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PROPOSED STANDARD

Internet Engineering Task Force (IETF)              L. Hornquist Astrand
Request for Comments: 8636                                    Apple, Inc
Updates: 4556                                                     L. Zhu
Category: Standards Track                             Oracle Corporation
ISSN: 2070-1721                                                M. Cullen
                                                       Painless Security
                                                               G. Hudson
                                                                     MIT
                                                               July 2019


Public Key Cryptography for Initial Authentication in Kerberos (PKINIT)
                           Algorithm Agility

Abstract

   This document updates the Public Key Cryptography for Initial
   Authentication in Kerberos (PKINIT) standard (RFC 4556) to remove
   protocol structures tied to specific cryptographic algorithms.  The
   PKINIT key derivation function is made negotiable, and the digest
   algorithms for signing the pre-authentication data and the client's
   X.509 certificates are made discoverable.

   These changes provide preemptive protection against vulnerabilities
   discovered in the future in any specific cryptographic algorithm and
   allow incremental deployment of newer algorithms.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8636.











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Copyright Notice

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   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.
























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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   4
   3.  paChecksum Agility  . . . . . . . . . . . . . . . . . . . . .   4
   4.  CMS Digest Algorithm Agility  . . . . . . . . . . . . . . . .   5
   5.  X.509 Certificate Signer Algorithm Agility  . . . . . . . . .   5
   6.  KDF Agility . . . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  Interoperability  . . . . . . . . . . . . . . . . . . . . . .  11
   8.  Test Vectors  . . . . . . . . . . . . . . . . . . . . . . . .  12
     8.1.  Common Inputs . . . . . . . . . . . . . . . . . . . . . .  12
     8.2.  Test Vector for SHA-1, enctype 18 . . . . . . . . . . . .  12
       8.2.1.  Specific Inputs . . . . . . . . . . . . . . . . . . .  12
       8.2.2.  Outputs . . . . . . . . . . . . . . . . . . . . . . .  12
     8.3.  Test Vector for SHA-256, enctype 18 . . . . . . . . . . .  13
       8.3.1.  Specific Inputs . . . . . . . . . . . . . . . . . . .  13
       8.3.2.  Outputs . . . . . . . . . . . . . . . . . . . . . . .  13
     8.4.  Test Vector for SHA-512, enctype 16 . . . . . . . . . . .  13
       8.4.1.  Specific Inputs . . . . . . . . . . . . . . . . . . .  13
       8.4.2.  Outputs . . . . . . . . . . . . . . . . . . . . . . .  13
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     11.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Appendix A.  PKINIT ASN.1 Module  . . . . . . . . . . . . . . . .  18
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   The Public Key Cryptography for Initial Authentication in Kerberos
   (PKINIT) standard [RFC4556] defines several protocol structures that
   are either tied to SHA-1 [RFC6234] or do not support negotiation or
   discovery but are instead based on local policy:

   o  The checksum algorithm in the authentication request is hardwired
      to use SHA-1.

   o  The acceptable digest algorithms for signing the authentication
      data are not discoverable.

   o  The key derivation function in Section 3.2.3.1 of [RFC4556] is
      hardwired to use SHA-1.

   o  The acceptable digest algorithms for signing the client X.509
      certificates are not discoverable.




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   In August 2004, Xiaoyun Wang's research group reported MD4 [RFC6150]
   collisions [WANG04], alongside attacks on later hash functions
   including MD5 [RFC1321] and SHA-1 [RFC6234].  These attacks and their
   consequences are discussed in [RFC6194].  These discoveries
   challenged the security of protocols relying on the collision-
   resistance properties of these hashes.

   The Internet Engineering Task Force (IETF) called for action to
   update existing protocols to provide crypto algorithm agility so that
   protocols support multiple cryptographic algorithms (including hash
   functions) and provide clean, tested transition strategies between
   algorithms, as recommended by BCP 201 [RFC7696].

   To address these concerns, new key derivation functions (KDFs),
   identified by object identifiers, are defined.  The PKINIT client
   provides a list of KDFs in the request, and the Key Distribution
   Center (KDC) picks one in the response.  Thus, a mutually supported
   KDF is negotiated.

   Furthermore, structures are defined to allow the client to discover
   the Cryptographic Message Syntax (CMS) [RFC5652] digest algorithms
   supported by the KDC for signing the pre-authentication data and the
   client X.509 certificate.

