (3) If the caller passes the value GSS_C_NO_BINDINGS instead of a
   valid channel binding structure, the Bnd field SHALL be set to 16
   zero-valued octets.

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   If the caller to GSS_Accept_sec_context [RFC-2743] passes in
   GSS_C_NO_CHANNEL_BINDINGS [RFC-2744] as the channel bindings then
   the acceptor MAY ignore any channel bindings supplied by the
   initiator, returning success even if the initiator did pass in
   channel bindings.

   If the application supply, in the channel bindings, a buffer with a
   length field larger than 4294967295 (2^32 - 1), the implementation
   of this mechanism MAY chose to reject the channel bindings
   altogether, using major status GSS_S_BAD_BINDINGS [RFC-2743].  In
   any case, the size of channel binding data buffers that can be used
   (interoperable, without extensions) with this specification is
   limited to 4294967295 octets.

4.2. Per-Message Tokens

   Two classes of tokens are defined in this section:  "MIC" tokens,
   emitted by calls to GSS_GetMIC() and consumed by calls to
   GSS_VerifyMIC(), "Wrap" tokens, emitted by calls to GSS_Wrap() and
   consumed by calls to GSS_Unwrap().

   The new per-message tokens introduced here do not include the
   generic GSS-API token framing used by the context establishment
   tokens.  These new tokens are designed to be used with newer crypto
   systems that can, for example, have variable-size checksums.

4.2.1. Sequence Number

   To distinguish intentionally-repeated messages from maliciously-
   replayed ones, per-message tokens contain a sequence number field,
   which is a 64 bit integer expressed in big endian order.  After
   sending a GSS_GetMIC() or GSS_Wrap() token, the sender's sequence
   numbers SHALL be incremented by one.

4.2.2. Flags Field

   The "Flags" field is a one-octet integer used to indicate a set of
   attributes for the protected message.  For example, one flag is
   allocated as the direction-indicator, thus preventing an adversary
   from sending back the same message in the reverse direction and
   having it accepted.

   The meanings of bits in this field (the least significant bit is
   bit 0) are as follows:

        Bit    Name             Description
        0   SentByAcceptor    When set, this flag indicates the sender
                              is the context acceptor.  When not set,
                              it indicates the sender is the context
        1   Sealed            When set in Wrap tokens, this flag
                              indicates confidentiality is provided
                              for.  It SHALL NOT be set in MIC tokens.
        2   AcceptorSubkey    A subkey asserted by the context acceptor
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                              is used to protect the message.

   The rest of available bits are reserved for future use and MUST be
   cleared.  The receiver MUST ignore unknown flags.

4.2.3. EC Field

   The "EC" (Extra Count) field is a two-octet integer field expressed
   in big endian order.

   In Wrap tokens with confidentiality, the EC field SHALL be used to
   encode the number of octets in the filler, as described in section

   In Wrap tokens without confidentiality, the EC field SHALL be used
   to encode the number of octets in the trailing checksum, as
   described in section 4.2.4.

4.2.4. Encryption and Checksum Operations

   The encryption algorithms defined by the crypto profiles provide for
   integrity protection [KCRYPTO].  Therefore no separate checksum is

   The result of decryption can be longer than the original plaintext
   [KCRYPTO] and the extra trailing octets are called "crypto-system
   garbage" in this document.  However, given the size of any plaintext
   data, one can always find a (possibly larger) size so that, when
   padding the to-be-encrypted text to that size, there will be no
   crypto-system garbage added [KCRYPTO].

   In Wrap tokens that provide for confidentiality, the first 16 octets
   of the Wrap token (the "header", as defined in section 4.2.6), SHALL
   be appended to the plaintext data before encryption.  Filler octets
   MAY be inserted between the plaintext data and the "header", and the
   values and size of the filler octets are chosen by implementations,
   such that there SHALL be no crypto-system garbage present after the
   decryption.  The resulting Wrap token is {"header" |
   encrypt(plaintext-data | filler | "header")}, where encrypt() is the
   encryption operation (which provides for integrity protection)
   defined in the crypto profile [KCRYPTO], and the RRC field (as
   defined in section 4.2.5) in the to-be-encrypted header contain the
   hex value 00 00.

