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INTERNET-DRAFT Brian Tung
draft-ietf-cat-kerberos-pk-init-17.txt Clifford Neuman
Updates: RFC 1510bis USC/ISI
expires May 31, 2004 Matthew Hur
Ari Medvinsky
Microsoft Corporation
Sasha Medvinsky
Motorola, Inc.
John Wray
Iris Associates, Inc.
Jonathan Trostle
Public Key Cryptography for Initial Authentication in Kerberos
0. Status Of This Memo
This document is an Internet-Draft and is in full conformance with
all provision 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
The distribution of this memo is unlimited. It is filed as
draft-ietf-cat-kerberos-pk-init-17.txt and expires May 31, 2004.
Please send comments to the authors.
1. Abstract
This draft describes protocol extensions (hereafter called PKINIT)
to the Kerberos protocol specification (RFC 1510bis [1]). These
extensions provide a method for integrating public key cryptography
into the initial authentication exchange, by passing cryptographic
certificates and associated authenticators in preauthentication data
fields.
2. Introduction
A client typically authenticates itself to a service in Kerberos
using three distinct though related exchanges. First, the client
requests a ticket-granting ticket (TGT) from the Kerberos
authentication server (AS). Then, it uses the TGT to request a
service ticket from the Kerberos ticket-granting server (TGS).
Usually, the AS and TGS are integrated in a single device known as
a Kerberos Key Distribution Center, or KDC. (In this draft, we will
refer to both the AS and the TGS as the KDC.) Finally, the client
uses the service ticket to authenticate itself to the service.
The advantage afforded by the TGT is that the user need only
explicitly request a ticket and expose his credentials once. The
TGT and its associated session key can then be used for any
subsequent requests. One implication of this is that all further
authentication is independent of the method by which the initial
authentication was performed. Consequently, initial authentication
provides a convenient place to integrate public-key cryptography
into Kerberos authentication.
As defined, Kerberos authentication exchanges use symmetric-key
cryptography, in part for performance. (Symmetric-key cryptography
is typically 10-100 times faster than public-key cryptography,
depending on the public-key operations. [c]) One cost of using
symmetric-key cryptography is that the keys must be shared, so that
before a user can authentication himself, he must already be
registered with the KDC.
Conversely, public-key cryptography--in conjunction with an
established certification infrastructure--permits authentication
without prior registration. Adding it to Kerberos allows the
widespread use of Kerberized applications by users without requiring
them to register first--a requirement that has no inherent security
benefit.
As noted above, a convenient and efficient place to introduce
public-key cryptography into Kerberos is in the initial
authentication exchange. This document describes the methods and
data formats for integrating public-key cryptography into Kerberos
initial authentication. Another document (PKCROSS) describes a
similar protocol for Kerberos cross-realm authentication.
3. Extensions
This section describes extensions to RFC 1510bis for supporting the
use of public-key cryptography in the initial request for a ticket
granting ticket (TGT).
Briefly, the following changes to RFC 1510bis are proposed:
1. If public-key authentication is indicated, the client sends
the user's public-key data and an authenticator in a
preauthentication field accompanying the usual request.
This authenticator is signed by the user's private
signature key.
2. The KDC verifies the client's request against its own
policy and certification authorities.
3. If the request passes the verification tests, the KDC
replies as usual, but the reply is encrypted using either:
a. a randomly generated key, signed using the KDC's
signature key and encrypted using the user's encryption
key; or
b. a key generated through a Diffie-Hellman exchange with
the client, signed using the KDC's signature key.
Any key data required by the client to obtain the encryption
key is returned in a preauthentication field accompanying
the usual reply.
4. The client obtains the encryption key, decrypts the reply,
and then proceeds as usual.
Section 3.1 of this document defines the necessary message formats.
Section 3.2 describes their syntax and use in greater detail.
Implementation of all specified formats and uses in these sections
is REQUIRED for compliance with PKINIT.
