rfc2875.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

Network Working Group                                  H. Prafullchandra
Request for Comments: 2875                             Critical Path Inc
Category: Standards Track                                      J. Schaad
                                                               July 2000


             Diffie-Hellman Proof-of-Possession Algorithms

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 (2000).  All Rights Reserved.

Abstract

   This document describes two methods for producing an integrity check
   value from a Diffie-Hellman key pair.  This behavior is needed for
   such operations as creating the signature of a PKCS #10 certification
   request.  These algorithms are designed to provide a proof-of-
   possession rather than general purpose signing.

1. Introduction

   PKCS #10 [RFC2314] defines a syntax for certification requests. It
   assumes that the public key being requested for certification
   corresponds to an algorithm that is capable of signing/encrypting.
   Diffie-Hellman (DH) is a key agreement algorithm and as such cannot
   be directly used for signing or encryption.

   This document describes two new proof-of-possession algorithms using
   the Diffie-Hellman key agreement process to provide a shared secret
   as the basis of an integrity check value.  In the first algorithm,
   the value is constructed for a specific recipient/verifier by using a
   public key of that verifier.  In the second algorithm, the value is
   constructed for arbitrary verifiers.









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2. Terminology

   The following definitions will be used in this document

   DH certificate = a certificate whose SubjectPublicKey is a DH public
   value and is signed with any signature algorithm (e.g. RSA or DSA).

3. Static DH Proof-of-Possession Process

   The steps for creating a DH POP are:

   1. An entity (E) chooses the group parameters for a DH key
      agreement.

      This is done simply by selecting the group parameters from a
      certificate for the recipient of the POP process.

      A certificate with the correct group parameters has to be
      available. Let these common DH parameters be g and p; and let
      this DH key-pair be known as the Recipient key pair (Rpub and
      Rpriv).

      Rpub = g^x mod p         (where x=Rpriv, the private DH value and
                                ^ denotes exponentiation)

   2. The entity generates a DH public/private key-pair using the
      parameters from step 1.

      For an entity E:

         Epriv = DH private value = y
         Epub  = DH public value  = g^y mod p

   3. The POP computation process will then consist of:

      a) The value to be signed is obtained. (For a RFC2314 object, the
         value is the DER encoded certificationRequestInfo field
         represented as an octet string.) This will be the `text'
         referred to in [RFC2104], the data to which HMAC-SHA1 is
         applied.

      b) A shared DH secret is computed, as follows,

                shared secret = ZZ = g^xy mod p







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         [This is done by the entity E as Rpub^y and by the Recipient
         as Epub^x, where Rpub is retrieved from the Recipient's DH
         certificate (or is the one that was locally generated by the
         Entity) and Epub is retrieved from the actual certification
         request.]

      c) A temporary key K is derived from the shared secret ZZ as
         follows:

            K = SHA1(LeadingInfo | ZZ | TrailingInfo),
               where "|" means concatenation.

            LeadingInfo ::= Subject Distinguished Name from certificate
            TrailingInfo ::= Issuer Distinguished Name from certificate

      d) Compute HMAC-SHA1 over the data `text' as per [RFC2104] as:

            SHA1(K XOR opad, SHA1(K XOR ipad, text))

         where,
            opad (outer pad) = the byte 0x36 repeated 64 times and
            ipad (inner pad) = the byte 0x5C repeated 64 times.

         Namely,

          (1)  Append zeros to the end of K to create a 64 byte string
               (e.g., if K is of length 16 bytes it will be appended
               with 48 zero bytes 0x00).
          (2)  XOR (bitwise exclusive-OR) the 64 byte string computed
               in step (1) with ipad.
          (3)  Append the data stream `text' to the 64 byte string
               resulting from step (2).
          (4)  Apply SHA1 to the stream generated in step (3).
          (5)  XOR (bitwise exclusive-OR) the 64 byte string computed
               in step (1) with opad.
          (6)  Append the SHA1 result from step (4) to the 64 byte
               string resulting from step (5).
          (7)  Apply SHA1 to the stream generated in step (6) and
               output the result.

         Sample code is also provided in [RFC2104].

      e) The output of (d) is encoded as a BIT STRING (the Signature
         value).







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   The POP verification process requires the Recipient to carry out
   steps (a) through (d) and then simply compare the result of step (d)
   with what it received as the signature component. If they match then
   the following can be concluded:

      a) The Entity possesses the private key corresponding to the
         public key in the certification request because it needed the
         private key to calculate the shared secret; and
      b) Only the Recipient that the entity sent the request to could
         actually verify the request because they would require their
         own private key to compute the same shared secret. In the case
         where the recipient is a Certification Authority, this
         protects the Entity from rogue CAs.

