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Internet Engineering Task Force (IETF) J. Schaad
Request for Comments: 6211 Soaring Hawk Consulting
Category: Standards Track April 2011
ISSN: 2070-1721
Cryptographic Message Syntax (CMS)
Algorithm Identifier Protection Attribute
Abstract
The Cryptographic Message Syntax (CMS), unlike X.509/PKIX
certificates, is vulnerable to algorithm substitution attacks. In an
algorithm substitution attack, the attacker changes either the
algorithm being used or the parameters of the algorithm in order to
change the result of a signature verification process. In X.509
certificates, the signature algorithm is protected because it is
duplicated in the TBSCertificate.signature field with the proviso
that the validator is to compare both fields as part of the signature
validation process. This document defines a new attribute that
contains a copy of the relevant algorithm identifiers so that they
are protected by the signature or authentication process.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6211.
Schaad Standards Track [Page 1]
RFC 6211 CMS Algorithm Attribute April 2011
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notation . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Attribute Structure . . . . . . . . . . . . . . . . . . . . . . 5
3. Verification Process . . . . . . . . . . . . . . . . . . . . . 7
3.1. Signed Data Verification Changes . . . . . . . . . . . . . 7
3.2. Authenticated Data Verification Changes . . . . . . . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Normative References . . . . . . . . . . . . . . . . . . . 8
6.2. Informational References . . . . . . . . . . . . . . . . . 9
Appendix A. 2008 ASN.1 Module . . . . . . . . . . . . . . . . . 10
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RFC 6211 CMS Algorithm Attribute April 2011
1. Introduction
The Cryptographic Message Syntax [CMS], unlike X.509/PKIX
certificates [RFC5280], is vulnerable to algorithm substitution
attacks. In an algorithm substitution attack, the attacker changes
either the algorithm being used or the parameters of the algorithm in
order to change the result of a signature verification process. In
X.509 certificates, the signature algorithm is protected because it
is duplicated in the TBSCertificate.signature field with the proviso
that the validator is to compare both fields as part of the signature
validation process. This document defines a new attribute that
contains a copy of the relevant algorithm identifiers so that they
are protected by the signature or authentication process.
In an algorithm substitution attack, the attacker looks for a
different algorithm that produces the same result as the algorithm
used by the signer. As an example, if the creator of the message
used SHA-1 as the digest algorithm to hash the message content, then
the attacker looks for a different hash algorithm that produces a
result that is of the same length, but with which it is easier to
find collisions. Examples of other algorithms that produce a hash
value of the same length would be SHA-0 or RIPEMD-160. Similar
attacks can be mounted against parameterized algorithm identifiers.
When looking at some of the proposed randomized hashing functions,
such as that in [RANDOM-HASH], the associated security proofs assume
that the parameters are solely under the control of the originator
and not subject to selection by the attacker.
Some algorithms have been internally designed to be more resistant to
this type of attack. Thus, an RSA PKCS #1 v.15 signature [RFC3447]
cannot have the associated hash algorithm changed because it is
encoded as part of the signature. The Digital Signature Algorithm
(DSA) was originally defined so that it would only work with SHA-1 as
a hash algorithm; thus, by knowing the public key from the
certificate, a validator can be assured that the hash algorithm
cannot be changed. There is a convention, undocumented as far as I
can tell, that the same hash algorithm should be used for both the
content digest and the signature digest. There are cases, such as
third-party signers that are only given a content digest, where such
a convention cannot be enforced.
As with all attacks, the attack is going to be desirable on items
that are both long term and high value. One would expect that these
attacks are more likely to be made on older documents, as the
algorithms being used when the message was signed would be more
likely to have degraded over time. Countersigning, the classic
method of protecting a signature, does not provide any additional
protection against an algorithm substitution attack because
Schaad Standards Track [Page 3]
RFC 6211 CMS Algorithm Attribute April 2011
countersignatures sign just the signature, but the algorithm
substitution attacks leave the signature value alone while changing
the algorithms being used.
Using the SignerInfo structure from CMS, let's take a more detailed
look at each of the fields in the structure and discuss what fields
are and are not protected by the signature. I have included a copy
of the ASN.1 here for convenience. A similar analysis of the
AuthenticatedData structure is left to the reader, but it can be done
in much the same way.
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. As many implementations
of CMS today ignore the value of this field, that is not a
problem. If the value is increased, then no changes in the
processing are expected. If the value is decreased,
implementations that respect the structure would fail to decode,
but an erroneous signature validation would not be completed
successfully.
sid can be protected using either version of the signing certificate
authenticated attribute. SigningCertificateV2 is defined in
[RFC5035]. SigningCertificate is defined in [ESS-BASE]. 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 has been implicitly protected by the fact that CMS
has only defined one digest algorithm for each hash value length.