3.1. Definitions
3.1.1. Required Algorithms
[What is the current list of required algorithm? --brian]
3.1.2. Defined Message and Encryption Types
PKINIT makes use of the following new preauthentication types:
PA-PK-AS-REQ TBD
PA-PK-AS-REP TBD
PKINIT introduces the following new error types:
KDC_ERR_CLIENT_NOT_TRUSTED 62
KDC_ERR_KDC_NOT_TRUSTED 63
KDC_ERR_INVALID_SIG 64
KDC_ERR_KEY_TOO_WEAK 65
KDC_ERR_CERTIFICATE_MISMATCH 66
KDC_ERR_CANT_VERIFY_CERTIFICATE 70
KDC_ERR_INVALID_CERTIFICATE 71
KDC_ERR_REVOKED_CERTIFICATE 72
KDC_ERR_REVOCATION_STATUS_UNKNOWN 73
KDC_ERR_CLIENT_NAME_MISMATCH 75
PKINIT uses the following typed data types for errors:
TD-DH-PARAMETERS 102
TD-TRUSTED-CERTIFIERS 104
TD-CERTIFICATE-INDEX 105
PKINIT defines the following encryption types, for use in the AS-REQ
message (to indicate acceptance of the corresponding encryption OIDs
in PKINIT):
dsaWithSHA1-CmsOID 9
md5WithRSAEncryption-CmsOID 10
sha1WithRSAEncryption-CmsOID 11
rc2CBC-EnvOID 12
rsaEncryption-EnvOID (PKCS1 v1.5) 13
rsaES-OAEP-ENV-OID (PKCS1 v2.0) 14
des-ede3-cbc-Env-OID 15
The above encryption types are used (in PKINIT) only within CMS [8]
structures within the PKINIT preauthentication fields. Their use
within Kerberos EncryptedData structures is unspecified.
3.1.3. Algorithm Identifiers
PKINIT does not define, but does make use of, the following
algorithm identifiers.
PKINIT uses the following algorithm identifier for Diffie-Hellman
key agreement [11]:
dhpublicnumber
PKINIT uses the following signature algorithm identifiers [8, 12]:
sha-1WithRSAEncryption (RSA with SHA1)
md5WithRSAEncryption (RSA with MD5)
id-dsa-with-sha1 (DSA with SHA1)
PKINIT uses the following encryption algorithm identifiers [12] for
encrypting the temporary key with a public key:
rsaEncryption (PKCS1 v1.5)
id-RSAES-OAEP (PKCS1 v2.0)
These OIDs are not to be confused with the encryption types listed
above.
PKINIT uses the following algorithm identifiers [8] for encrypting
the reply key with the temporary key:
des-ede3-cbc (three-key 3DES, CBC mode)
rc2-cbc (RC2, CBC mode)
Again, these OIDs are not to be confused with the encryption types
listed above.
3.2. PKINIT Preauthentication Syntax and Use
In this section, we describe the syntax and use of the various
preauthentication fields employed to implement PKINIT.
3.2.1. Client Request
The initial authentication request (AS-REQ) is sent as per RFC
1510bis, except that a preauthentication field containing data
signed by the user's private signature key accompanies the request,
as follows:
PA-PK-AS-REQ ::= SEQUENCE {
-- PAType TBD
signedAuthPack [0] ContentInfo,
-- Defined in CMS.
-- Type is SignedData.
-- Content is AuthPack
-- (defined below).
trustedCertifiers [1] SEQUENCE OF TrustedCAs OPTIONAL,
-- A list of CAs, trusted by
-- the client, used to certify
-- KDCs.
kdcCert [2] IssuerAndSerialNumber OPTIONAL,
-- Defined in CMS.
-- Identifies a particular KDC
-- certificate, if the client
-- already has it.
encryptionCert [3] IssuerAndSerialNumber OPTIONAL,
-- May identify the user's
-- Diffie-Hellman certificate,
-- or an RSA encryption key
-- certificate.
...
}
TrustedCAs ::= CHOICE {
caName [0] Name,
-- Fully qualified X.500 name
-- as defined in X.509 [11].
issuerAndSerial [1] IssuerAndSerialNumber,
-- Identifies a specific CA
-- certificate, if the client
-- only trusts one.
...
