have escrowed keys for the purposes of authentication. Because
signing-only certificates are normally not escrowed, by using DH-
based key delivery this is avoided.
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. By
using DH-based key delivery and reusing DH keys, the necessary crypto
processing cost per request can be minimized.
When the Diffie-Hellman key exchange method is used, additional pre-
authentication data [RFC4120] (in addition to the PA_PK_AS_REQ, as
defined in this specification) is not bound to the AS_REQ by the
mechanisms discussed in this specification (meaning it may be dropped
or added by attackers without being detected by either the client or
the KDC). Designers of additional pre-authentication data should
take that into consideration if such additional pre-authentication
data can be used in conjunction with the PA_PK_AS_REQ. The future
work of the Kerberos working group is expected to update the hash
algorithms specified in this document and provide a generic mechanism
to bind additional pre-authentication data with the accompanying
AS_REQ.
The key usage number 6 used by the asChecksum field is also used for
the authenticator checksum (cksum field of AP-REQ) contained in the
PA-TGS-REQ preauthentication data contained in a TGS-REQ [RFC4120].
This conflict is present for historical reasons; the reuse of key
usage numbers is strongly discouraged.
5. Acknowledgements
The following people have made significant contributions to this
document: Paul Leach, Stefan Santesson, Sam Hartman, Love Hornquist
Astrand, Ken Raeburn, Nicolas Williams, John Wray, Tom Yu, Jeffrey
Hutzelman, David Cross, Dan Simon, Karthik Jaganathan, Chaskiel M
Grundman, and Jeffrey Altman.
Andre Scedrov, Aaron D. Jaggard, Iliano Cervesato, Joe-Kai Tsay, and
Chris Walstad discovered a binding issue between the AS-REQ and AS-
REP in draft -26; the asChecksum field was added as the result.
Special thanks to Clifford Neuman, Matthew Hur, Ari Medvinsky, Sasha
Medvinsky, and Jonathan Trostle who wrote earlier versions of this
document.
The authors are indebted to the Kerberos working group chair, Jeffrey
Hutzelman, who kept track of various issues and was enormously
helpful during the creation of this document.
Some of the ideas on which this document 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
document approaches those goals primarily from the Kerberos
perspective.
Lastly, comments from groups working on similar ideas in DCE have
been invaluable.
6. References
6.1. Normative References
[IEEE1363] IEEE, "Standard Specifications for Public Key
Cryptography", IEEE 1363, 2000.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2412] Orman, H., "The OAKLEY Key Determination Protocol", RFC
2412, November 1998.
[RFC2631] Rescorla, E., "Diffie-Hellman Key Agreement Method", RFC
2631, June 1999.
[RFC3279] Bassham, L., Polk, W., and R. Housley, "Algorithms and
Identifiers for the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 3279, April 2002.
[RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[RFC3370] Housley, R., "Cryptographic Message Syntax (CMS)
Algorithms", RFC 3370, August 2002.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[RFC3526] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
Diffie-Hellman groups for Internet Key Exchange (IKE)",
RFC 3526, May 2003.
[RFC3560] Housley, R., "Use of the RSAES-OAEP Key Transport
Algorithm in Cryptographic Message Syntax (CMS)", RFC
3560, July 2003.
[RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For
Public Keys Used For Exchanging Symmetric Keys", BCP 86,
RFC 3766, April 2004.
[RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)", RFC
3852, July 2004.
[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", RFC 3961, February 2005.
[RFC3962] Raeburn, K., "Advanced Encryption Standard (AES)
Encryption for Kerberos 5", RFC 3962, February 2005.
[RFC4086] Eastlake, D., 3rd, Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
June 2005.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
[X680] ITU-T Recommendation X.680 (2002) | ISO/IEC 8824-1:2002,
Information technology - Abstract Syntax Notation One
(ASN.1): Specification of basic notation.
[X690] ITU-T Recommendation X.690 (2002) | ISO/IEC 8825-1:2002,
Information technology - ASN.1 encoding Rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER).
6.2. Informative References
[ODL99] Odlyzko, A., "Discrete logarithms: The past and the
future, Designs, Codes, and Cryptography (1999)". April
1999.
[RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
Version 5 Generic Security Service Application Program
Interface (GSS-API) Mechanism: Version 2", RFC 4121, July
2005.
[RFC4158] Cooper, M., Dzambasow, Y., Hesse, P., Joseph, S., and R.
Nicholas, "Internet X.509 Public Key Infrastructure:
Certification Path Building", RFC 4158, September 2005.
