draft-ietf-cat-kerberos-pk-init-31.txt [plain text]
NETWORK WORKING GROUP L. Zhu
Internet-Draft Microsoft Corporation
Expires: June 15, 2006 B. Tung
USC Information Sciences Institute
December 12, 2005
Public Key Cryptography for Initial Authentication in Kerberos
draft-ietf-cat-kerberos-pk-init-31
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document describes protocol extensions (hereafter called PKINIT)
to the Kerberos protocol specification. These extensions provide a
method for integrating public key cryptography into the initial
authentication exchange, by using asymmetric-key signature and/or
encryption algorithms in pre-authentication data fields.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. Extensions . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Definitions, Requirements, and Constants . . . . . . . . . 5
3.1.1. Required Algorithms . . . . . . . . . . . . . . . . . 5
3.1.2. Defined Message and Encryption Types . . . . . . . . . 5
3.1.3. Algorithm Identifiers . . . . . . . . . . . . . . . . 6
3.2. PKINIT Pre-authentication Syntax and Use . . . . . . . . . 7
3.2.1. Generation of Client Request . . . . . . . . . . . . . 7
3.2.2. Receipt of Client Request . . . . . . . . . . . . . . 12
3.2.3. Generation of KDC Reply . . . . . . . . . . . . . . . 16
3.2.4. Receipt of KDC Reply . . . . . . . . . . . . . . . . . 23
3.3. Interoperability Requirements . . . . . . . . . . . . . . 24
3.4. KDC Indication of PKINIT Support . . . . . . . . . . . . . 25
4. Security Considerations . . . . . . . . . . . . . . . . . . . 25
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
7.1. Normative References . . . . . . . . . . . . . . . . . . . 28
7.2. Informative References . . . . . . . . . . . . . . . . . . 30
Appendix A. PKINIT ASN.1 Module . . . . . . . . . . . . . . . . . 30
Appendix B. Test Vectors . . . . . . . . . . . . . . . . . . . . 35
Appendix C. Miscellaneous Information about Microsoft Windows
PKINIT Implementations . . . . . . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 39
Intellectual Property and Copyright Statements . . . . . . . . . . 40
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1. 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. Finally, the client uses
the service ticket to authenticate itself to the service.
The advantage afforded by the TGT is that the client exposes his
long-term secrets only once. The TGT and its associated session key
can then be used for any subsequent service ticket requests. One
result 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 in [RFC4120], Kerberos authentication exchanges use
symmetric-key cryptography, in part for performance. One
disadvantage of using symmetric-key cryptography is that the keys
must be shared, so that before a client can authenticate itself, he
must already be registered with the KDC.
Conversely, an established Public Key Infrastructure (PKI) already
provides key management and key distribution mechanisms that are
sufficient to establish authentication and secure communication.
Adding public-key cryptography to Kerberos allows Kerberized
applications to leverage these existing services. In addition, human
users can avoid the inconvenience of managing strong passwords, by
using randomly generated strong public and private key pairs.
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.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Both the AS and the TGS are referred to as the KDC.
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In this protocol, both the client and the KDC have a public-private
key pair in order to prove their identities to each other over the
open network. The client's key pair is used to authenticate the
client to the KDC, and the KDC's key pair is used to authenticate the
KDC to the client. The term "signature key" is used to refer to the
private key of the key pair being used.
In this document, the encryption key used to encrypt the enc-part
field of the KDC-REP in the AS-REP [RFC4120] is referred to as the AS
reply key.
In this document, an empty sequence in an optional field can be
either included or omitted: both encodings are permitted and
considered equivalent.
In this document, the term "Modular Exponential Diffie-Hellman" is
used to refer to the Diffie-Hellman key exchange as described in
[RFC2631], in order to differentiate it from other equivalent
representations of the same key agreement algorithm.
3. Extensions
This section describes extensions to [RFC4120] for supporting the use
of public-key cryptography in the initial request for a ticket.
Briefly, this document defines the following extensions to [RFC4120]:
1. The client indicates the use of public-key authentication by
including a special preauthenticator in the initial request. This
preauthenticator contains the client's public-key data and a
signature.
2. The KDC tests the client's request against its authentication
policy and trusted Certification Authorities (CAs).
3. If the request passes the verification tests, the KDC replies as
usual, but the reply is encrypted using either:
a. a key generated through a Diffie-Hellman (DH) key exchange
[RFC2631] [IEEE1363] with the client, signed using the KDC's
signature key; or
b. a symmetric encryption key, signed using the KDC's signature
key and encrypted using the client's public key.
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Any keying material required by the client to obtain the
encryption key for decrypting the KDC reply is returned in a pre-
authentication field accompanying the usual reply.
4. The client validates the KDC's signature, obtains the encryption
key, decrypts the reply, and then proceeds as usual.
Section 3.1 of this document enumerates the required algorithms and
necessary extension message types. Section 3.2 describes the
extension messages in greater detail.
3.1. Definitions, Requirements, and Constants
3.1.1. Required Algorithms
All PKINIT implementations MUST support the following algorithms:
o AS reply key enctype: aes128-cts-hmac-sha1-96 and aes256-cts-hmac-
sha1-96 [RFC3962].
o Signature algorithm: sha-1WithRSAEncryption [RFC3279].
o AS reply key delivery method: Diffie-Hellman key exchange
[RFC2631].
In addition, implementations of this specification MUST be capable of
processing the Extended Key Usage (EKU) extension and the id-pkinit-
san (as defined in Section 3.2.2) otherName of the Subject
Alternative Name (SAN) extension in X.509 certificates [RFC3280], if
present.