}
[Should we even allow principalName as a choice? --brian]
AuthPack ::= SEQUENCE {
pkAuthenticator [0] PKAuthenticator,
clientPublicValue [1] SubjectPublicKeyInfo OPTIONAL
-- Defined in X.509,
-- reproduced below.
-- Present only if the client
-- is using ephemeral-ephemeral
-- Diffie-Hellman.
}
PKAuthenticator ::= SEQUENCE {
cusec [0] INTEGER,
ctime [1] KerberosTime,
-- cusec and ctime are used as
-- in RFC 1510bis, for replay
-- prevention.
nonce [2] INTEGER,
-- Binds reply to request,
-- except is zero when client
-- will accept cached
-- Diffie-Hellman parameters
-- from KDC and MUST NOT be
-- zero otherwise.
paChecksum [3] Checksum,
-- Defined in RFC 1510bis.
-- Performed over KDC-REQ-BODY,
-- must be unkeyed.
...
}
SubjectPublicKeyInfo ::= SEQUENCE {
-- As defined in X.509.
algorithm AlgorithmIdentifier,
-- Equals dhpublicnumber (see
-- AlgorithmIdentifier, below)
-- for PKINIT.
subjectPublicKey BIT STRING
-- Equals public exponent
-- (INTEGER encoded as payload
-- of BIT STRING) for PKINIT.
}
AlgorithmIdentifier ::= SEQUENCE {
-- As defined in X.509.
algorithm OBJECT IDENTIFIER,
-- dhpublicnumber is
-- { iso (1) member-body (2)
-- US (840) ansi-x942 (10046)
-- number-type (2) 1 }
-- From RFC 2459 [11].
parameters ANY DEFINED BY algorithm OPTIONAL
-- Content is DomainParameters
-- (see below) for PKINIT.
}
DomainParameters ::= SEQUENCE {
-- As defined in RFC 2459.
p INTEGER,
-- p is the odd prime, equals
-- jq+1.
g INTEGER,
-- Generator.
q INTEGER,
-- Divides p-1.
j INTEGER OPTIONAL,
-- Subgroup factor.
validationParms ValidationParms OPTIONAL
}
ValidationParms ::= SEQUENCE {
-- As defined in RFC 2459.
seed BIT STRING,
-- Seed for the system parameter
-- generation process.
pgenCounter INTEGER
-- Integer value output as part
-- of the system parameter
-- generation process.
}
The ContentInfo in the signedAuthPack is filled out as follows:
1. The eContent field contains data of type AuthPack. It MUST
contain the pkAuthenticator, and MAY also contain the
user's Diffie-Hellman public value (clientPublicValue).
2. The eContentType field MUST contain the OID value for
pkauthdata: { iso (1) org (3) dod (6) internet (1)
security (5) kerberosv5 (2) pkinit (3) pkauthdata (1)}
3. The signerInfos field MUST contain the signature of the
AuthPack.
4. The certificates field MUST contain at least a signature
verification certificate chain that the KDC can use to
verify the signature on the AuthPack. Additionally, the
client may also insert an encryption certificate chain, if
(for example) the client is not using ephemeral-ephemeral
Diffie-Hellman.
5. If a Diffie-Hellman key is being used, the parameters SHOULD
be chosen from the First or Second defined Oakley Groups.
(See RFC 2409 [c].)
6. The KDC may wish to use cached Diffie-Hellman parameters.
To indicate acceptance of caching, the client sends zero in
the nonce field of the pkAuthenticator. Zero is not a valid
value for this field under any other circumstances. Since
zero is used to indicate acceptance of cached parameters,
message binding in this case is performed instead using the
nonce in the main request.
3.2.2. Validation of Client Request
Upon receiving the client's request, the KDC validates it. This
section describes the steps that the KDC MUST (unless otherwise
noted) take in validating the request.
The KDC must look for a user certificate in the signedAuthPack.
If it cannot find one signed by a CA it trusts, it sends back an
error of type KDC_ERR_CANT_VERIFY_CERTIFICATE. The accompanying
e-data for this error is a SEQUENCE OF TypedData:
TypedData ::= SEQUENCE {
-- As defined in RFC 1510bis.
data-type [0] INTEGER,
data-value [1] OCTET STRING
}
For this error, the data-type is TD-TRUSTED-CERTIFIERS, and the
data-value is an OCTET STRING containing the DER encoding of
TrustedCertifiers ::= SEQUENCE OF Name
If, while verifying the certificate chain, the KDC determines that
the signature on one of the certificates in the signedAuthPack is
invalid, it returns an error of type KDC_ERR_INVALID_CERTIFICATE.
