--- /dev/null
+
+<Network Working Group> Larry Zhu
+Internet Draft Karthik Jaganathan
+Updates: 1964 Microsoft
+Category: Standards Track Sam Hartman
+draft-ietf-krb-wg-gssapi-cfx-07.txt MIT
+ March 9, 2004
+ Expires: September 9, 2004
+
+ The Kerberos Version 5 GSS-API Mechanism: Version 2
+
+Status of this Memo
+
+ This document is an Internet-Draft and is in full conformance with
+ all provisions of Section 10 of [RFC-2026].
+
+ Internet-Drafts are working documents of the Internet Engineering
+ Task Force (IETF), its areas, and its working groups. Note that
+ other groups may also distribute working documents as Internet-
+ Drafts. Internet-Drafts are draft documents valid for a maximum of
+ six months and may be updated, replaced, or obsoleted by other
+ documents at any time. It is inappropriate to use Internet-Drafts
+ as reference material or to cite them other than as "work in
+ progress."
+
+ The list of current Internet-Drafts can be accessed at
+ http://www.ietf.org/ietf/1id-abstracts.txt.
+
+ The list of Internet-Draft Shadow Directories can be accessed at
+ http://www.ietf.org/shadow.html.
+
+ To learn the current status of any Internet-Draft, please check the
+ "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
+ Directories on ftp.ietf.org (US East Coast), nic.nordu.net (Europe),
+ ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).
+
+ The distribution of this memo is unlimited. It is filed as
+ draft-ietf-krb-wg-gssapi-cfx-07.txt, and expires on September 9
+ 2004. Please send comments to: ietf-krb-wg@anl.gov.
+
+Abstract
+
+ 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
+ profiles.
+
+Conventions used in this document
+
+Zhu 1 \f
+DRAFT Kerberos Version 5 GSS-API Expires September 2004
+
+
+ The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
+ "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
+ document are to be interpreted as described in [RFC-2119].
+
+ The term "little endian order" is used for brevity to refer to the
+ least-significant-octet-first encoding, while the term "big endian
+ order" is for the most-significant-octet-first encoding.
+
+Table of Contents
+
+ 1. Introduction ............................................... 2
+ 2. Key Derivation for Per-Message Tokens ...................... 3
+ 3. Quality of Protection ...................................... 4
+ 4. Definitions and Token Formats .............................. 4
+ 4.1. Context Establishment Tokens ............................. 4
+ 4.1.1. Authenticator Checksum ................................. 5
+ 4.2. Per-Message Tokens ....................................... 8
+ 4.2.1. Sequence Number ........................................ 8
+ 4.2.2. Flags Field ............................................ 8
+ 4.2.3. EC Field ............................................... 9
+ 4.2.4. Encryption and Checksum Operations ..................... 9
+ 4.2.5. RRC Field .............................................. 10
+ 4.2.6. Message Layouts ........................................ 10
+ 4.3. Context Deletion Tokens .................................. 11
+ 4.4. Token Identifier Assignment Considerations ............... 11
+ 5. Parameter Definitions ...................................... 12
+ 5.1. Minor Status Codes ....................................... 12
+ 5.1.1. Non-Kerberos-specific codes ............................ 12
+ 5.1.2. Kerberos-specific-codes ................................ 12
+ 5.2. Buffer Sizes ............................................. 13
+ 6. Backwards Compatibility Considerations ..................... 13
+ 7. Security Considerations .................................... 13
+ 8. Acknowledgments ............................................ 14
+ 9. Intellectual Property Statement ............................ 15
+ 10. References ................................................ 15
+ 10.1. Normative References .................................... 15
+ 10.2. Informative References .................................. 15
+ 11. Author's Address .......................................... 15
+ Full Copyright Statement ...................................... 17
+
+1. Introduction
+
+ [KCRYPTO] defines a generic framework for describing encryption and
+ checksum types to be used with the Kerberos protocol and associated
+ protocols.
+
+ [RFC-1964] describes the GSS-API mechanism for Kerberos Version 5.
+ It defines the format of context establishment, per-message and
+ context deletion tokens and uses algorithm identifiers for each
+ cryptosystem in per message and context deletion tokens.
+
+ The approach taken in this document obviates the need for algorithm
+ identifiers. This is accomplished by using the same encryption
+ algorithm, specified by the crypto profile [KCRYPTO] for the session
+ key or subkey that is created during context negotiation, and its
+ required checksum algorithm. Message layouts of the per-message
+Zhu 2 \f
+DRAFT Kerberos Version 5 GSS-API Expires September 2004
+
+ tokens are therefore revised to remove algorithm indicators and also
+ to add extra information to support the generic crypto framework
+ [KCRYPTO].
