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IETF RFC 4556
Public Key Cryptography for Initial Authentication in Kerberos (PKINIT)
Last modified on Friday, June 30th, 2006
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Network Working Group L. Zhu
Request for Comments: 4556 Microsoft Corporation
Category: Standards Track B. Tung
Aerospace Corporation
June 2006
Public Key Cryptography for
Initial Authentication in Kerberos (PKINIT)
 Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright © The Internet Society (2006).
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.
Table of Contents
1. Introduction ....................................................2
2. Conventions Used in This Document ...............................4
3. Extensions ......................................................5
3.1. Definitions, Requirements, and Constants ...................6
3.1.1. Required Algorithms .................................6
3.1.2. Recommended Algorithms ..............................6
3.1.3. Defined Message and Encryption Types ................7
3.1.4. Kerberos Encryption Types Defined for CMS
Algorithm Identifiers ...............................8
3.2. PKINIT Pre-authentication Syntax and Use ...................9
3.2.1. Generation of Client Request ........................9
3.2.2. Receipt of Client Request ..........................14
3.2.3. Generation of KDC Reply ............................18
3.2.3.1. Using Diffie-Hellman Key Exchange .........21
3.2.3.2. Using Public Key Encryption ...............23
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RFC 4556 PKINIT June 2006
3.2.4. Receipt of KDC Reply ...............................25
3.3. Interoperability Requirements .............................26
3.4. KDC Indication of PKINIT Support ..........................27
4. Security Considerations ........................................27
5. Acknowledgements ...............................................30
6. References .....................................................30
6.1. Normative References ......................................30
6.2. Informative References ....................................32
Appendix A. PKINIT ASN.1 Module ..................................33
Appendix B. Test Vectors .........................................38
Appendix C. Miscellaneous Information about Microsoft Windows
PKINIT Implementations ...............................40
1. Introduction
The Kerberos V5 protocol [RFC 4120] involves use of a trusted third
party known as the Key Distribution Center (KDC) to negotiate shared
session keys between clients and services and provide mutual
authentication between them.
The corner-stones of Kerberos V5 are the Ticket and the
Authenticator. A Ticket encapsulates a symmetric key (the ticket
session key) in an envelope (a public message) intended for a
specific service. The contents of the Ticket are encrypted with a
symmetric key shared between the service principal and the issuing
KDC. The encrypted part of the Ticket contains the client principal
name, among other items. An Authenticator is a record that can be
shown to have been recently generated using the ticket session key in
the associated Ticket. The ticket session key is known by the client
who requested the ticket. The contents of the Authenticator are
encrypted with the associated ticket session key. The encrypted part
of an Authenticator contains a timestamp and the client principal
name, among other items.
As shown in Figure 1, below, the Kerberos V5 protocol consists of the
following message exchanges between the client and the KDC, and the
client and the application service:
- The Authentication Service (AS) Exchange
The client obtains an "initial" ticket from the Kerberos
authentication server (AS), typically a Ticket Granting Ticket
(TGT). The AS-REQ message and the AS-REP message are the request
and the reply message, respectively, between the client and the
AS.
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RFC 4556 PKINIT June 2006
- The Ticket Granting Service (TGS) Exchange
The client subsequently uses the TGT to authenticate and request a
service ticket for a particular service, from the Kerberos
ticket-granting server (TGS). The TGS-REQ message and the TGS-REP
message are the request and the reply message respectively between
the client and the TGS.
- The Client/Server Authentication Protocol (AP) Exchange
The client then makes a request with an AP-REQ message, consisting
of a service ticket and an authenticator that certifies the
client's possession of the ticket session key. The server may
optionally reply with an AP-REP message. AP exchanges typically
negotiate session-specific symmetric keys.
Usually, the AS and TGS are integrated in a single device also known
as the KDC.