The accompanying e-data for this error is a SEQUENCE OF TypedData,
whose data-type is TD-CERTIFICATE-INDEX, and whose data-value is an
OCTET STRING containing the DER encoding of the index into the
CertificateSet field, ordered as sent by the client:
CertificateIndex ::= INTEGER
-- 0 = first certificate (in
-- order of encoding),
-- 1 = second certificate, etc.
If more than one signature is invalid, the KDC sends one TypedData
per invalid signature.
The KDC MAY also check whether any of the certificates in the user's
chain have been revoked. If any of them have been revoked, the KDC
returns an error of type KDC_ERR_REVOKED_CERTIFICATE; if the KDC
attempts to determine the revocation status but is unable to do so,
it returns an error of type KDC_ERR_REVOCATION_STATUS_UNKNOWN. In
either case, the certificate or certificates affected are identified
exactly as for an error of type KDC_ERR_INVALID_CERTIFICATE (see
above).
If the certificate chain is successfully validated, but the name in
the user's certificate does not match the name given in the request,
the KDC returns an error of type KDC_ERR_CLIENT_NAME_MISMATCH. There
is no accompanying e-data for this error.
Even if the chain is validated, and the names in the certificate and
the request match, the KDC may decide not to trust the client. For
example, the certificate may include (or not include) an Enhanced
Key Usage (EKU) OID in the extensions field. As a matter of local
policy, the KDC may decide to reject requests on the basis of the
absence or presence of specific EKU OIDs. In this case, the KDC
returns an error of type KDC_ERR_CLIENT_NOT_TRUSTED. For the
benefit of implementors, we define a PKINIT EKU OID as follows:
{ iso (1) org (3) dod (6) internet (1) security (5) kerberosv5 (2)
pkinit (3) pkekuoid (2) }.
If the certificate chain and usage check out, but the client's
signature on the signedAuthPack fails to verify, the KDC returns an
error of type KDC_ERR_INVALID_SIG. There is no accompanying e-data
for this error.
[What about the case when all this checks out but one or more
certificates is rejected for other reasons? For example, perhaps
the key is too short for local policy. --DRE]
The KDC must check the timestamp to ensure that the request is not
a replay, and that the time skew falls within acceptable limits. If
the check fails, the KDC returns an error of type KRB_AP_ERR_REPEAT
or KRB_AP_ERR_SKEW, respectively.
Finally, if the clientPublicValue is filled in, indicating that the
client wishes to use ephemeral-ephemeral Diffie-Hellman, the KDC
checks to see if the parameters satisfy its policy. If they do not,
it returns an error of type KDC_ERR_KEY_TOO_WEAK. The accompanying
e-data is a SEQUENCE OF TypedData, whose data-type is
TD-DH-PARAMETERS, and whose data-value is an OCTET STRING containing
the DER encoding of a DomainParameters (see above), including
appropriate Diffie-Hellman parameters with which to retry the
request.
[This makes no sense. For example, maybe the key is too strong for
local policy. --DRE]
In order to establish authenticity of the reply, the KDC will sign
some key data (either the random key used to encrypt the reply in
the case of a KDCDHKeyInfo, or the Diffie-Hellman parameters used to
generate the reply-encrypting key in the case of a ReplyKeyPack).
The signature certificate to be used is to be selected as follows:
1. If the client included a kdcCert field in the PA-PK-AS-REQ,
use the referred-to certificate, if the KDC has it. If it
does not, the KDC returns an error of type
KDC_ERR_CERTIFICATE_MISMATCH.
2. Otherwise, if the client did not include a kdcCert field,
but did include a trustedCertifiers field, and the KDC
possesses a certificate issued by one of the listed
certifiers, use that certificate. if it does not possess
one, it returns an error of type KDC_ERR_KDC_NOT_TRUSTED.
