





Network Working Group                                            J. Linn
Request for Comments: 1964                       OpenVision Technologies
Category: Standards Track                                      June 1996


                The Kerberos Version 5 GSS-API Mechanism

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.

ABSTRACT

   This specification defines protocols, procedures, and conventions to
   be employed by peers implementing the Generic Security Service
   Application Program Interface (as specified in RFCs 1508 and 1509)
   when using Kerberos Version 5 technology (as specified in RFC 1510).

ACKNOWLEDGMENTS

   Much of the material in this memo is based on working documents
   drafted by John Wray of Digital Equipment Corporation and on
   discussions, implementation activities, and interoperability testing
   involving Marc Horowitz, Ted Ts'o, and John Wray.  Particular thanks
   are due to each of these individuals for their contributions towards
   development and availability of GSS-API support within the Kerberos
   Version 5 code base.

1. Token Formats

   This section discusses protocol-visible characteristics of the GSS-
   API mechanism to be implemented atop Kerberos V5 security technology
   per RFC-1508 and RFC-1510; it defines elements of protocol for
   interoperability and is independent of language bindings per RFC-
   1509.

   Tokens transferred between GSS-API peers (for security context
   management and per-message protection purposes) are defined.  The
   data elements exchanged between a GSS-API endpoint implementation and
   the Kerberos KDC are not specific to GSS-API usage and are therefore
   defined within RFC-1510 rather than within this specification.






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   To support ongoing experimentation, testing, and evolution of the
   specification, the Kerberos V5 GSS-API mechanism as defined in this
   and any successor memos will be identified with the following Object
   Identifier, as defined in RFC-1510, until the specification is
   advanced to the level of Proposed Standard RFC:

   {iso(1), org(3), dod(5), internet(1), security(5), kerberosv5(2)}

   Upon advancement to the level of Proposed Standard RFC, the Kerberos
   V5 GSS-API mechanism will be identified by an Object Identifier
   having the value:

   {iso(1) member-body(2) United States(840) mit(113554) infosys(1)
   gssapi(2) krb5(2)}

1.1. Context Establishment Tokens

   Per RFC-1508, Appendix B, the initial context establishment token
   will be enclosed within framing as follows:

   InitialContextToken ::=
   [APPLICATION 0] IMPLICIT SEQUENCE {
           thisMech        MechType
                   -- MechType is OBJECT IDENTIFIER
                   -- representing "Kerberos V5"
           innerContextToken ANY DEFINED BY thisMech
                   -- contents mechanism-specific;
                   -- ASN.1 usage within innerContextToken
                   -- is not required
           }

   The innerContextToken of the initial context token will consist of a
   Kerberos V5 KRB_AP_REQ message, preceded by a two-byte token-id
   (TOK_ID) field, which shall contain the value 01 00.

   The above GSS-API framing shall be applied to all tokens emitted by
   the Kerberos V5 GSS-API mechanism, including KRB_AP_REP, KRB_ERROR,
   context-deletion, and per-message tokens, not just to the initial
   token in a context establishment sequence.  While not required by
   RFC-1508, this enables implementations to perform enhanced error-
   checking. The innerContextToken field of context establishment tokens
   for the Kerberos V5 GSS-API mechanism will contain a Kerberos message
   (KRB_AP_REQ, KRB_AP_REP or KRB_ERROR), preceded by a 2-byte TOK_ID
   field containing 01 00 for KRB_AP_REQ messages, 02 00 for KRB_AP_REP
   messages and 03 00 for KRB_ERROR messages.






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1.1.1. Initial Token

   Relevant KRB_AP_REQ syntax (from RFC-1510) is as follows:

   AP-REQ ::= [APPLICATION 14] SEQUENCE {
           pvno [0]        INTEGER,        -- indicates Version 5
           msg-type [1]    INTEGER,        -- indicates KRB_AP_REQ
           ap-options[2]   APOptions,
           ticket[3]       Ticket,
           authenticator[4]        EncryptedData
   }

   APOptions ::= BIT STRING {
           reserved (0),
           use-session-key (1),
           mutual-required (2)
   }

   Ticket ::= [APPLICATION 1] SEQUENCE {
           tkt-vno [0]     INTEGER,        -- indicates Version 5
           realm [1]       Realm,
           sname [2]       PrincipalName,
           enc-part [3]    EncryptedData
   }

   -- Encrypted part of ticket
   EncTicketPart ::= [APPLICATION 3] SEQUENCE {
           flags[0]        TicketFlags,
           key[1]          EncryptionKey,
           crealm[2]       Realm,
           cname[3]        PrincipalName,
           transited[4]    TransitedEncoding,
           authtime[5]     KerberosTime,
           starttime[6]    KerberosTime OPTIONAL,
           endtime[7]      KerberosTime,
           renew-till[8]   KerberosTime OPTIONAL,
           caddr[9]        HostAddresses OPTIONAL,
           authorization-data[10]  AuthorizationData OPTIONAL
   }

