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



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          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:

   - 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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   - 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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  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.




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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.
















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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].










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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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  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]













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  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



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                  -- hints to guide its selection of an appropriate
                  -- certificate chain to return to the client.
         kdcPkId                 [2] IMPLICIT OCTET STRING
                                     OPTIONAL,
                  -- Contains a CMS type SignerIdentifier encoded
                  -- according to [RFC3852].
                  -- Identifies, if present, a particular KDC
                  -- public key that the client already has.
         ...
      }

      DHNonce ::= OCTET STRING

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

      AuthPack ::= SEQUENCE {
         pkAuthenticator         [0] PKAuthenticator,
         clientPublicValue       [1] SubjectPublicKeyInfo OPTIONAL,
                  -- Type SubjectPublicKeyInfo is defined in
                  -- [RFC3280].
                  -- Specifies Diffie-Hellman domain parameters
                  -- and the client's public key value [IEEE1363].



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                  -- 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 (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]).









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  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



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      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



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      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:





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      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).



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  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.




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  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



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  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:






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      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



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                  -- according to [RFC3852].
                  -- The contentType field of the type ContentInfo is
                  -- id-envelopedData (1.2.840.113549.1.7.3).
                  -- The content field is an EnvelopedData.
                  -- The contentType field for the type EnvelopedData
                  -- is id-signedData (1.2.840.113549.1.7.2).
                  -- The eContentType field for the inner type
                  -- SignedData (when unencrypted) is
                  -- id-pkinit-rkeyData (1.3.6.1.5.2.3.3) and the
                  -- eContent field contains the DER encoding of the
                  -- type ReplyKeyPack.
                  -- ReplyKeyPack is defined in Section 3.2.3.2.
         ...
      }

      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



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                  -- 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



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      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:




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         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:




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  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.






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  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.




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  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.





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  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.





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  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.



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  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.





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  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.




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  [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.




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  [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.























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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



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                  -- according to [RFC3852].
                  -- The contentType field of the type ContentInfo
                  -- is id-signedData (1.2.840.113549.1.7.2),
                  -- and the content field is a SignedData.
                  -- The eContentType field for the type SignedData is
                  -- id-pkinit-authData (1.3.6.1.5.2.3.1), and the
                  -- eContent field contains the DER encoding of the
                  -- type AuthPack.
                  -- AuthPack is defined below.
         trustedCertifiers       [1] SEQUENCE OF
                     ExternalPrincipalIdentifier OPTIONAL,
                  -- Contains a list of CAs, trusted by the client,
                  -- that can be used to certify the KDC.
                  -- Each ExternalPrincipalIdentifier identifies a CA
                  -- or a CA certificate (thereby its public key).
                  -- The information contained in the
                  -- trustedCertifiers SHOULD be used by the KDC as
                  -- hints to guide its selection of an appropriate
                  -- certificate chain to return to the client.
         kdcPkId                 [2] IMPLICIT OCTET STRING
                                     OPTIONAL,
                  -- Contains a CMS type SignerIdentifier encoded
                  -- according to [RFC3852].
                  -- Identifies, if present, a particular KDC
                  -- public key that the client already has.
         ...
      }

      DHNonce ::= OCTET STRING

      ExternalPrincipalIdentifier ::= SEQUENCE {
         subjectName            [0] IMPLICIT OCTET STRING OPTIONAL,
                  -- Contains a PKIX type Name encoded according to
                  -- [RFC3280].
                  -- Identifies the certificate subject by the
                  -- distinguished subject name.
                  -- REQUIRED when there is a distinguished subject
                  -- name present in the certificate.
        issuerAndSerialNumber   [1] IMPLICIT OCTET STRING OPTIONAL,
                  -- Contains a CMS type IssuerAndSerialNumber encoded
                  -- according to [RFC3852].
                  -- Identifies a certificate of the subject.
                  -- REQUIRED for TD-INVALID-CERTIFICATES and
                  -- TD-TRUSTED-CERTIFIERS.
        subjectKeyIdentifier    [2] IMPLICIT OCTET STRING OPTIONAL,
                  -- Identifies the subject's public key by a key
                  -- identifier.  When an X.509 certificate is
                  -- referenced, this key identifier matches the X.509



