Network Working Group                                             L. Zhu
Request for Comments: 4121                                 K. Jaganathan
Updates: 1964                                                  Microsoft
Category: Standards Track                                     S. Hartman
                                                               July 2005

                        The Kerberos Version 5
   Generic Security Service Application Program Interface (GSS-API)
                         Mechanism: Version 2

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).


   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

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Table of Contents

   1. Introduction ....................................................2
   2. Key Derivation for Per-Message Tokens ...........................4
   3. Quality of Protection ...........................................4
   4. Definitions and Token Formats ...................................5
      4.1. Context Establishment Tokens ...............................5
           4.1.1. Authenticator Checksum ..............................6
      4.2. Per-Message Tokens .........................................9
           4.2.1. Sequence Number .....................................9
           4.2.2. Flags Field .........................................9
           4.2.3. EC Field ...........................................10
           4.2.4. Encryption and Checksum Operations .................10
           4.2.5. RRC Field ..........................................11
           4.2.6. Message Layouts ....................................12
      4.3. Context Deletion Tokens ...................................13
      4.4. Token Identifier Assignment Considerations ................13
   5. Parameter Definitions ..........................................14
      5.1. Minor Status Codes ........................................14
           5.1.1. Non-Kerberos-specific Codes ........................14
           5.1.2. Kerberos-specific Codes ............................15
      5.2. Buffer Sizes ..............................................15
   6. Backwards Compatibility Considerations .........................15
   7. Security Considerations ........................................16
   8. Acknowledgements................................................17
   9. References .....................................................18
      9.1. Normative References ......................................18
      9.2. Informative References ....................................18

1.  Introduction

   [RFC3961] defines a generic framework for describing encryption and
   checksum types to be used with the Kerberos protocol and associated

   [RFC1964] 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 [RFC3961] for the session
   key or subkey that is created during context negotiation, and its
   required checksum algorithm.  Message layouts of the per-message
   tokens are therefore revised to remove algorithm indicators and to
   add extra information to support the generic crypto framework

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   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) [RFC4120] are not specific to GSS-API
   usage and are therefore defined within [RFC4120] rather than this

   The new token formats specified in this document MUST be used with
   all "newer" encryption types [RFC4120] and MAY be used with
   encryption types that are not "newer", 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 [RFC3961],
   as defined in section 3.1.3 of [RFC4120].  The list of not-newer
   encryption types is as follows [RFC3961]:

           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

   Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   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.

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2.  Key Derivation for Per-Message Tokens

   To limit the exposure of a given key, [RFC3961] adopted "one-way"
   "entropy-preserving" derived keys, from a base key or protocol key,
   for different purposes or key usages.

   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 [RFC4120] created during the context

           Name                         Value
          KG-USAGE-ACCEPTOR-SEAL         22
          KG-USAGE-ACCEPTOR-SIGN         23
          KG-USAGE-INITIATOR-SEAL        24
          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
   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, while 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 in the AP-REP message.  If the acceptor asserts a
   subkey, the base key is the acceptor-asserted subkey and subsequent
   per-message tokens MUST be flagged with "AcceptorSubkey", as
   described in section 4.2.2.  Otherwise, if the initiator asserts a
   subkey in the AP-REQ message, the base key is this subkey;  if the
   initiator does not assert a subkey, the base key is the session key
   in the service ticket.

3.  Quality of Protection

   The GSS-API specification [RFC2743] provides 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 that do not use the default
   QOP are not guaranteed to be portable across implementations, or even
   to inter-operate with different deployment configurations of the same
   implementation.  Using a different algorithm than 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.

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   The encryption and checksum algorithms in per-message tokens are now
   implicitly defined by the algorithms associated with the session key
   or subkey.  Therefore, algorithm identifiers as described in
   [RFC1964] are no longer needed and are removed from the new token

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
   [RFC2743], as illustrated by the following pseudo-ASN.1 structures:



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

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


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

   Following are the TOK_ID values used in the context establishment

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

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   Where Kerberos message KRB_AP_REQUEST, KRB_AP_REPLY, and KRB_ERROR
   are defined in [RFC4120].

   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

   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 [RFC4120] 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
       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

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   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 [RFC2744].

           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() [RFC2743].  If the
   corresponding return state values [RFC2743] indicate that any of the
   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.

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

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   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 [RFC2743], section 1.1.6).

   When using C language bindings, channel bindings are communicated to
   the GSS-API using the following structure [RFC2744]:

         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 [RFC2744].

   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

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   (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.