   ASN Encoding

   The ASN.1 structures associated with the static Diffie-Hellman POP
   algorithm are:

      id-dhPop-static-HMAC-SHA1 OBJECT IDENTIFIER ::= { id-pkix
         id-alg(6) 3}

      DhPopStatic ::= SEQUENCE {
         issuerAndSerial IssuerAndSerialNumber OPTIONAL,
         hashValue       MessageDigest
      }

     issuerAndSerial is the issuer name and serial number of the
     certificate from which the public key was obtained.  The
     issuerAndSerial field is omitted if the public key did not come
     from a certificate.

     hashValue contains the result of the SHA-1 HMAC operation in step
     3d.

   DhPopStatic is encoded as a BIT STRING and is the signature value
   (i.e. encodes the above sequence instead of the raw output from 3d).

4. Discrete Logarithm Signature

   The use of a single set of parameters for an entire public key
   infrastructure allows all keys in the group to be attacked together.

   For this reason we need to create a proof of possession for Diffie-
   Hellman keys that does not require the use of a common set of
   parameters.





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   This POP is based on the Digital Signature Algorithm, but we have
   removed the restrictions imposed by the [FIPS-186] standard.  The use
   of this method does impose some additional restrictions on the set of
   keys that may be used, however if the key generation algorithm
   documented in [DH-X9.42] is used the required restrictions are met.
   The additional restrictions are the requirement for the existence of
   a q parameter. Adding the q parameter is generally accepted as a good
   practice as it allows for checking of small group attacks.

   The following definitions are used in the rest of this section:

      p is a large prime
      g = h(p-1)/q mod p ,
         where h is any integer 1 < h < p-1 such that h(p-1) mod q > 1
         (g has order q mod p)
      q is a large prime
      j is a large integer such that p = qj + 1

      x is a randomly or pseudo-randomly generated integer with
         1 < x < q
      y = g^x mod p

   Note: These definitions match the ones in [DH-X9.42].

4.1 Expanding the Digest Value

   Besides the addition of a q parameter, [FIPS-186] also imposes size
   restrictions on the parameters.  The length of q must be 160-bits
   (matching output of the SHA-1 digest algorithm) and length of p must
   be 1024-bits.  The size restriction on p is eliminated in this
   document, but the size restriction on q is replaced with the
   requirement that q must be at least 160-bits.  (The size restriction
   on q is identical with that in [DH-X9.42].)

   Given that there is not a random length-hashing algorithm, a hash
   value of the message will need to be derived such that the hash is in
   the range from 0 to q-1.  If the length of q is greater than 160-bits
   then a method must be provided to expand the hash length.

   The method for expanding the digest value used in this section does
   not add any additional security beyond the 160-bits provided by SHA-
   1.  The value being signed is increased mainly to enhance the
   difficulty of reversing the signature process.








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   This algorithm produces m the value to be signed.

   Let L = the size of q (i.e. 2^L <= q < 2^(L+1)).  Let M be the
   original message to be signed.

   1. Compute d = SHA-1(M), the SHA-1 digest of the original message.

   2. If L == 160 then m = d.

   3. If L > 160 then follow steps (a) through (d) below.

      a) Set n = L / 160, where / represents integer division,
         consequently, if L = 200, n = 1.
      b) Set m = d, the initial computed digest value.
      c) For i = 0 to n - 1
         m = m | SHA(m),  where "|" means concatenation.
      d) m = LEFTMOST(m, L-1), where LEFTMOST returns the L-1 left most
         bits of m.

   Thus the final result of the process meets the criteria that 0 <= m <
   q.

4.2 Signature Computation Algorithm

   The signature algorithm produces the pair of values (r, s), which is
   the signature. The signature is computed as follows:

   Given m, the value to be signed, as well as the parameters defined
   earlier in section 5.

   1. Generate a random or pseudorandom integer k, such that 0 < k^-1 <
      q.

   2. Compute r = (g^k mod p) mod q.

   3. If r is zero, repeat from step 1.

   4. Compute s = (k^-1 (m + xr)) mod q.

   5. If s is zero, repeat from step 1.

4.3 Signature Verification Algorithm

   The signature verification process is far more complicated than is
   normal for the Digital Signature Algorithm, as some assumptions about
   the validity of parameters cannot be taken for granted.





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   Given a message m to be validated, the signature value pair (r, s)
   and the parameters for the key.

   1. Perform a strong verification that p is a prime number.

   2. Perform a strong verification that q is a prime number.

   3. Verify that q is a factor of p-1, if any of the above checks fail
      then the signature cannot be verified and must be considered a
      failure.

   4. Verify that r and s are in the range [1, q-1].

   5. Compute w = (s^-1) mod q.

   6. Compute u1 = m*w mod q.

   7. Compute u2 = r*w mod q.

   8. Compute v = ((g^u1 * y^u2) mod p) mod q.

   9. Compare v and r, if they are the same then the signature verified
      correctly.

4.4 ASN Encoding

   The signature is encoded using

      id-alg-dhPOP OBJECT IDENTIFIER ::= {id-pkix id-alg(6) 4}

   The parameters for id-alg-dhPOP are encoded as DomainParameters
   (imported from [PROFILE]).  The parameters may be omitted in the
   signature, as they must exist in the associated key request.

   The signature value pair r and s are encoded using Dss-Sig-Value