+--------------+
+--------->| KDC |
AS-REQ / +-------| |
/ / +--------------+
/ / ^ |
/ |AS-REP / |
| | / TGS-REQ + TGS-REP
| | / /
| | / /
| | / +---------+
| | / /
| | / /
| | / /
| v / v
++-------+------+ +-----------------+
| Client +------------>| Application |
| | AP-REQ | Server |
| |<------------| |
+---------------+ AP-REP +-----------------+
Figure 1: The Message Exchanges in the Kerberos V5 Protocol
In the AS exchange, the KDC reply contains the ticket session key,
among other items, that is encrypted using a key (the AS reply key)
shared between the client and the KDC. The AS reply key is typically
derived from the client's password for human users. Therefore, for
human users, the attack resistance strength of the Kerberos protocol
is no stronger than the strength of their passwords.
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RFC 4556 PKINIT June 2006
The use of asymmetric cryptography in the form of X.509 certificates
[RFC 3280] is popular for facilitating data origin authentication and
perfect secrecy. An established Public Key Infrastructure (PKI)
provides key management and key distribution mechanisms that can be
used to establish authentication and secure communication. Adding
public-key cryptography to Kerberos provides a nice congruence to
public-key protocols, obviates the human users' burden to manage
strong passwords, and allows Kerberized applications to take
advantage of existing key services and identity management.
The advantage afforded by the Kerberos 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. In
addition, the use of symmetric cryptography after the initial
exchange is preferred for performance.
This document describes the methods and data formats using which the
client and the KDC can use public and private key pairs to mutually
authenticate in the AS exchange and negotiate the AS reply key, known
only by the client and the KDC, to encrypt the AS-REP sent by the
KDC.
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 [RFC 2119].
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 term "signature key" is used to refer to the
private key of the key pair being used.
The encryption key used to encrypt the enc-part field of the KDC-REP
in the AS-REP [RFC 4120] is referred to as the AS reply key.
An empty sequence in an optional field can be either included or
omitted: both encodings are permitted and considered equivalent.
The term "Modular Exponential Diffie-Hellman" is used to refer to the
Diffie-Hellman key exchange, as described in [RFC 2631], in order to
differentiate it from other equivalent representations of the same
key agreement algorithm.
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3. Extensions
This section describes extensions to [RFC 4120] for supporting the use
of public-key cryptography in the initial request for a ticket.
Briefly, this document defines the following extensions to [RFC 4120]:
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
[RFC 2631] [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.
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.
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RFC 4556 PKINIT June 2006
3.1. Definitions, Requirements, and Constants
3.1.1. Required Algorithms
All PKINIT implementations MUST support the following algorithms:
o AS reply key enctypes: aes128-cts-hmac-sha1-96 and aes256-cts-
hmac-sha1-96 [RFC 3962].
o Signature algorithm: sha-1WithRSAEncryption [RFC 3370].
o AS reply key delivery method: the Diffie-Hellman key delivery
method, as described in Section 3.2.3.1.
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 [RFC 3280].
3.1.2. Recommended Algorithms
All PKINIT implementations SHOULD support the following algorithm:
o AS reply key delivery method: the public key encryption key
delivery method, as described in Section 3.2.3.2.
For implementations that support the public key encryption key
delivery method, the following algorithms MUST be supported:
a) Key transport algorithms identified in the keyEncryptionAlgorithm
field of the type KeyTransRecipientInfo [RFC 3852] for encrypting
the temporary key in the encryptedKey field [RFC 3852] with a
public key, as described in Section 3.2.3.2: rsaEncryption (this
is the RSAES-PKCS1-v1_5 encryption scheme) [RFC 3370] [RFC 3447].
b) Content encryption algorithms identified in the
contentEncryptionAlgorithm field of the type EncryptedContentInfo
[RFC 3852] for encrypting the AS reply key with the temporary key
contained in the encryptedKey field of the type
KeyTransRecipientInfo [RFC 3852], as described in Section 3.2.3.2:
des-ede3-cbc (three-key 3DES, CBC mode) [RFC 3370].
