Network Working Group J. Schaad
Internet-Draft Soaring Hawk Consulting
Intended status: Standards Track November 22, 2010
Expires: May 26, 2011
Signer Info Algorithm Protection Attribute
draft-schaad-smime-algorithm-attribute-02
draft-schaad-smime-algorithm-attribute-03
Abstract
A new attribute is defined that allows for protection of the digest
and signature algorithm structures in an authenticated data or a
signer info structure. Using the attribute includes the algorithm
definition information in the integrity protection process.
Status of this Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notation . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Attribute Structure . . . . . . . . . . . . . . . . . . . . . 5
3. Verification Process . . . . . . . . . . . . . . . . . . . . . 7
3.1. Signed Data Verification Changes . . . . . . . . . . . . . 7
3.2. Authenticated Data Verification Changes . . . . . . . . . 7
4. Security IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. 10
6.1. Normative References . . . . . . . . . . . . . . . . . . . 9
5.2. 10
6.2. Informational References . . . . . . . . . . . . . . . . . 9 10
Appendix A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 10 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12 13
1. Introduction
In the current definition of [CMS], there are some fields that are
not protected in the process of doing either a signature validation
or an authentication validation. In this document a new signed or
authenticated attribute is defined which permits these fields to be
validated.
Taking the SignerInfo structure from CMS, let's look at each of the
fields and discuss what is and is not protected by the signature.
The ASN.1 is included here for convenience. (The analysis of
AuthenticatedData is similar.)
SignerInfo ::= SEQUENCE {
version CMSVersion,
sid SignerIdentifier,
digestAlgorithm DigestAlgorithmIdentifier,
signedAttrs [0] IMPLICIT SignedAttributes OPTIONAL,
signatureAlgorithm SignatureAlgorithmIdentifier,
signature SignatureValue,
unsignedAttrs [1] IMPLICIT UnsignedAttributes OPTIONAL }
version is not protected by the signature. Many implementations of
CMS today actually ignore the value of this field. If the
structure decodes then this is considered sufficient to continue
processing. Using most decoders on the market the value of this
field does not control how the decoding is actually processed.
sid can be protected by the use of either version of the signing
certificate authenticated attribute. SigningCertificateV2 is
defined in [RFC5035]. SigningCertificate is defined in [RFC2634].
In addition to allowing for the protection of the signer
identifier, the specific certificate is protected by including a
hash of the certificate to be used for validation.
digestAlgorithm the digest algorithm used has been implicitly
protected by the fact that CMS has only defined one digest
algorithm for each hash value length. (The algorithm RIPEM-160
was never standardized). If newer digest algorithms are defined
where there are multiple algorithms for a given hash length, or
where parameters are defined for a specific algorithm, this
implicit protection will no longer exist.
signedAttributes are directly protected by the signature when they
are present. The DER encoding of this value is what is actually
hashed for the signature computation.
signatureAlgorithm has been protected by implication in the past.
The use of an RSA public key implied that the RSA v 1.5 signature
algorithm was being used. The hash algorithm and this fact could
be checked by the internal padding defined. This is no longer
true with the addition of the RSA-PSS signature algorithms. The
use of a DSA public key implied the SHA-1 hash algorithm as that
was the only possible hash algorithm and the DSA was the public
signature algorithm. This is longer true with the addition of the
SHA2 signature algorithms.
signature is not directly protected by any other value unless a
counter signature is present. However this represents the
cryptographically computed value that protects the rest of the
signature information.
unsignedAttrs is not protected by the signature value. It is also
explicitly designed not to be protected by the signature value.
As can be seen above, the digestAlgorithm and signatureAlgorithm
fields have been indirectly rather than explicitly protected in the
past. With new algorithms that have been or are being defined this
will no longer be the case. This document defines and describes a
new attribute that will explicitly protect these fields along with
the macAlgorithm field of the AuthenticatedData structure.
1.1. Notation
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 [RFC2119].
2. Attribute Structure
The following defines the algorithm protection attribute:
The algorithm-protection attribute has the ASN.1 type
CMSAlgorithmProtection:
aa-cmsAlgorithmProtection ATTRIBUTE ::= {
TYPE CMSAlgorithmProtection
IDENTIFIED BY { id-aa-CMSAlgorithmProtection }
}
The following object identifier identifies the algorithm-protection
attribute:
id-aa-CMSAlgorithmProtection OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) 52 }
The algorithm-protection attribute uses the following ASN.1 type:
CMSAlgorithmProtection ::= SEQUENCE {
digestAlgorithm DigestAlgorithmIdentifier,
signatureAlgorithm [1] SignatureAlgorithmIdentifier OPTIONAL,
macAlgorithm [2] MessageAuthenticationCodeAlgorithm
OPTIONAL
}
(WITH COMPONENTS { signatureAlgorithm PRESENT,
macAlgorithm ABSENT } |
WITH COMPONENTS { signatureAlgorithm ABSENT,
macAlgorithm PRESENT })
The fields are defined as follows:
digestAlgorithm contains a copy of the SignerInfo.digestAlgorithm
field or the AuthenticatedData.digestAlgorithm field including any
parameters associated with it.
signatureAlgorithm contains a copy of the signature algorithm
identifier and any parameters associated with it. This field is
only populated if the attribute is placed in a
SignerInfo.signedAttrs sequence.
macAlgorithm contains a copy of the message authentication code
algorithm identifier and any parameters associated with it. This
field is only populated if the attribute is placed in an
AuthenticatedData.authAttrs sequence.
