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/rfc9709.¶

Copyright Notice

Copyright (c) 2025 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 (https://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 Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶

1. Introduction

This document specifies the derivation of the content-encryption key for the Cryptographic Message Syntax (CMS) enveloped-data content type [RFC5652], the content-encryption key for the CMS encrypted-data content type [RFC5652], or the content-authenticated-encryption key for the authenticated-enveloped-data content type [RFC5083].¶

The use of this mechanism provides protection against an attacker that manipulates the content-encryption algorithm identifier or the content-authenticated-encryption algorithm identifier. Johannes Roth and Falko Strenzke presented such an attack at IETF 118 [RS2023], where:¶

1. The attacker intercepts a CMS authenticated-enveloped-data content [RFC5083] that uses either AES-CCM or AES-GCM [RFC5084].¶

2. The attacker turns the intercepted content into a "garbage" CMS enveloped-data content (Section 6 of [RFC5652]) that is composed of AES-CBC guess blocks.¶

3. The attacker sends the "garbage" message to the victim, and the victim reveals the result of the decryption to the attacker.¶

4. If any of the transformed plaintext blocks match the guess for that block, then the attacker learns the plaintext for that block.¶

With highly structured messages, one block can reveal the only sensitive part of the original message.¶

This attack is thwarted if the encryption key depends upon the delivery of the unmodified algorithm identifier.¶

The mitigation for this attack has three parts:¶

1. Potential recipients include the id-alg-cek-hkdf-sha256 algorithm identifier (with no parameters) in S/MIME Capabilities to indicate support for this mitigation.¶
2. As a flag to the recipient that this mitigation is being used, carry the id-alg-cek-hkdf-sha256 algorithm identifier as the contentEncryptionAlgorithm in the EncryptedContentInfo structure. This structure is used in the enveloped-data content type, the encrypted-data content type, and the authenticated-enveloped-data content type. The parameters field of the id-alg-cek-hkdf-sha256 algorithm identifier identifies the content-encryption algorithm or the content-authenticated-encryption algorithm and any associated parameters.¶

3. Perform encryption with a derived content-encryption key or content-authenticated-encryption key:¶

         CEK' = HKDF(CEK, AlgorithmIdentifier)
   

   ¶

2. Use of HKDF with SHA-256 to Derive Encryption Keys

The mitigation uses the HMAC-based Extract-and-Expand Key Derivation Function (HKDF) [RFC5869] to derive output keying material (OKM) from input keying material (IKM). HKDF is used with the SHA-256 hash function [FIPS180]. The derivation includes the DER-encoded AlgorithmIdentifier as the optional info input value. The encoded value includes the ASN.1 tag for SEQUENCE (0x30), the length, and the value. This AlgorithmIdentifier is carried as the parameter to the id-alg-cek-hkdf-sha256 algorithm identifier. If an attacker were to change the originator-provided AlgorithmIdentifier, then the recipient will derive a different content-encryption key or content-authenticated-encryption key.¶

The CMS_CEK_HKDF_SHA256 function uses the HKDF-Extract and HKDF-Expand functions to derive the OKM from the IKM:¶

Inputs:

IKM

input keying material¶

info

DER-encoded AlgorithmIdentifier¶

Output:

OKM

output keying material (same size as IKM)¶

The output OKM is calculated as follows:¶

   OKM_SIZE = len(IKM)  /* length in octets */
   IF OKM_SIZE > 8160 THEN raise error

   salt = "The Cryptographic Message Syntax"
   PRK = HKDF-Extract(salt, IKM)

   OKM = HKDF-Expand(PRK, info, OKM_SIZE)

3. The id-alg-cek-hkdf-sha256 Algorithm Identifier

The id-alg-cek-hkdf-sha256 algorithm identifier indicates that the CMS_CEK_HKDF_SHA256 function defined in Section 2 is used to derive the content-encryption key or the content-authenticated-encryption key.¶

