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<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
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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
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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
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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
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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
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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.
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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.
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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
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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].
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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
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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.
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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" */
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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
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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.
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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.
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[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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