S/MIME Working Group J Schaad
Internet Draft Soaring Hawk Consulting
Document: draft-ietf-smime-pss-00.txt February 2003
Category: Standards
Use of the PSS Signature Algorithm in CMS
Status of this Memo
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Abstract
This document specifies the conventions for using the RSA
Probabilistic Signature Scheme (RSASSA-PSS) digital signature
algorithm [P1v2.1] with the Cryptographic Message Syntax (CMS) [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
[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].
NOTE: AT PRESENT THERE ARE INCOMPATILITIES BETWEEN THIS DOCUMENT AND
RSASSA-PSS [P1v2.1] and RFC XXX [RSA-ALGS]. These will be resolved
before final acceptance by the working group and reflect issues that
I believe need to be addressed in those documents.
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 ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-1(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 contain RSASSA-
PSS-params. Again, complete details can be found in [PKALGS].
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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2.2 Signature Identifiers
The algorithm identifier for RSASAA-PSS signatures is:
id-RSASSA-PSS-sig OBJECT IDENTIFER ::= {pkcs-1 XX }
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 SHOULD 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-sig. The algorithm
parameters field MUST contain RSASSA-PSS-params.
signature contains the single value resulting from the signing
operation.
When the subjectPublicKeyInfo algorithm identifier for the public
key in the certificate contains id-RSASSA-PSS, the signature
verification processing MUST compare the RSASSA-PSS-params field in
the signed message and the certificate subjectPublicKeyInfo
algorithm. If the hashAlgorithm and maskGenAlgorithm fields are not
the same, then signature verification MUST fail.
AUTHOR NOTE: Verify that these are the correct set of tests to be
validated. It may change in [RSA-ALGS].
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
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CMS and PSS Signature February 2003
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.
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",
Internet-Draft, August 2002.
<draft-jonsson-pkcs1-v2dot1-00.txt>
RSA-ALGS Housley, R. and B. Kaliski, "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-00.txt,
December 2002.
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.
SHA2 National Institute of Standards and Technology
(NIST), FIPS 180-2: Secure Hash Standard, 1
August 2002.
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.
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.
RANDOM Eastlake, D., S. Crocker and J. Schiller
"Randomness Recommendations for Security",
RFC 1750, December 1994.
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7. Author's Address
Jim Schaad
Soaring Hawk Consulting
PO Box 675
Gold Bar, WA 98251
Email: jimsch@exmsft.com
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