2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  paChecksum Agility

   The paChecksum defined in Section 3.2.1 of [RFC4556] provides a
   cryptographic binding between the client's pre-authentication data
   and the corresponding Kerberos request body.  This also prevents the
   KDC-REQ body from being tampered with.  SHA-1 is the only allowed
   checksum algorithm defined in [RFC4556].  This facility relies on the
   collision-resistance properties of the SHA-1 checksum [RFC6234].

   When the reply key delivery mechanism is based on public key
   encryption as described in Section 3.2.3.2 of [RFC4556], the
   asChecksum in the KDC reply provides integrity protection for the
   unauthenticated clear text in these messages and the binding between
   the pre-authentication and the ticket request and response messages.
   However, if the reply key delivery mechanism is based on the Diffie-
   Hellman key agreement as described in Section 3.2.3.1 of [RFC4556],



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   the security provided by using SHA-1 in the paChecksum is weak, and
   nothing else cryptographically binds the Authentication Service (AS)
   request to the ticket response.  In this case, the new KDF selected
   by the KDC, as described in Section 6, provides the cryptographic
   binding and integrity protection.

4.  CMS Digest Algorithm Agility

   Section 3.2.2 of [RFC4556] is updated to add optional typed data to
   the KDC_ERR_DIGEST_IN_SIGNED_DATA_NOT_ACCEPTED error.  When a KDC
   implementation conforming to this specification returns this error
   code, it MAY include a list of supported CMS types signifying the
   digest algorithms supported by the KDC in decreasing order of
   preference.  This is accomplished by including a
   TD_CMS_DATA_DIGEST_ALGORITHMS typed data element in the error data.

   td-cms-digest-algorithms INTEGER ::= 111

   The corresponding data for the TD_CMS_DATA_DIGEST_ALGORITHMS contains
   the TD-CMS-DIGEST-ALGORITHMS-DATA structure, which is ASN.1
   Distinguished Encoding Rules (DER) [X680] [X690] encoded and is
   defined as follows:

   TD-CMS-DIGEST-ALGORITHMS-DATA ::= SEQUENCE OF
       AlgorithmIdentifier
           -- Contains the list of CMS algorithm [RFC5652]
           -- identifiers indicating the digest algorithms
           -- acceptable to the KDC for signing CMS data in
           -- decreasing order of preference.

   The algorithm identifiers in TD-CMS-DIGEST-ALGORITHMS identify the
   digest algorithms supported by the KDC.

   This information sent by the KDC via TD_CMS_DATA_DIGEST_ALGORITHMS
   can facilitate troubleshooting when none of the digest algorithms
   supported by the client is supported by the KDC.

5.  X.509 Certificate Signer Algorithm Agility

   Section 3.2.2 of [RFC4556] is updated to add optional typed data to
   the KDC_ERR_DIGEST_IN_CERT_NOT_ACCEPTED error.  When a KDC conforming
   to this specification returns this error, it MAY send a list of
   digest algorithms acceptable to the KDC for use by the certification
   authority (CA) in signing the client's X.509 certificate in
   decreasing order of preference.  This is accomplished by including a
   TD_CERT_DIGEST_ALGORITHMS typed data element in the error data.  The
   corresponding data contains the ASN.1 DER encoding of the TD-CERT-
   DIGEST-ALGORITHMS-DATA structure defined as follows:



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   td-cert-digest-algorithms INTEGER ::= 112

   TD-CERT-DIGEST-ALGORITHMS-DATA ::= SEQUENCE {
           allowedAlgorithms [0] SEQUENCE OF AlgorithmIdentifier,
                      -- Contains the list of CMS algorithm [RFC5652]
                      -- identifiers indicating the digest algorithms
                      -- that are used by the CA to sign the client's
                      -- X.509 certificate and are acceptable to the KDC
                      -- in the process of validating the client's X.509
                      -- certificate in decreasing order of
                      -- preference.
           rejectedAlgorithm [1] AlgorithmIdentifier OPTIONAL,
                      -- This identifies the digest algorithm that was
                      -- used to sign the client's X.509 certificate and
                      -- has been rejected by the KDC in the process of
                      -- validating the client's X.509 certificate
                      -- [RFC5280].
           ...
   }

   The KDC fills in the allowedAlgorithm field with the list of
   algorithm [RFC5652] identifiers indicating digest algorithms that are
   used by the CA to sign the client's X.509 certificate and are
   acceptable to the KDC in the process of validating the client's X.509
   certificate in decreasing order of preference.  The rejectedAlgorithm
   field identifies the signing algorithm for use in signing the
   client's X.509 certificate that has been rejected by the KDC in the
   process of validating the client's certificate [RFC5280].