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3.1.3. 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:
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 77
KDC_ERR_DIGEST_IN_CERT_NOT_ACCEPTED 78
KDC_ERR_PA_CHECKSUM_MUST_BE_INCLUDED 79
KDC_ERR_DIGEST_IN_SIGNED_DATA_NOT_ACCEPTED 80
KDC_ERR_PUBLIC_KEY_ENCRYPTION_NOT_SUPPORTED 81
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 the
specifications defining each data structure; a reference to the
appropriate specification is provided for each data structure.
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RFC 4556 PKINIT June 2006
Interoperability note: Some implementations may not be able to decode
wrapped Cryptographic Message Syntax (CMS) [RFC 3852] 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.
3.1.4. Kerberos Encryption Types Defined for CMS Algorithm Identifiers
PKINIT defines the following Kerberos encryption type numbers
[RFC 3961], which can be used in the etype field of the AS-REQ
[RFC 4120] message to indicate to the KDC the client's acceptance of
the corresponding algorithms (including key transport algorithms
[RFC 3370], content encryption algorithms [RFC 3370], and signature
algorithms) for use with Cryptographic Message Syntax (CMS) [RFC 3852]
[RFC 3370].
Per [RFC 4120], the encryption types in the etype field are in the
decreasing preference order of the client. Note that there is no
significance in the relative order between any two of different types
of algorithms: key transport algorithms, content encryption
algorithms, and signature algorithms.
The presence of each of these encryption types in the etype field is
equivalent to the presence of the corresponding algorithm Object
Identifier (OID) in the supportedCMSTypes field as described in
Section 3.2.1. And the preference order expressed in the
supportedCMSTypes field would override the preference order listed in
the etype field.
Kerberos Encryption Type Name Num Corresponding Algorithm OID
============================== === ===============================
id-dsa-with-sha1-CmsOID 9 id-dsa-with-sha1 [RFC 3370]
md5WithRSAEncryption-CmsOID 10 md5WithRSAEncryption [RFC 3370]
sha-1WithRSAEncryption-CmsOID 11 sha-1WithRSAEncryption [RFC 3370]
rc2-cbc-EnvOID 12 rc2-cbc [RFC 3370]
rsaEncryption-EnvOID 13 rsaEncryption [RFC 3447][RFC 3370]
id-RSAES-OAEP-EnvOID 14 id-RSAES-OAEP [RFC 3447][RFC 3560]
des-ede3-cbc-EnvOID 15 des-ede3-cbc [RFC 3370]
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RFC 4556 PKINIT June 2006
The above encryption type numbers are used only to indicate support
for the use of the corresponding algorithms in PKINIT; they do not
correspond to actual Kerberos encryption types [RFC 3961] and MUST NOT
be used in the etype field of the Kerberos EncryptedData type
[RFC 4120]. The practice of assigning Kerberos encryption type
numbers to indicate support for CMS algorithms is considered
deprecated, and new numbers should not be assigned for this purpose.
Instead, the supportedCMSTypes field should be used to identify the
algorithms supported by the client and the preference order of the
client.
For maximum interoperability, however, PKINIT clients wishing to
indicate to the KDC the support for one or more of the algorithms
listed above SHOULD include the corresponding encryption type
number(s) in the etype field of the AS-REQ.
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 [RFC 4120];
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.
PA-PK-AS-REQ ::= SEQUENCE {
signedAuthPack [0] IMPLICIT OCTET STRING,
-- Contains a CMS type ContentInfo encoded
-- according to [RFC 3852].
-- 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
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RFC 4556 PKINIT June 2006
-- 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 [RFC 3852].
-- 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
-- [RFC 3280].
-- 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 [RFC 3852].
-- 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
-- [RFC 3280].
-- Specifies Diffie-Hellman domain parameters
-- and the client's public key value [IEEE1363].
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RFC 4556 PKINIT June 2006
-- The DH public key value is encoded as a BIT
-- STRING according to [RFC 3279].
-- 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
-- [RFC 3280].
-- List of CMS algorithm [RFC 3370] 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 (see Section 3.2.3.1).
...