3.1.2. Defined Message and Encryption Types
PKINIT makes use of the following new pre-authentication types:
PA_PK_AS_REQ 16
PA_PK_AS_REP 17
PKINIT also makes use of the following new authorization data type:
AD_INITIAL_VERIFIED_CAS 9
PKINIT introduces the following new error codes:
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KDC_ERR_CLIENT_NOT_TRUSTED 62
KDC_ERR_INVALID_SIG 64
KDC_ERR_DH_KEY_PARAMETERS_NOT_ACCEPTED 65
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
KDC_ERR_INCONSISTENT_KEY_PURPOSE 76
KDC_ERR_DIGEST_IN_CERT_NOT_ACCEPTED 77
KDC_ERR_HASH_IN_KDF_NOT_ACCEPTED 78
KDC_ERR_DIGEST_IN_SIGNED_DATA_NOT_ACCEPTED 79
PKINIT uses the following typed data types for errors:
TD_TRUSTED_CERTIFIERS 104
TD_INVALID_CERTIFICATES 105
TD_DH_PARAMETERS 109
The ASN.1 module for all structures defined in this document (plus
IMPORT statements for all imported structures) is given in
Appendix A.
All structures defined in or imported into this document MUST be
encoded using Distinguished Encoding Rules (DER) [X680] [X690]
(unless otherwise noted). All data structures carried in OCTET
STRINGs must be encoded according to the rules specified in
corresponding specifications.
Interoperability note: Some implementations may not be able to decode
wrapped CMS objects encoded with BER; specifically, they may not be
able to decode indefinite length encodings. To maximize
interoperability, implementers SHOULD encode CMS objects used in
PKINIT with DER.
3.1.3. Algorithm Identifiers
PKINIT does not define, but does make use of, the following algorithm
identifiers.
PKINIT uses the following algorithm identifier(s) for Modular
Exponential Diffie-Hellman key agreement [RFC2631] [RFC3279]:
dhpublicnumber (as described in [RFC3279])
PKINIT uses the following signature algorithm identifiers as defined
in [RFC3279]:
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sha-1WithRSAEncryption (RSA with SHA1)
md5WithRSAEncryption (RSA with MD5)
id-dsa-with-sha1 (DSA with SHA1)
PKINIT uses the following encryption algorithm identifiers as defined
in [RFC3447] for encrypting the temporary key with a public key:
rsaEncryption
id-RSAES-OAEP
PKINIT uses the following algorithm identifiers [RFC3370] [RFC3565]
for encrypting the AS reply key with the temporary key:
des-ede3-cbc (three-key 3DES, CBC mode, as defined in [RFC3370])
rc2-cbc (RC2, CBC mode, as defined in [RFC3370])
id-aes256-CBC (AES-256, CBC mode, as defined in [RFC3565])
PKINIT defines the following encryption types, for use in the etype
field of the AS-REQ [RFC4120] message to indicate acceptance of the
corresponding algorithms that can used by Cryptographic Message
Syntax (CMS) [RFC3852] messages in the reply:
id-dsa-with-sha1-CmsOID 9
-- Indicates that the client supports id-dsa-with-sha1.
md5WithRSAEncryption-CmsOID 10
-- Indicates that the client supports md5WithRSAEncryption.
sha-1WithRSAEncryption-CmsOID 11
-- Indicates that the client supports sha-1WithRSAEncryption.
rc2-cbc-EnvOID 12
-- Indicates that the client supports rc2-cbc.
rsaEncryption-EnvOID 13
-- Indicates that the client supports rsaEncryption.
id-RSAES-OAEP-EnvOID 14
-- Indicates that the client supports id-RSAES-OAEP.
des-ede3-cbc-EnvOID 15
-- Indicates that the client supports des-ede3-cbc.
3.2. PKINIT Pre-authentication Syntax and Use
This section defines the syntax and use of the various pre-
authentication fields employed by PKINIT.
3.2.1. Generation of Client Request
The initial authentication request (AS-REQ) is sent as per [RFC4120];
in addition, a pre-authentication data element, whose padata-type is
PA_PK_AS_REQ and whose padata-value contains the DER encoding of the
type PA-PK-AS-REQ, is included.
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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,
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-- 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 encryption types 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 (see Section 3.2.3.1).
...
}
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,
-- Contains the SHA1 checksum, performed over
-- KDC-REQ-BODY.
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...
}
The ContentInfo [RFC3852] structure contained in the signedAuthPack
field of the type PA-PK-AS-REQ is encoded according to [RFC3852] and
is filled out as follows:
1. The contentType field of the type ContentInfo is id-signedData
(as defined in [RFC3852]), and the content field is a SignedData
(as defined in [RFC3852]).
2. The eContentType field for the type SignedData is id-pkinit-
authData: { iso(1) org(3) dod(6) internet(1) security(5)
kerberosv5(2) pkinit(3) authData(1) }. Notes to CMS
implementers: the signed attribute content-type MUST be present
in this SignedData instance and its value is id-pkinit-authData
according to [RFC3852].
3. The eContent field for the type SignedData contains the DER
encoding of the type AuthPack.
4. The signerInfos field of the type SignedData contains a single
signerInfo, which contains the signature over the type AuthPack.
5. The AuthPack structure contains a PKAuthenticator, the client
public key information, the CMS encryption types supported by the
client and a DHNonce. The pkAuthenticator field certifies to the
KDC that the client has recent knowledge of the signing key that
authenticates the client. The clientPublicValue field specifies
Diffie-Hellman domain parameters and the client's public key
value. The DH public key value is encoded as a BIT STRING
according to [RFC3279]. The clientPublicValue field is present
only if the client wishes to use the Diffie-Hellman key agreement
method. The supportedCMSTypes field specifies the list of CMS
encryption types supported by the client in order of (decreasing)
preference. The clientDHNonce field is described later in this
section.
6. The ctime field in the PKAuthenticator structure contains the
current time on the client's host, and the cusec field contains
the microsecond part of the client's timestamp. The ctime and
cusec fields are used together to specify a reasonably accurate
timestamp [RFC4120]. The nonce field is chosen randomly. The
paChecksum field contains a SHA1 checksum that is performed over
the KDC-REQ-BODY [RFC4120].
