Internet DRAFT - draft-housley-lamps-cms-update-alg-id-protect
draft-housley-lamps-cms-update-alg-id-protect
Network Working Group R. Housley
Internet-Draft Vigil Security
Updates: 5652 (if approved) October 03, 2019
Intended status: Standards Track
Expires: April 5, 2020
Update to the Cryptographic Message Syntax (CMS) for Algorithm
Identifier Protection
draft-housley-lamps-cms-update-alg-id-protect-00
Abstract
This document updates the Cryptographic Message Syntax (CMS)
specified in RFC 5652 to ensure that algorithm identifiers are
adequately protected.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 5, 2020.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Require use the same hash algorithm . . . . . . . . . . . . . 3
3.1. RFC 5652, Section 5.3 . . . . . . . . . . . . . . . . . . 3
3.2. RFC 5652, Section 5.4 . . . . . . . . . . . . . . . . . . 4
3.3. RFC 5652, Section 5.6 . . . . . . . . . . . . . . . . . . 4
3.4. Backward Compatibility Considerations . . . . . . . . . . 5
3.5. Timestamp Compatibility Considerations . . . . . . . . . 5
4. Recommend inclusion of the CMSAlgorithmProtection attribute . 5
4.1. RFC 5652, Section 14 . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
This document updates the Cryptographic Message Syntax (CMS)
[RFC5652] to ensure that algorithm identifiers are adequately
protected.
The CMS Signed-data Content Type [RFC5652], unlike X.509 certificates
[RFC5280], can be vulnerable to algorithm substitution attacks. In
an algorithm substitution attack, the attacker changes either the
algorithm identifier or the parameters associated with the algorithm
identifier to change the verification process used by the recipient.
The X.509 certificate structure protects the algorithm identifier and
the associate parameters by signing them.
In an algorithm substitution attack, the attacker looks for a
different algorithm that produces the same result as the algorithm
used by the originator. As an example, if the signer of a message
used SHA-256 [SHS] as the digest algorithm to hash the message
content, then the attacker looks for a weaker hash algorithm that
produces a result that is of the same length. The attacker's goal is
to find a different message that results in the same hash value,
which is commonly called a collision. Today, there are many hash
functions that produce 256-bit results. One of them may be found to
be weak in the future.
Further, when a digest algorithm produces a larger result than is
needed by a digital signature algorithm, the digest value is reduced
to the size needed by the signature algorithm. This can be done both
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by truncation and modulo operations, with the simplest being
straightforward truncation. In this situation, the attacker needs to
find a collision with the reduced digest value. As an example, if
the message signer uses SHA-512 [SHS] as the digest algorithm and
ECDSA with the P-256 curve [DSS] as the signature algorithm, then the
attacker needs to find a collision with the first half of the digest.
Similar attacks can be mounted against parameterized algorithm
identifiers. When looking at randomized hash functions, such as the
example in [RFC6210], the algorithm identifier parameter includes a
random value that can be manipulated by an attacker looking for
collisions. Some other algorithm identifiers include complex
parameter structures, and each value provides another opportunity for
manipulation by an attacker.
This document makes two updates to CMS to provide similar protection
for the algorithm identifier. First, it mandates a convention
followed by many implementations by requiring the originator to use
the same hash algorithm to compute the digest of the message content
and the digest of signed attributes. Second, it recommends that the
originator include the CMSAlgorithmProtection attribute [RFC6211].
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Require use the same hash algorithm
This section updates [RFC5652] to require the originator to use the
same hash algorithm to compute the digest of the message content and
the digest of signed attributes.
3.1. RFC 5652, Section 5.3
Change the paragraph describing the digestAlgorithm as follows:
OLD:
digestAlgorithm identifies the message digest algorithm, and any
associated parameters, used by the signer. The message digest is
computed on either the content being signed or the content
together with the signed attributes using the process described in
Section 5.4. The message digest algorithm SHOULD be among those
listed in the digestAlgorithms field of the associated SignerData.
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Implementations MAY fail to validate signatures that use a digest
algorithm that is not included in the SignedData digestAlgorithms
set.
NEW:
digestAlgorithm identifies the message digest algorithm, and any
associated parameters, used by the signer. The message digest is
computed on either the content being signed or the content
together with the signed attributes using the process described in
Section 5.4. The message digest algorithm SHOULD be among those
listed in the digestAlgorithms field of the associated SignerData.
If signedAttrs are present in the SignerInfo, then the same digest
algorithm MUST be used to compute the digest of the SignedData
encapContentInfo eContent, which is carried in the message-digest
attribute, and to compute the digest of the DER-encoded SET OF
signed attributes, which is passed to the signature algorithm.
Implementations MAY fail to validate signatures that use a digest
algorithm that is not included in the SignedData digestAlgorithms
set.
3.2. RFC 5652, Section 5.4
Add the following paragraph as the second paragraph in Section 5.4:
ADD:
When the signedAttrs field is present, the same digest algorithm
MUST be used to compute the digest of the the encapContentInfo
eContent OCTET STRING, which is carried in the message-digest
attribute, and the collection of attributes that are signed.