}
PKAuthenticator ::= SEQUENCE {
cusec [0] INTEGER (0..999999),
ctime [1] KerberosTime,
-- cusec and ctime are used as in [RFC 4120], 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.
...
}
The ContentInfo [RFC 3852] structure contained in the signedAuthPack
field of the type PA-PK-AS-REQ is encoded according to [RFC 3852] and
is filled out as follows:
1. The contentType field of the type ContentInfo is id-signedData
(as defined in [RFC 3852]), and the content field is a SignedData
(as defined in [RFC 3852]).
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RFC 4556 PKINIT June 2006
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 [RFC 3852].
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 [RFC 3279]. 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 algorithm identifiers that are supported by the client in
order of (decreasing) preference, and can be used to identify a
signature algorithm or a key transport algorithm [RFC 3370] in the
keyEncryptionAlgorithm field of the type KeyTransRecipientInfo,
or a content encryption algorithm [RFC 3370] in the
contentEncryptionAlgorithm field of the type EncryptedContentInfo
[RFC 3852] when encrypting the AS reply key as described in
Section 3.2.3.2. However, there is no significance in the
relative order between any two of different types of algorithms:
key transport algorithms, content encryption algorithms, and
signature algorithms. 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 [RFC 4120]. The nonce field is chosen randomly. The
paChecksum field MUST be present and it contains a SHA1 checksum
that is performed over the KDC-REQ-BODY [RFC 4120]. In order to
ease future migration from the use of SHA1, the paChecksum field
is made optional syntactically: when the request is extended to
negotiate hash algorithms, the new client wishing not to use SHA1
will send the request in the extended message syntax without the
paChecksum field. The KDC conforming to this specification MUST
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RFC 4556 PKINIT June 2006
return a KRB-ERROR [RFC 4120] message with the code
KDC_ERR_PA_CHECKSUM_MUST_BE_INCLUDED (see Section 3.2.3). That
will allow a new client to retry with SHA1 if allowed by the
local policy.
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
[RFC 4158]. 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
[RFC 3279], and the client's Diffie-Hellman public key value is
mapped to a subjectPublicKey (a BIT STRING) according to
[RFC 3279]. When using the Diffie-Hellman key agreement method,
implementations MUST support Oakley 1024-bit Modular Exponential
(MODP) well-known group 2 [RFC 2412] and Oakley 2048-bit MODP
well-known group 14 [RFC 3526] and SHOULD support Oakley 4096-bit
MODP well-known group 16 [RFC 3526].
The Diffie-Hellman field size should be chosen so as to provide
sufficient cryptographic security [RFC 3766].
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 [RFC 3280]. This field identifies the certificate subject by
the distinguished subject name. This field is REQUIRED when
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RFC 4556 PKINIT June 2006
there is a distinguished subject name present in the certificate
being used.
2. The issuerAndSerialNumber field contains a CMS type
IssuerAndSerialNumber encoded according to [RFC 3852]. 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 [RFC 3852] 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 [RFC 3852]. 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 [RFC 3852].
If, while validating the client's X.509 certificate [RFC 3280], the
KDC cannot build a certification path to validate the client's
certificate, it sends back a KRB-ERROR [RFC 4120] message with the
code KDC_ERR_CANT_VERIFY_CERTIFICATE. The accompanying e-data for
this error message is a TYPED-DATA (as defined in [RFC 4120]) 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:
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RFC 4556 PKINIT June 2006
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).
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 [RFC 4120] 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 [RFC 3280].
Depending 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).
Zhu & Tung Standards Track PAGE 15
RFC 4556 PKINIT June 2006
Note that the TD_INVALID_CERTIFICATES error data is only used to
identify invalid certificates sent by the client in the request.
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 principal name 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
[RFC 3280], 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 Kerberos client name in the AS-REQ does not match a name bound
by the KDC (the binding can be in the certificate, for example, as
described above), or if there is no binding found by the KDC, 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.