6.  KDF Agility

   Section 3.2.3.1 of [RFC4556] is updated to define additional key
   derivation functions (KDFs) to derive a Kerberos protocol key based
   on the secret value generated by the Diffie-Hellman key exchange.
   Section 3.2.1 of [RFC4556] is updated to add a new field to the
   AuthPack structure to indicate which new KDFs are supported by the
   client.  Section 3.2.3 of [RFC4556] is updated to add a new field to
   the DHRepInfo structure to indicate which KDF is selected by the KDC.

   The KDF algorithm described in this document (based on [SP80056A])
   can be implemented using any cryptographic hash function.










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   A new KDF for PKINIT usage is identified by an object identifier.
   The following KDF object identifiers are defined:

   id-pkinit OBJECT IDENTIFIER ::=
            { iso(1) identified-organization(3) dod(6) internet(1)
              security(5) kerberosv5(2) pkinit (3) }
       -- Defined in RFC 4556 and quoted here for the reader.

   id-pkinit-kdf OBJECT IDENTIFIER      ::= { id-pkinit kdf(6) }
       -- PKINIT KDFs

   id-pkinit-kdf-ah-sha1 OBJECT IDENTIFIER
       ::= { id-pkinit-kdf sha1(1) }
       -- SP800-56A ASN.1 structured hash-based KDF using SHA-1

   id-pkinit-kdf-ah-sha256 OBJECT IDENTIFIER
       ::= { id-pkinit-kdf sha256(2) }
       -- SP800-56A ASN.1 structured hash-based KDF using SHA-256

   id-pkinit-kdf-ah-sha512 OBJECT IDENTIFIER
       ::= { id-pkinit-kdf sha512(3) }
       -- SP800-56A ASN.1 structured hash-based KDF using SHA-512

   id-pkinit-kdf-ah-sha384 OBJECT IDENTIFIER
       ::= { id-pkinit-kdf sha384(4) }
       -- SP800-56A ASN.1 structured hash-based KDF using SHA-384

   Where id-pkinit is defined in [RFC4556].  All key derivation
   functions specified above use the one-step key derivation method
   described in Section 5.8.2.1 of [SP80056A], choosing the ASN.1 format
   for FixedInfo, and Section 4.1 of [SP80056C], choosing option 1 for
   the auxiliary function H.  id-pkinit-kdf-ah-sha1 uses SHA-1 [RFC6234]
   as the hash function.  id-pkinit-kdf-ah-sha256, id-pkinit-kdf-ah-
   sha356, and id-pkinit-kdf-ah-sha512 use SHA-256 [RFC6234], SHA-384
   [RFC6234], and SHA-512 [RFC6234], respectively.

   To name the input parameters, an abbreviated version of the key
   derivation method is described below.

   1.  reps = ceiling(L/H_outputBits)

   2.  Initialize a 32-bit, big-endian bit string counter as 1.

   3.  For i = 1 to reps by 1, do the following:

       1.  Compute Hashi = H(counter || Z || OtherInfo).

       2.  Increment counter (not to exceed 2^32-1)



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   4.  Set key_material = Hash1 || Hash2 || ... so that the length of
       key_material is L bits, truncating the last block as necessary.

   5.  The above KDF produces a bit string of length L in bits as the
       keying material.  The AS reply key is the output of random-to-
       key() [RFC3961], using that keying material as the input.

   The input parameters for these KDFs are provided as follows:

   o  H_outputBits is 160 bits for id-pkinit-kdf-ah-sha1, 256 bits for
      id-pkinit-kdf-ah-sha256, 384 bits for id-pkinit-kdf-ah-sha384, and
      512 bits for id-pkinit-kdf-ah-sha512.

   o  max_H_inputBits is 2^64.

   o  The secret value (Z) is the shared secret value generated by the
      Diffie-Hellman exchange.  The Diffie-Hellman shared value is first
      padded with leading zeros such that the size of the secret value
      in octets is the same as that of the modulus, then represented as
      a string of octets in big-endian order.

   o  The key data length (L) is the key-generation seed length in bits
      [RFC3961] for the Authentication Service (AS) reply key.  The
      enctype of the AS reply key is selected according to [RFC4120].

   o  The algorithm identifier (algorithmID) input parameter is the
      identifier of the respective KDF.  For example, this is id-pkinit-
      kdf-ah-sha1 if the KDF uses SHA-1 as the hash.

   o  The initiator identifier (partyUInfo) contains the ASN.1 DER
      encoding of the KRB5PrincipalName [RFC4556] that identifies the
      client as specified in the AS-REQ [RFC4120] in the request.

   o  The recipient identifier (partyVInfo) contains the ASN.1 DER
      encoding of the KRB5PrincipalName [RFC4556] that identifies the
      ticket-granting server (TGS) as specified in the AS-REQ [RFC4120]
      in the request.

   o  The supplemental public information (suppPubInfo) is the ASN.1 DER
      encoding of the PkinitSuppPubInfo structure, as defined later in
      this section.

   o  The supplemental private information (suppPrivInfo) is absent.








