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rfc:rfc4556

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 (C) 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

Zhu & Tung Standards Track [Page 1] 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 [RFC4120] 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:
  1. 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.

Zhu & Tung Standards Track [Page 2] RFC 4556 PKINIT June 2006

  1. 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.
  1. 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.

Zhu & Tung Standards Track [Page 3] RFC 4556 PKINIT June 2006

 The use of asymmetric cryptography in the form of X.509 certificates
 [RFC3280] 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 [RFC2119].
 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 [RFC4120] 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 [RFC2631], in order to
 differentiate it from other equivalent representations of the same
 key agreement algorithm.

Zhu & Tung Standards Track [Page 4] RFC 4556 PKINIT June 2006

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.
    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.

Zhu & Tung Standards Track [Page 5] 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 [RFC3962].
 o  Signature algorithm: sha-1WithRSAEncryption [RFC3370].
 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 [RFC3280].

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 [RFC3852] for encrypting
    the temporary key in the encryptedKey field [RFC3852] with a
    public key, as described in Section 3.2.3.2: rsaEncryption (this
    is the RSAES-PKCS1-v1_5 encryption scheme) [RFC3370] [RFC3447].
 b) Content encryption algorithms identified in the
    contentEncryptionAlgorithm field of the type EncryptedContentInfo
    [RFC3852] for encrypting the AS reply key with the temporary key
    contained in the encryptedKey field of the type
    KeyTransRecipientInfo [RFC3852], as described in Section 3.2.3.2:
    des-ede3-cbc (three-key 3DES, CBC mode) [RFC3370].

Zhu & Tung Standards Track [Page 6] RFC 4556 PKINIT June 2006

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.

Zhu & Tung Standards Track [Page 7] RFC 4556 PKINIT June 2006

 Interoperability note: Some implementations may not be able to decode
 wrapped Cryptographic Message Syntax (CMS) [RFC3852] 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
 [RFC3961], which can be used in the etype field of the AS-REQ
 [RFC4120] message to indicate to the KDC the client's acceptance of
 the corresponding algorithms (including key transport algorithms
 [RFC3370], content encryption algorithms [RFC3370], and signature
 algorithms) for use with Cryptographic Message Syntax (CMS) [RFC3852]
 [RFC3370].
 Per [RFC4120], 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 [RFC3370]
  md5WithRSAEncryption-CmsOID    10  md5WithRSAEncryption [RFC3370]
  sha-1WithRSAEncryption-CmsOID  11  sha-1WithRSAEncryption [RFC3370]
  rc2-cbc-EnvOID                 12  rc2-cbc [RFC3370]
  rsaEncryption-EnvOID           13  rsaEncryption [RFC3447][RFC3370]
  id-RSAES-OAEP-EnvOID           14  id-RSAES-OAEP [RFC3447][RFC3560]
  des-ede3-cbc-EnvOID            15  des-ede3-cbc [RFC3370]

Zhu & Tung Standards Track [Page 8] 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 [RFC3961] and MUST NOT
 be used in the etype field of the Kerberos EncryptedData type
 [RFC4120].  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 [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.
     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

Zhu & Tung Standards Track [Page 9] RFC 4556 PKINIT June 2006

  1. - hints to guide its selection of an appropriate
  2. - certificate chain to return to the client.