Exactly one of signatureAlgorithm and macAlgorithm SHALL be present.
An algorithm-protection attribute MUST have a single attribute value,
even though the syntax is defined as a SET OF AttributeValue. There
MUST NOT be zero or multiple instances of AttributeValue present.
The algorithm-protection attribute MUST be a signed attribute or an
authenticated attribute; it MUST NOT be an unsigned attribute, an
unauthenticated attribute or an unprotected attribute.
The SignedAttributes and AuthAttributes syntax are each defined as a
SET of Attributes. The SignedAttributes in a signerInfo MUST include
only one instance of the algorithm protection attribute. Similarly,
the AuthAttributes in an AuthenticatedData MUST include only one
instance of the algorithm protection attribute.
3. Verification Process
The exact verification process depends on the structure being dealt
with.
When doing comparisons of the fields, a field whose value is a
default value and one which is explicitly provided MUST compare as
equivalent. It is not required that a field which is absent in one
case and present in another case be compared as equivalent. (This
means that an algorithm identifier with absent parameters and one
with NULL parameters are not expected to compare as equivalent.)
3.1. Signed Data Verification Changes
If a CMS validator supports this attribute, the following additional
verification steps MUST be performed:
1. The SignerInfo.digestAlgorithm field MUST be compared to the
digestAlgorithm field in the attribute. If the fields are not the
same (modulo encoding) then signature validation MUST fail.
2. The SignerInfo.signatureAlgorithm field MUST be compared to the
signatureAlgorithm field in the attribute. If the fields are not the
same (modulo encoding) then the signature validation MUST fail.
3.2. Authenticated Data Verification Changes
If a CMS validator supports this attribute, the following additional
verification steps MUST be performed:
1. The AuthenticatedData.digestAlgorithm field MUST be compared to
the digestAlgorithm field in the attribute. If the fields are not
same (modulo encoding) then signature validation MUST fail.
2. The AuthenticatedData.macAlgorithm field MUST be compared to the
macAlgorithm field in the attribute. If the fields are not the same
(modulo encoding) then the signature validation MUST fail.
4. IANA Considerations
There are no IANA considerations. All identifiers are assigned out
of the S/MIME OID arc.
5. Security Considerations
This document is designed to address the security issue of algorithm
substitutions of the algorithms used by the validator. At this time
there is no known method to exploit this type of attack. If the
attack could be successful, then either a weaker algorithm could be
substituted for a stronger algorithm or the parameters could be
modified by an attacker to change the behavior of the hashing
algorithm used. (One example would be changing the initial parameter
value for [I-D.schaad-smime-hash-experiment].)
The attribute defined in this document is to be placed in a location
that is protected by the signature or message authentication code.
This attribute does not provide any additional security if placed in
an un-signed or un-authenticated location.
5.
6. References
5.1.
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2634] Hoffman, P., "Enhanced Security Services for S/MIME",
RFC 2634, June 1999.
[RFC5035] Schaad, J., "Enhanced Security Services (ESS) Update:
Adding CertID Algorithm Agility", RFC 5035, August 2007.
[CMS] Housley, R., "Cryptographic Message Syntax (CMS)",
RFC 5652, September 2009.
[RFC5912] Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
June 2010.
5.2.
6.2. Informational References
[I-D.schaad-smime-hash-experiment]
Schaad, J., "Experiment: Hash functions with parameters in
CMS and S/MIME", draft-schaad-smime-hash-experiment-01
(work in progress), December 2009.
Appendix A. ASN.1 Module
CMSAlgorithmProtectionAttribute
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0)
id-mod-cms-algorithmProtect(52) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
IMPORTS
-- Cryptographic Message Syntax (CMS) [RFC5652] [CMS]
DigestAlgorithmIdentifier, MessageAuthenticationCodeAlgorithm,
SignatureAlgorithmIdentifier
FROM CryptographicMessageSyntax-2009
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) id-mod-cms-2004-02(41) }
-- Common PKIX structures [RFC5912]
ATTRIBUTE
FROM PKIX-CommonTypes-2009
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-pkixCommon-02(57)};
--
-- The CMS Algorithm Protection attribute is a Signed Attribute or
-- an Authenticated Attribute.
--
-- Add this attribute to SignedAttributesSet in [RFC5652] [CMS]
-- Add this attribute to AuthAttriuteSet AuthAttributeSet in [RFC5652] [CMS]
--
aa-cmsAlgorithmProtection ATTRIBUTE ::= {
TYPE CMSAlgorithmProtection
IDENTIFIED BY { id-aa-cmsAlgorithmProtect }
}
id-aa-cmsAlgorithmProtect OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
pkcs9(9) 52 }
CMSAlgorithmProtection ::= SEQUENCE {
digestAlgorithm DigestAlgorithmIdentifier,
signatureAlgorithm [1] SignatureAlgorithmIdentifier OPTIONAL,
macAlgorithm [2] MessageAuthenticationCodeAlgorithm
OPTIONAL
}
(WITH COMPONENTS { signatureAlgorithm PRESENT,
macAlgorithm ABSENT } |
WITH COMPONENTS { signatureAlgorithm ABSENT,
macAlgorithm PRESENT })
END
Author's Address
Jim Schaad
Soaring Hawk Consulting
PO Box 675
Gold Bar, WA 98251
Email: ietf@augustcellars.com