Network Working Group                                          J. Schaad
Request for Comments: 4056                       Soaring Hawk Consulting
Category: Standards Track                                    June 2005


               Use of the RSASSA-PSS Signature Algorithm
                 in Cryptographic Message Syntax (CMS)

 Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

 Copyright Notice

   Copyright © The Internet Society (2005).

 Abstract

   This document specifies the conventions for using the RSASSA-PSS (RSA
   Probabilistic Signature Scheme) digital signature algorithm with the
   Cryptographic Message Syntax (CMS).

1.  Overview

   This document specifies the conventions for using the RSA
   Probabilistic Signature Scheme (RSASSA-PSS) [P1v2.1] digital
   signature algorithm with the Cryptographic Message Syntax [CMS]
   signed-data content type.

   CMS values are generated using ASN.1 [X.208-88], using the Basic
   Encoding Rules (BER) [X.209-88] and the Distinguished Encoding Rules
   (DER) [X.509-88].

   This document is written to be used in conjunction with RFC 4055
   [RSA-ALGS].  All of the ASN.1 structures referenced in this document
   are defined in RFC 4055.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [STDWORDS].







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RFC 4056                 CMS and PSS Signature                 June 2005


1.1.  PSS Algorithm

   Although there are no known defects with the PKCS #1 v1.5 [P1v1.5]
   signature algorithm, RSASSA-PSS [P1v2.1] was developed in an effort
   to have more mathematically provable security.  PKCS #1 v1.5
   signatures were developed in an ad hoc manner; RSASSA-PSS was
   developed based on mathematical foundations.

2.  Algorithm Identifiers and Parameters

2.1.  Certificate Identifiers

   The RSASSA-PSS signature algorithm is defined in RFC 3447 [P1v2.1].
   Conventions for encoding the public key are defined in RFC 4055
   [RSA-ALGS].

   Two algorithm identifiers for RSA subject public keys in certificates
   are used.  These are:

      rsaEncryption OBJECT IDENTIFIER ::= { pkcs-1 1 }

   and

      id-RSASSA-PSS OBJECT IDENTIFIER ::= { pkcs-1 10 }

   When the rsaEncryption algorithm identifier is used for a public key,
   the AlgorithmIdentifier parameters field MUST contain NULL.  Complete
   details can be found in [RSA-ALGS].

   When the id-RSASSA-PSS algorithm identifier is used for a public key,
   the AlgorithmIdentifier parameters field MUST either be absent or
   contain RSASSA-PSS-params. Again, complete details can be found in
   [RSA-ALGS].

   In both cases, the RSA public key, which is composed of a modulus and
   a public exponent, MUST be encoded using the RSAPublicKey type.  The
   output of this encoding is carried in the certificate subject public
   key.

         RSAPublicKey ::= SEQUENCE {
            modulus INTEGER, -- n
            publicExponent INTEGER } -- e









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RFC 4056                 CMS and PSS Signature                 June 2005


2.2.  Signature Identifiers

   The algorithm identifier for RSASAA-PSS signatures is:

      id-RSASSA-PSS OBJECT IDENTIFIER ::= {pkcs-1 10 }

   When the id-RSASSA-PSS algorithm identifier is used for a signature,
   the AlgorithmIdentifier parameters field MUST contain RSASSA-PSS-
   params.  Information about RSASSA-PSS-params can be found in [RSA-
   ALGS].

   When signing, the RSA algorithm generates a single value, and that
   value is used directly as the signature value.

3.  Signed-data Conventions

   digestAlgorithms SHOULD contain the one-way hash function used to
   compute the message digest on the eContent value.

   The same one-way hash function SHOULD be used for computing the
   message digest on both the eContent and the signedAttributes value if
   signedAttributes exist.

   The same one-way hash function MUST be used for computing the message
   digest on the signedAttributes and as the hashAlgorithm in the RSA-
   PSS-params structure.

   signatureAlgorithm MUST contain id-RSASSA-PSS.  The algorithm
   parameters field MUST contain RSASSA-PSS-params.

   signature contains the single value resulting from the signing
   operation.