+
+ Tokens transferred between GSS-API peers for security context
+ establishment are also described in this document. The data
+ elements exchanged between a GSS-API endpoint implementation and the
+ Kerberos Key Distribution Center (KDC) [KRBCLAR] are not specific to
+ GSS-API usage and are therefore defined within [KRBCLAR] rather than
+ within this specification.
+
+ The new token formats specified in this document MUST be used with
+ all "newer" encryption types [KRBCLAR] and MAY be used with "older"
+ encryption types, provided that the initiator and acceptor know,
+ from the context establishment, that they can both process these new
+ token formats.
+
+ "Newer" encryption types are those which have been specified along
+ with or since the new Kerberos cryptosystem specification [KCRYPTO],
+ as defined in section 3.1.3 of [KRBCLAR]. The list of not-newer
+ encryption types is as follows [KCRYPTO]:
+
+ Encryption Type Assigned Number
+ ----------------------------------------------
+ des-cbc-crc 1
+ des-cbc-md4 2
+ des-cbc-md5 3
+ des3-cbc-md5 5
+ des3-cbc-sha1 7
+ dsaWithSHA1-CmsOID 9
+ md5WithRSAEncryption-CmsOID 10
+ sha1WithRSAEncryption-CmsOID 11
+ rc2CBC-EnvOID 12
+ rsaEncryption-EnvOID 13
+ rsaES-OAEP-ENV-OID 14
+ des-ede3-cbc-Env-OID 15
+ des3-cbc-sha1-kd 16
+ rc4-hmac 23
+
+2. Key Derivation for Per-Message Tokens
+
+ To limit the exposure of a given key, [KCRYPTO] adopted "one-way"
+ "entropy-preserving" derived keys, for different purposes or key
+ usages, from a base key or protocol key.
+
+ This document defines four key usage values below that are used to
+ derive a specific key for signing and sealing messages, from the
+ session key or subkey [KRBCLAR] created during the context
+ establishment.
+
+ Name Value
+ -------------------------------------
+ KG-USAGE-ACCEPTOR-SEAL 22
+ KG-USAGE-ACCEPTOR-SIGN 23
+ KG-USAGE-INITIATOR-SEAL 24
+
+Zhu 3 \f
+DRAFT Kerberos Version 5 GSS-API Expires September 2004
+
+ 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 4.2.6.1), and
+ KG-USAGE-ACCEPTOR-SEAL is used for Wrap tokens(as defined in section
+ 4.2.6.2); similarly when the sender is the context initiator, KG-
+ USAGE-INITIATOR-SIGN is used as the usage number in the key
+ derivation function for MIC tokens, KG-USAGE-INITIATOR-SEAL is used
+ for Wrap Tokens. Even if the Wrap token does not provide for
+ confidentiality the same usage values specified above are used.
+
+ During the context initiation and acceptance sequence, the acceptor
+ MAY assert a subkey, and if so, subsequent messages MUST use this
+ subkey as the protocol key and these messages MUST be flagged as
+ "AcceptorSubkey" as described in section 4.2.2.
+
+3. Quality of Protection
+
+ The GSS-API specification [RFC-2743] provides for Quality of
+ Protection (QOP) values that can be used by applications to request
+ a certain type of encryption or signing. A zero QOP value is used
+ to indicate the "default" protection; applications which do not use
+ the default QOP are not guaranteed to be portable across
+ implementations or even inter-operate with different deployment
+ configurations of the same implementation. Using an algorithm that
+ is different from the one for which the key is defined may not be
+ appropriate. Therefore, when the new method in this document is
+ used, the QOP value is ignored.
+
+ The encryption and checksum algorithms in per-message tokens are now
+ implicitly defined by the algorithms associated with the session key
+ or subkey. Algorithms identifiers as described in [RFC-1964] are
+ therefore no longer needed and removed from the new token headers.
+
+4. Definitions and Token Formats
+
+ This section provides terms and definitions, as well as descriptions
+ for tokens specific to the Kerberos Version 5 GSS-API mechanism.
