EUF-CMA for the Cryptographic Message Syntax (CMS) SignedData
draft-vangeest-lamps-cms-euf-cma-signeddata-00
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| Last updated | 2024-12-05 | ||
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draft-vangeest-lamps-cms-euf-cma-signeddata-00
Limited Additional Mechanisms for PKIX and SMIME D. Van Geest
Internet-Draft CryptoNext Security
Intended status: Standards Track F. Strenzke
Expires: 8 June 2025 MTG AG
5 December 2024
EUF-CMA for the Cryptographic Message Syntax (CMS) SignedData
draft-vangeest-lamps-cms-euf-cma-signeddata-00
Abstract
The Cryptographic Message Syntax (CMS) has different signature
verification behaviour based on whether signed attributes are present
or not. This results in a potential existential forgery
vulnerability in CMS and protocols which use CMS. This document
describes the vulnerability and lists a number of potential
mitigations for LAMPS working group discussion.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://danvangeest.github.io/cms-euf-cma-signeddata/draft-vangeest-
lamps-cms-euf-cma-signeddata.html. Status information for this
document may be found at https://datatracker.ietf.org/doc/draft-
vangeest-lamps-cms-euf-cma-signeddata/.
Discussion of this document takes place on the Limited Additional
Mechanisms for PKIX and SMIME Working Group mailing list
(mailto:spasm@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/spasm/. Subscribe at
https://www.ietf.org/mailman/listinfo/spasm/.
Source for this draft and an issue tracker can be found at
https://github.com/danvangeest/cms-euf-cma-signeddata.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on 8 June 2025.
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4
3. Potential Mitigations . . . . . . . . . . . . . . . . . . . . 4
3.1. Immediate Forced Use of Specific Signature Context
Strings . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Attribute-Specified Use of Implicit Signature Context
Strings . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2.1. Signing . . . . . . . . . . . . . . . . . . . . . . . 5
3.2.2. Verifying . . . . . . . . . . . . . . . . . . . . . . 5
3.3. Attribute-Specified Use of Explicit Signature Context
Strings . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Straw Mitigations . . . . . . . . . . . . . . . . . . . . . . 8
4.1. Attack Detection in CMS . . . . . . . . . . . . . . . . . 8
4.2. Always/Never use SignedAttributes in Your Protocol . . . 8
4.3. Attack Detection in Your Protocol . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . 10
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
The Cryptographic Message Syntax (CMS) [RFC5652] signed-data content
type allows any number of signers in parallel to sign any type of
content.
CMS gives a signer two options when generating a signature on some
content:
* Generate a signature on the whole content; or
* Compute a hash over the content, place this hash in the message-
digest attribute in the SignedAttributes type, and generate a
signature on the SignedAttributes.
The resulting signature does not commit to the presence of the
SignedAttributes type, allowing an attacker to influence verification
behaviour. An attacker can perform two different types of attacks:
1. Take an arbitrary CMS signed message M which was originally
signed with SignedAttributes present and remove the
SignedAttributes, thereby crafting a new message M' that was
never signed by the signer. M' has the DER-encoded
SignedAttributes of the original message as its content and
verifies correctly against the original signature of M.
2. Let the signer sign a message of the attacker's choice without
SignedAttributes. The attacker chooses this message to be a
valid DER-encoding of a SignedAttributes object. He can then add
this encoded SignedAttributes object to the signed message and
change the signed message to the one that was used to create the
messageDigest attribute within the SignedAttributes. The
signature created by the signer is valid for this arbitrary
attacker-chosen message.
This vulnerability was presented by Falko Strenzke at IETF 121
[LAMPS121] and is detailed in [Str23].
Due to the limited flexibility of either the signed or the forged
message in either attack variant, the fraction of vulnerable systems
can be assumed to be small. But due to the wide deployment of the
affected protocols, such instances cannot be excluded.
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2. Conventions and Definitions
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. Potential Mitigations
Potential mitigations are described in the following sub-sections as
input to the working group discussion. If this draft is adopted and
the working group has taken a decision which measure(s) should be
realized, we'll describe the chosen measures in detail.
The mitigations in this section make use of a context string which is
passed to the signature algorithm's sign and verify functions.
ML-DSA [FIPS204], SLH-DSA [FIPS205], Composite ML-DSA
[I-D.ietf-lamps-pq-composite-sigs], and Ed448 [RFC8032] take a
context string during signing and verification. The context string
may be up to 255 bytes long. By default the context string is the
empty string.
Sign(sk, M, ctx="")
Verify(sk, M, ctx="")
RSA, ECDSA and Ed25519 signatures do not take a context string and
would not be helped by these mitigations.
Ed448 can take a context string but does not currently in CMS
[RFC8419].