3. Otherwise, if the client included neither a kdcCert field
nor a trustedCertifiers field, and the KDC has only one
signature certificate, use that certificate. If it has
more than one certificate, it returns an error of type
KDC_ERR_CERTIFICATE_MISMATCH.
3.2.3. KDC Reply
Assuming that the client's request has been properly validated, the
KDC proceeds as per RFC 1510bis, except as follows.
The user's name as represented in the AS-REP must be derived from
the certificate provided in the client's request. If the KDC has
its own mapping from the name in the certificate to a Kerberos name,
it uses that Kerberos name.
Otherwise, if the certificate contains a subjectAltName extension
with PrincipalName, it uses that name. In this case, the realm in
the ticket is that of the local realm (or some other realm name
chosen by that realm). (OID and syntax for this extension to be
specified here.) Otherwise, the KDC returns an error of type
KDC_ERR_CLIENT_NAME_MISMATCH.
In addition, the certifiers in the certification path of the user's
certificate MUST be added to an authdata (to be specified at a later
time).
The AS-REP is otherwise unchanged from RFC 1510bis. The KDC then
encrypts the reply as usual, but not with the user's long-term key.
Instead, it encrypts it with either a random encryption key, or a
key derived through a Diffie-Hellman exchange. Which is the case is
indicated by the contents of the PA-PK-AS-REP (note tags):
PA-PK-AS-REP ::= CHOICE {
-- PAType YY (TBD)
dhSignedData [0] ContentInfo,
-- Type is SignedData.
-- Content is KDCDHKeyInfo
-- (defined below).
encKeyPack [1] ContentInfo,
-- Type is EnvelopedData.
-- Content is ReplyKeyPack
-- (defined below).
...
}
Note that PA-PK-AS-REP is a CHOICE: either a dhSignedData, or an
encKeyPack, but not both. The former contains data of type
KDCDHKeyInfo, and is used only when the reply is encrypted using a
Diffie-Hellman derived key:
KDCDHKeyInfo ::= SEQUENCE {
subjectPublicKey [0] BIT STRING,
-- Equals public exponent
-- (g^a mod p).
-- INTEGER encoded as payload
-- of BIT STRING.
nonce [1] INTEGER,
-- Binds reply to request.
-- Exception: A value of zero
-- indicates that the KDC is
-- using cached values.
dhKeyExpiration [2] KerberosTime OPTIONAL,
-- Expiration time for KDC's
-- cached values.
...
}
The fields of the ContentInfo for dhSignedData are to be filled in
as follows:
1. The eContent field contains data of type KDCDHKeyInfo.
2. The eContentType field contains the OID value for
pkdhkeydata: { iso (1) org (3) dod (6) internet (1)
security (5) kerberosv5 (2) pkinit (3) pkdhkeydata (2) }
3. The signerInfos field contains a single signerInfo, which is
the signature of the KDCDHKeyInfo.
4. The certificates field contains a signature verification
certificate chain that the client may use to verify the
KDC's signature over the KDCDHKeyInfo.) It may only be left
empty if the client did not include a trustedCertifiers
field in the PA-PK-AS-REQ, indicating that it has the KDC's
certificate.
5. If the client and KDC agree to use cached parameters, the
KDC SHOULD return a zero in the nonce field and include the
expiration time of the cached values in the dhKeyExpiration
field. If this time is exceeded, the client SHOULD NOT use
the reply. If the time is absent, the client SHOULD NOT use
the reply and MAY resubmit a request with a non-zero nonce,
thus indicating non-acceptance of the cached parameters.
The key is derived as follows: Both the KDC and the client calculate
the value g^(ab) mod p, where a and b are the client and KDC's
private exponents, respectively. They both take the first N bits of
this secret value and convert it into a reply key, where N depends
on the key type.
1. For example, if the key type is DES, N = 64 bits, where some
of the bits are replaced with parity bits, according to FIPS
PUB 74 [c].
2. If the key type is (three-key) 3DES, N = 192 bits, where
some of the bits are replaced with parity bits, again
according to FIPS PUB 74.