+
+4.1. Context Establishment Tokens
+
+ All context establishment tokens emitted by the Kerberos Version 5
+ GSS-API mechanism SHALL have the framing described in section 3.1 of
+ [RFC-2743], as illustrated by the following pseudo-ASN.1 structures:
+
+ GSS-API DEFINITIONS ::=
+
+ BEGIN
+
+ MechType ::= OBJECT IDENTIFIER
+ -- representing Kerberos V5 mechanism
+
+ GSSAPI-Token ::=
+ -- option indication (delegation, etc.) indicated within
+Zhu 4 \f
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+
+ -- mechanism-specific token
+ [APPLICATION 0] IMPLICIT SEQUENCE {
+ thisMech MechType,
+ innerToken ANY DEFINED BY thisMech
+ -- contents mechanism-specific
+ -- ASN.1 structure not required
+ }
+
+ END
+
+ Where the innerToken field starts with a two-octet token-identifier
+ (TOK_ID) expressed in big endian order, followed by a Kerberos
+ message.
+
+ Here are the TOK_ID values used in the context establishment tokens:
+
+ Token TOK_ID Value in Hex
+ -----------------------------------------
+ KRB_AP_REQ 01 00
+ KRB_AP_REP 02 00
+ KRB_ERROR 03 00
+
+ Where Kerberos message KRB_AP_REQUEST, KRB_AP_REPLY, and KRB_ERROR
+ are defined in [KRBCLAR].
+
+ If an unknown token identifier (TOK_ID) is received in the initial
+ context establishment token, the receiver MUST return
+ GSS_S_CONTINUE_NEEDED major status, and the returned output token
+ MUST contain a KRB_ERROR message with the error code
+ KRB_AP_ERR_MSG_TYPE [KRBCLAR].
+
+4.1.1. Authenticator Checksum
+
+ The authenticator in the KRB_AP_REQ message MUST include the
+ optional sequence number and the checksum field. The checksum field
+ is used to convey service flags, channel bindings, and optional
+ delegation information.
+
+ The checksum type MUST be 0x8003. When delegation is used, a ticket-
+ granting ticket will be transferred in a KRB_CRED message. This
+ ticket SHOULD have its forwardable flag set. The EncryptedData
+ field of the KRB_CRED message [KRBCLAR] MUST be encrypted in the
+ session key of the ticket used to authenticate the context.
+
+ The authenticator checksum field SHALL have the following format:
+
+ Octet Name Description
+ -----------------------------------------------------------------
+ 0..3 Lgth Number of octets in Bnd field; Represented
+ in little-endian order; Currently contains
+ hex value 10 00 00 00 (16).
+ 4..19 Bnd Channel binding information, as described in
+ section 4.1.1.2.
+ 20..23 Flags Four-octet context-establishment flags in
+ little-endian order as described in section
+Zhu 5 \f
+DRAFT Kerberos Version 5 GSS-API Expires September 2004
+
+ 4.1.1.1.
+ 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 4.1.1.1.
+ 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)
+ [optional].
+ 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 4.1.1.1), and
+ at least 28 octets plus Dlgth octets when GSS_C_DELEG_FLAG is set.
+ When GSS_C_DELEG_FLAG is set, the DlgOpt, Dlgth and Deleg fields
+ of the checksum data MUST immediately follow the Flags field. The
+ optional trailing octets (namely the "Exts" field) facilitate
+ future extensions to this mechanism. When delegation is not used
+ but the Exts field is present, the Exts field starts at octet 24
+ (DlgOpt, Dlgth and Deleg are absent).
+
+ Initiators that do not support the extensions MUST NOT include more
+ than 24 octets in the checksum field, when GSS_C_DELEG_FLAG is not
+ set, or more than 28 octets plus the KRB_CRED in the Deleg field,
+ when GSS_C_DELEG_FLAG is set. Acceptors that do not understand the
+ extensions MUST ignore any octets past the Deleg field of the
+ checksum data, when GSS_C_DELEG_FLAG is set, or past the Flags field
+ of the checksum data, when GSS_C_DELEG_FLAG is not set.
+
+4.1.1.1. Checksum Flags Field
+
+ The checksum "Flags" field is used to convey service options or
+ extension negotiation information.
+
+ The following context establishment flags are defined in [RFC-2744].
+
+ Flag Name Value
+ ---------------------------------
+ GSS_C_DELEG_FLAG 1
+ GSS_C_MUTUAL_FLAG 2
+ GSS_C_REPLAY_FLAG 4
+ GSS_C_SEQUENCE_FLAG 8
+ GSS_C_CONF_FLAG 16
+ GSS_C_INTEG_FLAG 32
+
+ Context establishment flags are exposed to the calling application.
