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PROPOSED STANDARD
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 (C) 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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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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