   -- Unencrypted authenticator
   Authenticator ::= [APPLICATION 2] SEQUENCE  {
           authenticator-vno[0]    INTEGER,
           crealm[1]               Realm,
           cname[2]                PrincipalName,
           cksum[3]                Checksum OPTIONAL,
           cusec[4]                INTEGER,
           ctime[5]                KerberosTime,



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           subkey[6]               EncryptionKey OPTIONAL,
           seq-number[7]           INTEGER OPTIONAL,
           authorization-data[8]   AuthorizationData OPTIONAL
   }

   For purposes of this specification, the authenticator shall include
   the optional sequence number, and the checksum field shall be used to
   convey channel binding, service flags, and optional delegation
   information.  The checksum will have a type of 0x8003 (a value being
   registered within the Kerberos protocol specification), and a value
   field of at least 24 bytes in length.  The length of the value field
   is extended beyond 24 bytes if and only if an optional facility to
   carry a Kerberos-defined KRB_CRED message for delegation purposes is
   supported by an implementation and active on a context. When
   delegation is active, a TGT with its FORWARDABLE flag set will be
   transferred within the KRB_CRED message.

   The checksum value field's format is as follows:

   Byte    Name    Description
   0..3    Lgth    Number of bytes in Bnd field;
                   Currently contains hex 10 00 00 00
                   (16, represented in little-endian form)
   4..19   Bnd     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.
   20..23  Flags   Bit vector of context-establishment flags,
                   with values consistent with RFC-1509, p. 41:
                           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
                   The resulting bit vector is encoded into bytes 20..23
                   in little-endian form.
   24..25  DlgOpt  The Delegation Option identifier (=1) [optional]
   26..27  Dlgth   The length of the Deleg field. [optional]
   28..n   Deleg   A KRB_CRED message (n = Dlgth + 29) [optional]

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

        (1) Each integer field shall be formatted into four bytes, using
        little-endian byte ordering, for purposes of MD5 hash
        computation.



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        (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 bindings structure, the Bnd field shall be set
        to 16 zero-valued bytes.

   In the initial Kerberos V5 GSS-API mechanism token (KRB_AP_REQ token)
   from initiator to target, the GSS_C_DELEG_FLAG, GSS_C_MUTUAL_FLAG,
   GSS_C_REPLAY_FLAG, and GSS_C_SEQUENCE_FLAG values shall each be set
   as the logical AND of the initiator's corresponding request flag to
   GSS_Init_sec_context() and a Boolean indicator of whether that
   optional service is available to GSS_Init_sec_context()'s caller.
   GSS_C_CONF_FLAG and GSS_C_INTEG_FLAG, for which no corresponding
   context-level input indicator flags to GSS_Init_sec_context() exist,
   shall each be set to indicate whether their respective per-message
   protection services are available for use on the context being
   established.

   When input source address channel binding values are provided by a
   caller (i.e., unless the input argument is GSS_C_NO_BINDINGS or the
   source address specifier value within the input structure is
   GSS_C_NULL_ADDRTYPE), and the corresponding token received from the
   context's peer bears address restrictions, it is recommended that an
   implementation of the Kerberos V5 GSS-API mechanism should check that
   the source address as provided by the caller matches that in the
   received token, and should return the GSS_S_BAD_BINDINGS major_status
   value if a mismatch is detected. Note: discussion is ongoing about
   the strength of recommendation to be made in this area, and on the
   circumstances under which such a recommendation should be applicable;
   implementors are therefore advised that changes on this matter may be
   included in subsequent versions of this specification.

1.1.2. Response Tokens

   A context establishment sequence based on the Kerberos V5 mechanism
   will perform one-way authentication (without confirmation or any
   return token from target to initiator in response to the initiator's
   KRB_AP_REQ) if the mutual_req bit is not set in the application's
   call to GSS_Init_sec_context().  Applications requiring confirmation
   that their authentication was successful should request mutual
   authentication, resulting in a "mutual-required" indication within
   KRB_AP_REQ APoptions and the setting of the mutual_req bit in the



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   flags field of the authenticator checksum.  In response to such a
   request, the context target will reply to the initiator with a token
   containing either a KRB_AP_REP or KRB_ERROR, completing the mutual
   context establishment exchange.

   Relevant KRB_AP_REP syntax is as follows:

   AP-REP ::= [APPLICATION 15] SEQUENCE {
           pvno [0]        INTEGER,        -- represents Kerberos V5
           msg-type [1]    INTEGER,        -- represents KRB_AP_REP
           enc-part [2]    EncryptedData
   }

   EncAPRepPart ::= [APPLICATION 27] SEQUENCE {
           ctime [0]       KerberosTime,
           cusec [1]       INTEGER,
           subkey [2]      EncryptionKey OPTIONAL,
           seq-number [3]  INTEGER OPTIONAL
   }

   The optional seq-number element within the AP-REP's EncAPRepPart
   shall be included.