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   OtherInfo is the ASN.1 DER encoding of the following sequence:

   OtherInfo ::= SEQUENCE {
           algorithmID   AlgorithmIdentifier,
           partyUInfo     [0] OCTET STRING,
           partyVInfo     [1] OCTET STRING,
           suppPubInfo    [2] OCTET STRING OPTIONAL,
           suppPrivInfo   [3] OCTET STRING OPTIONAL
   }

   The PkinitSuppPubInfo structure is defined as follows:

   PkinitSuppPubInfo ::= SEQUENCE {
          enctype           [0] Int32,
              -- The enctype of the AS reply key.
          as-REQ            [1] OCTET STRING,
              -- The DER encoding of the AS-REQ [RFC4120] from the
              -- client.
          pk-as-rep         [2] OCTET STRING,
              -- The DER encoding of the PA-PK-AS-REP [RFC4556] in the
              -- KDC reply.
          ...
   }

   The PkinitSuppPubInfo structure contains mutually known public
   information specific to the authentication exchange.  The enctype
   field is the enctype of the AS reply key as selected according to
   [RFC4120].  The as-REQ field contains the DER encoding of the AS-REQ
   type [RFC4120] in the request sent from the client to the KDC.  Note
   that the as-REQ field does not include the wrapping 4-octet length
   when TCP is used.  The pk-as-rep field contains the DER encoding of
   the PA-PK-AS-REP [RFC4556] type in the KDC reply.  The
   PkinitSuppPubInfo provides a cryptographic binding between the pre-
   authentication data and the corresponding ticket request and
   response, thus addressing the concerns described in Section 3.

   The KDF is negotiated between the client and the KDC.  The client
   sends an unordered set of supported KDFs in the request, and the KDC
   picks one from the set in the reply.












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   To accomplish this, the AuthPack structure in [RFC4556] is extended
   as follows:

   AuthPack ::= SEQUENCE {
          pkAuthenticator   [0] PKAuthenticator,
          clientPublicValue [1] SubjectPublicKeyInfo OPTIONAL,
          supportedCMSTypes [2] SEQUENCE OF AlgorithmIdentifier
                   OPTIONAL,
          clientDHNonce     [3] DHNonce OPTIONAL,
          ...,
          supportedKDFs     [4] SEQUENCE OF KDFAlgorithmId OPTIONAL,
              -- Contains an unordered set of KDFs supported by the
              -- client.
          ...
   }

   KDFAlgorithmId ::= SEQUENCE {
          kdf-id            [0] OBJECT IDENTIFIER,
              -- The object identifier of the KDF
          ...
   }

   The new supportedKDFs field contains an unordered set of KDFs
   supported by the client.

   The KDFAlgorithmId structure contains an object identifier that
   identifies a KDF.  The algorithm of the KDF and its parameters are
   defined by the corresponding specification of that KDF.

   The DHRepInfo structure in [RFC4556] is extended as follows:

   DHRepInfo ::= SEQUENCE {
           dhSignedData         [0] IMPLICIT OCTET STRING,
           serverDHNonce        [1] DHNonce OPTIONAL,
           ...,
           kdf                  [2] KDFAlgorithmId OPTIONAL,
               -- The KDF picked by the KDC.
           ...
   }

   The new kdf field in the extended DHRepInfo structure identifies the
   KDF picked by the KDC.  If the supportedKDFs field is present in the
   request, a KDC conforming to this specification MUST choose one of
   the KDFs supported by the client and indicate its selection in the
   kdf field in the reply.  If the supportedKDFs field is absent in the
   request, the KDC MUST omit the kdf field in the reply and use the key





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   derivation function from Section 3.2.3.1 of [RFC4556].  If none of
   the KDFs supported by the client is acceptable to the KDC, the KDC
   MUST reply with the new error code KDC_ERR_NO_ACCEPTABLE_KDF:

   o  KDC_ERR_NO_ACCEPTABLE_KDF 100

   If the client fills the supportedKDFs field in the request but the
   kdf field in the reply is not present, the client can deduce that the
   KDC is not updated to conform with this specification, or that the
   exchange was subjected to a downgrade attack.  It is a matter of
   local policy on the client whether to reject the reply when the kdf
   field is absent in the reply; if compatibility with non-updated KDCs
   is not a concern, the reply should be rejected.