Ed25519ctx [RFC8032] takes a context string but is not specified for
use in CMS.
3.1. Immediate Forced Use of Specific Signature Context Strings
Immediately update [I-D.ietf-lamps-cms-ml-dsa],
[I-D.ietf-lamps-cms-sphincs-plus], and
[I-D.ietf-lamps-pq-composite-sigs] to require a context string, with
a different value for use with and without signated attributes.
When signed attributes are present:
Sign(sk, M, "signed-attributes")
Verify(sk, M, "signed-attributes")
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When signed attributes are absent:
Sign(sk, M, "no-signed-attributes")
Verify(sk, M, "no-signed-attributes")
Unlike the following mitigations, Ed448 cannot be addressed by this
mitigation because it is already published and in use.
3.2. Attribute-Specified Use of Implicit Signature Context Strings
Like Section 3.1, but the use of the signature context string is
indicated by a new, empty (or attribute value ignored), sign-with-
context-implicit unsigned attribute.
[I-D.ietf-lamps-cms-ml-dsa], [I-D.ietf-lamps-cms-sphincs-plus], and
[I-D.ietf-lamps-pq-composite-sigs] can be published using the default
signature context string. ML-DSA, SLH-DSA, Composite-ML-DSA, and
Ed448 only use the non-default context string when the new attribute
is used.
3.2.1. Signing
When signed attributes are present:
unsigned-attributes.add(sign-with-context-implicit)
Sign(sk, M, "signed-attributes")
When signed attributes are absent:
unsigned-attributes.add(sign-with-context-implicit)
Sign(sk, M, "no-signed-attributes")
3.2.2. Verifying
When signed attributes are present:
IF unsigned-attributes.contains(sign-with-context-implicit)
THEN Verify(sk, M, "signed-attributes")
ELSE Verify(sk, M, "")
When signed attributes are absent:
IF unsigned-attributes.contains(sign-with-context-implicit)
THEN Verify(sk, M, "no-signed-attributes")
ELSE Verify(sk, M, "")
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3.3. Attribute-Specified Use of Explicit Signature Context Strings
Like Section 3.2 but the new unsigned attribute (sign-with-context-
explict) contains a semi-colon-delimited list of keyword (and
optional value) strings. This addresses the possibility of future
CMS features that require context parameters.
ctx = "<keyword_1>[=value1];...;<keyword_n>[=value]"
The list is ordered alphabetically by type string. This list is
validated by the verifier and used as the signature context string.
(alternative: the SHA-256 hash of the list is used as the signature
context string to avoid it getting too long)
A proposed list of initial signature context string keywords follows:
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+================+==================+=============================+
| keyword | value | comment |
+================+==================+=============================+
| "IETF/CMS" | | REQUIRED to be in the sign- |
| | | with-context-implicit |
| | | attribute, to differentiate |
| | | a signature in CMS from a |
| | | signature with the same |
| | | private key over some other |
| | | data. |
+----------------+------------------+-----------------------------+
| "signed-attrs" | | Present if signed |
| | | attributes are used, not |
| | | present if signed |
| | | attributes are not used. |
| | | Alternative: always |
| | | present, value = 0/1, yes/ |
| | | no depending on whether |
| | | signed attributes are |
| | | present or not. |
+----------------+------------------+-----------------------------+
| "app-ctx" | base64( SHA-256( | Allows the protocol using |
| | protocol_context | CMS to specify a context. |
| | ) ) | SHA-256 is applied so that |
| | | the length available to the |
| | | protocol context isn't |
| | | dependent on the other |
| | | context values used in CMS. |
| | | (alternative: no SHA-256 |
| | | here, apply SHA-256 to the |
| | | whole CMS context). |
| | | base64-encoding is applied |
| | | so the app context doesn't |
| | | introduce semi-colons to |
| | | mess up CMS' parsing of |
| | | this string. |
+----------------+------------------+-----------------------------+
Table 1: Potential Context String Keywords
When a verifier processes a SignerInfo containing the sign-with-
context-explicit attribute, it MUST perform the following consistency
checks:
* If the "signed-attrs" keyword is present and SignedAttributes is
not present in the SignerInfo, fail verification.
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* If the "signed-attrs" keyword is not present and SignedAttributes
is present in the SignerInfo, fail verification.
If the consistency checks pass, the signature is verified using the
string in the sign-with-context-explicit attribute as the signature
context (alternative: using SHA-256 of the string in the sign-with-
context-explicit attribute).
When a verifier processes a SignerInfo without the sign-with-context-
explicit attribute, they MUST verify the signature using the default
signature context value ("").