   The syntax of KRB_ERROR is as follows:

   KRB-ERROR ::=   [APPLICATION 30] SEQUENCE {
           pvno[0]         INTEGER,
           msg-type[1]     INTEGER,
           ctime[2]        KerberosTime OPTIONAL,
           cusec[3]        INTEGER OPTIONAL,
           stime[4]        KerberosTime,
           susec[5]        INTEGER,
           error-code[6]   INTEGER,
           crealm[7]       Realm OPTIONAL,
           cname[8]        PrincipalName OPTIONAL,
           realm[9]        Realm, -- Correct realm
           sname[10]       PrincipalName, -- Correct name
           e-text[11]      GeneralString OPTIONAL,
           e-data[12]      OCTET STRING OPTIONAL
   }

   Values to be transferred in the error-code field of a KRB-ERROR
   message are defined in [RFC-1510], not in this specification.








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1.2. Per-Message and Context Deletion Tokens

   Three classes of tokens are defined in this section: "MIC" tokens,
   emitted by calls to GSS_GetMIC() (formerly GSS_Sign()) and consumed
   by calls to GSS_VerifyMIC() (formerly GSS_Verify()), "Wrap" tokens,
   emitted by calls to GSS_Wrap() (formerly GSS_Seal()) and consumed by
   calls to GSS_Unwrap() (formerly GSS_Unseal()), and context deletion
   tokens, emitted by calls to GSS_Delete_sec_context() and consumed by
   calls to GSS_Process_context_token().  Note: References to GSS-API
   per-message routines in the remainder of this specification will be
   based on those routines' newer recommended names rather than those
   names' predecessors.

   Several variants of cryptographic keys are used in generation and
   processing of per-message tokens:

        (1) context key: uses Kerberos session key (or subkey, if
        present in authenticator emitted by context initiator) directly

        (2) confidentiality key: forms variant of context key by
        exclusive-OR with the hexadecimal constant f0f0f0f0f0f0f0f0.

        (3) MD2.5 seed key: forms variant of context key by reversing
        the bytes of the context key (i.e. if the original key is the
        8-byte sequence {aa, bb, cc, dd, ee, ff, gg, hh}, the seed key
        will be {hh, gg, ff, ee, dd, cc, bb, aa}).

1.2.1. Per-message Tokens - MIC

Use of the GSS_GetMIC() call yields a token, 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:

   Byte no          Name           Description
    0..1           TOK_ID          Identification field.
                                   Tokens emitted by GSS_GetMIC() contain
                                   the hex value 01 01 in this field.
    2..3           SGN_ALG         Integrity algorithm indicator.
                                   00 00 - DES MAC MD5
                                   01 00 - MD2.5
                                   02 00 - DES MAC
    4..7           Filler          Contains ff ff ff ff
    8..15          SND_SEQ         Sequence number field.
    16..23         SGN_CKSUM       Checksum of "to-be-signed data",
                                   calculated according to algorithm
                                   specified in SGN_ALG field.





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   GSS-API tokens must be encapsulated within the higher-level protocol
   by the application; no embedded length field is necessary.

1.2.1.1. Checksum

   Checksum calculation procedure (common to all algorithms): Checksums
   are calculated over the data field, logically prepended by the first
   8 bytes of the plaintext packet header.  The resulting value binds
   the data to the packet type and signature algorithm identifier
   fields.

   DES MAC MD5 algorithm: The checksum is formed by computing an MD5
   [RFC-1321] hash over the plaintext data, and then computing a DES-CBC
   MAC on the 16-byte MD5 result.  A standard 64-bit DES-CBC MAC is
   computed per [FIPS-PUB-113], employing the context key and a zero IV.
   The 8-byte result is stored in the SGN_CKSUM field.

   MD2.5 algorithm: The checksum is formed by first DES-CBC encrypting a
   16-byte zero-block, using a zero IV and a key formed by reversing the
   bytes of the context key (i.e. if the original key is the 8-byte
   sequence {aa, bb, cc, dd, ee, ff, gg, hh}, the checksum key will be
   {hh, gg, ff, ee, dd, cc, bb, aa}).   The resulting 16-byte value is
   logically prepended to the to-be-signed data.  A standard MD5
   checksum is calculated over the combined data, and the first 8 bytes
   of the result are stored in the SGN_CKSUM field.  Note 1: we refer to
   this algorithm informally as "MD2.5" to connote the fact that it uses
   half of the 128 bits generated by MD5; use of only a subset of the
   MD5 bits is intended to protect against the prospect that data could
   be postfixed to an existing message with corresponding modifications
   being made to the checksum.  Note 2: This algorithm is fairly novel
   and has received more limited evaluation than that to which other
   integrity algorithms have been subjected.  An initial, limited
   evaluation indicates that it may be significantly weaker than DES MAC
   MD5.

