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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.

Linn Standards Track [Page 1] RFC 1964 Kerberos Version 5 GSS-API June 1996

 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.

Linn Standards Track [Page 2] RFC 1964 Kerberos Version 5 GSS-API June 1996

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
 }
  1. - 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
 }
  1. - 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,

Linn Standards Track [Page 3] RFC 1964 Kerberos Version 5 GSS-API June 1996

         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.

Linn Standards Track [Page 4] RFC 1964 Kerberos Version 5 GSS-API June 1996

      (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

Linn Standards Track [Page 5] RFC 1964 Kerberos Version 5 GSS-API June 1996

 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.

Linn Standards Track [Page 6] RFC 1964 Kerberos Version 5 GSS-API June 1996

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.

Linn Standards Track [Page 7] RFC 1964 Kerberos Version 5 GSS-API June 1996

 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

Linn Standards Track [Page 8] RFC 1964 Kerberos Version 5 GSS-API June 1996

 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

Linn Standards Track [Page 9] RFC 1964 Kerberos Version 5 GSS-API June 1996

 (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.

Linn Standards Track [Page 10] RFC 1964 Kerberos Version 5 GSS-API June 1996

 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:

Linn Standards Track [Page 11] RFC 1964 Kerberos Version 5 GSS-API June 1996

 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

Linn Standards Track [Page 12] RFC 1964 Kerberos Version 5 GSS-API June 1996

 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.

Linn Standards Track [Page 13] RFC 1964 Kerberos Version 5 GSS-API June 1996

      (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

Linn Standards Track [Page 14] RFC 1964 Kerberos Version 5 GSS-API June 1996

 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.

Linn Standards Track [Page 15] RFC 1964 Kerberos Version 5 GSS-API June 1996

 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.

Linn Standards Track [Page 16] RFC 1964 Kerberos Version 5 GSS-API June 1996

 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.

Linn Standards Track [Page 17] RFC 1964 Kerberos Version 5 GSS-API June 1996

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

Linn Standards Track [Page 18] RFC 1964 Kerberos Version 5 GSS-API June 1996

 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.

Linn Standards Track [Page 19] RFC 1964 Kerberos Version 5 GSS-API June 1996

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

Linn Standards Track [Page 20]

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