Use of the RSASSA-PSS Signature Algorithm in Cryptographic Message Syntax (CMS)
draft-ietf-smime-pss-03

S/MIME Working Group                                         J Schaad 
Internet Draft                                Soaring Hawk Consulting 
Document: draft-ietf-smime-pss-03.txt                   December 2003 
Category: Standards                                                   
 
 
             Use of the RSA PSS Signature Algorithm in CMS 
 
 
Status of this Memo 
    
   This document is an Internet-Draft and is in full conformance with 
   all provisions of Section 10 of RFC2026 [1].  
    
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Abstract 
    
   This document specifies the conventions for using the RSA 
   Probabilistic Signature Scheme (RSASSA-PSS) digital signature 
   algorithm with the Cryptographic Message Syntax (CMS). 
    
    
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 RFC 2119 
   [STDWORDS]. 
    
    
1. Overview 
    
   This document specifies the conventions for using the RSASSA-PSS (RSA

   Signature Scheme with Appendix - Probabilistic Signature Scheme) 
   [P1v2.1] digital signature algorithm with the Cryptographic Message 
   Syntax [CMS] signed-data content type. 
 
  
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   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 XXX [RSA-
   ALGS].  All of the ASN.1 structures referenced in this document are 
   defined in RFC XXX. 
    
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 XXX [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 
 
 
2.2 Signature Identifiers 
     
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   The algorithm identifier for RSASAA-PSS signatures is: 
    
       id-RSASSA-PSS OBJECT IDENTIFER ::= {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. 
   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. 
    
     
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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. 
    
5. Normative References 
    
   CMS      Housley, R, "Cryptographic Message Syntax",  
            RFC 3369, August 2002. 
    
   P1v2.1   Jonsson, J., and B. Kaliski, "PKCS #1: RSA 
            Cryptography Specification Version 2.1", 
            RFC 3447, February 2003.  
    
   RSA-ALGS Schaad, J., B. Kaliski 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",  
            draft-ietf-pkix-rsa-pkalgs-01.txt,  
            November 2003. 
    
   STDWORDS S. Bradner, "Key Words for Use in RFCs to 
            Indicate Requirement Levels", RFC 2119, March 
            1997. 
    
   X.208-88 CCITT Recommendation X.208: Specification of 
            Abstract Syntax Notation One (ASN.1), 1998. 
     
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   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. Informational References 
    
   P1v1.5   Kaliski, B. and J. Staddon, "PKCS #1: RSA Encryption, 
            Version 2.0, RFC 2437, October 1998. 
    
   PKALGS   Polk, W, R Housley, L. Bassham, "Algorithms and Identifiers 
            for the Internet X.509 Public Key Infrastructure 
            Certificate and Certificate Revocation List (CRL) Profile", 
            RFC 3279, April 2002. 
    
   RANDOM   Eastlake, D., S. Crocker and J. Schiller 
            "Randomness Recommendations for Security",  
            RFC 1750, December 1994. 
 
 
7. Author's Address 
    
   Jim Schaad 
   Soaring Hawk Consulting 
   PO Box 675 
   Gold Bar, WA 98251 
    
   Email: jimsch@exmsft.com 
    
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