   DES-MAC algorithm: A standard 64-bit DES-CBC MAC is computed on the
   plaintext data per [FIPS-PUB-113], employing the context key and a
   zero IV. Padding procedures to accomodate plaintext data lengths
   which may not be integral multiples of 8 bytes are defined in [FIPS-
   PUB-113].  The result is an 8-byte value, which is stored in the
   SGN_CKSUM field.  Support for this algorithm may not be present in
   all implementations.

1.2.1.2. Sequence Number

   Sequence number field: The 8 byte plaintext sequence number field is
   formed from the sender's four-byte sequence number as follows.  If
   the four bytes of the sender's sequence number are named s0, s1, s2



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   and s3 (from least to most significant), the plaintext sequence
   number field is the 8 byte sequence: (s0, s1, s2, s3, di, di, di,
   di), where 'di' is the direction-indicator (Hex 0 - sender is the
   context initiator, Hex FF - sender is the context acceptor).  The
   field is then DES-CBC encrypted using the context key and an IV
   formed from the first 8 bytes of the previously calculated SGN_CKSUM
   field. After sending a GSS_GetMIC() or GSS_Wrap() token, the sender's
   sequence number is incremented by one.

   The receiver of the token will first verify the SGN_CKSUM field.  If
   valid, the sequence number field may be decrypted and compared to the
   expected sequence number.  The repetition of the (effectively 1-bit)
   direction indicator within the sequence number field provides
   redundancy so that the receiver may verify that the decryption
   succeeded.

   Since the checksum computation is used as an IV to the sequence
   number decryption, attempts to splice a checksum and sequence number
   from different messages will be detected.  The direction indicator
   will detect packets that have been maliciously reflected.

   The sequence number provides a basis for detection of replayed
   tokens.  Replay detection can be performed using state information
   retained on received sequence numbers, interpreted in conjunction
   with the security context on which they arrive.

   Provision of per-message replay and out-of-sequence detection
   services is optional for implementations of the Kerberos V5 GSS-API
   mechanism.  Further, it is recommended that implementations of the
   Kerberos V5 GSS-API mechanism which offer these services should honor
   a caller's request that the services be disabled on a context.
   Specifically, if replay_det_req_flag is input FALSE, replay_det_state
   should be returned FALSE and the GSS_DUPLICATE_TOKEN and
   GSS_OLD_TOKEN stati should not be indicated as a result of duplicate
   detection when tokens are processed; if sequence_req_flag is input
   FALSE, sequence_state should be returned FALSE and
   GSS_DUPLICATE_TOKEN, GSS_OLD_TOKEN, and GSS_UNSEQ_TOKEN stati should
   not be indicated as a result of out-of-sequence detection when tokens
   are processed.

1.2.2. Per-message Tokens - Wrap

   Use of the GSS_Wrap() call yields a token which encapsulates the
   input user data (optionally encrypted) along with associated
   integrity check quantities. The token emitted by GSS_Wrap() consists
   of an integrity header whose format is identical to that emitted by
   GSS_GetMIC() (except that the TOK_ID field contains the value 02 01),
   followed by a body portion that contains either the plaintext data



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   (if SEAL_ALG = ff ff) or encrypted data for any other supported value
   of SEAL_ALG.  Currently, only SEAL_ALG = 00 00 is supported, and
   means that DES-CBC encryption is being used to protect the data.

   The GSS_Wrap() token has the following format:

   Byte no          Name           Description
    0..1           TOK_ID          Identification field.
                                   Tokens emitted by GSS_Wrap() contain
                                   the hex value 02 01 in this field.
    2..3           SGN_ALG         Checksum algorithm indicator.
                                   00 00 - DES MAC MD5
                                   01 00 - MD2.5
                                   02 00 - DES MAC
    4..5           SEAL_ALG        ff ff - none
                                   00 00 - DES
    6..7           Filler          Contains ff ff
    8..15          SND_SEQ         Encrypted sequence number field.
    16..23         SGN_CKSUM       Checksum of plaintext padded data,
                                   calculated according to algorithm
                                   specified in SGN_ALG field.
    24..last       Data            encrypted or plaintext padded data

   GSS-API tokens must be encapsulated within the higher-level protocol
   by the application; no embedded length field is necessary.

1.2.2.1. Checksum

   Checksum calculation procedure (common to all algorithms): Checksums
   are calculated over the plaintext padded data field, logically
   prepended by the first 8 bytes of the plaintext packet header.  The
   resulting signature binds the data to the packet type, protocol
   version, and signature algorithm identifier fields.