   Implementations conforming to this specification MUST support
   id-pkinit-kdf-ah-sha256.

7.  Interoperability

   An old client interoperating with a new KDC will not recognize a
   TD-CMS-DIGEST-ALGORITHMS-DATA element in a
   KDC_ERR_DIGEST_IN_SIGNED_DATA_NOT_ACCEPTED error or a TD-CERT-DIGEST-
   ALGORITHMS-DATA element in a KDC_ERR_DIGEST_IN_CERT_NOT_ACCEPTED
   error.  Because the error data is encoded as typed data, the client
   will ignore the unrecognized elements.

   An old KDC interoperating with a new client will not include a
   TD-CMS-DIGEST-ALGORITHMS-DATA element in a
   KDC_ERR_DIGEST_IN_SIGNED_DATA_NOT_ACCEPTED error or a TD-CERT-DIGEST-
   ALGORITHMS-DATA element in a KDC_ERR_DIGEST_IN_CERT_NOT_ACCEPTED
   error.  To the client, this appears just as if a new KDC elected not
   to include a list of digest algorithms.

   An old client interoperating with a new KDC will not include the
   supportedKDFs field in the request.  The KDC MUST omit the kdf field
   in the reply and use the [RFC4556] KDF as expected by the client or
   reject the request if local policy forbids use of the old KDF.

   A new client interoperating with an old KDC will include the
   supportedKDFs field in the request; this field will be ignored as an
   unknown extension by the KDC.  The KDC will omit the kdf field in the
   reply and will use the [RFC4556] KDF.  The client can deduce from the
   omitted kdf field that the KDC is not updated to conform to this
   specification or that the exchange was subjected to a downgrade
   attack.  The client MUST use the [RFC4556] KDF or reject the reply if
   local policy forbids the use of the old KDF.





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8.  Test Vectors

   This section contains test vectors for the KDF defined above.

8.1.  Common Inputs

Z: Length = 256 bytes, Hex Representation = (All Zeros)
00000000 00000000 00000000 00000000 000000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 000000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 000000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 000000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 000000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 000000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 000000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 000000000 00000000 00000000 00000000

client: Length = 9 bytes, ASCII Representation = lha@SU.SE

server: Length = 18 bytes, ASCII Representation = krbtgt/SU.SE@SU.SE

as-req: Length = 10 bytes, Hex Representation =
AAAAAAAA AAAAAAAA AAAA

pk-as-rep:  Length = 9 bytes, Hex Representation =
BBBBBBBB BBBBBBBB BB

ticket: Length =  55 bytes, Hex Representation =
61353033 A0030201 05A1071B 0553552E 5345A210 300EA003 020101A1 0730051B
036C6861 A311300F A0030201 12A20804 0668656A 68656A

8.2.  Test Vector for SHA-1, enctype 18

8.2.1.  Specific Inputs

   algorithm-id: (id-pkinit-kdf-ah-sha1) Length = 8 bytes, Hex
   Representation = 2B060105 02030601

   enctype: (aes256-cts-hmac-sha1-96) Length = 1 byte, Decimal
   Representation = 18

8.2.2.  Outputs

 key-material: Length = 32 bytes, Hex Representation =
 E6AB38C9 413E035B B079201E D0B6B73D 8D49A814 A737C04E E6649614 206F73AD

 key: Length = 32 bytes, Hex Representation =
 E6AB38C9 413E035B B079201E D0B6B73D 8D49A814 A737C04E E6649614 206F73AD




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8.3.  Test Vector for SHA-256, enctype 18

8.3.1.  Specific Inputs

   algorithm-id: (id-pkinit-kdf-ah-sha256) Length = 8 bytes, Hex
   Representation = 2B060105 02030602

   enctype: (aes256-cts-hmac-sha1-96) Length = 1 byte, Decimal
   Representation = 18

8.3.2.  Outputs

 key-material: Length = 32 bytes, Hex Representation =
 77EF4E48 C420AE3F EC75109D 7981697E ED5D295C 90C62564 F7BFD101 FA9bC1D5

 key: Length = 32 bytes, Hex Representation =
 77EF4E48 C420AE3F EC75109D 7981697E ED5D295C 90C62564 F7BFD101 FA9bC1D5