kdcPkId [2] IMPLICIT OCTET STRING

                                    OPTIONAL,
                 -- Contains a CMS type SignerIdentifier encoded
                 -- according to [RFC3852].
                 -- Identifies, if present, a particular KDC
                 -- public key that the client already has.
        ...
     }
     DHNonce ::= OCTET STRING
     ExternalPrincipalIdentifier ::= SEQUENCE {
        subjectName            [0] IMPLICIT OCTET STRING OPTIONAL,
                 -- Contains a PKIX type Name encoded according to
                 -- [RFC3280].
                 -- Identifies the certificate subject by the
                 -- distinguished subject name.
                 -- REQUIRED when there is a distinguished subject
                 -- name present in the certificate.
       issuerAndSerialNumber   [1] IMPLICIT OCTET STRING OPTIONAL,
                 -- Contains a CMS type IssuerAndSerialNumber encoded
                 -- according to [RFC3852].
                 -- Identifies a certificate of the subject.
                 -- REQUIRED for TD-INVALID-CERTIFICATES and
                 -- TD-TRUSTED-CERTIFIERS.
       subjectKeyIdentifier    [2] IMPLICIT OCTET STRING OPTIONAL,
                 -- Identifies the subject's public key by a key
                 -- identifier.  When an X.509 certificate is
                 -- referenced, this key identifier matches the X.509
                 -- subjectKeyIdentifier extension value.  When other
                 -- certificate formats are referenced, the documents
                 -- that specify the certificate format and their use
                 -- with the CMS must include details on matching the
                 -- key identifier to the appropriate certificate
                 -- field.
                 -- RECOMMENDED for TD-TRUSTED-CERTIFIERS.
        ...
     }
     AuthPack ::= SEQUENCE {
        pkAuthenticator         [0] PKAuthenticator,
        clientPublicValue       [1] SubjectPublicKeyInfo OPTIONAL,
                 -- Type SubjectPublicKeyInfo is defined in
                 -- [RFC3280].
                 -- Specifies Diffie-Hellman domain parameters
                 -- and the client's public key value [IEEE1363].

Zhu & Tung Standards Track [Page 10] RFC 4556 PKINIT June 2006

  1. - The DH public key value is encoded as a BIT
  2. - STRING according to [RFC3279].
  3. - This field is present only if the client wishes
  4. - to use the Diffie-Hellman key agreement method.

supportedCMSTypes [2] SEQUENCE OF AlgorithmIdentifier

                                    OPTIONAL,
                 -- Type AlgorithmIdentifier is defined in
                 -- [RFC3280].
                 -- List of CMS algorithm [RFC3370] identifiers
                 -- that identify key transport algorithms, or
                 -- content encryption algorithms, or signature
                 -- algorithms supported by the client in order of
                 -- (decreasing) preference.
        clientDHNonce           [3] DHNonce OPTIONAL,
                 -- Present only if the client indicates that it
                 -- wishes to reuse DH keys or to allow the KDC to
                 -- do so (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 OPTIONAL,
                 -- MUST be present.
                 -- Contains the SHA1 checksum, performed over
                 -- KDC-REQ-BODY.
        ...
     }
 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]).

Zhu & Tung Standards Track [Page 11] 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 [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 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 [RFC3370] in the
     keyEncryptionAlgorithm field of the type KeyTransRecipientInfo,
     or a content encryption algorithm [RFC3370] in the
     contentEncryptionAlgorithm field of the type EncryptedContentInfo
     [RFC3852] 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 [RFC4120].  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 [RFC4120].  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

Zhu & Tung Standards Track [Page 12] RFC 4556 PKINIT June 2006

     return a KRB-ERROR [RFC4120] 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
     [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

Zhu & Tung Standards Track [Page 13] 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 [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:

Zhu & Tung Standards Track [Page 14] 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 [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].
 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
    [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 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 [RFC3280] 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 [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 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 [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
 those 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

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 [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

 If the paChecksum filed in the request is not present, the KDC
 conforming to this specification MUST return a KRB-ERROR [RFC4120]
 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 [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:

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
 [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's 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

Zhu & Tung Standards Track [Page 19] RFC 4556 PKINIT June 2006

  1. - according to [RFC3852].
  2. - The contentType field of the type ContentInfo is
  3. - id-envelopedData (1.2.840.113549.1.7.3).
  4. - The content field is an EnvelopedData.
  5. - The contentType field for the type EnvelopedData
  6. - is id-signedData (1.2.840.113549.1.7.2).
  7. - The eContentType field for the inner type
  8. - SignedData (when unencrypted) is
  9. - id-pkinit-rkeyData (1.3.6.1.5.2.3.3) and the
  10. - eContent field contains the DER encoding of the
  11. - type ReplyKeyPack.
  12. - ReplyKeyPack is defined in Section 3.2.3.2.