   DES MAC MD5 algorithm: The checksum is formed by computing an MD5
   hash over the plaintext padded data, and then computing a DES-CBC MAC
   on the 16-byte MD5 result.  A standard 64-bit DES-CBC MAC is computed
   per [FIPS-PUB-113], employing the context key and a zero IV. The 8-
   byte result is stored in the SGN_CKSUM field.

   MD2.5 algorithm: The checksum is formed by first DES-CBC encrypting a
   16-byte zero-block, using a zero IV and a key formed by reversing the
   bytes of the context key (i.e., if the original key is the 8-byte
   sequence {aa, bb, cc, dd, ee, ff, gg, hh}, the checksum key will be
   {hh, gg, ff, ee, dd, cc, bb, aa}). The resulting 16-byte value is
   logically pre-pended to the "to-be-signed data".  A standard MD5
   checksum is calculated over the combined data, and the first 8 bytes
   of the result are stored in the SGN_CKSUM field.



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   DES-MAC algorithm: A standard 64-bit DES-CBC MAC is computed on the
   plaintext padded data per [FIPS-PUB-113], employing the context key
   and a zero IV. The plaintext padded data is already assured to be an
   integral multiple of 8 bytes; no additional padding is required or
   applied in order to accomplish MAC calculation.  The result is an 8-
   byte value, which is stored in the SGN_CKSUM field.  Support for this
   lgorithm may not be present in all implementations.

1.2.2.2. Sequence Number

   Sequence number field: The 8 byte plaintext sequence number field is
   formed from the sender's four-byte sequence number as follows.  If
   the four bytes of the sender's sequence number are named s0, s1, s2
   and s3 (from least to most significant), the plaintext sequence
   number field is the 8 byte sequence: (s0, s1, s2, s3, di, di, di,
   di), where 'di' is the direction-indicator (Hex 0 - sender is the
   context initiator, Hex FF - sender is the context acceptor).

   The field is then DES-CBC encrypted using the context key and an IV
   formed from the first 8 bytes of the SEAL_CKSUM field.

   After sending a GSS_GetMIC() or GSS_Wrap() token, the sender's
   sequence numbers are incremented by one.

1.2.2.3. Padding

   Data padding: Before encryption and/or signature calculation,
   plaintext data is padded to the next highest multiple of 8 bytes, by
   appending between 1 and 8 bytes, the value of each such byte being
   the total number of pad bytes.  For example, given data of length 20
   bytes, four pad bytes will be appended, and each byte will contain
   the hex value 04.  An 8-byte random confounder is prepended to the
   data, and signatures are calculated over the resulting padded
   plaintext.

   After padding, the data is encrypted according to the algorithm
   specified in the SEAL_ALG field.  For SEAL_ALG=DES (the only non-null
   algorithm currently supported), the data is encrypted using DES-CBC,
   with an IV of zero.  The key used is derived from the established
   context key by XOR-ing the context key with the hexadecimal constant
   f0f0f0f0f0f0f0f0.

1.2.3. Context deletion token

   The token emitted by GSS_Delete_sec_context() is based on the packet
   format for tokens emitted by GSS_GetMIC().  The context-deletion
   token has the following format:




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   Byte no          Name           Description
    0..1           TOK_ID          Identification field.
                                   Tokens emitted by
                                   GSS_Delete_sec_context() contain
                                   the hex value 01 02 in this field.
    2..3           SGN_ALG         Integrity algorithm indicator.
                                   00 00 - DES MAC MD5
                                   01 00 - MD2.5
                                   02 00 - DES MAC
    4..7           Filler          Contains ff ff ff ff
    8..15          SND_SEQ         Sequence number field.
    16..23         SGN_CKSUM       Checksum of "to-be-signed data",
                                   calculated according to algorithm
                                   specified in SGN_ALG field.

   SGN_ALG and SND_SEQ will be calculated as for tokens emitted by
   GSS_GetMIC().  The SGN_CKSUM will be calculated as for tokens emitted
   by GSS_GetMIC(), except that the user-data component of the "to-be-
   signed" data will be a zero-length string.

2. Name Types and Object Identifiers

   This section discusses the name types which may be passed as input to
   the Kerberos V5 GSS-API mechanism's GSS_Import_name() call, and their
   associated identifier values.  It defines interface elements in
   support of portability, and assumes use of C language bindings per
   RFC-1509.  In addition to specifying OID values for name type
   identifiers, symbolic names are included and recommended to GSS-API
   implementors in the interests of convenience to callers.  It is
   understood that not all implementations of the Kerberos V5 GSS-API
   mechanism need support all name types in this list, and that
   additional name forms will likely be added to this list over time.
   Further, the definitions of some or all name types may later migrate
   to other, mechanism-independent, specifications. The occurrence of a
   name type in this specification is specifically not intended to
   suggest that the type may be supported only by an implementation of
   the Kerberos V5 mechanism.   In particular, the occurrence of the
   string "_KRB5_" in the symbolic name strings constitutes a means to
   unambiguously register the name strings, avoiding collision with
   other documents; it is not meant to limit the name types' usage or
   applicability.