+ If the calling application desires a particular service option then
+ it requests that option via GSS_Init_sec_context() [RFC-2743]. If
+ the corresponding return state values [RFC-2743] indicate that any
+ of above optional context level services will be active on the
+ context, the corresponding flag values in the table above MUST be
+ set in the checksum Flags field.
+
+
+Zhu 6 \f
+DRAFT Kerberos Version 5 GSS-API Expires September 2004
+
+ Flag values 4096..524288 (2^12, 2^13, ..., 2^19) are reserved for
+ use with legacy vendor-specific extensions to this mechanism.
+
+ All other flag values not specified herein are reserved for future
+ use. Future revisions of this mechanism may use these reserved
+ flags and may rely on implementations of this version to not use
+ such flags in order to properly negotiate mechanism versions.
+ Undefined flag values MUST be cleared by the sender, and unknown
+ flags MUST be ignored by the receiver.
+
+4.1.1.2. Channel Binding Information
+
+ These tags are intended to be used to identify the particular
+ communications channel for which the GSS-API security context
+ establishment tokens are intended, thus limiting the scope within
+ which an intercepted context establishment token can be reused by an
+ attacker (see [RFC-2743], section 1.1.6).
+
+ When using C language bindings, channel bindings are communicated
+ to the GSS-API using the following structure [RFC-2744]:
+
+ typedef struct gss_channel_bindings_struct {
+ OM_uint32 initiator_addrtype;
+ gss_buffer_desc initiator_address;
+ OM_uint32 acceptor_addrtype;
+ gss_buffer_desc acceptor_address;
+ gss_buffer_desc application_data;
+ } *gss_channel_bindings_t;
+
+ The member fields and constants used for different address types
+ are defined in [RFC-2744].
+
+ The "Bnd" field contains the MD5 hash of channel bindings, taken
+ over all non-null components of bindings, in order of declaration.
+ Integer fields within channel bindings are represented in little-
+ endian order for the purposes of the MD5 calculation.
+
+ In computing the contents of the Bnd field, the following detailed
+ points apply:
+
+ (1) For purposes of MD5 hash computation, each integer field and
+ input length field SHALL be formatted into four octets, using
+ little endian octet ordering.
+
+ (2) All input length fields within gss_buffer_desc elements of a
+ gss_channel_bindings_struct even those which are zero-valued, SHALL
+ be included in the hash calculation; the value elements of
+ gss_buffer_desc elements SHALL be dereferenced, and the resulting
+ data SHALL be included within the hash computation, only for the
+ case of gss_buffer_desc elements having non-zero length specifiers.
+
+ (3) If the caller passes the value GSS_C_NO_BINDINGS instead of a
+ valid channel binding structure, the Bnd field SHALL be set to 16
+ zero-valued octets.
+
+Zhu 7 \f
+DRAFT Kerberos Version 5 GSS-API Expires September 2004
+
+ If the caller to GSS_Accept_sec_context [RFC-2743] passes in
+ GSS_C_NO_CHANNEL_BINDINGS [RFC-2744] as the channel bindings then
+ the acceptor MAY ignore any channel bindings supplied by the
+ initiator, returning success even if the initiator did pass in
+ channel bindings.
+
+ If the application supply, in the channel bindings, a buffer with a
+ length field larger than 4294967295 (2^32 - 1), the implementation
+ of this mechanism MAY chose to reject the channel bindings
+ altogether, using major status GSS_S_BAD_BINDINGS [RFC-2743]. In
+ any case, the size of channel binding data buffers that can be used
+ (interoperable, without extensions) with this specification is
+ limited to 4294967295 octets.
+
+4.2. Per-Message Tokens
+
+ Two classes of tokens are defined in this section: "MIC" tokens,
+ emitted by calls to GSS_GetMIC() and consumed by calls to
+ GSS_VerifyMIC(), "Wrap" tokens, emitted by calls to GSS_Wrap() and
+ consumed by calls to GSS_Unwrap().
+
+ The new per-message tokens introduced here do not include the
+ generic GSS-API token framing used by the context establishment
+ tokens. These new tokens are designed to be used with newer crypto
+ systems that can, for example, have variable-size checksums.
+
+4.2.1. Sequence Number
+
+ To distinguish intentionally-repeated messages from maliciously-
+ replayed ones, per-message tokens contain a sequence number field,
+ which is a 64 bit integer expressed in big endian order. After
+ sending a GSS_GetMIC() or GSS_Wrap() token, the sender's sequence
+ numbers SHALL be incremented by one.
