Network Working Group L. Zhu

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

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

Status of This Memo

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

Copyright Notice

Copyright (C) The Internet Society (2005).

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.

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RFC 4121 Kerberos Version 5 GSS-API July 2005

Table of Contents

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

1. Introduction

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

[RFC1964] describes the GSS-API mechanism for Kerberos Version 5. It
defines the format of context establishment, per-message and context
deletion tokens, and uses algorithm identifiers for each cryptosystem
in per-message and context deletion tokens.

The approach taken in this document obviates the need for algorithm
identifiers. This is accomplished by using the same encryption
algorithm, specified by the crypto profile [RFC3961] for the session
key or subkey that is created during context negotiation, and its
required checksum algorithm. Message layouts of the per-message
tokens are therefore revised to remove algorithm indicators and to
add extra information to support the generic crypto framework
[RFC3961].

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Tokens transferred between GSS-API peers for security context
establishment are also described in this document. The data elements
exchanged between a GSS-API endpoint implementation and the Kerberos
Key Distribution Center (KDC) [RFC4120] are not specific to GSS-API
usage and are therefore defined within [RFC4120] rather than this
specification.

The new token formats specified in this document MUST be used with
all "newer" encryption types [RFC4120] and MAY be used with
encryption types that are not "newer", provided that the initiator
and acceptor know from the context establishment that they can both
process these new token formats.

"Newer" encryption types are those which have been specified along
with or since the new Kerberos cryptosystem specification [RFC3961],
as defined in section 3.1.3 of [RFC4120]. The list of not-newer
encryption types is as follows [RFC3961]:

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

Conventions used in this document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].

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

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2. Key Derivation for Per-Message Tokens

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

This document defines four key usage values below that are used to
derive a specific key for signing and sealing messages from the
session key or subkey [RFC4120] created during the context
establishment.

Name Value
-------------------------------------
KG-USAGE-ACCEPTOR-SEAL 22
KG-USAGE-ACCEPTOR-SIGN 23
KG-USAGE-INITIATOR-SEAL 24
KG-USAGE-INITIATOR-SIGN 25

When the sender is the context acceptor, KG-USAGE-ACCEPTOR-SIGN is
used as the usage number in the key derivation function for deriving
keys to be used in MIC tokens (as defined in section 4.2.6.1).
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, while KG-USAGE-INITIATOR-SEAL is
used for Wrap tokens. Even if the Wrap token does not provide for
confidentiality, the same usage values specified above are used.

During the context initiation and acceptance sequence, the acceptor
MAY assert a subkey in the AP-REP message. If the acceptor asserts a
subkey, the base key is the acceptor-asserted subkey and subsequent
per-message tokens MUST be flagged with "AcceptorSubkey", as
described in section 4.2.2. Otherwise, if the initiator asserts a
subkey in the AP-REQ message, the base key is this subkey; if the
initiator does not assert a subkey, the base key is the session key
in the service ticket.

3. Quality of Protection

The GSS-API specification [RFC2743] provides Quality of Protection
(QOP) values that can be used by applications to request a certain
type of encryption or signing. A zero QOP value is used to indicate
the "default" protection; applications that do not use the default
QOP are not guaranteed to be portable across implementations, or even
to inter-operate with different deployment configurations of the same
implementation. Using a different algorithm than the one for which
the key is defined may not be appropriate. Therefore, when the new
method in this document is used, the QOP value is ignored.

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The encryption and checksum algorithms in per-message tokens are now
implicitly defined by the algorithms associated with the session key
or subkey. Therefore, algorithm identifiers as described in
[RFC1964] are no longer needed and are removed from the new token
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
[RFC2743], 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
-- mechanism-specific token
[APPLICATION 0] IMPLICIT SEQUENCE {
thisMech MechType,
innerToken ANY DEFINED BY thisMech
-- contents mechanism-specific
-- ASN.1 structure not required
}

END

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

Following 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

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

If an unknown token identifier (TOK_ID) is received in the initial
context establishment token, the receiver MUST return
GSS_S_CONTINUE_NEEDED major status, and the returned output token
MUST contain a KRB_ERROR message with the error code
KRB_AP_ERR_MSG_TYPE [RFC4120].

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 [RFC4120] MUST be encrypted in the
session key of the ticket used to authenticate the context.