   For purposes of clarification to GSS-API implementors, this section's
   discussion of some name forms describes means through which those
   forms can be supported with existing Kerberos technology.  These
   discussions are not intended to preclude alternative implementation
   strategies for support of the name forms within Kerberos mechanisms
   or mechanisms based on other technologies.  To enhance application



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   portability, implementors of mechanisms are encouraged to support
   name forms as defined in this section, even if their mechanisms are
   independent of Kerberos V5.

2.1. Mandatory Name Forms

   This section discusses name forms which are to be supported by all
   conformant implementations of the Kerberos V5 GSS-API mechanism.

2.1.1. Kerberos Principal Name Form

   This name form shall be represented by the Object Identifier {iso(1)
   member-body(2) United States(840) mit(113554) infosys(1) gssapi(2)
   krb5(2) krb5_name(1)}.  The recommended symbolic name for this type
   is "GSS_KRB5_NT_PRINCIPAL_NAME".

   This name type corresponds to the single-string representation of a
   Kerberos name.  (Within the MIT Kerberos V5 implementation, such
   names are parseable with the krb5_parse_name() function.)  The
   elements included within this name representation are as follows,
   proceeding from the beginning of the string:

        (1) One or more principal name components; if more than one
        principal name component is included, the components are
        separated by `/`.  Arbitrary octets may be included within
        principal name components, with the following constraints and
        special considerations:

           (1a) Any occurrence of the characters `@` or `/` within a
           name component must be immediately preceded by the `\`
           quoting character, to prevent interpretation as a component
           or realm separator.

           (1b) The ASCII newline, tab, backspace, and null characters
           may occur directly within the component or may be
           represented, respectively, by `\n`, `\t`, `\b`, or `\0`.

           (1c) If the `\` quoting character occurs outside the contexts
           described in (1a) and (1b) above, the following character is
           interpreted literally.  As a special case, this allows the
           doubled representation `\\` to represent a single occurrence
           of the quoting character.

           (1d) An occurrence of the `\` quoting character as the last
           character of a component is illegal.






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        (2) Optionally, a `@` character, signifying that a realm name
        immediately follows. If no realm name element is included, the
        local realm name is assumed.  The `/` , `:`, and null characters
        may not occur within a realm name; the `@`, newline, tab, and
        backspace characters may be included using the quoting
        conventions described in (1a), (1b), and (1c) above.

2.1.2. Host-Based Service Name Form

   This name form has been incorporated at the mechanism-independent
   GSS-API level as of GSS-API, Version 2.  This subsection retains the
   Object Identifier and symbolic name assignments previously made at
   the Kerberos V5 GSS-API mechanism level, and adopts the definition as
   promoted to the mechanism-independent level.

   This name form shall be represented by the Object Identifier {iso(1)
   member-body(2) United States(840) mit(113554) infosys(1) gssapi(2)
   generic(1) service_name(4)}.  The previously recommended symbolic
   name for this type is "GSS_KRB5_NT_HOSTBASED_SERVICE_NAME".  The
   currently preferred symbolic name for this type is
   "GSS_C_NT_HOSTBASED_SERVICE".

   This name type is used to represent services associated with host
   computers.  This name form is constructed using two elements,
   "service" and "hostname", as follows:

      service@hostname

   When a reference to a name of this type is resolved, the "hostname"
   is canonicalized by attempting a DNS lookup and using the fully-
   qualified domain name which is returned, or by using the "hostname"
   as provided if the DNS lookup fails.  The canonicalization operation
   also maps the host's name into lower-case characters.

   The "hostname" element may be omitted. If no "@" separator is
   included, the entire name is interpreted as the service specifier,
   with the "hostname" defaulted to the canonicalized name of the local
   host.

   Values for the "service" element will be registered with the IANA.

2.1.3. Exported Name Object Form for Kerberos V5 Mechanism

   Support for this name form is not required for GSS-V1
   implementations, but will be required for use in conjunction with the
   GSS_Export_name() call planned for GSS-API Version 2.  Use of this
   name form will be signified by a "GSS-API Exported Name Object" OID
   value which will be defined at the mechanism-independent level for



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   GSS-API Version 2.

   This name type represents a self-describing object, whose framing
   structure will be defined at the mechanism-independent level for
   GSS-API Version 2.  When generated by the Kerberos V5 mechanism, the
   Mechanism OID within the exportable name shall be that of the
   Kerberos V5 mechanism.  The name component within the exportable name
   shall be a contiguous string with structure as defined for the
   Kerberos Principal Name Form.