   In Wrap tokens that do not provide for confidentiality, the checksum
   SHALL be calculated first over the to-be-signed plaintext data, and
   then the first 16 octets of the Wrap token (the "header", as defined
   in section 4.2.6).  Both the EC field and the RRC field in the token
   header SHALL be filled with zeroes for the purpose of calculating
   the checksum.  The resulting Wrap token is {"header" | plaintext-
   data | get_mic(plaintext-data | "header")},  where get_mic() is the
   checksum operation for the required checksum mechanism of the chosen
   encryption mechanism defined in the crypto profile [KCRYPTO].

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   The parameters for the key and the cipher-state in the encrypt() and
   get_mic() operations have been omitted for brevity.

   For MIC tokens, the checksum SHALL be calculated as follows: the
   checksum operation is calculated first over the to-be-signed
   plaintext data, and then the first 16 octets of the MIC token, where
   the checksum mechanism is the required checksum mechanism of the
   chosen encryption mechanism defined in the crypto profile [KCRYPTO].

   The resulting Wrap and MIC tokens bind the data to the token header,
   including the sequence number and the direction indicator.

4.2.5. RRC Field

   The "RRC" (Right Rotation Count) field in Wrap tokens is added to
   allow the data to be encrypted in-place by existing SSPI (Security
   Service Provider Interface) [SSPI] applications that do not provide
   an additional buffer for the trailer (the cipher text after the in-
   place-encrypted data) in addition to the buffer for the header (the
   cipher text before the in-place-encrypted data).  The resulting Wrap
   token in the previous section, excluding the first 16 octets of the
   token header, is rotated to the right by "RRC" octets.  The net
   result is that "RRC" octets of trailing octets are moved toward the
   header.  Consider the following as an example of this rotation
   operation:  Assume that the RRC value is 3 and the token before the
   rotation is {"header" | aa | bb | cc | dd | ee | ff | gg | hh}, the
   token after rotation would be {"header" | ff | gg | hh | aa | bb |
   cc | dd | ee }, where {aa | bb | cc |...| hh} is used to indicate
   the octet sequence.

   The RRC field is expressed as a two-octet integer in big endian

   The rotation count value is chosen by the sender based on
   implementation details, and the receiver MUST be able to interpret
   all possible rotation count values, including rotation counts
   greater than the length of the token.

4.2.6. Message Layouts

   Per-message tokens start with a two-octet token identifier (TOK_ID)
   field, expressed in big endian order.  These tokens are defined
   separately in subsequent sub-sections. MIC Tokens

   Use of the GSS_GetMIC() call yields a token (referred as the MIC
   token in this document), separate from the user
   data being protected, which can be used to verify the integrity of
   that data as received.  The token has the following format:

      Octet no   Name        Description
       0..1     TOK_ID     Identification field.  Tokens emitted by
                           GSS_GetMIC() contain the hex value 04 04
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                           expressed in big endian order in this field.
       2        Flags      Attributes field, as described in section
       3..7     Filler     Contains five octets of hex value FF.
       8..15    SND_SEQ    Sequence number field in clear text,
                           expressed in big endian order.
       16..last SGN_CKSUM  Checksum of the "to-be-signed" data and
                           octet 0..15, as described in section 4.2.4.

   The Filler field is included in the checksum calculation for
   simplicity. Wrap Tokens

   Use of the GSS_Wrap() call yields a token (referred as the Wrap
   token in this document), which consists of a descriptive header,
   followed by a body portion that contains either the input user data
   in plaintext concatenated with the checksum, or the input user data
   encrypted.  The GSS_Wrap() token SHALL have the following format:

      Octet no   Name        Description
       0..1     TOK_ID     Identification field.  Tokens emitted by
                           GSS_Wrap() contain the the hex value 05 04
                           expressed in big endian order in this field.
       2        Flags      Attributes field, as described in section
       3        Filler     Contains the hex value FF.
       4..5     EC         Contains the "extra count" field, in big
                           endian order as described in section 4.2.3.
       6..7     RRC        Contains the "right rotation count" in big
                           endian order, as described in section 4.2.5.
       8..15    SND_SEQ    Sequence number field in clear text,
                           expressed in big endian order.
       16..last Data       Encrypted data for Wrap tokens with
                           confidentiality, or plaintext data followed
                           by the checksum for Wrap tokens without
                           confidentiality, as described in section

4.3. Context Deletion Tokens

   Context deletion tokens are empty in this mechanism.  Both peers to
   a security context invoke GSS_Delete_sec_context() [RFC-2743]
   independently, passing a null output_context_token buffer to
   indicate that no context_token is required.  Implementations of
   GSS_Delete_sec_context() should delete relevant locally-stored
   context information.