[I-D.ietf-lamps-cms-ml-dsa], [I-D.ietf-lamps-cms-sphincs-plus], and
[I-D.ietf-lamps-pq-composite-sigs] can be published using the default
signature context string. ML-DSA, SLH-DSA, Composite-ML-DSA, and
Ed448 only use the non-default context string when the new attribute
is used.
4. Straw Mitigations
The following mitigations might not be good ideas but are included
just in case there's a seed of genius in them.
4.1. Attack Detection in CMS
If SignedAttributes is not present, check if the signed message is a
valid DER-encoded SignedAttributes structure and fail if it is. The
mandatory contentType and messageDigest attributes, with their
respective OIDs, should give a low probability of a legitimate
message being flagged.
If an application protocol deliberately uses such a signed messages,
verification would fail.
This mitigation does not address the inverse problem where a protocol
doesn't used SignedAttributes but for some reason often sends
messages which happen to be formatted like valid SignedAttributes
encodings, with attacker-controlled bytes where the message digest
attribute would be.
4.2. Always/Never use SignedAttributes in Your Protocol
Individually update each protocol which use CMS to always require or
forbid signed attributes.
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4.3. Attack Detection in Your Protocol
Section 4.1 but specified in the protocol that uses CMS rather than
CMS itself.
5. Security Considerations
TODO Security
The vulnerability is not present in systems where the use of
SignedAttributes is mandatory, for example: SCEP, Certificate
Transparency, RFC 4018 firmware update, German Smart Metering CMS
data format. However, this security relies on a correct
implementation of the verification routine that ensures the presence
of SignedAttributes.
The vulnerability is also not present when the message is signed and
then encrypted, as the attacker cannot learn the signature.
Conceivably vulnerable systems (TODO: describe these better):
* Unencrypted firmware update denial of service
* Dense message space
* Signing unstructured data
* External signatures over unstructured data
* Systems with permissive parsers
6. IANA Considerations
This document has no IANA actions.
7. References
7.1. 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/rfc/rfc2119>.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009,
<https://www.rfc-editor.org/rfc/rfc5652>.
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[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/rfc/rfc8174>.
7.2. Informative References
[FIPS204] "Module-lattice-based digital signature standard",
National Institute of Standards and Technology (U.S.),
DOI 10.6028/nist.fips.204, August 2024,
<https://doi.org/10.6028/nist.fips.204>.
[FIPS205] "Stateless hash-based digital signature standard",
National Institute of Standards and Technology (U.S.),
DOI 10.6028/nist.fips.205, August 2024,
<https://doi.org/10.6028/nist.fips.205>.
[I-D.ietf-lamps-cms-ml-dsa]
S, B., R, A., and D. Van Geest, "Use of the ML-DSA
Signature Algorithm in the Cryptographic Message Syntax
(CMS)", Work in Progress, Internet-Draft, draft-ietf-
lamps-cms-ml-dsa-01, 22 November 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-lamps-
cms-ml-dsa-01>.
[I-D.ietf-lamps-cms-sphincs-plus]
Housley, R., Fluhrer, S., Kampanakis, P., and B.
Westerbaan, "Use of the SLH-DSA Signature Algorithm in the
Cryptographic Message Syntax (CMS)", Work in Progress,
Internet-Draft, draft-ietf-lamps-cms-sphincs-plus-17, 30
November 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-lamps-cms-sphincs-plus-17>.
[I-D.ietf-lamps-pq-composite-sigs]
Ounsworth, M., Gray, J., Pala, M., Klaußner, J., and S.
Fluhrer, "Composite ML-DSA For use in X.509 Public Key
Infrastructure and CMS", Work in Progress, Internet-Draft,
draft-ietf-lamps-pq-composite-sigs-03, 21 October 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-lamps-
pq-composite-sigs-03>.
[LAMPS121] Strenzke, F., "EUF-CMA for CMS SignedData", 6 November
2024, <https://datatracker.ietf.org/meeting/121/materials/
slides-121-lamps-cms-euf-cma-00>.
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017,
<https://www.rfc-editor.org/rfc/rfc8032>.
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[RFC8419] Housley, R., "Use of Edwards-Curve Digital Signature
Algorithm (EdDSA) Signatures in the Cryptographic Message
Syntax (CMS)", RFC 8419, DOI 10.17487/RFC8419, August
2018, <https://www.rfc-editor.org/rfc/rfc8419>.
[Str23] Strenzke, F., "ForgedAttributes: An Existential Forgery
Vulnerability of CMS and PKCS#7 Signatures", 22 November
2023, <https://ia.cr/2023/1801>.
Acknowledgments
TODO acknowledge.
Authors' Addresses
Daniel Van Geest
CryptoNext Security
Email: daniel.vangeest@cryptonext-security.com
Falko Strenzke
MTG AG
Email: falko.strenzke@mtg.de
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