8.4.  Test Vector for SHA-512, enctype 16

8.4.1.  Specific Inputs

   algorithm-id: (id-pkinit-kdf-ah-sha512) Length = 8 bytes, Hex
   Representation = 2B060105 02030603

   enctype: (des3-cbc-sha1-kd) Length = 1 byte, Decimal
   Representation = 16

8.4.2.  Outputs

   key-material: Length = 24 bytes, Hex Representation =
   D3C78A79 D65213EF E9A826F7 5DFB01F7 2362FB16 FB01DAD6

   key: Length = 32 bytes, Hex Representation =
   D3C78A79 D65213EF E9A826F7 5DFB01F7 2362FB16 FB01DAD6

9.  Security Considerations

   This document describes negotiation of checksum types, key derivation
   functions, and other cryptographic functions.  If a given negotiation
   is unauthenticated, care must be taken to accept only secure values;
   to do otherwise allows an active attacker to perform a downgrade
   attack.

   The discovery method described in Section 4 uses a Kerberos error
   message, which is unauthenticated in a typical exchange.  An attacker
   may attempt to downgrade a client to a weaker CMS type by forging a
   KDC_ERR_DIGEST_IN_SIGNED_DATA_NOT_ACCEPTED error.  It is a matter of



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RFC 8636                PKINIT Algorithm Agility               July 2019


   local policy whether a client accepts a downgrade to a weaker CMS
   type and whether the KDC accepts the weaker CMS type.  A client may
   reasonably assume that the real KDC implements all hash functions
   used in the client's X.509 certificate, and so the client may refuse
   attempts to downgrade to weaker hash functions.

   The discovery method described in Section 5 also uses a Kerberos
   error message.  An attacker may attempt to downgrade a client to a
   certificate using a weaker signing algorithm by forging a
   KDC_ERR_DIGEST_IN_CERT_NOT_ACCEPTED error.  It is a matter of local
   policy whether a client accepts a downgrade to a weaker certificate
   and whether the KDC accepts the weaker certificate.  This attack is
   only possible if the client device possesses multiple client
   certificates of varying strengths.

   In the KDF negotiation method described in Section 6, the client
   supportedKDFs value is protected by the signature on the
   signedAuthPack field in the request.  If this signature algorithm is
   vulnerable to collision attacks, an attacker may attempt to downgrade
   the negotiation by substituting an AuthPack with a different or
   absent supportedKDFs value, using a PKINIT freshness token [RFC8070]
   to partially control the legitimate AuthPack value.  A client that is
   performing anonymous PKINIT [RFC8062] does not sign the AuthPack, so
   an attacker can easily remove the supportedKDFs value in this case.
   Finally, the kdf field in the DHRepInfo of the KDC response is
   unauthenticated and could be altered or removed by an attacker,
   although this alteration will likely result in a decryption failure
   by the client rather than a successful downgrade.  It is a matter of
   local policy whether a client accepts a downgrade to the old KDF and
   whether the KDC allows the use of the old KDF.

   The paChecksum field, which binds the client pre-authentication data
   to the Kerberos request body, remains fixed at SHA-1.  If an attacker
   substitutes a different request body using an attack against SHA-1 (a
   second preimage attack is likely required as the attacker does not
   control any part of the legitimate request body), the KDC will not
   detect the substitution.  Instead, if a new KDF is negotiated, the
   client will detect the substitution by failing to decrypt the reply.

   An attacker may attempt to impersonate the KDC to the client via an
   attack on the hash function used in the dhSignedData signature,
   substituting the attacker's subjectPublicKey for the legitimate one
   without changing the hash value.  It is a matter of local policy
   which hash function the KDC uses in its signature and which hash
   functions the client will accept in the KDC signature.  A KDC may
   reasonably assume that the client implements all hash functions used
   in the KDF algorithms listed the supportedKDFs field of the request.




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RFC 8636                PKINIT Algorithm Agility               July 2019


10.  IANA Considerations

   IANA has made the following assignments in the Kerberos "Pre-
   authentication and Typed Data" registry created by Section 7.1 of RFC
   6113.

               TD-CMS-DIGEST-ALGORITHMS   111  [RFC8636]
               TD-CERT-DIGEST-ALGORITHMS  112  [RFC8636]

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3961]  Raeburn, K., "Encryption and Checksum Specifications for
              Kerberos 5", RFC 3961, DOI 10.17487/RFC3961, February
              2005, <https://www.rfc-editor.org/info/rfc3961>.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              DOI 10.17487/RFC4120, July 2005,
              <https://www.rfc-editor.org/info/rfc4120>.

