Use of the DH-Based KEM (DHKEM) in the Cryptographic Message Syntax (CMS)

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Table of Contents

1. Introduction

The Cryptographic Message Syntax (CMS) enveloped-data content type [RFC5652] and the CMS authenticated-enveloped-data content type [RFC5083] support both key transport and key agreement algorithms to establish the key used to encrypt the content. In recent years, cryptographers have be specifying Key Encapsulation Mechanism (KEM) algorithms, including quantum-secure KEM algorithms. This document defines conventions for wrapping Diffie-Hellman Ephemeral-Static (E-S) Diffie-Hellman (DH) and Elliptic Curve Diffie-Hellman (ECDH) to fit the KEM interface for the CMS enveloped-data content type and the CMS authenticated-enveloped-data content type as defined in [I-D.ietf-lamps-cms-kemri]. This is a parallel mechanism to [RFC5990] which does the same for RSA. The benefit is to allow forward-compatibility of older DH-based ciphers into new mechanisms that only support KEMs.¶

A KEM algorithm is a one-pass (store-and-forward) mechanism for transporting random keying material to a recipient using the recipient's public key. The recipient's private key is needed to recover the random keying material, which is then treated as a pairwise shared secret between the originator and recipient. A KEM algorithm provides three functions:¶

• KeyGen() -> (pk, sk):¶

• Generate the public key (pk) and a private key (sk).¶

• Encapsulate(pk) -> (ct, ss):¶

• Given the recipient's public key (pk), produce a ciphertext (ct) to be passed to the recipient and shared secret (ss) for the originator.¶

• Decapsulate(sk, ct) -> ss:¶

• Given the private key (sk) and the ciphertext (ct), produce the shared secret (ss) for the recipient.¶

To support a particular KEM algorithm, the CMS originator MUST implement Encapsulate().¶

To support a particular KEM algorithm, the CMS recipient MUST implement KeyGen() and Decapsulate(). The recipient's public key is usually carried in a certificate [RFC5280].¶

This draft follows the DH-Based KEM (DHKEM) construction defined in [RFC9180] whereby the Encapsulate() operation includes the generation of an ephemeral key and the usage of that key against the recipient's static public key.¶

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. Cryptographic dependencies

4. DH-Based KEM (DHKEM)

This is a straightforward application of the DHKEM construction from [RFC9180] section 4.1 which is to be used unmodified.¶

CMS encrypt operations performed by the sender are to use Encap(pkR). CMS decrypt operations performed by the received are to use Decap(enc, skR).¶

The authenticated modes, AuthEncap(pkR, skS) and AuthDecap(enc, skR, pkS) do not apply to CMS.¶

5. ASN.1 Module

In order to carry a DHKEM inside a CMS KEMRecipientInfo [I-D.ietf-lamps-cms-kemri], we define id-kem-dhkem, kema-dhkem, and DHKemParameters.¶

CMS-DHKEM-2023
    { iso(1) member-body(2) us(840) rsadsi(113549)
      pkcs(1) pkcs-9(9) smime(16) modules(0)
      id-mod-cms-dhkem-2023(TBDMOD) }

  DEFINITIONS IMPLICIT TAGS ::=
  BEGIN
  -- EXPORTS ALL;

  IMPORTS

  AlgorithmIdentifier{}, KEY-AGREE, KEY-DERIVATION
    FROM AlgorithmInformation-2009
      { iso(1) identified-organization(3) dod(6) internet(1)
        security(5) mechanisms(5) pkix(7) id-mod(0)
        id-mod-algorithmInformation-02(58) }

   KEM-ALGORITHM
     FROM KEMAlgorithmInformation-2023 -- [I-D.ietf-lamps-cms-kemri]
       { iso(1) identified-organization(3) dod(6) internet(1)
         security(5) mechanisms(5) pkix(7) id-mod(0)
         id-mod-kemAlgorithmInformation-2023(99) }

   pk-dh, pk-ec
     FROM PKIXAlgs-2009
       { iso(1) identified-organization(3) dod(6) internet(1)
         security(5) mechanisms(5) pkix(7) id-mod(0)
         id-mod-pkix1-algorithms2008-02(56) }

  pk-X25519, pk-X448
    FROM Safecurves-pkix-18
      { iso(1) identified-organization(3) dod(6) internet(1)
        security(5) mechanisms(5) pkix(7) id-mod(0)
        id-mod-safecurves-pkix(93) } ;


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

  kema-dhkem KEM-ALGORITHM ::= {
      IDENTIFIER id-alg-dhkem
      PARAMS TYPE DHKemParameters
      PUBLIC-KEYS { pk-dh | pk-ec | pk-X25519 | pk-X448 }
      UKM ARE optional
      SMIME-CAPS { TYPE DHKemParameters IDENTIFIED BY id-kem-dhkem } }

  DHKemParameters ::= SEQUENCE {
      dh         KeyAgreeAlgorithmIdentifier,
      kdf        KeyDerivationFunction,
      keyLength  KeyLength }

  KeyAgreeAlgorithmIdentifier ::= AlgorithmIdentifier{ KEY-AGREE, {...} }

  KeyDerivationFunction ::= AlgorithmIdentifier { KEY-DERIVATION, {...} }

  KeyLength ::= INTEGER (1..MAX)

END

EDNOTE: The other way to define this would be to call out a toplevel DHKEM for each one: id-kema-dhkem-dh id-kema-dhkem-ecdh, id-kema-dhkem-x25519, id-kema-dhkem-x448. EDNOTE: This approach adds a layer of wrapping for the benefit of agility and future-proofing. I would be happy to write them each out if that's considered better.¶

6. Security Considerations

This document does not add any security considerations above those already present for the Ephemeral-Static mode of the underlying (EC)DH primitive and in [RFC9180].¶

7. IANA Considerations

This document registers the OID id-alg-dhkem¶

The IANA is requested to allocate a value from the "SMI Security for S/MIME Module Identifier" registry for the included ASN.1 module, and allocate values from "SMI Security for S/MIME Algorithms" to identify the new algorithm defined within.¶

Acknowledgments

TODO acknowledge.¶

Authors' Addresses