   If the subjectPublicKeyInfo algorithm identifier for the public key
   in the certificate is id-RSASSA-PSS and the parameters field is
   present, the following additional steps MUST be done as part of
   signature validation:

   1.  The hashAlgorithm field in the certificate
       subjectPublicKey.algorithm parameters and the signatureAlgorithm
       parameters MUST be the same.

   2.  The maskGenAlgorithm field in the certificate
       subjectPublicKey.algorithm parameters and the signatureAlgorithm
       parameters MUST be the same.






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RFC 4056                 CMS and PSS Signature                 June 2005


   3.  The saltLength in the signatureAlgorithm parameters MUST be
       greater or equal to the saltLength in the certificate
       subjectPublicKey.algorithm parameters.

   4.  The trailerField in the certificate subjectPublicKey.algorithm
       parameters and signatureAlgorithm parameters MUST be the same.

   In doing the above comparisons, default values are considered to be
   the same as extant values.  If any of the above four steps is not
   true, the signature checking algorithm MUST fail validation.

4.  Security Considerations

   Implementations must protect the RSA private key.  Compromise of the
   RSA private key may result in the ability to forge signatures.

   The generation of RSA private key relies on random numbers.  The use
   of inadequate pseudo-random number generators (PRNGs) to generate
   these values can result in little or no security.  An attacker may
   find it much easier to reproduce the PRNG environment that produced
   the keys, searching the resulting small set of possibilities, rather
   than brute force searching the whole key space.  The generation of
   quality random numbers is difficult.  RFC 1750 [RANDOM] offers
   important guidance in this area.

   Using the same private key for different algorithms has the potential
   of allowing an attacker to get extra information about the key.  It
   is strongly suggested that the same key not be used for both the PKCS
   #1 v1.5 and RSASSA-PSS signature algorithms.

   When computing signatures, the same hash function should be used for
   all operations.  This reduces the number of failure points in the
   signature process.

   The parameter checking procedures outlined in section 3 are of
   special importance.  It is possible to forge signatures by changing
   (especially to weaker values) these parameter values.  Signers using
   this algorithm should take care that only one set of parameter values
   is used as this decreases the possibility of leaking information.












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RFC 4056                 CMS and PSS Signature                 June 2005


5.  Normative References

   [CMS]       Housley, R., "Cryptographic Message Syntax (CMS)", RFC
               3852, July 2004.

   [P1v2.1]    Jonsson, J. and B. Kaliski, "Public-Key Cryptography
               Standards (PKCS) #1: RSA Cryptography Specifications
               Version 2.1", RFC 3447, February 2003.

   [RSA-ALGS]  Schaad, J., Kaliski, B., and R. Housley, "Additional
               Algorithms and Identifiers for RSA Cryptography for use
               in the Internet X.509 Public Key Infrastructure
               Certificate and Certificate Revocation List (CRL)
               Profile", RFC 4055, June 2005.

   [STDWORDS]  Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [X.208-88]  CCITT Recommendation X.208: Specification of Abstract
               Syntax Notation One (ASN.1), 1998.

   [X.209-88]  CCITT Recommendation X.209: Specification of Basic
               Encoding Rules for Abstract Syntax Notation One (ASN.1),
               1988.

   [X.509-88]  CCITT Recommendation X.509: The Directory Authentication
               Framework, 1988.

6.  Informative References

   [P1v1.5]    Kaliski, B., "PKCS #1: RSA Encryption Version 1.5", RFC
               2313, March 1998.

   [RANDOM]    Eastlake 3rd, D., Crocker, S., and J. Schiller,
               "Randomness Recommendations for Security", RFC 1750,
               December 1994.

Author' Address

   Jim Schaad
   Soaring Hawk Consulting
   PO Box 675
   Gold Bar, WA 98251

   EMail: jimsch@exmsft.com






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RFC 4056                 CMS and PSS Signature                 June 2005


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   Copyright © The Internet Society (2005).

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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.







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Use of the RSASSA-PSS Signature Algorithm in Cryptographic Message Syntax (CMS)

RFC TOTAL SIZE: 11514 bytes
PUBLICATION DATE: Wednesday, June 15th, 2005
LEGAL RIGHTS: The IETF Trust (see BCP 78)