     }
     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

Zhu & Tung Standards Track [Page 20] RFC 4556 PKINIT June 2006

  1. - 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.
 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 [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

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
     [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
     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:

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 [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:
         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)

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:

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 [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) }.
     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 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 [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.
     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.

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 [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
 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].

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 [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

 The security of cryptographic algorithms is dependent on generating
 secret quantities [RFC4086].  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 [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
 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 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 [RFC3766].
 PKINIT requires that keys for symmetric cryptosystems 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
 have escrowed keys for the purposes of authentication.  Because
 signing-only certificates are normally not escrowed, by using DH-
 based key delivery this is avoided.
 PKINIT does not provide for a "return routability" test to prevent
 attackers from mounting a denial-of-service attack on the KDC by
 causing it to perform unnecessary and expensive public-key
 operations.  Strictly speaking, this is also true of standard
 Kerberos, although the potential cost is not as great, because
 standard Kerberos does not make use of public-key cryptography.  By
 using DH-based key delivery and reusing DH keys, the necessary crypto
 processing cost per request can be minimized.
 When the Diffie-Hellman key exchange method is used, additional pre-
 authentication data [RFC4120] (in addition to the PA_PK_AS_REQ, as
 defined in this specification) is not bound to the AS_REQ by the
 mechanisms discussed in this specification (meaning it may be dropped
 or added by attackers without being detected by either the client or
 the KDC).  Designers of additional pre-authentication data should
 take that into consideration if such additional pre-authentication
 data can be used in conjunction with the PA_PK_AS_REQ.  The future
 work of the Kerberos working group is expected to update the hash
 algorithms specified in this document and provide a generic mechanism
 to bind additional pre-authentication data with the accompanying
 AS_REQ.

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 [RFC4120].
 This conflict is present for historical reasons; the reuse of key
 usage numbers is strongly discouraged.

5. Acknowledgements

 The following people have made significant contributions to this
 document: Paul Leach, Stefan Santesson, Sam Hartman, Love Hornquist
 Astrand, Ken Raeburn, Nicolas Williams, John Wray, Tom Yu, Jeffrey
 Hutzelman, David Cross, Dan Simon, Karthik Jaganathan, Chaskiel M
 Grundman, and Jeffrey Altman.
 Andre Scedrov, Aaron D. Jaggard, Iliano Cervesato, Joe-Kai Tsay, and
 Chris Walstad discovered a binding issue between the AS-REQ and AS-
 REP in draft -26; the asChecksum field was added as the result.
 Special thanks to Clifford Neuman, Matthew Hur, Ari Medvinsky, Sasha
 Medvinsky, and Jonathan Trostle who wrote earlier versions of this
 document.
 The authors are indebted to the Kerberos working group chair, Jeffrey
 Hutzelman, who kept track of various issues and was enormously
 helpful during the creation of this document.
 Some of the ideas on which this document is based arose during
 discussions over several years between members of the SAAG, the IETF
 CAT working group, and the PSRG, regarding integration of Kerberos
 and SPX.  Some ideas have also been drawn from the DASS system.
 These changes are by no means endorsed by these groups.  This is an
 attempt to revive some of the goals of those groups, and this
 document approaches those goals primarily from the Kerberos
 perspective.
 Lastly, comments from groups working on similar ideas in DCE have
 been invaluable.

6. References

6.1. Normative References

 [IEEE1363] IEEE, "Standard Specifications for Public Key
            Cryptography", IEEE 1363, 2000.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.

