RFC 8933: Update to the Cryptographic Message Syntax (CMS) for Algorithm Identifier Protection

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 7841.¶

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8933.¶

Copyright Notice

Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved.¶

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1. Introduction

This document updates the Cryptographic Message Syntax (CMS) [RFC5652] to ensure that algorithm identifiers in signed-data and authenticated-data content types 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 associated 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 called a cross-algorithm 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 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 the Elliptic Curve Digital Signature Algorithm (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 randomized hash functions are employed, 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 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. Required Use of 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.¶

4. Recommended 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.¶

5. IANA Considerations

This document has no IANA actions.¶

6. Security Considerations

The security properties of the CMS [RFC5652] signed-data and authenticated-data content types are updated to offer protection for algorithm identifiers, which makes algorithm substitution attacks significantly more difficult.¶

For the signed-data content type, the improvements specified in this document force an attacker to mount a hash algorithm substitution attack on the overall signature, not just on the message digest of the encapContentInfo eContent.¶

Some digital signature algorithms have prevented hash function substitutions by including a digest algorithm identifier as an input to the signature algorithm. As discussed in [HASHID], such a "firewall" may not be effective or even possible with newer signature algorithms. For example, RSASSA-PKCS1-v1_5 [RFC8017] protects the digest algorithm identifier, but RSASSA-PSS [RFC8017] does not. Therefore, it remains important that a signer have a way to signal to a recipient which digest algorithms are allowed to be used in conjunction with the verification of an overall signature. This signaling can be done as part of the specification of the signature algorithm in an X.509v3 certificate extension [RFC5280] or some other means. The Digital Signature Standard (DSS) [DSS] takes the first approach by requiring the use of an "approved" one-way hash algorithm.¶

For the authenticated-data content type, the improvements specified in this document force an attacker to mount a MAC algorithm substitution attack, which is difficult because the attacker does not know the authentication key.¶

The CMSAlgorithmProtection attribute [RFC6211] offers protection for the algorithm identifiers used in the signed-data and authenticated-data content types. However, no protection is provided for the algorithm identifiers in the enveloped-data, digested-data, or encrypted-data content types. Likewise, the CMSAlgorithmProtection attribute provides no protection for the algorithm identifiers used in the authenticated-enveloped-data content type defined in [RFC5083]. A mechanism for algorithm identifier protection for these content types is work for the future.¶

Acknowledgements

Many thanks to Jim Schaad and Peter Gutmann; without knowing it, they motivated me to write this document. Thanks to Roman Danyliw, Ben Kaduk, and Peter Yee for their careful review and editorial suggestions.¶

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