The following object identifier identifies the id-alg-cek-hkdf-sha256 algorithm:¶

   id-alg-cek-hkdf-sha256 OBJECT IDENTIFIER ::= { iso(1)
      member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
      id-smime(16) alg(3) 31 }

The id-alg-cek-hkdf-sha256 parameters field has an ASN.1 type of AlgorithmIdentifier.¶

Using the conventions from [RFC5911], the id-alg-cek-hkdf-sha256 algorithm identifier is defined as:¶

  ContentEncryptionAlgorithmIdentifier ::=
    AlgorithmIdentifier{CONTENT-ENCRYPTION, { ... } }

  cea-CEKHKDFSHA256 CONTENT-ENCRYPTION ::= {
    IDENTIFIER id-alg-cek-hkdf-sha256
    PARAMS TYPE ContentEncryptionAlgorithmIdentifier ARE required
    SMIME-CAPS { IDENTIFIED BY id-alg-cek-hkdf-sha256 } }

4. SMIMECapabilities Attribute Conventions

The SMIMECapabilities attribute is defined in Section 2.5.2 of [RFC8551]. An S/MIME client announces the set of cryptographic functions it supports using the SMIMECapabilities attribute.¶

If an S/MIME client supports the mechanism in this document, the id-alg-cek-hkdf-sha256 object identifier SHOULD be included in the set of cryptographic functions. The parameter with this encoding MUST be absent.¶

The encoding for id-alg-cek-hkdf-sha256, in hexadecimal, is:¶

   30 0d 06 0b 2a 86 48 86 f7 0d 01 09 10 03 1f

5. Use of HKDF with SHA-256 with CMS

This section describes the originator and recipient processing to implement this mitigation for each of the CMS encrypting content types.¶

6. Security Considerations

This mitigation always uses HKDF with SHA-256. One KDF algorithm was selected to avoid the need for negotiation. In the future, if a weakness is found in the KDF algorithm, a new attribute will need to be assigned for use with an alternative KDF algorithm.¶

If the attacker removes the id-alg-cek-hkdf-sha256 object identifier from the contentEncryptionAlgorithm.algorithm field of the EncryptedContentInfo structure prior to delivery to the recipient, then the recipient will not attempt to derive CEK', which will deny the recipient access to the content but will not assist the attacker in recovering the plaintext content.¶

If the attacker changes contentEncryptionAlgorithm.parameters field of the EncryptedContentInfo structure prior to delivery to the recipient, then the recipient will derive a different CEK', which will not assist the attacker in recovering the plaintext content. Providing the object identifier as an input to the key derivation function is sufficient to mitigate the attack described in [RS2023], but this mitigation includes both the object identifier and the parameters to protect against some yet-to-be-discovered attack that only manipulates the parameters.¶

Implementations MUST protect the content-encryption keys and content-authenticated-encryption keys, including the CEK and CEK'. Compromise of a content-encryption key may result in disclosure of the associated encrypted content. Compromise of a content-authenticated-encryption key may result in disclosure of the associated encrypted content or allow modification of the authenticated content and the AAD.¶

Implementations MUST randomly generate content-encryption keys and content-authenticated-encryption keys. Using an inadequate pseudorandom number generator (PRNG) to generate cryptographic keys can result in little or no security. An attacker may find it much easier to reproduce the PRNG environment that produced the keys and then search the resulting small set of possibilities, rather than brute-force searching the whole key space. The generation of quality random numbers is difficult. [RFC4086] offers important guidance on this topic.¶

7. Privacy Considerations

If the message-digest attribute is included in the AuthAttributes, then the attribute value will contain the unencrypted one-way hash value of the plaintext of the content. Disclosure of this hash value enables content tracking, and it can be used to determine if the content matches one or more candidates. For these reasons, the AuthAttributes SHOULD NOT contain the message-digest attribute.¶