+
+4.2.2. Flags Field
+
+ The "Flags" field is a one-octet integer used to indicate a set of
+ attributes for the protected message. For example, one flag is
+ allocated as the direction-indicator, thus preventing an adversary
+ from sending back the same message in the reverse direction and
+ having it accepted.
+
+ The meanings of bits in this field (the least significant bit is
+ bit 0) are as follows:
+
+ Bit Name Description
+ ---------------------------------------------------------------
+ 0 SentByAcceptor When set, this flag indicates the sender
+ is the context acceptor. When not set,
+ it indicates the sender is the context
+ initiator.
+ 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
+Zhu 8 \f
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+
+ is used to protect the message.
+
+ The rest of available bits are reserved for future use and MUST be
+ cleared. The receiver MUST ignore unknown flags.
+
+4.2.3. EC Field
+
+ The "EC" (Extra Count) field is a two-octet integer field expressed
+ in big endian order.
+
+ In Wrap tokens with confidentiality, the EC field SHALL be used to
+ encode the number of octets in the filler, as described in section
+ 4.2.4.
+
+ In Wrap tokens without confidentiality, the EC field SHALL be used
+ to encode the number of octets in the trailing checksum, as
+ described in section 4.2.4.
+
+4.2.4. Encryption and Checksum Operations
+
+ The encryption algorithms defined by the crypto profiles provide for
+ integrity protection [KCRYPTO]. Therefore no separate checksum is
+ needed.
+
+ The result of decryption can be longer than the original plaintext
+ [KCRYPTO] 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 so that, when
+ padding the to-be-encrypted text to that size, there will be no
+ crypto-system residue added [KCRYPTO].
+
+ In Wrap tokens that provide for confidentiality, the first 16 octets
+ of the Wrap token (the "header", as defined in section 4.2.6), SHALL
+ be appended to the plaintext data before encryption. Filler octets
+ MAY be inserted between the plaintext data and the "header", and the
+ values and size of the filler octets are chosen by implementations,
+ such that there SHALL be no crypto-system 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 [KCRYPTO], and the RRC field (as
+ defined in section 4.2.5) in the to-be-encrypted header contain the
+ hex value 00 00.
+
+ In Wrap tokens that do not provide for confidentiality, the checksum
+ SHALL be calculated first over the to-be-signed plaintext data, and
+ then the first 16 octets of the Wrap token (the "header", as defined
+ in section 4.2.6). Both the EC field and the RRC field in the token
+ header SHALL be filled with zeroes for the purpose of calculating
+ the checksum. The resulting Wrap token is {"header" | plaintext-
+ data | get_mic(plaintext-data | "header")}, where get_mic() is the
+ checksum operation for the required checksum mechanism of the chosen
+ encryption mechanism defined in the crypto profile [KCRYPTO].
+
+
+Zhu 9 \f
+DRAFT Kerberos Version 5 GSS-API Expires September 2004
+
+ The parameters for the key and the cipher-state in the encrypt() and
+ get_mic() operations have been omitted for brevity.
+
+ For MIC tokens, the checksum SHALL be calculated as follows: the
+ checksum operation is calculated first over the to-be-signed
+ plaintext data, and then the first 16 octets of the MIC token, where
+ the checksum mechanism is the required checksum mechanism of the
+ chosen encryption mechanism defined in the crypto profile [KCRYPTO].
+
+ The resulting Wrap and MIC tokens bind the data to the token header,
+ including the sequence number and the direction indicator.
+
+4.2.5. RRC Field
+
+ The "RRC" (Right Rotation Count) field in Wrap tokens is added to
+ allow the data to be encrypted in-place by existing SSPI (Security
+ Service Provider Interface) [SSPI] applications that do not provide
+ an additional buffer for the trailer (the cipher text after the in-
+ place-encrypted data) in addition to the buffer for the header (the
+ cipher text before the in-place-encrypted data). The resulting Wrap
+ token in the previous section, excluding the first 16 octets of the
+ token header, is rotated to the right by "RRC" octets. The net
+ result is that "RRC" octets of trailing octets are moved toward the
+ header. Consider the following as an example of this rotation
+ operation: Assume that the RRC value is 3 and the token before the
+ rotation is {"header" | aa | bb | cc | dd | ee | ff | gg | hh}, the
+ token after rotation would be {"header" | ff | gg | hh | aa | bb |
+ cc | dd | ee }, where {aa | bb | cc |...| hh} is used to indicate
+ the octet sequence.
+
+ The RRC field is expressed as a two-octet integer in big endian
+ order.