Zhu & Tung Standards Track [Page 30] RFC 4556 PKINIT June 2006

 [RFC2412]  Orman, H., "The OAKLEY Key Determination Protocol", RFC
            2412, November 1998.
 [RFC2631]  Rescorla, E., "Diffie-Hellman Key Agreement Method", RFC
            2631, June 1999.
 [RFC3279]  Bassham, L., Polk, W., and R. Housley, "Algorithms and
            Identifiers for the Internet X.509 Public Key
            Infrastructure Certificate and Certificate Revocation List
            (CRL) Profile", RFC 3279, April 2002.
 [RFC3280]  Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
            X.509 Public Key Infrastructure Certificate and
            Certificate Revocation List (CRL) Profile", RFC 3280,
            April 2002.
 [RFC3370]  Housley, R., "Cryptographic Message Syntax (CMS)
            Algorithms", RFC 3370, August 2002.
 [RFC3447]  Jonsson, J. and B. Kaliski, "Public-Key Cryptography
            Standards (PKCS) #1: RSA Cryptography Specifications
            Version 2.1", RFC 3447, February 2003.
 [RFC3526]  Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
            Diffie-Hellman groups for Internet Key Exchange (IKE)",
            RFC 3526, May 2003.
 [RFC3560]  Housley, R., "Use of the RSAES-OAEP Key Transport
            Algorithm in Cryptographic Message Syntax (CMS)", RFC
            3560, July 2003.
 [RFC3766]  Orman, H. and P. Hoffman, "Determining Strengths For
            Public Keys Used For Exchanging Symmetric Keys", BCP 86,
            RFC 3766, April 2004.
 [RFC3852]  Housley, R., "Cryptographic Message Syntax (CMS)", RFC
            3852, July 2004.
 [RFC3961]  Raeburn, K., "Encryption and Checksum Specifications for
            Kerberos 5", RFC 3961, February 2005.
 [RFC3962]  Raeburn, K., "Advanced Encryption Standard (AES)
            Encryption for Kerberos 5", RFC 3962, February 2005.
 [RFC4086]  Eastlake, D., 3rd, Schiller, J., and S. Crocker,
            "Randomness Requirements for Security", BCP 106, RFC 4086,
            June 2005.

Zhu & Tung Standards Track [Page 31] RFC 4556 PKINIT June 2006

 [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
            Kerberos Network Authentication Service (V5)", RFC 4120,
            July 2005.
 [X680]     ITU-T Recommendation X.680 (2002) | ISO/IEC 8824-1:2002,
            Information technology - Abstract Syntax Notation One
            (ASN.1): Specification of basic notation.
 [X690]     ITU-T Recommendation X.690 (2002) | ISO/IEC 8825-1:2002,
            Information technology - ASN.1 encoding Rules:
            Specification of Basic Encoding Rules (BER), Canonical
            Encoding Rules (CER) and Distinguished Encoding Rules
            (DER).

6.2. Informative References

 [ODL99]    Odlyzko, A., "Discrete logarithms: The past and the
            future, Designs, Codes, and Cryptography (1999)".  April
            1999.
 [RFC4121]  Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
            Version 5 Generic Security Service Application Program
            Interface (GSS-API) Mechanism: Version 2", RFC 4121, July
            2005.
 [RFC4158]  Cooper, M., Dzambasow, Y., Hesse, P., Joseph, S., and R.
            Nicholas, "Internet X.509 Public Key Infrastructure:
            Certification Path Building", RFC 4158, September 2005.

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

  1. - according to [RFC3852].
  2. - The contentType field of the type ContentInfo
  3. - is id-signedData (1.2.840.113549.1.7.2),
  4. - and the content field is a SignedData.
  5. - The eContentType field for the type SignedData is
  6. - id-pkinit-authData (1.3.6.1.5.2.3.1), and the
  7. - eContent field contains the DER encoding of the
  8. - type AuthPack.
  9. - AuthPack is defined below.