If the KDC and client are not using Diffie-Hellman, the KDC encrypts
the reply with an encryption key, packed in the encKeyPack, which
contains data of type ReplyKeyPack:
ReplyKeyPack ::= SEQUENCE {
replyKey [0] EncryptionKey,
-- Defined in RFC 1510bis.
-- Used to encrypt main reply.
-- MUST be at least as strong as
-- enctype of session key.
nonce [1] INTEGER,
-- Binds reply to request.
...
}
[What exactly does "at least as strong" mean? --DRE]
The fields of the ContentInfo for encKeyPack MUST be filled in as
follows:
1. The innermost data is of type SignedData. The eContent for
this data is of type ReplyKeyPack.
2. The eContentType for this data contains the OID value for
pkrkeydata: { iso (1) org (3) dod (6) internet (1)
security (5) kerberosv5 (2) pkinit (3) pkrkeydata (3) }
3. The signerInfos field contains a single signerInfo, which is
the signature of the ReplyKeyPack.
4. The certificates field contains a signature verification
certificate chain, which the client may use to verify the
KDC's signature over the ReplyKeyPack.) It may only be left
empty if the client did not include a trustedCertifiers
field in the PA-PK-AS-REQ, indicating that it has the KDC's
certificate.
5. The outer data is of type EnvelopedData. The
encryptedContent for this data is the SignedData described
in items 1 through 4, above.
6. The encryptedContentType for this data contains the OID
value for id-signedData: { iso (1) member-body (2) us (840)
rsadsi (113549) pkcs (1) pkcs7 (7) signedData (2) }
7. The recipientInfos field is a SET which MUST contain exactly
one member of type KeyTransRecipientInfo. The encryptedKey
for this member contains the temporary key which is
encrypted using the client's public key.
8. Neither the unprotectedAttrs field nor the originatorInfo
field is required for PKINIT.
3.2.4. Validation of KDC Reply
Upon receipt of the KDC's reply, the client proceeds as follows. If
the PA-PK-AS-REP contains a dhSignedData, the client obtains and
verifies the Diffie-Hellman parameters, and obtains the shared key
as described above. Otherwise, the message contains an encKeyPack,
and the client decrypts and verifies the temporary encryption key.
In either case, the client then decrypts the main reply with the
resulting key, and then proceeds as described in RFC 1510bis.
4. Security Considerations
PKINIT raises certain security considerations beyond those that can
be regulated strictly in protocol definitions. We will address them
in this section.
PKINIT extends the cross-realm model to the public-key
infrastructure. Anyone using PKINIT must be aware of how the
certification infrastructure they are linking to works.
Also, as in standard Kerberos, PKINIT presents the possibility of
interactions between cryptosystems of varying strengths, and this
now includes public-key cryptosystems. Many systems, for example,
allow the use of 512-bit public keys. Using such keys to wrap data
encrypted under strong conventional cryptosystems, such as 3DES, may
be inappropriate.
PKINIT calls for randomly generated keys for conventional
cryptosystems. Many such systems contain systematically "weak"
keys. For recommendations regarding these weak keys, see RFC
1510bis.
Care should be taken in how certificates are chosen for the purposes
of authentication using PKINIT. Some local policies may require
that key escrow be applied for certain certificate types. People
deploying PKINIT should be aware of the implications of using
certificates that have escrowed keys for the purposes of
authentication.
PKINIT does not provide for a "return routability" test to prevent
attackers from mounting a denial-of-service attack on the KDC by
causing it to perform unnecessary and expensive public-key
operations. Strictly speaking, this is also true of standard
Kerberos, although the potential cost is not as great, because
standard Kerberos does not make use of public-key cryptography.
5. Acknowledgements
Some of the ideas on which this proposal is based arose during
discussions over several years between members of the SAAG, the IETF
CAT working group, and the PSRG, regarding integration of Kerberos
and SPX. Some ideas have also been drawn from the DASS system.
These changes are by no means endorsed by these groups. This is an
attempt to revive some of the goals of those groups, and this
proposal approaches those goals primarily from the Kerberos
perspective. Lastly, comments from groups working on similar ideas
in DCE have been invaluable.
6. Expiration Date
This draft expires May 31, 2004.