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

      AuthPack ::= SEQUENCE {
         pkAuthenticator         [0] PKAuthenticator,
         clientPublicValue       [1] SubjectPublicKeyInfo OPTIONAL,
                  -- Type SubjectPublicKeyInfo is defined in
                  -- [RFC3280].
                  -- Specifies Diffie-Hellman domain parameters
                  -- and the client's public key value [IEEE1363].
                  -- The DH public key value is encoded as a BIT
                  -- STRING according to [RFC3279].
                  -- This field is present only if the client wishes
                  -- to use the Diffie-Hellman key agreement method.
         supportedCMSTypes       [2] SEQUENCE OF AlgorithmIdentifier
                                     OPTIONAL,
                  -- Type AlgorithmIdentifier is defined in
                  -- [RFC3280].
                  -- List of CMS algorithm [RFC3370] identifiers
                  -- that identify key transport algorithms, or
                  -- content encryption algorithms, or signature
                  -- algorithms supported by the client in order of
                  -- (decreasing) preference.
         clientDHNonce           [3] DHNonce OPTIONAL,
                  -- Present only if the client indicates that it
                  -- wishes to reuse DH keys or to allow the KDC to
                  -- do so.
         ...
      }

      PKAuthenticator ::= SEQUENCE {
         cusec                   [0] INTEGER (0..999999),
         ctime                   [1] KerberosTime,
                  -- cusec and ctime are used as in [RFC4120], for
                  -- replay prevention.
         nonce                   [2] INTEGER (0..4294967295),
                  -- Chosen randomly; this nonce does not need to
                  -- match with the nonce in the KDC-REQ-BODY.
         paChecksum              [3] OCTET STRING OPTIONAL,
                  -- MUST be present.
                  -- Contains the SHA1 checksum, performed over



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                  -- 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.
         ...



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      }

      DHRepInfo ::= SEQUENCE {
         dhSignedData            [0] IMPLICIT OCTET STRING,
                  -- Contains a CMS type ContentInfo encoded according
                  -- to [RFC3852].
                  -- The contentType field of the type ContentInfo is
                  -- id-signedData (1.2.840.113549.1.7.2), and the
                  -- content field is a SignedData.
                  -- The eContentType field for the type SignedData is
                  -- id-pkinit-DHKeyData (1.3.6.1.5.2.3.2), and the
                  -- eContent field contains the DER encoding of the
                  -- type KDCDHKeyInfo.
                  -- KDCDHKeyInfo is defined below.
         serverDHNonce           [1] DHNonce OPTIONAL,
                  -- Present if and only if dhKeyExpiration is
                  -- present.
         ...
      }

      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.



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

      TD-DH-PARAMETERS ::= SEQUENCE OF AlgorithmIdentifier
                  -- Each AlgorithmIdentifier specifies a set of
                  -- Diffie-Hellman domain parameters [IEEE1363].
                  -- This list is in decreasing preference order.
      END

Appendix B.  Test Vectors

  Function octetstring2key() is defined in Section 3.2.3.1.  This
  section describes a few sets of test vectors that would be useful for
  implementers of octetstring2key().

  Set 1:
  =====
  Input octet string x is:

    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    00 00 00 00 00 00 00 00 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




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  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




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  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: [email protected]


  Brian Tung
  Aerospace Corporation
  2350 E. El Segundo Blvd.
  El Segundo, CA  90245
  US

  EMail: [email protected]

































Zhu & Tung                  Standards Track                    [Page 41]

RFC 4556                         PKINIT                        June 2006


Full Copyright Statement

  Copyright (C) The Internet Society (2006).

  This document is subject to the rights, licenses and restrictions
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Acknowledgement

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Zhu & Tung                  Standards Track                    [Page 42]


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