trustedCertifiers [1] SEQUENCE OF

                    ExternalPrincipalIdentifier OPTIONAL,
                 -- Contains a list of CAs, trusted by the client,
                 -- that can be used to certify the KDC.
                 -- Each ExternalPrincipalIdentifier identifies a CA
                 -- or a CA certificate (thereby its public key).
                 -- The information contained in the
                 -- trustedCertifiers SHOULD be used by the KDC as
                 -- hints to guide its selection of an appropriate
                 -- certificate chain to return to the client.
        kdcPkId                 [2] IMPLICIT OCTET STRING
                                    OPTIONAL,
                 -- Contains a CMS type SignerIdentifier encoded
                 -- according to [RFC3852].
                 -- Identifies, if present, a particular KDC
                 -- public key that the client already has.
        ...
     }
     DHNonce ::= OCTET STRING
     ExternalPrincipalIdentifier ::= SEQUENCE {
        subjectName            [0] IMPLICIT OCTET STRING OPTIONAL,
                 -- Contains a PKIX type Name encoded according to
                 -- [RFC3280].
                 -- Identifies the certificate subject by the
                 -- distinguished subject name.
                 -- REQUIRED when there is a distinguished subject
                 -- name present in the certificate.
       issuerAndSerialNumber   [1] IMPLICIT OCTET STRING OPTIONAL,
                 -- Contains a CMS type IssuerAndSerialNumber encoded
                 -- according to [RFC3852].
                 -- Identifies a certificate of the subject.
                 -- REQUIRED for TD-INVALID-CERTIFICATES and
                 -- TD-TRUSTED-CERTIFIERS.
       subjectKeyIdentifier    [2] IMPLICIT OCTET STRING OPTIONAL,
                 -- Identifies the subject's public key by a key
                 -- identifier.  When an X.509 certificate is
                 -- referenced, this key identifier matches the X.509

Zhu & Tung Standards Track [Page 34] RFC 4556 PKINIT June 2006

  1. - subjectKeyIdentifier extension value. When other
  2. - certificate formats are referenced, the documents
  3. - that specify the certificate format and their use
  4. - with the CMS must include details on matching the
  5. - key identifier to the appropriate certificate
  6. - field.
  7. - RECOMMENDED for TD-TRUSTED-CERTIFIERS.

     }
     AuthPack ::= SEQUENCE {
        pkAuthenticator         [0] PKAuthenticator,
        clientPublicValue       [1] SubjectPublicKeyInfo OPTIONAL,
                 -- Type SubjectPublicKeyInfo is defined in
                 -- [RFC3280].
                 -- Specifies Diffie-Hellman domain parameters
                 -- and the client's public key value [IEEE1363].
                 -- The DH public key value is encoded as a BIT
                 -- STRING according to [RFC3279].
                 -- This field is present only if the client wishes
                 -- to use the Diffie-Hellman key agreement method.
        supportedCMSTypes       [2] SEQUENCE OF AlgorithmIdentifier
                                    OPTIONAL,
                 -- Type AlgorithmIdentifier is defined in
                 -- [RFC3280].
                 -- List of CMS algorithm [RFC3370] identifiers
                 -- that identify key transport algorithms, or
                 -- content encryption algorithms, or signature
                 -- algorithms supported by the client in order of
                 -- (decreasing) preference.
        clientDHNonce           [3] DHNonce OPTIONAL,
                 -- Present only if the client indicates that it
                 -- wishes to reuse DH keys or to allow the KDC to
                 -- do so.
        ...
     }
     PKAuthenticator ::= SEQUENCE {
        cusec                   [0] INTEGER (0..999999),
        ctime                   [1] KerberosTime,
                 -- cusec and ctime are used as in [RFC4120], for
                 -- replay prevention.
        nonce                   [2] INTEGER (0..4294967295),
                 -- Chosen randomly; this nonce does not need to
                 -- match with the nonce in the KDC-REQ-BODY.
        paChecksum              [3] OCTET STRING OPTIONAL,
                 -- MUST be present.
                 -- Contains the SHA1 checksum, performed over

Zhu & Tung Standards Track [Page 35] RFC 4556 PKINIT June 2006

  1. - KDC-REQ-BODY.