7. Bibliography
[1] J. Kohl, C. Neuman. The Kerberos Network Authentication Service
(V5). Request for Comments 1510.
[2] B.C. Neuman, Theodore Ts'o. Kerberos: An Authentication Service
for Computer Networks, IEEE Communications, 32(9):33-38. September
1994.
[3] M. Sirbu, J. Chuang. Distributed Authentication in Kerberos
Using Public Key Cryptography. Symposium On Network and Distributed
System Security, 1997.
[4] B. Cox, J.D. Tygar, M. Sirbu. NetBill Security and Transaction
Protocol. In Proceedings of the USENIX Workshop on Electronic
Commerce, July 1995.
[5] T. Dierks, C. Allen. The TLS Protocol, Version 1.0. Request
for Comments 2246, January 1999.
[6] B.C. Neuman, Proxy-Based Authorization and Accounting for
Distributed Systems. In Proceedings of the 13th International
Conference on Distributed Computing Systems, May 1993.
[7] ITU-T (formerly CCITT) Information technology - Open Systems
Interconnection - The Directory: Authentication Framework
Recommendation X.509 ISO/IEC 9594-8
[8] R. Housley. Cryptographic Message Syntax.
draft-ietf-smime-cms-13.txt, April 1999, approved for publication as
RFC.
[9] PKCS #7: Cryptographic Message Syntax Standard. An RSA
Laboratories Technical Note Version 1.5. Revised November 1, 1993
[10] R. Rivest, MIT Laboratory for Computer Science and RSA Data
Security, Inc. A Description of the RC2(r) Encryption Algorithm.
March 1998. Request for Comments 2268.
[11] R. Housley, W. Ford, W. Polk, D. Solo. Internet X.509 Public
Key Infrastructure, Certificate and CRL Profile, January 1999.
Request for Comments 2459.
[12] B. Kaliski, J. Staddon. PKCS #1: RSA Cryptography
Specifications, October 1998. Request for Comments 2437.
[13] ITU-T (formerly CCITT) Information Processing Systems - Open
Systems Interconnection - Specification of Abstract Syntax Notation
One (ASN.1) Rec. X.680 ISO/IEC 8824-1
[14] PKCS #3: Diffie-Hellman Key-Agreement Standard, An RSA
Laboratories Technical Note, Version 1.4, Revised November 1, 1993.
8. Authors
Brian Tung
Clifford Neuman
USC Information Sciences Institute
4676 Admiralty Way Suite 1001
Marina del Rey CA 90292-6695
Phone: +1 310 822 1511
E-mail: {brian,bcn}@isi.edu
Matthew Hur
Ari Medvinsky
Microsoft Corporation
One Microsoft Way
Redmond WA 98052
Phone: +1 425 707 3336
E-mail: matthur@microsoft.com, arimed@windows.microsoft.com
Sasha Medvinsky
Motorola, Inc.
6450 Sequence Drive
San Diego, CA 92121
+1 858 404 2367
E-mail: smedvinsky@motorola.com
John Wray
Iris Associates, Inc.
5 Technology Park Dr.
Westford, MA 01886
E-mail: John_Wray@iris.com
Jonathan Trostle
E-mail: jtrostle@world.std.com

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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-04.txt MIT
November 21, 2003
Expires: May 21, 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.
Abstract
This memo defines protocols, procedures, and conventions to be
employed by peers implementing the Generic Security Service
Application Program Interface (GSS-API as specified in [RFC-2743])
when using the Kerberos Version 5 mechanism (as specified in
[KRBCLAR]).
[RFC-1964] is updated and incremental changes are proposed in
response to recent developments such as the introduction of Kerberos
crypto framework [KCRYPTO]. These changes support the inclusion of
new cryptosystems based on crypto profiles [KCRYPTO], by defining
new per-message tokens along with their encryption and checksum
algorithms.
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 [RFC-2119].
1. Introduction
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[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
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 KDC 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 memo 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
Note that in this document, the term "little endian order" is used
for brevity to refer to the least-significant-octet-first encoding,
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while the term "big endian order" is for the most-significant-octet-
first encoding.