(The algorithm RIPEMD-160 was never standardized.) There is also
an unwritten convention that the same digest algorithm should be
used both here and for the signature algorithm. If newer digest
algorithms are defined so that there are multiple algorithms for a
given hash length (it is expected that the SHA-3 project will do
so), or that parameters are defined for a specific algorithm, much
of the implicit protection will be lost.
signedAttributes are directly protected by the signature when they
are present. The Distinguished Encoding Rules (DER) encoding of
this value is what is hashed for the signature computation.
Schaad Standards Track [Page 4]
RFC 6211 CMS Algorithm Attribute April 2011
signatureAlgorithm has been protected by implication in the past.
The use of an RSA public key implied that the RSA v1.5 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 still somewhat true as there is an
implicit tie between the length of the DSA public key and the
length of the hash algorithm to be used, but this is known by
convention and there is no explicit enforcement for this.
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. The
SignedData structure was explicitly designed that unsignedAttrs
are not 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:
Schaad Standards Track [Page 5]
RFC 6211 CMS Algorithm Attribute April 2011
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
(SignerInfo.signatureAlgorithm). This field is populated only 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
(AuthenticatedData.macAlgorithm). This field is populated only if
the attribute is placed in an AuthenticatedData.authAttrs
sequence.
Exactly one of signatureAlgorithm or 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.
Schaad Standards Track [Page 6]
RFC 6211 CMS Algorithm Attribute April 2011
3. Verification Process
While the exact verification steps depend on the structure that is
being validated, there are some common rules to be followed when
comparing the two algorithm structures:
o A field with a default value MUST compare as identical,
independently of whether the value is defaulted or is explicitly
provided. This implies that a binary compare of the encoded bytes
is insufficient.
o For some algorithms, such as SHA-1, the parameter value of NULL
can be included in the ASN.1 encoding by some implementations and
be omitted by other implementations. It is left to the
implementer of this attribute to decide the comparison for
equality is satisfied in this case. As a general rule, the same
implementation is expected to produce both encoded values thus
making it unlikely that this corner case should exist. This is an
issue because some implementations will omit a NULL element, while
others will encode a NULL element for some digest algorithms such
as SHA-1 (see the comments in Section 2.1 of [RFC3370]). The
issue is even worse because the NULL is absent in some cases
(e.g., [RFC3370]), but is required in other cases (e.g.,
[RFC4056]).
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 authentication MUST fail.
Schaad Standards Track [Page 7]
RFC 6211 CMS Algorithm Attribute April 2011
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 authentication MUST fail.
4. 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 [RFC6210].)
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 unsigned or unauthenticated location.
6. References
6.1. Normative References
[ASN.1-2008] ITU-T, "ITU-T Recommendations X.680, X.681, X.682, and
X.683", 2008.
[CMS] Housley, R., "Cryptographic Message Syntax (CMS)",
RFC 5652, September 2009.
[ESS-BASE] Hoffman, P., "Enhanced Security Services for S/MIME",
RFC 2634, June 1999.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5035] Schaad, J., "Enhanced Security Services (ESS) Update:
Adding CertID Algorithm Agility", RFC 5035,
August 2007.
[RFC5912] Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
Public Key Infrastructure Using X.509 (PKIX)",
RFC 5912, June 2010.
Schaad Standards Track [Page 8]
RFC 6211 CMS Algorithm Attribute April 2011
6.2. Informative References
[RANDOM-HASH] Halevi, S. and H. Krawczyk, "Strengthening Digital
Signatures via Random Hashing", January 2007,
<http://webee.technion.ac.il/~hugo/rhash/rhash.pdf>.
[RFC3370] Housley, R., "Cryptographic Message Syntax (CMS)
Algorithms", RFC 3370, August 2002.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[RFC4056] Schaad, J., "Use of the RSASSA-PSS Signature Algorithm
in Cryptographic Message Syntax (CMS)", RFC 4056,
June 2005.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation
List (CRL) Profile", RFC 5280, May 2008.
[RFC6210] Schaad, J., "Experiment: Hash Functions with
Parameters in the Cryptographic Message Syntax (CMS)
and S/MIME", RFC 6210, April 2011.
Schaad Standards Track [Page 9]
RFC 6211 CMS Algorithm Attribute April 2011
Appendix A. 2008 ASN.1 Module
The ASN.1 module defined uses the 2008 ASN.1 definitions found in
[ASN.1-2008]. This module contains the ASN.1 module that contains
the required definitions for the types and values defined in this
document. The module uses the ATTRIBUTE class defined in [RFC5912].
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) [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 [CMS]
-- Add this attribute to AuthAttributeSet in [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 }
Schaad Standards Track [Page 10]
RFC 6211 CMS Algorithm Attribute April 2011
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
EMail: ietf@augustcellars.com
Schaad Standards Track [Page 11]