   If the caller to GSS_Accept_sec_context [RFC2743] passes in
   GSS_C_NO_CHANNEL_BINDINGS [RFC2744] 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 supplies, in the channel bindings, a buffer with a
   length field larger than 4294967295 (2^32 - 1), the implementation of
   this mechanism MAY choose to reject the channel bindings altogether,
   using major status GSS_S_BAD_BINDINGS [RFC2743].  In any case, the
   size of channel-binding data buffers that can be used (interoperable,
   without extensions) with this specification is limited to 4294967295

4.2.  Per-Message Tokens

   Two classes of tokens are defined in this section: (1) "MIC" tokens,
   emitted by calls to GSS_GetMIC() and consumed by calls to
   GSS_VerifyMIC(), and (2) "Wrap" tokens, emitted by calls to
   GSS_Wrap() and consumed by calls to GSS_Unwrap().

   These new per-message tokens 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 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 the acceptance
   of the same message sent back in the reverse direction by an

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   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
                               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 [RFC3961].  Therefore, no separate checksum is

   The result of decryption can be longer than the original plaintext
   [RFC3961] and the extra trailing octets are called "crypto-system
   residue" in this document.  However, given the size of any plaintext
   data, one can always find a (possibly larger) size, such that when
   padding the to-be-encrypted text to that size, there will be no
   crypto-system residue added [RFC3961].

   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."  The

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   values and size of the filler octets are chosen by implementations,
   such that there SHALL be no crypto-system residue 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 [RFC3961], and the RRC field (as
   defined in section 4.2.5) in the to-be-encrypted header contains 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 over 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 [RFC3961].

   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 over 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

   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).  Excluding the first
   16 octets of the token header, the resulting Wrap token in the
   previous section is rotated to the right by "RRC" octets.  The net
   result is that "RRC" octets of trailing octets are moved toward the

   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

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   rotation would be {"header" | ff | gg | hh | aa | bb | cc | dd | ee
   }, where {aa | bb | cc |...| hh} would be used to indicate the octet

   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.  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 the following 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
                             expressed in big-endian order in this
         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

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   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 hex value 05 04
                             expressed in big-endian order in this
          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
          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() [RFC2743]
   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 that support portability,
   and assumes use of C language bindings per [RFC2744].

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 that a particular GSS-API implementation uses to represent the
   minor_status values specified in this section.

   This list may grow over time and the need for additional minor_status
   codes, specific to particular implementations, may arise.  However,
   it is recommended that implementations should return a minor_status
   value as defined on a mechanism-wide basis within this section when
   that code accurately represents 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" */

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5.1.2.  Kerberos-specific Codes

                 /* "Client principal in credentials does not match
                    specified name" */
                 /* "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().  They MUST also 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 [RFC1964] when
   the key type corresponds to not "newer" enctypes.  As an alternative,
   one might retry sending the message with the sign or seal algorithm
   explicitly defined as in [RFC1964].  However, this would require
   either the use of a mechanism such as [RFC2478] to securely negotiate
   the method, or the use of an out-of-band mechanism to choose the
   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.

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   GSS_Unwrap() or GSS_VerifyMIC() can process a message token as
   follows: it can look at the first octet of the token header, and 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 (1) channel bindings are
   not used by GSS_Accept_sec_context [RFC2743], and (2) 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 considered by designers
   of applications that would use channel bindings, whether to limit the
   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.  Therefore, the KDC could 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 has a
   drawback for cases in which a service principal name is used for both
   GSSAPI-based and non-GSSAPI-based communication (most notably the
   "host" service key), if the GSSAPI implementation does not understand
   (for example) AES [RFC3962], but the Kerberos implementation does.
   This 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.

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8.  Acknowledgements

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

   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.

   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.

   John Linn wrote the original Kerberos Version 5 mechanism
   specification [RFC1964], of which some text has been retained.

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9.  References

9.1.  Normative References

   [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.

   [RFC2744]  Wray, J., "Generic Security Service API Version 2:
              C-bindings", RFC 2744, January 2000.

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

   [RFC3961]  Raeburn, K., "Encryption and Checksum Specifications for
              Kerberos 5", RFC 3961, February 2005.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              July 2005.

9.2.  Informative References

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

   [RFC3962]  Raeburn, K., "Advanced Encryption Standard (AES)
              Encryption for Kerberos 5", RFC 3962, February 2005.

   [RFC2478]  Baize, E. and D. Pinkas, "The Simple and Protected GSS-API
              Negotiation Mechanism", RFC 2478, December 1998.

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Authors' Addresses

   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-ietf@mit.edu

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Full Copyright Statement

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