   In order to achieve a distinguished encoding for comparison purposes,
   the following additional constraints are imposed on the export
   operation:

        (1) all occurrences of the characters `@`,  `/`, and `\` within
        principal components or realm names shall be quoted with an
        immediately-preceding `\`.

        (2) all occurrences of the null, backspace, tab, or newline
        characters within principal components or realm names will be
        represented, respectively, with `\0`, `\b`, `\t`, or `\n`.

        (3) the `\` quoting character shall not be emitted within an
        exported name except to accomodate cases (1) and (2).

2.2. Optional Name Forms

   This section discusses additional name forms which may optionally be
   supported by implementations of the Kerberos V5 GSS-API mechanism.
   It is recognized that some of the name forms cited here are derived
   from UNIX(tm) operating system platforms; some listed forms may be
   irrelevant to non-UNIX platforms, and definition of additional forms
   corresponding to such platforms may also be appropriate.  It is also
   recognized that OS-specific functions outside GSS-API are likely to
   exist in order to perform translations among these forms, and that
   GSS-API implementations supporting these forms may themselves be
   layered atop such OS-specific functions.  Inclusion of this support
   within GSS-API implementations is intended as a convenience to
   applications.

2.2.1. User Name Form

   This name form shall be represented by the Object Identifier {iso(1)
   member-body(2) United States(840) mit(113554) infosys(1) gssapi(2)
   generic(1) user_name(1)}.  The recommended symbolic name for this
   type is "GSS_KRB5_NT_USER_NAME".

   This name type is used to indicate a named user on a local system.



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   Its interpretation is OS-specific.  This name form is constructed as:

      username

   Assuming that users' principal names are the same as their local
   operating system names, an implementation of GSS_Import_name() based
   on Kerberos V5 technology can process names of this form by
   postfixing an "@" sign and the name of the local realm.

2.2.2. Machine UID Form

   This name form shall be represented by the Object Identifier {iso(1)
   member-body(2) United States(840) mit(113554) infosys(1) gssapi(2)
   generic(1) machine_uid_name(2)}.  The recommended symbolic name for
   this type is "GSS_KRB5_NT_MACHINE_UID_NAME".

   This name type is used to indicate a numeric user identifier
   corresponding to a user on a local system.  Its interpretation is
   OS-specific.  The gss_buffer_desc representing a name of this type
   should contain a locally-significant uid_t, represented in host byte
   order.  The GSS_Import_name() operation resolves this uid into a
   username, which is then treated as the User Name Form.

2.2.3. String UID Form

   This name form shall be represented by the Object Identifier {iso(1)
   member-body(2) United States(840) mit(113554) infosys(1) gssapi(2)
   generic(1) string_uid_name(3)}.  The recommended symbolic name for
   this type is "GSS_KRB5_NT_STRING_UID_NAME".

   This name type is used to indicate a string of digits representing
   the numeric user identifier of a user on a local system.  Its
   interpretation is OS-specific. This name type is similar to the
   Machine UID Form, except that the buffer contains a string
   representing the uid_t.

3. Credentials Management

   The Kerberos V5 protocol uses different credentials (in the GSSAPI
   sense) for initiating and accepting security contexts.  Normal
   clients receive a ticket-granting ticket (TGT) and an associated
   session key at "login" time; the pair of a TGT and its corresponding
   session key forms a credential which is suitable for initiating
   security contexts.  A ticket-granting ticket, its session key, and
   any other (ticket, key) pairs obtained through use of the ticket-
   granting-ticket, are typically stored in a Kerberos V5 credentials
   cache, sometimes known as a ticket file.




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   The encryption key used by the Kerberos server to seal tickets for a
   particular application service forms the credentials suitable for
   accepting security contexts.  These service keys are typically stored
   in a Kerberos V5 key table, or srvtab file.  In addition to their use
   as accepting credentials, these service keys may also be used to
   obtain initiating credentials for their service principal.

   The Kerberos V5 mechanism's credential handle may contain references
   to either or both types of credentials.  It is a local matter how the
   Kerberos V5 mechanism implementation finds the appropriate Kerberos
   V5 credentials cache or key table.

   However, when the Kerberos V5 mechanism attempts to obtain initiating
   credentials for a service principal which are not available in a
   credentials cache, and the key for that service principal is
   available in a Kerberos V5 key table, the mechanism should use the
   service key to obtain initiating credentials for that service.  This
   should be accomplished by requesting a ticket-granting-ticket from
   the Kerberos Key Distribution Center (KDC), and decrypting the KDC's
   reply using the service key.

4. 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-1509.

4.1. 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 implementors 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.