+
+ The rotation count value is chosen by the sender based on
+ implementation details, and the receiver MUST be able to interpret
+ all possible rotation count values, including rotation counts
+ greater than the length of the token.
+
+4.2.6. Message Layouts
+
+ Per-message tokens start with a two-octet token identifier (TOK_ID)
+ field, expressed in big endian order. These tokens are defined
+ separately in subsequent sub-sections.
+
+4.2.6.1. 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
+Zhu 10 \f
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+
+ expressed in big endian order in this field.
+ 2 Flags Attributes field, as described in section
+ 4.2.2.
+ 3..7 Filler Contains five octets of hex value FF.
+ 8..15 SND_SEQ Sequence number field in clear text,
+ expressed in big endian order.
+ 16..last SGN_CKSUM Checksum of the "to-be-signed" data and
+ octet 0..15, as described in section 4.2.4.
+
+ The Filler field is included in the checksum calculation for
+ simplicity.
+
+4.2.6.2. Wrap Tokens
+
+ Use of the GSS_Wrap() call yields a token (referred as the Wrap
+ token in this document), which consists of a descriptive header,
+ followed by a body portion that contains either the input user data
+ in plaintext concatenated with the checksum, or the input user data
+ encrypted. The GSS_Wrap() token SHALL have the following format:
+
+ Octet no Name Description
+ ---------------------------------------------------------------
+ 0..1 TOK_ID Identification field. Tokens emitted by
+ GSS_Wrap() contain the the hex value 05 04
+ expressed in big endian order in this field.
+ 2 Flags Attributes field, as described in section
+ 4.2.2.
+ 3 Filler Contains the hex value FF.
+ 4..5 EC Contains the "extra count" field, in big
+ endian order as described in section 4.2.3.
+ 6..7 RRC Contains the "right rotation count" in big
+ endian order, as described in section 4.2.5.
+ 8..15 SND_SEQ Sequence number field in clear text,
+ expressed in big endian order.
+ 16..last Data Encrypted data for Wrap tokens with
+ confidentiality, or plaintext data followed
+ by the checksum for Wrap tokens without
+ confidentiality, as described in section
+ 4.2.4.
+
+4.3. Context Deletion Tokens
+
+ Context deletion tokens are empty in this mechanism. Both peers to
+ a security context invoke GSS_Delete_sec_context() [RFC-2743]
+ independently, passing a null output_context_token buffer to
+ indicate that no context_token is required. Implementations of
+ GSS_Delete_sec_context() should delete relevant locally-stored
+ context information.
+
+4.4. Token Identifier Assignment Considerations
+
+ Token identifiers (TOK_ID) from 0x60 0x00 through 0x60 0xFF
+ inclusive are reserved and SHALL NOT be assigned. Thus by examining
+ the first two octets of a token, one can tell unambiguously if it is
+ wrapped with the generic GSS-API token framing.
+Zhu 11 \f
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+
+
+5. Parameter Definitions
+
+ This section defines parameter values used by the Kerberos V5 GSS-
+ API mechanism. It defines interface elements in support of
+ portability, and assumes use of C language bindings per [RFC-2744].
+
+5.1. Minor Status Codes
+
+ This section recommends common symbolic names for minor_status
+ values to be returned by the Kerberos V5 GSS-API mechanism. Use of
+ these definitions will enable independent implementers to enhance
+ application portability across different implementations of the
+ mechanism defined in this specification. (In all cases,
+ implementations of GSS_Display_status() will enable callers to
+ convert minor_status indicators to text representations.) Each
+ implementation should make available, through include files or other
+ means, a facility to translate these symbolic names into the
+ concrete values which a particular GSS-API implementation uses to
+ represent the minor_status values specified in this section.
+
+ It is recognized that this list may grow over time, and that the
+ need for additional minor_status codes specific to particular
+ implementations may arise. It is recommended, however, that
+ implementations should return a minor_status value as defined on a
+ mechanism-wide basis within this section when that code is
+ accurately representative of reportable status rather than using a
+ separate, implementation-defined code.