Zhu & Tung Standards Track PAGE 16
RFC 4556 PKINIT June 2006
The KDC MAY require the presence of an Extended Key Usage (EKU)
KeyPurposeId [RFC 3280] id-pkinit-KPClientAuth in the extensions field
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 [RFC 3280] 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 [RFC 3280], 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 that 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 OIDs.
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 [RFC 4120] 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
those specified above, the KDC returns a KRB-ERROR [RFC 4120] 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 [RFC 4120] 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
Zhu & Tung Standards Track PAGE 17
RFC 4556 PKINIT June 2006
not, it MUST return an error message with the code
KDC_ERR_DH_KEY_PARAMETERS_NOT_ACCEPTED. The accompanying e-data is a
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 [RFC 3280] and is
filled in according to [RFC 3279]. More specifically, Section 2.3.3
of [RFC 3279] 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 [RFC 4120]
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
If the paChecksum filed in the request is not present, the KDC
conforming to this specification MUST return a KRB-ERROR [RFC 4120]
message with the code KDC_ERR_PA_CHECKSUM_MUST_BE_INCLUDED. The
accompanying e-data MUST be encoded in TYPED-DATA (no error data is
defined by this specification).
Assuming that the client's request has been properly validated, the
KDC proceeds as per [RFC 4120], except as follows.
The KDC MUST set the initial flag and include an authorization data
element of ad-type [RFC 4120] AD_INITIAL_VERIFIED_CAS in the issued
ticket. The ad-data [RFC 4120] field contains the DER encoding of the
type AD-INITIAL-VERIFIED-CAS:
Zhu & Tung Standards Track PAGE 18
RFC 4556 PKINIT June 2006
AD-INITIAL-VERIFIED-CAS ::= SEQUENCE OF
ExternalPrincipalIdentifier
-- Identifies the certification path with 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 with which the client certificate was validated. Each
ExternalPrincipalIdentifier (as defined in Section 3.2.1) in the AD-
INITIAL-VERIFIED-CAS structure identifies a CA or a CA certificate
(thereby its public key).
Note that 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.
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
[RFC 4120]). 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's checking the
transited field when the TRANSITED-POLICY-CHECKED flag has not been
set [RFC 4120].) 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 [RFC 4120].
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
Zhu & Tung Standards Track PAGE 19
RFC 4556 PKINIT June 2006
-- according to [RFC 3852].
-- 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.
...
}
DHRepInfo ::= SEQUENCE {
dhSignedData [0] IMPLICIT OCTET STRING,
-- Contains a CMS type ContentInfo encoded according
-- to [RFC 3852].
-- 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 [RFC 3279].
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
Zhu & Tung Standards Track PAGE 20
RFC 4556 PKINIT June 2006
-- field MUST also be omitted.
...
}
The content of the AS-REP is otherwise unchanged from [RFC 4120]. 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.
If the client does not wish to use the Diffie-Hellman key delivery
method (the clientPublicValue field is not present in the request)
and the KDC does not support the public key encryption key delivery
method, the KDC MUST return an error message with the code
KDC_ERR_PUBLIC_KEY_ENCRYPTION_NOT_SUPPORTED. There is no
accompanying e-data for this error message.
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
period in X.509 certificates [RFC 3280], 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 [RFC 3852] structure for the dhSignedData field is
filled in as follows:
1. The contentType field of the type ContentInfo is id-signedData
(as defined in [RFC 3852]), and the content field is a SignedData
(as defined in [RFC 3852]).
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 [RFC 3852].
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
Zhu & Tung Standards Track PAGE 21
RFC 4556 PKINIT June 2006
KDCDHKeyInfo field identifies KDC's DH public key. This DH
public key value is encoded as a BIT STRING according to
[RFC 3279]. 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
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 [RFC 4158]. 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:
Zhu & Tung Standards Track PAGE 22
RFC 4556 PKINIT June 2006
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 [RFC 2631].
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 [RFC 3961] of the AS reply
key whose enctype is selected according to [RFC 4120].