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7. The certificates field of the type SignedData contains
certificates intended to facilitate certification path
construction, so that the KDC can verify the signature over the
type AuthPack. For path validation, these certificates SHOULD be
sufficient to construct at least one certification path from the
client certificate to one trust anchor acceptable by the KDC
[RFC4158]. The client MUST be capable of including such a set of
certificates if configured to do so. The certificates field MUST
NOT contain "root" CA certificates.
8. The client's Diffie-Hellman public value (clientPublicValue) is
included if and only if the client wishes to use the Diffie-
Hellman key agreement method. The Diffie-Hellman domain
parameters [IEEE1363] for the client's public key are specified
in the algorithm field of the type SubjectPublicKeyInfo [RFC3279]
and the client's Diffie-Hellman public key value is mapped to a
subjectPublicKey (a BIT STRING) according to [RFC3279]. When
using the Diffie-Hellman key agreement method, implementations
MUST support Oakley 1024-bit Modular Exponential (MODP) well-
known group 2 [RFC2412] and Oakley 2048-bit MODP well-known group
14 [RFC3526], and SHOULD support Oakley 4096-bit MODP well-known
group 16 [RFC3526].
The Diffie-Hellman field size should be chosen so as to provide
sufficient cryptographic security [RFC3766].
When MODP Diffie-Hellman is used, the exponents should have at
least twice as many bits as the symmetric keys that will be
derived from them [ODL99].
9. The client may wish to reuse DH keys or to allow the KDC to do so
(see Section 3.2.3.1). If so, then the client includes the
clientDHNonce field. This nonce string MUST be as long as the
longest key length of the symmetric key types that the client
supports. This nonce MUST be chosen randomly.
The ExternalPrincipalIdentifier structure is used in this document to
identify the subject's public key thereby the subject principal.
This structure is filled out as follows:
1. The subjectName field contains a PKIX type Name encoded according
to [RFC3280]. This field identifies the certificate subject by
the distinguished subject name. This field is REQUIRED when
there is a distinguished subject name present in the certificate
being used.
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2. The issuerAndSerialNumber field contains a CMS type
IssuerAndSerialNumber encoded according to [RFC3852]. This field
identifies a certificate of the subject. This field is REQUIRED
for TD-INVALID-CERTIFICATES and TD-TRUSTED-CERTIFIERS (both
structures are defined in Section 3.2.2).
3. The subjectKeyIdentifier [RFC3852] field 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. This field is RECOMMENDED for TD-TRUSTED-
CERTIFIERS (as defined in Section 3.2.2).
The trustedCertifiers field of the type PA-PK-AS-REQ 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 kdcPkId field of the type PA-PK-AS-REQ contains a CMS type
SignerIdentifier encoded according to [RFC3852]. This field
identifies, if present, a particular KDC public key that the client
already has.
3.2.2. Receipt 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 verifies the client's signature in the signedAuthPack field
according to [RFC3852].
If, while validating the client's X.509 certificate [RFC3280], the
KDC cannot build a certification path to validate the client's
certificate, it sends back a KRB-ERROR [RFC4120] message with the
code KDC_ERR_CANT_VERIFY_CERTIFICATE. The accompanying e-data for
this error message is a TYPED-DATA (as defined in [RFC4120]) that
contains an element whose data-type is TD_TRUSTED_CERTIFIERS, and
whose data-value contains the DER encoding of the type TD-TRUSTED-
CERTIFIERS:
TD-TRUSTED-CERTIFIERS ::= SEQUENCE OF
ExternalPrincipalIdentifier
-- Identifies a list of CAs trusted by the KDC.
-- Each ExternalPrincipalIdentifier identifies a CA
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-- or a CA certificate (thereby its public key).
Each ExternalPrincipalIdentifier (as defined in Section 3.2.1) in the
TD-TRUSTED-CERTIFIERS structure identifies a CA or a CA certificate
(thereby its public key) trusted by the KDC.
Upon receiving this error message, the client SHOULD retry only if it
has a different set of certificates (from those of the previous
requests) that form a certification path (or a partial path) from one
of the trust anchors acceptable by the KDC to its own certificate.
If, while processing the certification path, the KDC determines that
the signature on one of the certificates in the signedAuthPack field
is invalid, it returns a KRB-ERROR [RFC4120] message with the code
KDC_ERR_INVALID_CERTIFICATE. The accompanying e-data for this error
message is a TYPED-DATA that contains an element whose data-type is
TD_INVALID_CERTIFICATES, and whose data-value contains the DER
encoding of the type TD-INVALID-CERTIFICATES:
TD-INVALID-CERTIFICATES ::= SEQUENCE OF
ExternalPrincipalIdentifier
-- Each ExternalPrincipalIdentifier identifies a
-- certificate (sent by the client) with an invalid
-- signature.
Each ExternalPrincipalIdentifier (as defined in Section 3.2.1) in the
TD-INVALID-CERTIFICATES structure identifies a certificate (that was
sent by the client) with an invalid signature.
If more than one X.509 certificate signature is invalid, the KDC MAY
include one IssuerAndSerialNumber per invalid signature within the
TD-INVALID-CERTIFICATES.
The client's X.509 certificate is validated according to [RFC3280].
Based on local policy, the KDC may also check whether any X.509
certificates in the certification path validating the client's
certificate have been revoked. If any of them have been revoked, the
KDC MUST return an error message with the code
KDC_ERR_REVOKED_CERTIFICATE; if the KDC attempts to determine the
revocation status but is unable to do so, it SHOULD return an error
message with the code KDC_ERR_REVOCATION_STATUS_UNKNOWN. The
certificate or certificates affected are identified exactly as for
the error code KDC_ERR_INVALID_CERTIFICATE (see above).
Note that the TD_INVALID_CERTIFICATES error data is only used to
identify invalid certificates sent by the client in the request.
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The client's public key is then used to verify the signature. If the
signature fails to verify, the KDC MUST return an error message with
the code KDC_ERR_INVALID_SIG. There is no accompanying e-data for
this error message.