4.4. Token Identifier Assignment Considerations

   Token identifiers (TOK_ID) from 0x60 0x00 through 0x60 0xFF
   inclusive are reserved and SHALL NOT be assigned.  Thus by examining
   the first two octets of a token, one can tell unambiguously if it is
   wrapped with the generic GSS-API token framing.
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5. Parameter Definitions

   This section defines parameter values used by the Kerberos V5 GSS-
   API mechanism.  It defines interface elements in support of
   portability, and assumes use of C language bindings per [RFC-2744].

5.1. Minor Status Codes

   This section recommends common symbolic names for minor_status
   values to be returned by the Kerberos V5 GSS-API mechanism.  Use of
   these definitions will enable independent implementers to enhance
   application portability across different implementations of the
   mechanism defined in this specification.  (In all cases,
   implementations of GSS_Display_status() will enable callers to
   convert minor_status indicators to text representations.)  Each
   implementation should make available, through include files or other
   means, a facility to translate these symbolic names into the
   concrete values which a particular GSS-API implementation uses to
   represent the minor_status values specified in this section.

   It is recognized that this list may grow over time, and that the
   need for additional minor_status codes specific to particular
   implementations may arise.  It is recommended, however, that
   implementations should return a minor_status value as defined on a
   mechanism-wide basis within this section when that code is
   accurately representative of reportable status rather than using a
   separate, implementation-defined code.

5.1.1. Non-Kerberos-specific codes

              /* "No @ in SERVICE-NAME name string" */
              /* "STRING-UID-NAME contains nondigits" */
              /* "UID does not resolve to username" */
              /* "Validation error" */
              /* "Couldn't allocate gss_buffer_t data" */
              /* "Message context invalid" */
              /* "Buffer is the wrong size" */
              /* "Credential usage type is unknown" */
              /* "Unknown quality of protection specified" */

5.1.2. Kerberos-specific-codes

              /* "Client principal in credentials does not match
                 specified name" */
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              /* "No key available for specified service principal" */
              /* "No Kerberos ticket-granting ticket available" */
              /* "Authenticator has no subkey" */
              /* "Context is already fully established" */
              /* "Unknown signature type in token" */
              /* "Invalid field length in token" */
              /* "Attempt to use incomplete security context" */

5.2. Buffer Sizes

   All implementations of this specification MUST be capable of
   accepting buffers of at least 16K octets as input to GSS_GetMIC(),
   GSS_VerifyMIC(), and GSS_Wrap(), and MUST be capable of accepting
   the output_token generated by GSS_Wrap() for a 16K octet input
   buffer as input to GSS_Unwrap().  Implementations SHOULD support 64K
   octet input buffers, and MAY support even larger input buffer sizes.

6. Backwards Compatibility Considerations

   The new token formats defined in this document will only be
   recognized by new implementations.  To address this, implementations
   can always use the explicit sign or seal algorithm in [RFC-1964]
   when the key type corresponds to "older" enctypes.  An alternative
   approach might be to retry sending the message with the sign or seal
   algorithm explicitly defined as in [RFC-1964].  However this would
   require either the use of a mechanism such as [RFC-2478] to securely
   negotiate the method or the use out of band mechanism to choose
   appropriate mechanism.  For this reason, it is RECOMMENDED that the
   new token formats defined in this document SHOULD be used only if
   both peers are known to support the new mechanism during context
   negotiation because of, for example, the use of "new" enctypes.

   GSS_Unwrap() or GSS_VerifyMIC() can process a message token as
   follows: it can look at the first octet of the token header, if it
   is 0x60 then the token must carry the generic GSS-API pseudo ASN.1
   framing, otherwise the first two octets of the token contain the
   TOK_ID that uniquely identify the token message format.

7. Security Considerations

   Channel bindings are validated by the acceptor.  The acceptor can
   ignore the channel bindings restriction supplied by the initiator
   and carried in the authenticator checksum, if channel bindings are
   not used by GSS_Accept_sec_context [RFC-2743], and the acceptor does
   not prove to the initiator that it has the same channel bindings as
   the initiator, even if the client requested mutual authentication.
   This limitation should be taken into consideration by designers of
   applications that would use channel bindings, whether to limit the
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   use of GSS-API contexts to nodes with specific network addresses, to
   authenticate other established, secure channels using Kerberos
   Version 5, or for any other purpose.