3.3. RFC 5652, Section 5.6
Change the paragraph discussing the signedAttributes as follows:
OLD:
The recipient MUST NOT rely on any message digest values computed
by the originator. If the SignedData signerInfo includes
signedAttributes, then the content message digest MUST be
calculated as described in Section 5.4. For the signature to be
valid, the message digest value calculated by the recipient MUST
be the same as the value of the messageDigest attribute included
in the signedAttributes of the SignedData signerInfo.
NEW:
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The recipient MUST NOT rely on any message digest values computed
by the originator. If the SignedData signerInfo includes
signedAttributes, then the content message digest MUST be
calculated as described in Section 5.4, using the same digest
algorithm to compute the digest of the the encapContentInfo
eContent OCTET STRING and the message-digest attribute. For the
signature to be valid, the message digest value calculated by the
recipient MUST be the same as the value of the messageDigest
attribute included in the signedAttributes of the SignedData
signerInfo.
3.4. Backward Compatibility Considerations
The new requirement introduced above might lead to compatibility with
an implementation that allowed different digest algorithms to be used
to compute the digest of the message content and the digest of signed
attributes. The signatures produced by such an implementation when
two different digest algorithms are used will be considered invalid
by an implementation that follows this specification. However, most,
if not all, implementations already require the originator to use the
same digest algorithm for both operations.
READER:
If you have an implementation that allows different digest
algorithms to be used to compute the digest of the message content
and the digest of signed attributes, please tell us on the
spasm@ietf.org mail list.
3.5. Timestamp Compatibility Considerations
The new requirement introduced above might lead to compatibility
issues for timestamping systems when the originator does not wish to
share the message content with the Time Stamp Authority (TSA)
[RFC3161]. In this situation, the originator sends a TimeStampReq to
the TSA that includes a MessageImprint, which consists of a digest
algorithm identifier and a digest value, then the TSA uses the digest
in the MessageImprint. As a result, the signature algorithm used by
the TSA needs to be compatible with the digest algorithm selected by
the originator for the MessageImprint.
4. Recommend inclusion of the CMSAlgorithmProtection attribute
This section updates [RFC5652] to recommend that the originator
include the CMSAlgorithmProtection attribute [RFC6211] whenever
signed attributes or authenticated attributes are present.
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4.1. RFC 5652, Section 14
Add the following paragraph as the eighth paragraph in Section 14:
ADD:
While no known algorithm substitution attacks are known at this
time, the inclusion of the algorithm identifiers used by the
originator as a signed attribute or an authenticated attribute
makes such an attack significantly more difficult. Therefore, the
originator of a Signed-data content type that includes signed
attributes SHOULD include the CMSAlgorithmProtection attribute
[RFC6211] as one of the signed attributes. Likewise, the
originator of an Authenticated-data content type that includes
authenticated attributes SHOULD include the CMSAlgorithmProtection
attribute [RFC6211] as one of the authenticated attributes.
5. IANA Considerations
This document makes no requests of the IANA.
6. Security Considerations
The security considerations of [RFC5652] are updated ensure that
algorithm identifiers are adequately protected, which makes algorithm
substitution attacks significantly more difficult.
The CMSAlgorithmProtection attribute [RFC6211] offers protection the
algorithm identifiers used in the signed-data and authenticated-data
content types. There is not currently protection mechanism for the
algorithm identifiers used in the enveloped-data, digested-data, or
encrypted-data content types. Likewise there us not currently
protection mechanism for the algorithm identifiers used in the
authenticated-enveloped-data content type defined in [RFC5083].
7. Acknowledgements
Many thanks to Jim Schaad and Peter Gutmann; without knowing it, they
motivated me to write this document.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009,
<https://www.rfc-editor.org/info/rfc5652>.
[RFC6211] Schaad, J., "Cryptographic Message Syntax (CMS) Algorithm
Identifier Protection Attribute", RFC 6211,
DOI 10.17487/RFC6211, April 2011,
<https://www.rfc-editor.org/info/rfc6211>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
8.2. Informative References
[DSS] National Institute of Standards and Technology (NIST),
"Digital Signature Standard (DSS)", FIPS
Publication 186-3, June 2009.
[RFC3161] Adams, C., Cain, P., Pinkas, D., and R. Zuccherato,
"Internet X.509 Public Key Infrastructure Time-Stamp
Protocol (TSP)", RFC 3161, DOI 10.17487/RFC3161, August
2001, <https://www.rfc-editor.org/info/rfc3161>.
[RFC5083] Housley, R., "Cryptographic Message Syntax (CMS)
Authenticated-Enveloped-Data Content Type", RFC 5083,
DOI 10.17487/RFC5083, November 2007,
<https://www.rfc-editor.org/info/rfc5083>.
[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, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC6210] Schaad, J., "Experiment: Hash Functions with Parameters in
the Cryptographic Message Syntax (CMS) and S/MIME",
RFC 6210, DOI 10.17487/RFC6210, April 2011,
<https://www.rfc-editor.org/info/rfc6210>.
[SHS] National Institute of Standards and Technology (NIST),
"Secure Hash Standard", FIPS Publication 180-3, October
2008.
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Author's Address
Russ Housley
Vigil Security
516 Dranesville Road
Herndon, VA 20170
US
Email: housley@vigilsec.com
Housley Expires April 5, 2020 [Page 8]