3. Define the function octetstring2key() as follows:
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 [RFC 3961] 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)
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 [RFC 3852] structure for the encKeyPack field is
filled in as follows:
Zhu & Tung Standards Track PAGE 23
RFC 4556 PKINIT June 2006
1. The contentType field of the type ContentInfo is id-envelopedData
(as defined in [RFC 3852]), and the content field is an
EnvelopedData (as defined in [RFC 3852]).
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) }.
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 [RFC 3852].
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 [RFC 4158]. 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 that
MUST contain exactly one member of type KeyTransRecipientInfo.
The encryptedKey of this member contains the temporary key that
is encrypted using the client's public key.
8. The unprotectedAttrs or originatorInfo fields of the type
EnvelopedData MAY be present.
Zhu & Tung Standards Track PAGE 24
RFC 4556 PKINIT June 2006
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 [RFC 4120].
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 [RFC 3961] of the checksum operation is the replyKey,
and the key usage number is 6. If the replyKey's enctype is "newer"
[RFC 4120] [RFC 4121], the checksum operation is the required checksum
operation [RFC 3961] of that enctype.
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 [RFC 3961] of the checksum is the
-- replyKey and the key usage number is 6.
-- If the replyKey's enctype is "newer" [RFC 4120]
-- [RFC 4121], the checksum is the required
-- checksum operation [RFC 3961] 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.
Zhu & Tung Standards Track PAGE 25
RFC 4556 PKINIT June 2006
In either case, the client MUST verify the signature in the
SignedData according to [RFC 3852]. The KDC's X.509 certificate MUST
be validated according to [RFC 3280]. 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
[RFC 3280] 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 [RFC 4120]).
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
certificate contains the EKU KeyPurposeId [RFC 3280] 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 [RFC 3280] 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 [RFC 4120].
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 if the KDC cannot
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.
Zhu & Tung Standards Track PAGE 26
RFC 4556 PKINIT June 2006
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
certificates in the reply.
3.4. KDC Indication of PKINIT Support
If pre-authentication is required but was not present in the request,
per [RFC 4120] 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
The security of cryptographic algorithms is dependent on generating
secret quantities [RFC 4086]. The number of truly random bits is
extremely important in determining the attack resistance strength of
the cryptosystem; for example, the secret Diffie-Hellman exponents
must be chosen based on n truly random bits (where n is the system
security requirement). The security of the overall system is
significantly weakened by using insufficient random inputs: a
sophisticated attacker may find it easier to reproduce the
environment that produced the secret quantities and to search the
resulting small set of possibilities than to locate the quantities in
the whole of the potential number space.
Zhu & Tung Standards Track PAGE 27
RFC 4556 PKINIT June 2006
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 [RFC 3766].
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
and procedures appropriate to the use of Public Key Infrastructures
[RFC 3280].
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 [RFC 3280], and, for each CA or trust
anchor, the realms for which it is allowed to issue certificates.
In addition, if any CA that 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.
Zhu & Tung Standards Track PAGE 28
RFC 4556 PKINIT June 2006
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. When
using such weak asymmetric keys to protect/exchange stronger
symmetric Keys, the attack resistant strength of the overall system
is no better than that of these weak keys [RFC 3766].
PKINIT requires that keys for symmetric cryptosystems be generated.
Some such systems contain "weak" keys. For recommendations regarding
these weak keys, see [RFC 4120].
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 [RFC 3447]; 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
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 [RFC 4120] (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.
Zhu & Tung Standards Track PAGE 29
RFC 4556 PKINIT June 2006
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 [RFC 4120].
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.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Zhu & Tung Standards Track PAGE 30
RFC 4556 PKINIT June 2006
[RFC 2412] Orman, H., "The OAKLEY Key Determination Protocol", RFC
2412, November 1998.
[RFC 2631] Rescorla, E., "Diffie-Hellman Key Agreement Method", RFC
2631, June 1999.
[RFC 3279] 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.
[RFC 3280] 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.