The authenticator checksum field SHALL have the following format:

Octet Name Description
-----------------------------------------------------------------
0..3 Lgth Number of octets in Bnd field; Represented
in little-endian order; Currently contains
hex value 10 00 00 00 (16).
4..19 Bnd Channel binding information, as described in
section 4.1.1.2.
20..23 Flags Four-octet context-establishment flags in
little-endian order as described in section
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

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

Initiators that do not support the extensions MUST NOT include more
than 24 octets in the checksum field (when GSS_C_DELEG_FLAG is not
set) or more than 28 octets plus the KRB_CRED in the Deleg field
(when GSS_C_DELEG_FLAG is set). Acceptors that do not understand the

Extensions MUST ignore any octets past the Deleg field of the
checksum data (when GSS_C_DELEG_FLAG is set) or past the Flags field
of the checksum data (when GSS_C_DELEG_FLAG is not set).

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 [RFC2744].

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

Context establishment flags are exposed to the calling application.
If the calling application desires a particular service option, then
it requests that option via GSS_Init_sec_context() [RFC2743]. If the
corresponding return state values [RFC2743] indicate that any of the
above optional context level services will be active on the context,
the corresponding flag values in the table above MUST be set in the
checksum Flags field.

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

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

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 [RFC2743], section 1.1.6).

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

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

The member fields and constants used for different address types are
defined in [RFC2744].

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

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

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

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

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

If the caller to GSS_Accept_sec_context [RFC2743] passes in
GSS_C_NO_CHANNEL_BINDINGS [RFC2744] as the channel bindings, then the
acceptor MAY ignore any channel bindings supplied by the initiator,
returning success even if the initiator did pass in channel bindings.

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

4.2. Per-Message Tokens

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

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

4.2.1. Sequence Number

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

4.2.2. Flags Field

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

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

Bit Name Description
--------------------------------------------------------------
0 SentByAcceptor When set, this flag indicates the sender
is the context acceptor. When not set,
it indicates the sender is the context
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
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 [RFC3961]. Therefore, no separate checksum is
needed.

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

In Wrap tokens that provide for confidentiality, the first 16 octets
of the Wrap token (the "header", as defined in section 4.2.6), SHALL
be appended to the plaintext data before encryption. Filler octets
MAY be inserted between the plaintext data and the "header." The

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values and size of the filler octets are chosen by implementations,
such that there SHALL be no crypto-system residue present after the
decryption. The resulting Wrap token is {"header" |
encrypt(plaintext-data | filler | "header")}, where encrypt() is the
encryption operation (which provides for integrity protection)
defined in the crypto profile [RFC3961], and the RRC field (as
defined in section 4.2.5) in the to-be-encrypted header contains the
hex value 00 00.

In Wrap tokens that do not provide for confidentiality, the checksum
SHALL be calculated first over the to-be-signed plaintext data, and
then over the first 16 octets of the Wrap token (the "header", as
defined in section 4.2.6). Both the EC field and the RRC field in
the token header SHALL be filled with zeroes for the purpose of
calculating the checksum. The resulting Wrap token is {"header" |
plaintext-data | get_mic(plaintext-data | "header")}, where get_mic()
is the checksum operation for the required checksum mechanism of the
chosen encryption mechanism defined in the crypto profile [RFC3961].

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

For MIC tokens, the checksum SHALL be calculated as follows: the
checksum operation is calculated first over the to-be-signed
plaintext data, and then over the first 16 octets of the MIC token,
where the checksum mechanism is the required checksum mechanism of
the chosen encryption mechanism defined in the crypto profile
[RFC3961].

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

4.2.5. RRC Field

The "RRC" (Right Rotation Count) field in Wrap tokens is added to
allow the data to be encrypted in-place by existing SSPI (Security
Service Provider Interface) [SSPI] applications that do not provide
an additional buffer for the trailer (the cipher text after the in-
place-encrypted data) in addition to the buffer for the header (the
cipher text before the in-place-encrypted data). Excluding the first
16 octets of the token header, the resulting Wrap token in the
previous section is rotated to the right by "RRC" octets. The net
result is that "RRC" octets of trailing octets are moved toward the
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

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rotation would be {"header" | ff | gg | hh | aa | bb | cc | dd | ee
}, where {aa | bb | cc |...| hh} would be used to indicate the octet
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. The receiver MUST be able to interpret all
possible rotation count values, including rotation counts greater
than the length of the token.