   [RFC4556]  Zhu, L. and B. Tung, "Public Key Cryptography for Initial
              Authentication in Kerberos (PKINIT)", RFC 4556,
              DOI 10.17487/RFC4556, June 2006,
              <https://www.rfc-editor.org/info/rfc4556>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <https://www.rfc-editor.org/info/rfc5652>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <https://www.rfc-editor.org/info/rfc6234>.





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RFC 8636                PKINIT Algorithm Agility               July 2019


   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [SP80056A] Barker, E., Chen, L., Roginsky, A., Vassilev, A., and R.
              Davis, "Recommendation for Pair-Wise Key-Establishment
              Schemes Using Discrete Logarithm Cryptography", NIST
              Special Publications 800-56A, Revision 3,
              DOI 10.6028/NIST.SP.800-56Ar3, April 2018,
              <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
              NIST.SP.800-56Ar3.pdf>.

   [SP80056C] Barker, E., Chen, L., and R. Davis, "Recommendation for
              Key-Derivation Methods in Key-Establishment Schemes", NIST
              Special Publications 800-56C, Revision 1,
              DOI 10.6028/NIST.SP.800-56Cr1, April 2018,
              <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
              NIST.SP.800-56Cr1.pdf>.

   [X680]     ITU-T, "Information technology - Abstract Syntax Notation
              One (ASN.1): Specification of basic notation", ITU-T
              Recommendation X.680, August 2015,
              <https://www.itu.int/rec/T-REC-X.680-201508-I/en>.

   [X690]     ITU-T, "Information technology - ASN.1 encoding Rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER)", ITU-T Recommendation X.690, August 2015,
              <https://www.itu.int/rec/T-REC-X.690-201508-I/en>.

11.2.  Informative References

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              DOI 10.17487/RFC1321, April 1992,
              <https://www.rfc-editor.org/info/rfc1321>.

   [RFC6150]  Turner, S. and L. Chen, "MD4 to Historic Status",
              RFC 6150, DOI 10.17487/RFC6150, March 2011,
              <https://www.rfc-editor.org/info/rfc6150>.

   [RFC6194]  Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
              Considerations for the SHA-0 and SHA-1 Message-Digest
              Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,
              <https://www.rfc-editor.org/info/rfc6194>.







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RFC 8636                PKINIT Algorithm Agility               July 2019


   [RFC7696]  Housley, R., "Guidelines for Cryptographic Algorithm
              Agility and Selecting Mandatory-to-Implement Algorithms",
              BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
              <https://www.rfc-editor.org/info/rfc7696>.

   [RFC8062]  Zhu, L., Leach, P., Hartman, S., and S. Emery, Ed.,
              "Anonymity Support for Kerberos", RFC 8062,
              DOI 10.17487/RFC8062, February 2017,
              <https://www.rfc-editor.org/info/rfc8062>.

   [RFC8070]  Short, M., Ed., Moore, S., and P. Miller, "Public Key
              Cryptography for Initial Authentication in Kerberos
              (PKINIT) Freshness Extension", RFC 8070,
              DOI 10.17487/RFC8070, February 2017,
              <https://www.rfc-editor.org/info/rfc8070>.

   [WANG04]   Wang, X., Lai, X., Feng, D., Chen, H., and X. Yu,
              "Cryptanalysis of the Hash Functions MD4 and RIPEMD",
              Advances in Cryptology - EUROCRYPT 2005,
              DOI 10.1007/11426639_1, August 2004.































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Appendix A.  PKINIT ASN.1 Module

   KerberosV5-PK-INIT-Agility-SPEC {
          iso(1) identified-organization(3) dod(6) internet(1)
          security(5) kerberosV5(2) modules(4) pkinit(5) agility (1)
   } DEFINITIONS EXPLICIT TAGS ::= BEGIN

   IMPORTS
      AlgorithmIdentifier, SubjectPublicKeyInfo
          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 5280.

      Ticket, Int32, 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.

      PKAuthenticator, DHNonce, id-pkinit
          FROM KerberosV5-PK-INIT-SPEC {
            iso(1) identified-organization(3) dod(6) internet(1)
            security(5) kerberosV5(2) modules(4) pkinit(5) };
            -- as defined in RFC 4556.