+
+5.1.1. Non-Kerberos-specific codes
+
+ 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" */
+
+5.1.2. Kerberos-specific-codes
+
+ GSS_KRB5_S_KG_CCACHE_NOMATCH
+ /* "Client principal in credentials does not match
+ specified name" */
+Zhu 12 \f
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+
+ GSS_KRB5_S_KG_KEYTAB_NOMATCH
+ /* "No key available for specified service principal" */
+ GSS_KRB5_S_KG_TGT_MISSING
+ /* "No Kerberos ticket-granting ticket available" */
+ 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" */
+
+5.2. Buffer Sizes
+
+ All implementations of this specification MUST be capable of
+ accepting buffers of at least 16K octets as input to GSS_GetMIC(),
+ GSS_VerifyMIC(), and GSS_Wrap(), and MUST be capable of accepting
+ the output_token generated by GSS_Wrap() for a 16K octet input
+ buffer as input to GSS_Unwrap(). Implementations SHOULD support 64K
+ octet input buffers, and MAY support even larger input buffer sizes.
+
+6. Backwards Compatibility Considerations
+
+ The new token formats defined in this document will only be
+ recognized by new implementations. To address this, implementations
+ can always use the explicit sign or seal algorithm in [RFC-1964]
+ when the key type corresponds to "older" enctypes. An alternative
+ approach might be to retry sending the message with the sign or seal
+ algorithm explicitly defined as in [RFC-1964]. However this would
+ require either the use of a mechanism such as [RFC-2478] to securely
+ negotiate the method or the use out of band mechanism to choose
+ appropriate mechanism. For this reason, it is RECOMMENDED that the
+ new token formats defined in this document SHOULD be used only if
+ both peers are known to support the new mechanism during context
+ negotiation because of, for example, the use of "new" enctypes.
+
+ GSS_Unwrap() or GSS_VerifyMIC() can process a message token as
+ follows: it can look at the first octet of the token header, if it
+ is 0x60 then the token must carry the generic GSS-API pseudo ASN.1
+ framing, otherwise the first two octets of the token contain the
+ TOK_ID that uniquely identify the token message format.
+
+7. Security Considerations
+
+ Channel bindings are validated by the acceptor. The acceptor can
+ ignore the channel bindings restriction supplied by the initiator
+ and carried in the authenticator checksum, if channel bindings are
+ not used by GSS_Accept_sec_context [RFC-2743], and the acceptor does
+ not prove to the initiator that it has the same channel bindings as
+ the initiator, even if the client requested mutual authentication.
+ This limitation should be taken into consideration by designers of
+ applications that would use channel bindings, whether to limit the
+Zhu 13 \f
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+
+ use of GSS-API contexts to nodes with specific network addresses, to
+ authenticate other established, secure channels using Kerberos
+ Version 5, or for any other purpose.
+
+ Session key types are selected by the KDC. Under the current
+ mechanism, no negotiation of algorithm types occurs, so server-side
+ (acceptor) implementations cannot request that clients not use
+ algorithm types not understood by the server. However,
+ administrators can control what enctypes can be used for session
+ keys for this mechanism by controlling the set of the ticket session
+ key enctypes which the KDC is willing to use in tickets for a given
+ acceptor principal. The KDC could therefore be given the task of
+ limiting session keys for a given service to types actually
+ supported by the Kerberos and GSSAPI software on the server. This
+ does have a drawback for cases where a service principal name is
+ used both for GSSAPI-based and non-GSSAPI-based communication (most
+ notably the "host" service key), if the GSSAPI implementation does
+ not understand (for example) AES [AES-KRB5] but the Kerberos
+ implementation does. It means that AES session keys cannot be
+ issued for that service principal, which keeps the protection of
+ non-GSSAPI services weaker than necessary. KDC administrators
+ desiring to limit the session key types to support interoperability
+ with such GSSAPI implementations should carefully weigh the
+ reduction in protection offered by such mechanisms against the
+ benefits of interoperability.
+
+8. Acknowledgments
+
+ Ken Raeburn and Nicolas Williams corrected many of our errors in the
+ use of generic profiles and were instrumental in the creation of
+ this document.
+
+ The text for security considerations was contributed by Nicolas
+ Williams and Ken Raeburn.
+
+ Sam Hartman and Ken Raeburn suggested the "floating trailer" idea,
+ namely the encoding of the RRC field.
+
+ Sam Hartman and Nicolas Williams recommended the replacing our
+ earlier key derivation function for directional keys with different
+ key usage numbers for each direction as well as retaining the
+ directional bit for maximum compatibility.
+
+ Paul Leach provided numerous suggestions and comments.
+
+ Scott Field, Richard Ward, Dan Simon, Kevin Damour, and Simon
+ Josefsson also provided valuable inputs on this document.
+
+ Jeffrey Hutzelman provided comments and clarifications for the text
+ related to the channel bindings.
+
+ Jeffrey Hutzelman and Russ Housley suggested many editorial changes.