8. Operational Considerations

CMS is often used to provide encryption in messaging environments, where various forms of unsolicited messages (such as spam and phishing) represent a significant volume of unwanted traffic. Mitigation strategies for unwanted message traffic involve analysis of plaintext message content. When recipients accept unsolicited encrypted messages, they become even more vulnerable to unwanted traffic since many mitigation strategies will be unable to access the plaintext message content. Therefore, software that receives messages that have been encrypted using CMS ought to provide alternate mechanisms to handle the unwanted message traffic. One approach that does not require disclosure of keying material to a server is to reject or discard encrypted messages unless they purport to come from a member of a previously approved originator list.¶

9. IANA Considerations

For the ASN.1 module in Appendix A of this document, IANA has assigned the following object identifier (OID) in the "SMI Security for S/MIME Module Identifier (1.2.840.113549.1.9.16.0)" registry:¶

IANA has allocated the id-alg-cek-hkdf-sha256 algorithm identifier as specified in Section 3 in the "SMI Security for S/MIME Algorithms (1.2.840.113549.1.9.16.3)" registry as follows:¶

10. References

Appendix A. ASN.1 Module

This ASN.1 module builds upon the conventions established in [RFC5911].¶

<CODE BEGINS>
CMS-CEK-HKDF-SHA256-Module-2023
  { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
    id-smime(16) id-mod(0) id-mod-CMS-CEK-HKDF-SHA256-2023(80) }

DEFINITIONS IMPLICIT TAGS ::= BEGIN

EXPORTS ALL;

IMPORTS
  AlgorithmIdentifier{}, CONTENT-ENCRYPTION, SMIME-CAPS
  FROM AlgorithmInformation-2009 -- in RFC 5911
    { iso(1) identified-organization(3) dod(6) internet(1)
      security(5) mechanisms(5) pkix(7) id-mod(0)
      id-mod-algorithmInformation-02(58) } ;


--
-- CEK-HKDF-SHA256 Algorithm
--

id-alg-cek-hkdf-sha256 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) pkcs(1) pkcs-9(9) id-smime(16) alg(3) 31 }

ContentEncryptionAlgorithmIdentifier ::=
    AlgorithmIdentifier{CONTENT-ENCRYPTION, { ... } }

cea-CEKHKDFSHA256 CONTENT-ENCRYPTION ::= {
    IDENTIFIER id-alg-cek-hkdf-sha256
    PARAMS TYPE ContentEncryptionAlgorithmIdentifier ARE required
    SMIME-CAPS { IDENTIFIED BY id-alg-cek-hkdf-sha256 } }

--
-- S/MIME Capability for CEK-HKDF-SHA256 Algorithm
--

SMimeCaps SMIME-CAPS ::= { cap-CMSCEKHKDFSHA256, ... }

cap-CMSCEKHKDFSHA256 SMIME-CAPS ::=
    { -- No value -- IDENTIFIED BY id-alg-cek-hkdf-sha256 }

END

<CODE ENDS>

Appendix B. CMS_CEK_HKDF_SHA256 Function Examples

This appendix provides two test vectors for the CMS_CEK_HKDF_SHA256 function.¶

IKM = c702e7d0a9e064b09ba55245fb733cf3

The AES-128-GCM AlgorithmIdentifier:
 algorithm=2.16.840.1.101.3.4.1.6
 parameters=GCMParameters:
  aes-nonce=0x5c79058ba2f43447639d29e2
  aes-ICVlen is omitted; it indicates the DEFAULT of 12

DER-encoded AlgorithmIdentifier:
  301b0609608648016503040106300e040c5c79058ba2f43447639d29e2

OKM = 2124ffb29fac4e0fbbc7d5d87492bff3

IKM = c702e7d0a9e064b09ba55245fb733cf3

The AES-128-CBC AlgorithmIdentifier:
 algorithm=2.16.840.1.101.3.4.1.2
 parameters=AES-IV=0x651f722ffd512c52fe072e507d72b377

DER-encoded AlgorithmIdentifier:
  301d06096086480165030401020410651f722ffd512c52fe072e507d72b377

OKM = 9cd102c52f1e19ece8729b35bfeceb50

Acknowledgements

Thanks to Mike Ounsworth, Carl Wallace, and Joe Mandel their careful review and constructive comments.¶

Author's Address