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
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, and KG-USAGE-ACCEPTOR-SEAL is used
for Wrap tokens; 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
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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 V5 GSS-API
mechanism will have the framing shown below:
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
Where the notation and encoding of this pseudo ASN.1 header, which
is referred as the generic GSS-API token framing later in this
document, are described in [RFC-2743], and 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_REQUEST 01 00
KRB_AP_REPLY 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 estalishment 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
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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. The
length of the checksum MUST be 24 octets when delegation is not
used. 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 KRB_CRED message MUST be encrypted in the
session key of the ticket used to authenticate the context.
The format of the authenticator checksum field is as follows.
Octet Name Description
-----------------------------------------------------------------
0..3 Lgth Number of octets in Bnd field; Currently
contains hex value 10 00 00 00 (16, represented
in little-endian order)
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) [optional]
26..27 Dlgth The length of the Deleg field [optional]
28..n Deleg A KRB_CRED message (n = Dlgth + 29) [optional]
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]. An
implementation that supports a particular option or extension SHOULD
then set the appropriate flag in the checksum Flags field.
The most significant eight bits of the checksum flags are reserved
for future use. The receiver MUST ignore unknown checksum flags.
4.1.1.2. Channel Binding Information
Channel bindings are user-specified tags to identify a given context
to the peer application. These tags are intended to be used to
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identify the particular communications channel that carries the
context [RFC-2743] [RFC-2744].
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) Each integer field shall be formatted into four octets, using
little endian octet ordering, for purposes of MD5 hash computation.
(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.
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
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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 are 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
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 is 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 is 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
garbage". However, given the size of any plaintext data, one can
always find the next (possibly larger) size so that, when padding
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the to-be-encrypted text to that size, there will be no crypto-
system garbage 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), are
appended to the plaintext data before encryption. Filler octets can
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 is no crypto-system garbage 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 in the
to-be-encrypted header contains the hex value 00 00.
In Wrap tokens that do not provide for confidentiality, the checksum
is 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 are 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].
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 is first calculated 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]
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.
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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.
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, 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 octet 0..15 and the "to-be-
signed" data, 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, 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 has 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
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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.
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
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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" */
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 shall be capable of
accepting buffers of at least 16K octets as input to GSS_GetMIC(),
GSS_VerifyMIC(), and GSS_Wrap(), and shall be capable of accepting
the output_token generated by GSS_Wrap() for a 16K octet input
buffer as input to GSS_Unwrap(). Support for larger buffer sizes is
optional but recommended.
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]
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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_Verify_MIC() 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
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, administration of the server's Kerberos data (e.g., the
service key) has to be done in communication with the KDC, and it is
from the KDC that the client will request credentials. 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
memo.
The text for security considerations was contributed by 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
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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, and Kevin Damour also provided
valuable inputs on this memo.
Jeffrey Hutzelman provided comments on channel bindings and suggested
many editorial changes.
Luke Howard provided implementations of this memo 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 memo.
Martin Rex provided suggestions of TOK_ID assignment recommendations
thus the token tagging in this memo is unambiguous if the token is
wrapped with the pseudo ASN.1 header.
This document retains some of the text of RFC-1964 in relevant
sections.
9. References
9.1. 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.
[KCRYPTO] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", draft-ietf-krb-wg-crypto-05.txt, June, 2003. Work in
progress.
[KRBCLAR] Neuman, C., Kohl, J., Ts'o T., Yu T., Hartman, S.,
Raeburn, K., "The Kerberos Network Authentication Service (V5)",
draft-ietf-krb-wg-kerberos-clarifications-04.txt, February 2002.
Work in progress.
[AES-KRB5] Raeburn, K., "AES Encryption for Kerberos 5", draft-
raeburn-krb-rijndael-krb-05.txt, June 2003. Work in progress.
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Kerberos Version 5 GSS-API November 2003
[RFC-2478] Baize, E., Pinkas D., "The Simple and Protected GSS-API
Negotiation Mechanism", RFC 2478, December 1998.
9.2. Informative References
[SSPI] Leach, P., "Security Service Provider Interface", Microsoft
Developer Network (MSDN), April 2003.
10. 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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Kerberos Version 5 GSS-API November 2003
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