In addition to validating the client's signature, the KDC MUST also
check that the client's public key used to verify the client's
signature is bound to the client's principal name as specified in the
AS-REQ as follows:
1. If the KDC has its own binding between either the client's
signature-verification public key or the client's certificate and
the client's Kerberos principal name, it uses that binding.
2. Otherwise, if the client's X.509 certificate contains a Subject
Alternative Name (SAN) extension carrying a KRB5PrincipalName
(defined below) in the otherName field of the type GeneralName
[RFC3280], it binds the client's X.509 certificate to that name.
The type of the otherName field is AnotherName. The type-id field
of the type AnotherName is id-pkinit-san:
id-pkinit-san OBJECT IDENTIFIER ::=
{ iso(1) org(3) dod(6) internet(1) security(5) kerberosv5(2)
x509SanAN (2) }
And the value field of the type AnotherName is a
KRB5PrincipalName.
KRB5PrincipalName ::= SEQUENCE {
realm [0] Realm,
principalName [1] PrincipalName
}
If the KDC does not have its own binding and there is no
KRB5PrincipalName name present in the client's X.509 certificate, or
if the Kerberos name in the request does not match the
KRB5PrincipalName in the client's X.509 certificate (including the
realm name), the KDC MUST return an error message with the code
KDC_ERR_CLIENT_NAME_MISMATCH. There is no accompanying e-data for
this error message.
Even if the certification path is validated and the certificate is
mapped to the client's principal name, the KDC may decide not to
accept the client's certificate, depending on local policy.
The KDC MAY require the presence of an Extended Key Usage (EKU)
KeyPurposeId [RFC3280] id-pkinit-KPClientAuth in the extensions field
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of the client's X.509 certificate:
id-pkinit-KPClientAuth OBJECT IDENTIFIER ::=
{ iso(1) org(3) dod(6) internet(1) security(5) kerberosv5(2)
pkinit(3) keyPurposeClientAuth(4) }
-- PKINIT client authentication.
-- Key usage bits that MUST be consistent:
-- digitalSignature.
The digitalSignature key usage bit [RFC3280] MUST be asserted when
the intended purpose of the client's X.509 certificate is restricted
with the id-pkinit-KPClientAuth EKU.
If this EKU KeyPurposeId is required but it is not present or if the
client certificate is restricted not to be used for PKINIT client
authentication per Section 4.2.1.13 of [RFC3280], the KDC MUST return
an error message of the code KDC_ERR_INCONSISTENT_KEY_PURPOSE. There
is no accompanying e-data for this error message. KDCs implementing
this requirement SHOULD also accept the EKU KeyPurposeId id-ms-kp-sc-
logon (1.3.6.1.4.1.311.20.2.2) as meeting the requirement, as there
are a large number of X.509 client certificates deployed for use with
PKINIT which have this EKU.
As a matter of local policy, the KDC MAY decide to reject requests on
the basis of the absence or presence of other specific EKU OID's.
If the digest algorithm used in generating the CA signature for the
public key in any certificate of the request is not acceptable by the
KDC, the KDC MUST return a KRB-ERROR [RFC4120] message with the code
KDC_ERR_DIGEST_IN_CERT_NOT_ACCEPTED. The accompanying e-data MUST be
encoded in TYPED-DATA although none is defined at this point.
If the client's public key is not accepted with reasons other than
what were specified above, the KDC returns a KRB-ERROR [RFC4120]
message with the code KDC_ERR_CLIENT_NOT_TRUSTED. There is no
accompanying e-data currently defined for this error message.
The KDC MUST check the timestamp to ensure that the request is not a
replay, and that the time skew falls within acceptable limits. The
recommendations for clock skew times in [RFC4120] apply here. If the
check fails, the KDC MUST return error code KRB_AP_ERR_REPEAT or
KRB_AP_ERR_SKEW, respectively.
If the clientPublicValue is filled in, indicating that the client
wishes to use the Diffie-Hellman key agreement method, the KDC SHOULD
check to see if the key parameters satisfy its policy. If they do
not, it MUST return an error message with the code
KDC_ERR_DH_KEY_PARAMETERS_NOT_ACCEPTED. The accompanying e-data is a
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TYPED-DATA that contains an element whose data-type is
TD_DH_PARAMETERS, and whose data-value contains the DER encoding of
the type TD-DH-PARAMETERS:
TD-DH-PARAMETERS ::= SEQUENCE OF AlgorithmIdentifier
-- Each AlgorithmIdentifier specifies a set of
-- Diffie-Hellman domain parameters [IEEE1363].
-- This list is in decreasing preference order.
TD-DH-PARAMETERS contains a list of Diffie-Hellman domain parameters
that the KDC supports in decreasing preference order, from which the
client SHOULD pick one to retry the request.
The AlgorithmIdentifier structure is defined in [RFC3280] and is
filled in according to [RFC3279]. More specifically Section 2.3.3 of
[RFC3279] describes how to fill in the AlgorithmIdentifier structure
in the case where MODP Diffie-Hellman key exchange is used.
If the client included a kdcPkId field in the PA-PK-AS-REQ and the
KDC does not possess the corresponding key, the KDC MUST ignore the
kdcPkId field as if the client did not include one.
If the digest algorithm used by the id-pkinit-authData is not
acceptable by the KDC, the KDC MUST return a KRB-ERROR [RFC4120]
message with the code KDC_ERR_DIGEST_IN_SIGNED_DATA_NOT_ACCEPTED.
The accompanying e-data MUST be encoded in TYPED-DATA although none
is defined at this point.
3.2.3. Generation of KDC Reply
Assuming that the client's request has been properly validated, the
KDC proceeds as per [RFC4120], except as follows.
The KDC MUST set the initial flag and include an authorization data
element of ad-type [RFC4120] AD_INITIAL_VERIFIED_CAS in the issued
ticket. The ad-data [RFC4120] field contains the DER encoding of the
type AD-INITIAL-VERIFIED-CAS:
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).