Appendix A. PKINIT ASN.1 Module
KerberosV5-PK-INIT-SPEC {
iso(1) identified-organization(3) dod(6) internet(1)
security(5) kerberosV5(2) modules(4) pkinit(5)
} DEFINITIONS EXPLICIT TAGS ::= BEGIN
IMPORTS
SubjectPublicKeyInfo, AlgorithmIdentifier
FROM PKIX1Explicit88 { iso (1)
identified-organization (3) dod (6) internet (1)
security (5) mechanisms (5) pkix (7) id-mod (0)
id-pkix1-explicit (18) }
-- As defined in RFC 3280.
KerberosTime, PrincipalName, Realm, EncryptionKey, Checksum
FROM KerberosV5Spec2 { iso(1) identified-organization(3)
dod(6) internet(1) security(5) kerberosV5(2)
modules(4) krb5spec2(2) };
-- as defined in RFC 4120.
id-pkinit OBJECT IDENTIFIER ::=
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) kerberosv5(2) pkinit (3) }
id-pkinit-authData OBJECT IDENTIFIER ::= { id-pkinit 1 }
id-pkinit-DHKeyData OBJECT IDENTIFIER ::= { id-pkinit 2 }
id-pkinit-rkeyData OBJECT IDENTIFIER ::= { id-pkinit 3 }
id-pkinit-KPClientAuth OBJECT IDENTIFIER ::= { id-pkinit 4 }
id-pkinit-KPKdc OBJECT IDENTIFIER ::= { id-pkinit 5 }
id-pkinit-san OBJECT IDENTIFIER ::=
{ iso(1) org(3) dod(6) internet(1) security(5) kerberosv5(2)
x509SanAN (2) }
pa-pk-as-req INTEGER ::= 16
pa-pk-as-rep INTEGER ::= 17
ad-initial-verified-cas INTEGER ::= 9
td-trusted-certifiers INTEGER ::= 104
td-invalid-certificates INTEGER ::= 105
td-dh-parameters INTEGER ::= 109
PA-PK-AS-REQ ::= SEQUENCE {
signedAuthPack [0] IMPLICIT OCTET STRING,
-- Contains a CMS type ContentInfo encoded
-- according to [RFC3852].
-- The contentType field of the type ContentInfo
-- is id-signedData (1.2.840.113549.1.7.2),
-- and the content field is a SignedData.
-- The eContentType field for the type SignedData is
-- id-pkinit-authData (1.3.6.1.5.2.3.1), and the
-- eContent field contains the DER encoding of the
-- type AuthPack.
-- AuthPack is defined below.
trustedCertifiers [1] SEQUENCE OF
ExternalPrincipalIdentifier OPTIONAL,
-- Contains a list of CAs, trusted by the client,
-- that can be used to certify the KDC.
-- Each ExternalPrincipalIdentifier identifies a CA
-- or a CA certificate (thereby its public key).
-- The information contained in the
-- trustedCertifiers SHOULD be used by the KDC as
-- hints to guide its selection of an appropriate
-- certificate chain to return to the client.
kdcPkId [2] IMPLICIT OCTET STRING
OPTIONAL,
-- Contains a CMS type SignerIdentifier encoded
-- according to [RFC3852].
-- Identifies, if present, a particular KDC
-- public key that the client already has.
...
}
DHNonce ::= OCTET STRING
ExternalPrincipalIdentifier ::= SEQUENCE {
subjectName [0] IMPLICIT OCTET STRING OPTIONAL,
-- Contains a PKIX type Name encoded according to
-- [RFC3280].
-- Identifies the certificate subject by the
-- distinguished subject name.
-- REQUIRED when there is a distinguished subject
-- name present in the certificate.
issuerAndSerialNumber [1] IMPLICIT OCTET STRING OPTIONAL,
-- Contains a CMS type IssuerAndSerialNumber encoded
-- according to [RFC3852].
-- Identifies a certificate of the subject.
-- REQUIRED for TD-INVALID-CERTIFICATES and
-- TD-TRUSTED-CERTIFIERS.
subjectKeyIdentifier [2] IMPLICIT OCTET STRING OPTIONAL,
-- Identifies the subject’s public key by a key
-- identifier. When an X.509 certificate is
-- referenced, this key identifier matches the X.509
-- subjectKeyIdentifier extension value. When other
-- certificate formats are referenced, the documents
-- that specify the certificate format and their use
-- with the CMS must include details on matching the
-- key identifier to the appropriate certificate
-- field.
-- RECOMMENDED for TD-TRUSTED-CERTIFIERS.
...
}
AuthPack ::= SEQUENCE {
pkAuthenticator [0] PKAuthenticator,
clientPublicValue [1] SubjectPublicKeyInfo OPTIONAL,
-- Type SubjectPublicKeyInfo is defined in
-- [RFC3280].
-- Specifies Diffie-Hellman domain parameters
-- and the client’s public key value [IEEE1363].
-- The DH public key value is encoded as a BIT
-- STRING according to [RFC3279].