[RFC 3370] Housley, R., "Cryptographic Message Syntax (CMS)
Algorithms", RFC 3370, August 2002.
[RFC 3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[RFC 3526] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
Diffie-Hellman groups for Internet Key Exchange (IKE)",
RFC 3526, May 2003.
[RFC 3560] Housley, R., "Use of the RSAES-OAEP Key Transport
Algorithm in Cryptographic Message Syntax (CMS)", RFC
3560, July 2003.
[RFC 3766] Orman, H. and P. Hoffman, "Determining Strengths For
Public Keys Used For Exchanging Symmetric Keys", BCP 86,
RFC 3766, April 2004.
[RFC 3852] Housley, R., "Cryptographic Message Syntax (CMS)", RFC
3852, July 2004.
[RFC 3961] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", RFC 3961, February 2005.
[RFC 3962] Raeburn, K., "Advanced Encryption Standard (AES)
Encryption for Kerberos 5", RFC 3962, February 2005.
[RFC 4086] Eastlake, D., 3rd, Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
June 2005.
Zhu & Tung Standards Track PAGE 31
RFC 4556 PKINIT June 2006
[RFC 4120] 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.
[RFC 4121] 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.
[RFC 4158] Cooper, M., Dzambasow, Y., Hesse, P., Joseph, S., and R.
Nicholas, "Internet X.509 Public Key Infrastructure:
Certification Path Building", RFC 4158, September 2005.
Zhu & Tung Standards Track PAGE 32
RFC 4556 PKINIT June 2006
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
Zhu & Tung Standards Track PAGE 33
RFC 4556 PKINIT June 2006
-- according to [RFC 3852].
-- 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 [RFC 3852].
-- 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
-- [RFC 3280].
-- 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 [RFC 3852].
-- 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
Zhu & Tung Standards Track PAGE 34
RFC 4556 PKINIT June 2006
-- 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
-- [RFC 3280].
-- 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 [RFC 3279].
-- 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
-- [RFC 3280].
-- List of CMS algorithm [RFC 3370] 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 [RFC 4120], 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
Zhu & Tung Standards Track PAGE 35
RFC 4556 PKINIT June 2006
-- 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 [RFC 3852].
-- 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.
...
Zhu & Tung Standards Track PAGE 36
RFC 4556 PKINIT June 2006
}
DHRepInfo ::= SEQUENCE {
dhSignedData [0] IMPLICIT OCTET STRING,
-- Contains a CMS type ContentInfo encoded according
-- to [RFC 3852].
-- 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 [RFC 3279].
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.
Zhu & Tung Standards Track PAGE 37
RFC 4556 PKINIT June 2006
-- The protocol key [RFC 3961] of the checksum is the
-- replyKey and the key usage number is 6.
-- If the replyKey's enctype is "newer" [RFC 4120]
-- [RFC 4121], the checksum is the required
-- checksum operation [RFC 3961] 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
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 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
Zhu & Tung Standards Track PAGE 38
RFC 4556 PKINIT June 2006
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
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
Zhu & Tung Standards Track PAGE 39
RFC 4556 PKINIT June 2006
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 interoperate better 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 [RFC 3280].
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 will 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
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 uppercase. In addition, the
UserPrincipalName attribute is globally unique in Windows 2000 and
Windows 2003.
Zhu & Tung Standards Track PAGE 40
RFC 4556 PKINIT June 2006
Authors' Addresses
Larry Zhu
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
EMail: lzhu@microsoft.com
Brian Tung
Aerospace Corporation
2350 E. El Segundo Blvd.
El Segundo, CA 90245
US
EMail: brian@aero.org
Zhu & Tung Standards Track PAGE 41
RFC 4556 PKINIT June 2006
Full Copyright Statement
Copyright © The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
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OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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Acknowledgement
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Zhu & Tung Standards Track PAGE 42
Public Key Cryptography for Initial Authentication in Kerberos (PKINIT)
RFC TOTAL SIZE: 100339 bytes
PUBLICATION DATE: Friday, June 30th, 2006
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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