The AD-INITIAL-VERIFIED-CAS structure identifies the certification
path based on which the client certificate was validated. Each
ExternalPrincipalIdentifier (as defined in Section 3.2.1) in the AD-
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INITIAL-VERIFIED-CAS structure identifies a CA or a CA certificate
(thereby its public key).
The AS wraps any AD-INITIAL-VERIFIED-CAS data in AD-IF-RELEVANT
containers if the list of CAs satisfies the AS' realm's local policy
(this corresponds to the TRANSITED-POLICY-CHECKED ticket flag
[RFC4120]). Furthermore, any TGS MUST copy such authorization data
from tickets used within a PA-TGS-REQ of the TGS-REQ into the
resulting ticket. If the list of CAs satisfies the local KDC's
realm's policy, the TGS MAY wrap the data into the AD-IF-RELEVANT
container, otherwise it MAY unwrap the authorization data out of the
AD-IF-RELEVANT container.
Application servers that understand this authorization data type
SHOULD apply local policy to determine whether a given ticket bearing
such a type *not* contained within an AD-IF-RELEVANT container is
acceptable. (This corresponds to the AP server checking the
transited field when the TRANSITED-POLICY-CHECKED flag has not been
set [RFC4120].) If such a data type is contained within an AD-IF-
RELEVANT container, AP servers MAY apply local policy to determine
whether the authorization data is acceptable.
A pre-authentication data element, whose padata-type is PA_PK_AS_REP
and whose padata-value contains the DER encoding of the type PA-PK-
AS-REP (defined below), is included in the AS-REP [RFC4120].
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 in Section 3.2.3.2.
...
}
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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 in the KDCDHKeyInfo.
...
}
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.
...
}
The content of the AS-REP is otherwise unchanged from [RFC4120]. The
KDC encrypts the reply as usual, but not with the client's long-term
key. Instead, it encrypts it with either a shared key derived from a
Diffie-Hellman exchange, or a generated encryption key. The contents
of the PA-PK-AS-REP indicate which key delivery method is used.
In addition, the lifetime of the ticket returned by the KDC MUST NOT
exceed that of the client's public-private key pair. The ticket
lifetime, however, can be shorter than that of the client's public-
private key pair. For the implementations of this specification, the
lifetime of the client's public-private key pair is the validity
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period in X.509 certificates [RFC3280], unless configured otherwise.
3.2.3.1. Using Diffie-Hellman Key Exchange
In this case, the PA-PK-AS-REP contains a DHRepInfo structure.
The ContentInfo [RFC3852] structure for the dhSignedData field is
filled in as follows:
1. The contentType field of the type ContentInfo is id-signedData
(as defined in [RFC3852]), and the content field is a SignedData
(as defined in [RFC3852]).
2. The eContentType field for the type SignedData is the OID value
for id-pkinit-DHKeyData: { iso(1) org(3) dod(6) internet(1)
security(5) kerberosv5(2) pkinit(3) DHKeyData(2) }. Notes to CMS
implementers: the signed attribute content-type MUST be present
in this SignedData instance and its value is id-pkinit-DHKeyData
according to [RFC3852].
3. The eContent field for the type SignedData contains the DER
encoding of the type KDCDHKeyInfo.
4. The KDCDHKeyInfo structure contains the KDC's public key, a nonce
and optionally the expiration time of the KDC's DH key being
reused. The subjectPublicKey field of the type KDCDHKeyInfo
field identifies KDC's DH public key. This DH public key value
is encoded as a BIT STRING according to [RFC3279]. The nonce
field contains the nonce in the pkAuthenticator field in the
request if the DH keys are NOT reused. The value of this nonce
field is 0 if the DH keys are reused. The dhKeyExpiration field
is present if and only if the DH keys are reused. If the
dhKeyExpiration field is present, the KDC's public key in this
KDCDHKeyInfo structure MUST NOT be used past the point of this
expiration time. If this field is omitted then the serverDHNonce
field MUST also be omitted.
5. The signerInfos field of the type SignedData contains a single
signerInfo, which contains the signature over the type
KDCDHKeyInfo.
6. The certificates field of the type SignedData contains
certificates intended to facilitate certification path
construction, so that the client can verify the KDC's signature
over the type KDCDHKeyInfo. The information contained in the
trustedCertifiers in the request SHOULD be used by the KDC as
hints to guide its selection of an appropriate certificate chain
to return to the client. This field may be left empty if the KDC
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public key specified by the kdcPkId field in the PA-PK-AS-REQ was
used for signing. Otherwise, for path validation, these
certificates SHOULD be sufficient to construct at least one
certification path from the KDC certificate to one trust anchor
acceptable by the client [RFC4158]. The KDC MUST be capable of
including such a set of certificates if configured to do so. The
certificates field MUST NOT contain "root" CA certificates.
7. If the client included the clientDHNonce field, then the KDC may
choose to reuse its DH keys. If the server reuses DH keys then
it MUST include an expiration time in the dhKeyExpiration field.
Past the point of the expiration time, the signature over the
type DHRepInfo is considered expired/invalid. When the server
reuses DH keys then it MUST include a serverDHNonce at least as
long as the length of keys for the symmetric encryption system
used to encrypt the AS reply. Note that including the
serverDHNonce changes how the client and server calculate the key
to use to encrypt the reply; see below for details. The KDC
SHOULD NOT reuse DH keys unless the clientDHNonce field is
present in the request.
The AS reply key is derived as follows:
1. Both the KDC and the client calculate the shared secret value as
follows:
a) When MODP Diffie-Hellman is used, let DHSharedSecret be the
shared secret value. DHSharedSecret is the value ZZ as
described in Section 2.1.1 of [RFC2631].
DHSharedSecret is first padded with leading zeros such that the
size of DHSharedSecret in octets is the same as that of the
modulus, then represented as a string of octets in big-endian
order.
Implementation note: Both the client and the KDC can cache the
triple (ya, yb, DHSharedSecret), where ya is the client's public
key and yb is the KDC's public key. If both ya and yb are the
same in a later exchange, the cached DHSharedSecret can be used.