4.2.6. Message Layouts

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

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
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.

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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 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() [RFC2743]
independently, passing a null output_context_token buffer to indicate
that no context_token is required. Implementations of
GSS_Delete_sec_context() should delete relevant locally-stored
context information.

4.4. Token Identifier Assignment Considerations

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

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5. Parameter Definitions

This section defines parameter values used by the Kerberos V5 GSS-API
mechanism. It defines interface elements that support portability,
and assumes use of C language bindings per [RFC2744].

5.1. Minor Status Codes

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

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

5.1.1. Non-Kerberos-specific Codes

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" */

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5.1.2. Kerberos-specific Codes

GSS_KRB5_S_KG_CCACHE_NOMATCH
/* "Client principal in credentials does not match
specified name" */
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(). They MUST also be capable of
accepting the output_token generated by GSS_Wrap() for a 16K octet
input buffer as input to GSS_Unwrap(). Implementations SHOULD
support 64K octet input buffers, and MAY support even larger input
buffer sizes.

6. Backwards Compatibility Considerations

The new token formats defined in this document will only be
recognized by new implementations. To address this, implementations
can always use the explicit sign or seal algorithm in [RFC1964] when
the key type corresponds to not "newer" enctypes. As an alternative,
one might retry sending the message with the sign or seal algorithm
explicitly defined as in [RFC1964]. However, this would require
either the use of a mechanism such as [RFC2478] to securely negotiate
the method, or the use of an out-of-band mechanism to choose the
appropriate mechanism. For this reason, it is RECOMMENDED that the
new token formats defined in this document SHOULD be used only if
both peers are known to support the new mechanism during context
negotiation because of, for example, the use of "new" enctypes.

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RFC 4121 Kerberos Version 5 GSS-API July 2005

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

7. Security Considerations

Channel bindings are validated by the acceptor. The acceptor can
ignore the channel bindings restriction supplied by the initiator and
carried in the authenticator checksum, if (1) channel bindings are
not used by GSS_Accept_sec_context [RFC2743], and (2) the acceptor
does not prove to the initiator that it has the same channel bindings
as the initiator (even if the client requested mutual
authentication). This limitation should be considered by designers
of applications that would use channel bindings, whether to limit the
use of GSS-API contexts to nodes with specific network addresses, to
authenticate other established, secure channels using Kerberos
Version 5, or for any other purpose.

Session key types are selected by the KDC. Under the current
mechanism, no negotiation of algorithm types occurs, so server-side
(acceptor) implementations cannot request that clients not use
algorithm types not understood by the server. However,
administrators can control what enctypes can be used for session keys
for this mechanism by controlling the set of the ticket session key
enctypes which the KDC is willing to use in tickets for a given
acceptor principal. Therefore, the KDC could be given the task of
limiting session keys for a given service to types actually supported
by the Kerberos and GSSAPI software on the server. This has a
drawback for cases in which a service principal name is used for both
GSSAPI-based and non-GSSAPI-based communication (most notably the
"host" service key), if the GSSAPI implementation does not understand
(for example) AES [RFC3962], but the Kerberos implementation does.
This means that AES session keys cannot be issued for that service
principal, which keeps the protection of non-GSSAPI services weaker
than necessary. KDC administrators desiring to limit the session key
types to support interoperability with such GSSAPI implementations
should carefully weigh the reduction in protection offered by such
mechanisms against the benefits of interoperability.

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RFC 4121 Kerberos Version 5 GSS-API July 2005

8. Acknowledgements

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

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

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

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

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RFC 4121 Kerberos Version 5 GSS-API July 2005

9. References

9.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.

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

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

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

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

9.2. Informative References

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

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

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

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RFC 4121 Kerberos Version 5 GSS-API July 2005

Authors' Addresses

Larry Zhu
One Microsoft Way
Redmond, WA 98052 - USA

EMail: [email protected]

Karthik Jaganathan
One Microsoft Way
Redmond, WA 98052 - USA

EMail: [email protected]

Sam Hartman
Massachusetts Institute of Technology
77 Massachusetts Avenue
Cambridge, MA 02139 - USA

EMail: [email protected]

Zhu, et al. Standards Track [Page 19]

RFC 4121 Kerberos Version 5 GSS-API July 2005

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