     }
     TD-TRUSTED-CERTIFIERS ::= SEQUENCE OF
                    ExternalPrincipalIdentifier
                 -- Identifies a list of CAs trusted by the KDC.
                 -- Each ExternalPrincipalIdentifier identifies a CA
                 -- or a CA certificate (thereby its public key).
     TD-INVALID-CERTIFICATES ::= SEQUENCE OF
                    ExternalPrincipalIdentifier
                 -- Each ExternalPrincipalIdentifier identifies a
                 -- certificate (sent by the client) with an invalid
                 -- signature.
     KRB5PrincipalName ::= SEQUENCE {
         realm                   [0] Realm,
         principalName           [1] PrincipalName
     }
     AD-INITIAL-VERIFIED-CAS ::= SEQUENCE OF
                    ExternalPrincipalIdentifier
                 -- Identifies the certification path based on which
                 -- the client certificate was validated.
                 -- Each ExternalPrincipalIdentifier identifies a CA
                 -- or a CA certificate (thereby its public key).
     PA-PK-AS-REP ::= CHOICE {
        dhInfo                  [0] DHRepInfo,
                 -- Selected when Diffie-Hellman key exchange is
                 -- used.
        encKeyPack              [1] IMPLICIT OCTET STRING,
                 -- Selected when public key encryption is used.
                 -- Contains a CMS type ContentInfo encoded
                 -- according to [RFC3852].
                 -- The contentType field of the type ContentInfo is
                 -- id-envelopedData (1.2.840.113549.1.7.3).
                 -- The content field is an EnvelopedData.
                 -- The contentType field for the type EnvelopedData
                 -- is id-signedData (1.2.840.113549.1.7.2).
                 -- The eContentType field for the inner type
                 -- SignedData (when unencrypted) is
                 -- id-pkinit-rkeyData (1.3.6.1.5.2.3.3) and the
                 -- eContent field contains the DER encoding of the
                 -- type ReplyKeyPack.
                 -- ReplyKeyPack is defined below.
        ...

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 [RFC3852].
                 -- The contentType field of the type ContentInfo is
                 -- id-signedData (1.2.840.113549.1.7.2), and the
                 -- content field is a SignedData.
                 -- The eContentType field for the type SignedData is
                 -- id-pkinit-DHKeyData (1.3.6.1.5.2.3.2), and the
                 -- eContent field contains the DER encoding of the
                 -- type KDCDHKeyInfo.
                 -- KDCDHKeyInfo is defined below.
        serverDHNonce           [1] DHNonce OPTIONAL,
                 -- Present if and only if dhKeyExpiration is
                 -- present.
        ...
     }
     KDCDHKeyInfo ::= SEQUENCE {
        subjectPublicKey        [0] BIT STRING,
                 -- The KDC's DH public key.
                 -- The DH public key value is encoded as a BIT
                 -- STRING according to [RFC3279].
        nonce                   [1] INTEGER (0..4294967295),
                 -- Contains the nonce in the pkAuthenticator field
                 -- in the request if the DH keys are NOT reused,
                 -- 0 otherwise.
        dhKeyExpiration         [2] KerberosTime OPTIONAL,
                 -- Expiration time for KDC's key pair,
                 -- present if and only if the DH keys are reused.
                 -- If present, the KDC's DH public key MUST not be
                 -- used past the point of this expiration time.
                 -- If this field is omitted then the serverDHNonce
                 -- field MUST also be omitted.
        ...
     }
     ReplyKeyPack ::= SEQUENCE {
        replyKey                [0] EncryptionKey,
                 -- Contains the session key used to encrypt the
                 -- enc-part field in the AS-REP, i.e., the
                 -- AS reply key.
        asChecksum              [1] Checksum,
                -- Contains the checksum of the AS-REQ
                -- corresponding to the containing AS-REP.
                -- The checksum is performed over the type AS-REQ.

Zhu & Tung Standards Track [Page 37] RFC 4556 PKINIT June 2006

  1. - The protocol key [RFC3961] of the checksum is the
  2. - replyKey and the key usage number is 6.
  3. - If the replyKey's enctype is "newer" [RFC4120]
  4. - [RFC4121], the checksum is the required
  5. - checksum operation [RFC3961] for that enctype.
  6. - The client MUST verify this checksum upon receipt
  7. - 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 [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 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

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Zhu & Tung Standards Track [Page 42]

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