2. Let K be the key-generation seed length [RFC3961] of the AS reply
key whose enctype is selected according to [RFC4120].
3. Define the function octetstring2key() as follows:
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octetstring2key(x) == random-to-key(K-truncate(
SHA1(0x00 | x) |
SHA1(0x01 | x) |
SHA1(0x02 | x) |
...
))
where x is an octet string; | is the concatenation operator; 0x00,
0x01, 0x02, etc., are each represented as a single octet; random-
to-key() is an operation that generates a protocol key from a
bitstring of length K; and K-truncate truncates its input to the
first K bits. Both K and random-to-key() are as defined in the
kcrypto profile [RFC3961] for the enctype of the AS reply key.
4. When DH keys are reused, let n_c be the clientDHNonce, and n_k be
the serverDHNonce; otherwise, let both n_c and n_k be empty octet
strings.
5. The AS reply key k is:
k = octetstring2key(DHSharedSecret | n_c | n_k)
If the hash algorithm used in the key derivation function (currently
only octetstring2key() is defined) is not acceptable by the KDC, the
KDC MUST return a KRB-ERROR [RFC4120] message with the code
KDC_ERR_HASH_IN_KDF_NOT_ACCEPTED. The accompanying e-data MUST be
encoded in TYPED-DATA although none is defined at this point.
3.2.3.2. Using Public Key Encryption
In this case, the PA-PK-AS-REP contains the encKeyPack field where
the AS reply key is encrypted.
The ContentInfo [RFC3852] structure for the encKeyPack field is
filled in as follows:
1. The contentType field of the type ContentInfo is id-envelopedData
(as defined in [RFC3852]), and the content field is an
EnvelopedData (as defined in [RFC3852]).
2. The contentType field for the type EnvelopedData is id-
signedData: { iso (1) member-body (2) us (840) rsadsi (113549)
pkcs (1) pkcs7 (7) signedData (2) }.
3. The eContentType field for the inner type SignedData (when
decrypted from the encryptedContent field for the type
EnvelopedData) is id-pkinit-rkeyData: { iso(1) org(3) dod(6)
internet(1) security(5) kerberosv5(2) pkinit(3) rkeyData(3) }.
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Notes to CMS implementers: the signed attribute content-type MUST
be present in this SignedData instance and its value is id-
pkinit-rkeyData according to [RFC3852].
4. The eContent field for the inner type SignedData contains the DER
encoding of the type ReplyKeyPack (as described below).
5. The signerInfos field of the inner type SignedData contains a
single signerInfo, which contains the signature for the type
ReplyKeyPack.
6. The certificates field of the inner type SignedData contains
certificates intended to facilitate certification path
construction, so that the client can verify the KDC's signature
for the type ReplyKeyPack. The information contained in the
trustedCertifiers in the request SHOULD be used by the KDC as
hints to guide its selection of an appropriate certificate chain
to return to the client. This field may be left empty if the KDC
public key specified by the kdcPkId field in the PA-PK-AS-REQ was
used for signing. Otherwise, for path validation, these
certificates SHOULD be sufficient to construct at least one
certification path from the KDC certificate to one trust anchor
acceptable by the client [RFC4158]. The KDC MUST be capable of
including such a set of certificates if configured to do so. The
certificates field MUST NOT contain "root" CA certificates.
7. The recipientInfos field of the type EnvelopedData is a SET which
MUST contain exactly one member of type KeyTransRecipientInfo.
The encryptedKey of this member contains the temporary key which
is encrypted using the client's public key.
8. The unprotectedAttrs or originatorInfo fields of the type
EnvelopedData MAY be present.
If there is a supportedCMSTypes field in the AuthPack, the KDC must
check to see if it supports any of the listed types. If it supports
more than one of the types, the KDC SHOULD use the one listed first.
If it does not support any of them, it MUST return an error message
with the code KDC_ERR_ETYPE_NOSUPP [RFC4120].
Furthermore the KDC computes the checksum of the AS-REQ in the client
request. This checksum is performed over the type AS-REQ and the
protocol key [RFC3961] of the checksum operation is the replyKey and
the key usage number is 6. If the replyKey's enctype is "newer"
[RFC4120] [RFC4121], the checksum operation is the required checksum
operation [RFC3961] of that enctype.
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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.
-- The protocol key [RFC3961] of the checksum is the
-- replyKey and the key usage number is 6.
-- If the replyKey's enctype is "newer" [RFC4120]
-- [RFC4121], the checksum is the required
-- checksum operation [RFC3961] for that enctype.
-- The client MUST verify this checksum upon receipt
-- of the AS-REP.
...
}
Implementations of this RSA encryption key delivery method are
RECOMMENDED to support RSA keys at least 2048 bits in size.
3.2.4. Receipt of KDC Reply
Upon receipt of the KDC's reply, the client proceeds as follows. If
the PA-PK-AS-REP contains the dhSignedData field, the client derives
the AS reply key using the same procedure used by the KDC as defined
in Section 3.2.3.1. Otherwise, the message contains the encKeyPack
field, and the client decrypts and extracts the temporary key in the
encryptedKey field of the member KeyTransRecipientInfo, and then uses
that as the AS reply key.
If the public key encryption method is used, the client MUST verify
the asChecksum contained in the ReplyKeyPack.
In either case, the client MUST verify the signature in the
SignedData according to [RFC3852]. The KDC's X.509 certificate MUST
be validated according to [RFC3280]. In addition, unless the client
can otherwise verify that the public key used to verify the KDC's
signature is bound to the KDC of the target realm, the KDC's X.509
certificate MUST contain a Subject Alternative Name extension
[RFC3280] carrying an AnotherName whose type-id is id-pkinit-san (as
defined in Section 3.2.2) and whose value is a KRB5PrincipalName that
matches the name of the TGS of the target realm (as defined in
Section 7.3 of [RFC4120]).