-- This field is present only if the client wishes
-- to use the Diffie-Hellman key agreement method.
supportedCMSTypes [2] SEQUENCE OF AlgorithmIdentifier
OPTIONAL,
-- Type AlgorithmIdentifier is defined in
-- [RFC3280].
-- List of CMS algorithm [RFC3370] identifiers
-- that identify key transport algorithms, or
-- content encryption algorithms, or signature
-- algorithms supported by the client in order of
-- (decreasing) preference.
clientDHNonce [3] DHNonce OPTIONAL,
-- Present only if the client indicates that it
-- wishes to reuse DH keys or to allow the KDC to
-- do so.
...
}
PKAuthenticator ::= SEQUENCE {
cusec [0] INTEGER (0..999999),
ctime [1] KerberosTime,
-- cusec and ctime are used as in [RFC4120], for
-- replay prevention.
nonce [2] INTEGER (0..4294967295),
-- Chosen randomly; this nonce does not need to
-- match with the nonce in the KDC-REQ-BODY.
paChecksum [3] OCTET STRING OPTIONAL,
-- MUST be present.
-- Contains the SHA1 checksum, performed over
-- KDC-REQ-BODY.
...
}
TD-TRUSTED-CERTIFIERS ::= SEQUENCE OF
ExternalPrincipalIdentifier
-- Identifies a list of CAs trusted by the KDC.
-- Each ExternalPrincipalIdentifier identifies a CA
-- or a CA certificate (thereby its public key).
TD-INVALID-CERTIFICATES ::= SEQUENCE OF
ExternalPrincipalIdentifier
-- Each ExternalPrincipalIdentifier identifies a
-- certificate (sent by the client) with an invalid
-- signature.
KRB5PrincipalName ::= SEQUENCE {
realm [0] Realm,
principalName [1] PrincipalName
}
AD-INITIAL-VERIFIED-CAS ::= SEQUENCE OF
ExternalPrincipalIdentifier
-- Identifies the certification path based on which
-- the client certificate was validated.
-- Each ExternalPrincipalIdentifier identifies a CA
-- or a CA certificate (thereby its public key).
PA-PK-AS-REP ::= CHOICE {
dhInfo [0] DHRepInfo,
-- Selected when Diffie-Hellman key exchange is
-- used.
encKeyPack [1] IMPLICIT OCTET STRING,
-- Selected when public key encryption is used.
-- Contains a CMS type ContentInfo encoded
-- according to [RFC3852].
-- The contentType field of the type ContentInfo is
-- id-envelopedData (1.2.840.113549.1.7.3).
-- The content field is an EnvelopedData.
-- The contentType field for the type EnvelopedData
-- is id-signedData (1.2.840.113549.1.7.2).
-- The eContentType field for the inner type
-- SignedData (when unencrypted) is
-- id-pkinit-rkeyData (1.3.6.1.5.2.3.3) and the
-- eContent field contains the DER encoding of the
-- type ReplyKeyPack.
-- ReplyKeyPack is defined below.
...
}
DHRepInfo ::= SEQUENCE {
dhSignedData [0] IMPLICIT OCTET STRING,
-- Contains a CMS type ContentInfo encoded according
-- to [RFC3852].
-- The contentType field of the type ContentInfo is
-- id-signedData (1.2.840.113549.1.7.2), and the
-- content field is a SignedData.
-- The eContentType field for the type SignedData is
-- id-pkinit-DHKeyData (1.3.6.1.5.2.3.2), and the
-- eContent field contains the DER encoding of the
-- type KDCDHKeyInfo.
-- KDCDHKeyInfo is defined below.
serverDHNonce [1] DHNonce OPTIONAL,
-- Present if and only if dhKeyExpiration is
-- present.
...
}
KDCDHKeyInfo ::= SEQUENCE {
subjectPublicKey [0] BIT STRING,
-- The KDC’s DH public key.
-- The DH public key value is encoded as a BIT
-- STRING according to [RFC3279].
nonce [1] INTEGER (0..4294967295),
-- Contains the nonce in the pkAuthenticator field
-- in the request if the DH keys are NOT reused,
-- 0 otherwise.
dhKeyExpiration [2] KerberosTime OPTIONAL,
-- Expiration time for KDC’s key pair,
-- present if and only if the DH keys are reused.
-- If present, the KDC’s DH public key MUST not be
-- used past the point of this expiration time.
-- If this field is omitted then the serverDHNonce
-- field MUST also be omitted.
...
}
ReplyKeyPack ::= SEQUENCE {
replyKey [0] EncryptionKey,
-- Contains the session key used to encrypt the
-- enc-part field in the AS-REP, i.e., the
-- AS reply key.
asChecksum [1] Checksum,
-- Contains the checksum of the AS-REQ
-- corresponding to the containing AS-REP.
-- The checksum is performed over the type AS-REQ.