+
+
+
+Zhu 14 \f
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+
+ Luke Howard provided implementations of this document for the
+ Heimdal code base, and helped inter-operability testing with the
+ Microsoft code base, together with Love Hornquist Astrand. These
+ experiments formed the basis of this document.
+
+ Martin Rex provided suggestions of TOK_ID assignment recommendations
+ thus the token tagging in this document is unambiguous if the token
+ is wrapped with the pseudo ASN.1 header.
+
+ John Linn wrote the original Kerberos Version 5 mechanism
+ specification [RFC-1964], of which some of the text has been retained
+ in this document.
+
+9. Intellectual Property Statement
+
+ The IETF takes no position regarding the validity or scope of any
+ intellectual property or other rights that might be claimed to
+ pertain to the implementation or use of the technology described in
+ this document or the extent to which any license under such rights
+ might or might not be available; neither does it represent that it
+ has made any effort to identify any such rights. Information on the
+ IETF's procedures with respect to rights in standards-track and
+ standards-related documentation can be found in BCP-11. Copies of
+ claims of rights made available for publication and any assurances
+ of licenses to be made available, or the result of an attempt made
+ to obtain a general license or permission for the use of such
+ proprietary rights by implementers or users of this specification
+ can be obtained from the IETF Secretariat.
+
+ The IETF invites any interested party to bring to its attention any
+ copyrights, patents or patent applications, or other proprietary
+ rights which may cover technology that may be required to practice
+ this standard. Please address the information to the IETF Executive
+ Director.
+
+10. References
+
+10.1. Normative References
+
+ [RFC-2026] Bradner, S., "The Internet Standards Process -- Revision
+ 3", BCP 9, RFC 2026, October 1996.
+
+ [RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate
+ Requirement Levels", BCP 14, RFC 2119, March 1997.
+
+ [RFC-2743] Linn, J., "Generic Security Service Application Program
+ Interface Version 2, Update 1", RFC 2743, January 2000.
+
+ [RFC-2744] Wray, J., "Generic Security Service API Version 2: C-
+ bindings", RFC 2744, January 2000.
+
+ [RFC-1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
+ RFC 1964, June 1996.
+
+Zhu 15 \f
+DRAFT Kerberos Version 5 GSS-API Expires September 2004
+
+ [KCRYPTO] RFC-Editor: To be replaced by RFC number for draft-ietf-
+ krb-wg-crypto. Work in Progress.
+
+ [KRBCLAR] RFC-Editor: To be replaced by RFC number for draft-ietf-
+ krb-wg-kerberos-clarifications. Work in Progress.
+
+10.2. Informative References
+
+ [SSPI] Leach, P., "Security Service Provider Interface", Microsoft
+ Developer Network (MSDN), April 2003.
+
+ [AES-KRB5] RFC-Editor: To be replaced by RFC number for draft-
+ raeburn-krb-rijndael-krb. Work in Progress.
+
+ [RFC-2478] Baize, E., Pinkas D., "The Simple and Protected GSS-API
+ Negotiation Mechanism", RFC 2478, December 1998.
+
+11. Author's Address
+
+ Larry Zhu
+ One Microsoft Way
+ Redmond, WA 98052 - USA
+ EMail: LZhu@microsoft.com
+
+ Karthik Jaganathan
+ One Microsoft Way
+ Redmond, WA 98052 - USA
+ EMail: karthikj@microsoft.com
+
+ Sam Hartman
+ Massachusetts Institute of Technology
+ 77 Massachusetts Avenue
+ Cambridge, MA 02139 - USA
+ Email: hartmans@MIT.EDU
+
+
+
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+
+Full Copyright Statement
+
+ Copyright (C) The Internet Society (date). All Rights Reserved.
+
+ This document and translations of it may be copied and furnished to
+ others, and derivative works that comment on or otherwise explain it
+ or assist in its implementation may be prepared, copied, published
+ and distributed, in whole or in part, without restriction of any
+ kind, provided that the above copyright notice and this paragraph
+ are included on all such copies and derivative works. However, this
+ document itself may not be modified in any way, such as by removing
+ the copyright notice or references to the Internet Society or other
+ Internet organizations, except as needed for the purpose of
+ developing Internet standards in which case the procedures for
+ copyrights defined in the Internet Standards process must be
+ followed, or as required to translate it into languages other than
+ English.
+
+ The limited permissions granted above are perpetual and will not be
+ revoked by the Internet Society or its successors or assigns.
+
+ This document and the information contained herein is provided on an
+ "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
+ TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
+ BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
+ HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
+ MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+
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