Unless the client knows by some other means that the KDC certificate
is intended for a Kerberos KDC, the client MUST require that the KDC
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certificate contains the EKU KeyPurposeId [RFC3280] id-pkinit-KPKdc:
id-pkinit-KPKdc OBJECT IDENTIFIER ::=
{ iso(1) org(3) dod(6) internet(1) security(5) kerberosv5(2)
pkinit(3) keyPurposeKdc(5) }
-- Signing KDC responses.
-- Key usage bits that MUST be consistent:
-- digitalSignature.
The digitalSignature key usage bit [RFC3280] MUST be asserted when
the intended purpose of the KDC's X.509 certificate is restricted
with the id-pkinit-KPKdc EKU.
If the KDC certificate contains the Kerberos TGS name encoded as an
id-pkinit-san SAN, this certificate is certified by the issuing CA as
a KDC certificate, therefore the id-pkinit-KPKdc EKU is not required.
If all applicable checks are satisfied, the client then decrypts the
enc-part field of the KDC-REP in the AS-REP using the AS reply key,
and then proceeds as described in [RFC4120].
Implementation note: CAs issuing KDC certificates SHOULD place all
"short" and "fully-qualified" Kerberos realm names of the KDC (one
per GeneralName [RFC3280]) into the KDC certificate to allow maximum
flexibility.
3.3. Interoperability Requirements
The client MUST be capable of sending a set of certificates
sufficient to allow the KDC to construct a certification path for the
client's certificate, if the correct set of certificates is provided
through configuration or policy.
If the client sends all the X.509 certificates on a certification
path to a trust anchor acceptable by the KDC, and the KDC can not
verify the client's public key otherwise, the KDC MUST be able to
process path validation for the client's certificate based on the
certificates in the request.
The KDC MUST be capable of sending a set of certificates sufficient
to allow the client to construct a certification path for the KDC's
certificate, if the correct set of certificates is provided through
configuration or policy.
If the KDC sends all the X.509 certificates on a certification path
to a trust anchor acceptable by the client, and the client can not
verify the KDC's public key otherwise, the client MUST be able to
process path validation for the KDC's certificate based on the
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certificates in the reply.
3.4. KDC Indication of PKINIT Support
If pre-authentication is required, but was not present in the
request, per [RFC4120] an error message with the code
KDC_ERR_PREAUTH_FAILED is returned and a METHOD-DATA object will be
stored in the e-data field of the KRB-ERROR message to specify which
pre-authentication mechanisms are acceptable. The KDC can then
indicate the support of PKINIT by including an empty element whose
padata-type is PA_PK_AS_REQ in that METHOD-DATA object.
Otherwise if it is required by the KDC's local policy that the client
must be pre-authenticated using the pre-authentication mechanism
specified in this document, but no PKINIT pre-authentication was
present in the request, an error message with the code
KDC_ERR_PREAUTH_FAILED SHOULD be returned.
KDCs MUST leave the padata-value field of the PA_PK_AS_REQ element in
the KRB-ERROR's METHOD-DATA empty (i.e., send a zero-length OCTET
STRING), and clients MUST ignore this and any other value. Future
extensions to this protocol may specify other data to send instead of
an empty OCTET STRING.
4. Security Considerations
Kerberos error messages are not integrity protected, as a result, the
domain parameters sent by the KDC as TD-DH-PARAMETERS can be tampered
with by an attacker so that the set of domain parameters selected
could be either weaker or not mutually preferred. Local policy can
configure sets of domain parameters acceptable locally, or disallow
the negotiation of DH domain parameters.
The symmetric reply key size and Diffie-Hellman field size or RSA
modulus size should be chosen so as to provide sufficient
cryptographic security [RFC3766].
When MODP Diffie-Hellman is used, the exponents should have at least
twice as many bits as the symmetric keys that will be derived from
them [ODL99].
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. Users of PKINIT must understand security policies
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and procedures appropriate to the use of Public Key Infrastructures
[RFC3280].
In order to trust a KDC certificate that is certified by a CA as a
KDC certificate for a target realm (for example, by asserting the TGS
name of that Kerberos realm as an id-pkinit-san SAN and/or
restricting the certificate usage by using the id-pkinit-KPKdc EKU,
as described in Section 3.2.4), the client MUST verify that the KDC
certificate's issuing CA is authorized to issue KDC certificates for
that target realm. Otherwise, the binding between the KDC
certificate and the KDC of the target realm is not established.
How to validate this authorization is a matter of local policy. A
way to achieve this is the configuration of specific sets of
intermediary CAs and trust anchors, one of which must be on the KDC
certificate's certification path [RFC3280]; and for each CA or trust
anchor the realms for which it is allowed to issue certificates.
In addition, if any CA is trusted to issue KDC certificates can also
issue other kinds of certificates, then local policy must be able to
distinguish between them: for example, it could require that KDC
certificates contain the id-pkinit-KPKdc EKU or that the realm be
specified with the id-pkinit-san SAN.
It is the responsibility of the PKI administrators for an
organization to ensure that KDC certificates are only issued to KDCs,
and that clients can ascertain this using their local policy.
Standard Kerberos allows the possibility of interactions between
cryptosystems of varying strengths; this document adds interactions
with public-key cryptosystems to Kerberos. Some administrative
policies may allow the use of relatively weak public keys. Using
such keys to wrap data encrypted under stronger conventional
cryptosystems may be inappropriate.
PKINIT requires keys for symmetric cryptosystems to be generated.
Some such systems contain "weak" keys. For recommendations regarding
these weak keys, see [RFC4120].
PKINIT allows the use of the same RSA key pair for encryption and
signing when doing RSA encryption based key delivery. This is not
recommended usage of RSA keys [RFC3447], by using DH based key
delivery this is avoided.
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 used for certain certificate types. Deployers of
PKINIT should be aware of the implications of using certificates that
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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.
The syntax for the AD-INITIAL-VERIFIED-CAS authorization data does
permit empty SEQUENCEs to be encoded. Such empty sequences may only
be used if the KDC itself vouches for the user's certificate.
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.