   Session key types are selected by the KDC.  Under the current
   mechanism, no negotiation of algorithm types occurs, so server-side
   (acceptor) implementations cannot request that clients not use
   algorithm types not understood by the server.  However,
   administrators can control what enctypes can be used for session
   keys for this mechanism by controlling the set of the ticket session
   key enctypes which the KDC is willing to use in tickets for a given
   acceptor principal.  The KDC could therefore be given the task of
   limiting session keys for a given service to types actually
   supported by the Kerberos and GSSAPI software on the server.  This
   does have a drawback for cases where a service principal name is
   used both for GSSAPI-based and non-GSSAPI-based communication (most
   notably the "host" service key), if the GSSAPI implementation does
   not understand (for example) AES [AES-KRB5] but the Kerberos
   implementation does.  It means that AES session keys cannot be
   issued for that service principal, which keeps the protection of
   non-GSSAPI services weaker than necessary.  KDC administrators
   desiring to limit the session key types to support interoperability
   with such GSSAPI implementations should carefully weigh the
   reduction in protection offered by such mechanisms against the
   benefits of interoperability.

8. Acknowledgments

  Ken Raeburn and Nicolas Williams corrected many of our errors in the
  use of generic profiles and were instrumental in the creation of
  this document.

  The text for security considerations was contributed by Nicolas
  Williams and Ken Raeburn.

  Sam Hartman and Ken Raeburn suggested the "floating trailer" idea,
  namely the encoding of the RRC field.

  Sam Hartman and Nicolas Williams recommended the replacing our
  earlier key derivation function for directional keys with different
  key usage numbers for each direction as well as retaining the
  directional bit for maximum compatibility.

  Paul Leach provided numerous suggestions and comments.

  Scott Field, Richard Ward, Dan Simon, Kevin Damour, and Simon
  Josefsson also provided valuable inputs on this document.

  Jeffrey Hutzelman provided comments and clarifications for the text
  related to the channel bindings.

  Jeffrey Hutzelman and Russ Housley suggested many editorial changes.

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  Luke Howard provided implementations of this document for the
  Heimdal code base, and helped inter-operability testing with the
  Microsoft code base, together with Love Hornquist Astrand.  These
  experiments formed the basis of this document.

  Martin Rex provided suggestions of TOK_ID assignment recommendations
  thus the token tagging in this document is unambiguous if the token
  is wrapped with the pseudo ASN.1 header.

  This document retains some of the text of RFC-1964 in relevant

9. Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   intellectual property 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; neither does it represent that it
   has made any effort to identify any such rights.  Information on the
   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11.  Copies of
   claims of rights made available for publication 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 Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive

10. References

10.1. Normative References

   [RFC-2026] Bradner, S., "The Internet Standards Process -- Revision
   3", BCP 9, RFC 2026, October 1996.

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

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

   [RFC-2744] Wray, J., "Generic Security Service API Version 2: C-
   bindings", RFC 2744, January 2000.

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

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   [KCRYPTO] RFC-Editor: To be replaced by RFC number for draft-ietf-
   krb-wg-crypto.  Work in Progress.

   [KRBCLAR] RFC-Editor: To be replaced by RFC number for draft-ietf-
   krb-wg-kerberos-clarifications.  Work in Progress.

10.2. Informative References

   [SSPI] Leach, P., "Security Service Provider Interface", Microsoft
   Developer Network (MSDN), April 2003.

   [AES-KRB5] RFC-Editor: To be replaced by RFC number for draft-
   raeburn-krb-rijndael-krb.  Work in Progress.

   [RFC-2478] Baize, E., Pinkas D., "The Simple and Protected GSS-API
   Negotiation Mechanism", RFC 2478, December 1998.

11. Author's Address

   Larry Zhu
   One Microsoft Way
   Redmond, WA 98052 - USA
   EMail: LZhu@microsoft.com

   Karthik Jaganathan
   One Microsoft Way
   Redmond, WA 98052 - USA
   EMail: karthikj@microsoft.com

   Sam Hartman
   Massachusetts Institute of Technology
   77 Massachusetts Avenue
   Cambridge, MA 02139 - USA
   Email: hartmans@MIT.EDU

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