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4.1.1. Non-Kerberos-specific codes

   GSS_KRB5_S_G_BAD_SERVICE_NAME
           /* "No @ in SERVICE-NAME name string" */
   GSS_KRB5_S_G_BAD_STRING_UID
           /* "STRING-UID-NAME contains nondigits" */
   GSS_KRB5_S_G_NOUSER
           /* "UID does not resolve to username" */
   GSS_KRB5_S_G_VALIDATE_FAILED
           /* "Validation error" */
   GSS_KRB5_S_G_BUFFER_ALLOC
           /* "Couldn't allocate gss_buffer_t data" */
   GSS_KRB5_S_G_BAD_MSG_CTX
           /* "Message context invalid" */
   GSS_KRB5_S_G_WRONG_SIZE
           /* "Buffer is the wrong size" */
   GSS_KRB5_S_G_BAD_USAGE
           /* "Credential usage type is unknown" */
   GSS_KRB5_S_G_UNKNOWN_QOP
           /* "Unknown quality of protection specified" */

4.1.2. Kerberos-specific-codes

   GSS_KRB5_S_KG_CCACHE_NOMATCH
           /* "Principal in credential cache does not match desired name" */
   GSS_KRB5_S_KG_KEYTAB_NOMATCH
           /* "No principal in keytab matches desired name" */
   GSS_KRB5_S_KG_TGT_MISSING
           /* "Credential cache has no TGT" */
   GSS_KRB5_S_KG_NO_SUBKEY
           /* "Authenticator has no subkey" */
   GSS_KRB5_S_KG_CONTEXT_ESTABLISHED
           /* "Context is already fully established" */
   GSS_KRB5_S_KG_BAD_SIGN_TYPE
           /* "Unknown signature type in token" */
   GSS_KRB5_S_KG_BAD_LENGTH
           /* "Invalid field length in token" */
   GSS_KRB5_S_KG_CTX_INCOMPLETE
           /* "Attempt to use incomplete security context" */

4.2. Quality of Protection Values

   This section defines Quality of Protection (QOP) values to be used
   with the Kerberos V5 GSS-API mechanism as input to GSS_Wrap() and
   GSS_GetMIC() routines in order to select among alternate integrity
   and confidentiality algorithms. Additional QOP values may be added in
   future versions of this specification.  Non-overlapping bit positions
   are and will be employed in order that both integrity and



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   confidentiality QOP may be selected within a single parameter, via
   inclusive-OR of the specified integrity and confidentiality values.

4.2.1. Integrity Algorithms

   The following Quality of Protection (QOP) values are currently
   defined for the Kerberos V5 GSS-API mechanism, and are used to select
   among alternate integrity checking algorithms.

   GSS_KRB5_INTEG_C_QOP_MD5        (numeric value: 1)
           /* Integrity using partial MD5 ("MD2.5") of plaintext */

   GSS_KRB5_INTEG_C_QOP_DES_MD5    (numeric value: 2)
           /* Integrity using DES MAC of MD5 of plaintext */

   GSS_KRB5_INTEG_C_QOP_DES_MAC    (numeric value: 3)
           /* Integrity using DES MAC of plaintext */

4.2.2. Confidentiality Algorithms

   Only one confidentiality QOP value is currently defined for the
   Kerberos V5 GSS-API mechanism:

   GSS_KRB5_CONF_C_QOP_DES         (numeric value: 0)
           /* Confidentiality with DES */

   Note: confidentiality QOP should be indicated only by GSS-API calls
   capable of providing confidentiality services. If non-zero
   confidentiality QOP values are defined in future to represent
   different algorithms, therefore, the bit positions containing those
   values should be cleared before being returned by implementations of
   GSS_GetMIC() and GSS_VerifyMIC().

4.3. Buffer Sizes

   All implementations of this specification shall be capable of
   accepting buffers of at least 16 Kbytes as input to GSS_GetMIC(),
   GSS_VerifyMIC(), and GSS_Wrap(), and shall be capable of accepting
   the output_token generated by GSS_Wrap() for a 16 Kbyte input buffer
   as input to GSS_Unwrap(). Support for larger buffer sizes is optional
   but recommended.










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5. Security Considerations

   Security issues are discussed throughout this memo.

6. References


   [RFC-1321]: Rivest, R., "The MD5 Message-Digest Algorithm", RFC
   1321, April 1992.

   [RFC-1508]: Linn, J., "Generic Security Service Application Program
   Interface", RFC 1508, September 1993.

   [RFC-1509]: Wray, J., "Generic Security Service Application Program
   Interface: C-bindings", RFC 1509, September 1993.

   [RFC-1510]: Kohl, J., and C. Neuman, "The Kerberos Network
   Authentication Service (V5)", RFC 1510, September 1993.

   [FIPS-PUB-113]: National Bureau of Standards, Federal Information
   Processing Standard 113, "Computer Data Authentication", May 1985.

AUTHOR'S ADDRESS

   John Linn
   OpenVision Technologies
   One Main St.
   Cambridge, MA  02142  USA

   Phone: +1 617.374.2245
   EMail: John.Linn@ov.com




















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