   id-pkinit-kdf OBJECT IDENTIFIER      ::= { id-pkinit kdf(6) }
       -- PKINIT KDFs

   id-pkinit-kdf-ah-sha1 OBJECT IDENTIFIER
       ::= { id-pkinit-kdf sha1(1) }
       -- SP800-56A ASN.1 structured hash-based KDF using SHA-1

   id-pkinit-kdf-ah-sha256 OBJECT IDENTIFIER
       ::= { id-pkinit-kdf sha256(2) }
       -- SP800-56A ASN.1 structured hash-based KDF using SHA-256

   id-pkinit-kdf-ah-sha512 OBJECT IDENTIFIER
       ::= { id-pkinit-kdf sha512(3) }
       -- SP800-56A ASN.1 structured hash-based KDF using SHA-512

   id-pkinit-kdf-ah-sha384 OBJECT IDENTIFIER
       ::= { id-pkinit-kdf sha384(4) }
       -- SP800-56A ASN.1 structured hash-based KDF using SHA-384






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RFC 8636                PKINIT Algorithm Agility               July 2019


   TD-CMS-DIGEST-ALGORITHMS-DATA ::= SEQUENCE OF
       AlgorithmIdentifier
           -- Contains the list of CMS algorithm [RFC5652]
           -- identifiers indicating the digest algorithms
           -- acceptable to the KDC for signing CMS data in
           -- decreasing order of preference.

   TD-CERT-DIGEST-ALGORITHMS-DATA ::= SEQUENCE {
          allowedAlgorithms [0] SEQUENCE OF AlgorithmIdentifier,
              -- Contains the list of CMS algorithm [RFC5652]
              -- identifiers indicating the digest algorithms
              -- that are used by the CA to sign the client's
              -- X.509 certificate and are acceptable to the KDC
              -- in the process of validating the client's X.509
              -- certificate in decreasing order of
              -- preference.
          rejectedAlgorithm [1] AlgorithmIdentifier OPTIONAL,
              -- This identifies the digest algorithm that was
              -- used to sign the client's X.509 certificate and
              -- has been rejected by the KDC in the process of
              -- validating the client's X.509 certificate
              -- [RFC5280].
          ...
   }

   OtherInfo ::= SEQUENCE {
           algorithmID   AlgorithmIdentifier,
           partyUInfo     [0] OCTET STRING,
           partyVInfo     [1] OCTET STRING,
           suppPubInfo    [2] OCTET STRING OPTIONAL,
           suppPrivInfo   [3] OCTET STRING OPTIONAL
   }

   PkinitSuppPubInfo ::= SEQUENCE {
          enctype           [0] Int32,
              -- The enctype of the AS reply key.
          as-REQ            [1] OCTET STRING,
              -- The DER encoding of the AS-REQ [RFC4120] from the
              -- client.
          pk-as-rep         [2] OCTET STRING,
              -- The DER encoding of the PA-PK-AS-REP [RFC4556] in the
              -- KDC reply.
          ...
   }







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RFC 8636                PKINIT Algorithm Agility               July 2019


   AuthPack ::= SEQUENCE {
          pkAuthenticator   [0] PKAuthenticator,
          clientPublicValue [1] SubjectPublicKeyInfo OPTIONAL,
          supportedCMSTypes [2] SEQUENCE OF AlgorithmIdentifier
                   OPTIONAL,
          clientDHNonce     [3] DHNonce OPTIONAL,
          ...,
          supportedKDFs     [4] SEQUENCE OF KDFAlgorithmId OPTIONAL,
              -- Contains an unordered set of KDFs supported by the
              -- client.
          ...
   }

   KDFAlgorithmId ::= SEQUENCE {
          kdf-id            [0] OBJECT IDENTIFIER,
              -- The object identifier of the KDF
          ...
   }

   DHRepInfo ::= SEQUENCE {
          dhSignedData      [0] IMPLICIT OCTET STRING,
          serverDHNonce     [1] DHNonce OPTIONAL,
          ...,
          kdf               [2] KDFAlgorithmId OPTIONAL,
              -- The KDF picked by the KDC.
          ...
   }
   END























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RFC 8636                PKINIT Algorithm Agility               July 2019


Acknowledgements

   Jeffery Hutzelman, Shawn Emery, Tim Polk, Kelley Burgin, Ben Kaduk,
   Scott Bradner, and Eric Rescorla reviewed the document and provided
   suggestions for improvements.

Authors' Addresses

   Love Hornquist Astrand
   Apple, Inc
   Cupertino, CA
   United States of America

   Email: lha@apple.com


   Larry Zhu
   Oracle Corporation
   500 Oracle Parkway
   Redwood Shores, CA  94065
   United States of America

   Email: larryzhu@live.com


   Margaret Cullen
   Painless Security
   4 High St, Suite 134
   North Andover, MA  01845
   United States of America

   Phone: +1 781-405-7464
   Email: margaret@painless-security.com


   Greg Hudson
   MIT

   Email: ghudson@mit.edu












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