5. Acknowledgements
The following people have made significant contributions to this
draft: 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, 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
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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. IANA Considerations
This document has no actions for IANA.
7. References
7.1. 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)
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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.
[RFC3565] Schaad, J., "Use of the Advanced Encryption Standard (AES)
Encryption Algorithm in Cryptographic Message Syntax
(CMS)", RFC 3565, 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.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
[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.
[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).
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7.2. Informative References
[LENSTRA] Lenstra, A. and E. Verheul, "Selecting Cryptographic Key
Sizes", Journal of Cryptology 14 (2001) 255-293.
[ODL99] Odlyzko, A., "Discrete logarithms: The past and the
future, Designs, Codes, and Cryptography (1999)".
[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
FROM KerberosV5Spec2 { iso(1) identified-organization(3)
dod(6) internet(1) security(5) kerberosV5(2)
modules(4) krb5spec2(2) } ;
id-pkinit OBJECT IDENTIFIER ::=
{ iso (1) org (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) }
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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.
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-- 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 encryption types 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),
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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,
-- 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
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-- 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 {
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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.
-- The protocol key [RFC3961] of the checksum is the
-- replyKey and the key usage number is 6.
-- If the replyKey's enctype is "newer" [RFC4120]
-- [RFC4121], the checksum is the required
-- checksum operation [RFC3961] for that enctype.
-- The client MUST verify this checksum upon receipt
-- of the AS-REP.
...
}
TD-DH-PARAMETERS ::= SEQUENCE OF AlgorithmIdentifier
-- Each AlgorithmIdentifier specifies a set of
-- Diffie-Hellman domain parameters [IEEE1363].
-- This list is in decreasing preference order.
END
Appendix B. Test Vectors
Function octetstring2key() is defined in Section 3.2.3.1. This
section describes a few sets of test vectors that would be useful for
implementers of octetstring2key().
Set 1
=====
Input octet string x is:
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Output of K-truncate() when the key size is 32 octets:
5e e5 0d 67 5c 80 9f e5 9e 4a 77 62 c5 4b 65 83
75 47 ea fb 15 9b d8 cd c7 5f fc a5 91 1e 4c 41
Set 2:
=====
Input octet string x is:
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Output of K-truncate() when the key size is 32 octets:
ac f7 70 7c 08 97 3d df db 27 cd 36 14 42 cc fb
a3 55 c8 88 4c b4 72 f3 7d a6 36 d0 7d 56 78 7e
Set 3:
======
Input octet string x is:
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
10 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e
0f 10 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d
0e 0f 10 00 01 02 03 04 05 06 07 08 09 0a 0b 0c
0d 0e 0f 10 00 01 02 03 04 05 06 07 08 09 0a 0b
0c 0d 0e 0f 10 00 01 02 03 04 05 06 07 08 09 0a
0b 0c 0d 0e 0f 10 00 01 02 03 04 05 06 07 08 09
0a 0b 0c 0d 0e 0f 10 00 01 02 03 04 05 06 07 08
Output of K-truncate() when the key size is 32 octets:
c4 42 da 58 5f cb 80 e4 3b 47 94 6f 25 40 93 e3
73 29 d9 90 01 38 0d b7 83 71 db 3a cf 5c 79 7e
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Set 4:
=====
Input octet string x is:
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
10 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e
0f 10 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d
0e 0f 10 00 01 02 03 04 05 06 07 08 09 0a 0b 0c
0d 0e 0f 10 00 01 02 03 04 05 06 07 08
Output of K-truncate() when the key size is 32 octets:
00 53 95 3b 84 c8 96 f4 eb 38 5c 3f 2e 75 1c 4a
59 0e d6 ff ad ca 6f f6 4f 47 eb eb 8d 78 0f fc
Appendix C. Miscellaneous Information about Microsoft Windows PKINIT
Implementations
Earlier revisions of the PKINIT I-D were implemented in various
releases of Microsoft Windows and deployed in fairly large numbers.
To enable the community to better interoperate with systems running
those releases, the following information may be useful.
KDC certificates issued by Windows 2000 Enterprise CAs contain a
dNSName SAN with the DNS name of the host running the KDC, and the
id-kp-serverAuth EKU [RFC3280].
KDC certificates issued by Windows 2003 Enterprise CAs contain a
dNSName SAN with the DNS name of the host running the KDC, the id-kp-
serverAuth EKU and the id-ms-kp-sc-logon EKU.
It is anticipated that the next release of Windows is already too far
along to allow it to support the issuing KDC certificates with id-
pkinit-san SAN as specified in this RFC. Instead, they will have a
dNSName SAN containing the domain name of the KDC and the intended
purpose of these KDC certificates be restricted by the presence of
the id-pkinit-KPKdc EKU and id-kp-serverAuth EKU.
In addition to checking that the above are present in a KDC
certificate, Windows clients verify that the issuer of the KDC
certificate is one of a set of allowed issuers of such certificates,
so those wishing to issue KDC certificates need to configure their
Windows clients appropriately.
Client certificates accepted by Windows 2000 and Windows 2003 Server
KDCs must contain an id-ms-san-sc-logon-upn (1.3.6.1.4.1.311.20.2.3)
SAN and the id-ms-kp-sc-logon EKU. The id-ms-san-sc-logon-upn SAN
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contains a UTF8 encoded string whose value is that of the Directory
Service attribute UserPrincipalName of the client account object, and
the purpose of including the id-ms-san-sc-logon-upn SAN in the client
certificate is to validate the client mapping (in other words, the
client's public key is bound to the account that has this
UserPrincipalName value).
It should be noted that all Microsoft Kerberos realm names are domain
style realm names and strictly in upper case. In addition, the
UserPrincipalName attribute is globally unique in Windows 2000 and
Windows 2003.
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Authors' Addresses
Larry Zhu
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
Email: lzhu@microsoft.com
Brian Tung
USC Information Sciences Institute
4676 Admiralty Way Suite 1001
Marina del Rey, CA 90292
US
Email: brian@isi.edu
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