Use of Hybrid Public-Key Encryption (HPKE) with CBOR Object Signing and Encryption (COSE)

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Table of Contents

1. Introduction

Hybrid public-key encryption (HPKE) [I-D.irtf-cfrg-hpke] is a scheme that provides public key encryption of arbitrary-sized plaintexts given a recipient's public key. HPKE utilizes a non-interactive ephemeral-static Diffie-Hellman exchange to establish a shared secret. The motivation for standardizing a public key encryption scheme is explained in the introduction of [I-D.irtf-cfrg-hpke].¶

The HPKE specification defines several features for use with public key encryption and a subset of those features is applied to COSE [RFC8152]. Since COSE provides constructs for authentication, those are not re-used from the HPKE specification. This specification uses the "base" mode, as it is called in HPKE specification language.¶

2. Conventions and 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.¶

This specification uses the following abbreviations and terms: - Content-encryption key (CEK), a term defined in CMS [RFC2630]. - Hybrid Public Key Encryption (HPKE) is defined in [I-D.irtf-cfrg-hpke]. - pkR is the public key of the recipient, as defined in [I-D.irtf-cfrg-hpke]. - skR is the private key of the recipient, as defined in [I-D.irtf-cfrg-hpke].¶

3. HPKE for COSE

COSE_Encrypt_Tagged = #6.96(COSE_Encrypt)

HPKE_Encryption_Info = COSE_Encrypt_Tagged

; Layer 0
COSE_Encrypt = [
  Headers,
  ciphertext : bstr / nil,
  recipients : + COSE_recipient
]

; Layer 1
COSE_recipient = [
  protected   : bstr .cbor header_map, ; must contain alg parameter
  unprotected : header_map, ; must contain kid and ephemeral public key
  encCEK      : bstr, ; CEK encrypted with HPKE-derived shared secret
]

header_map = {
  Generic_Headers,
  * label =values,
}

   PartyInfo = (
       identity : bstr / nil,
       nonce : bstr / int / nil,
       other : bstr / nil
   )

   COSE_KDF_Context = [
       AlgorithmID : int / tstr,
       PartyUInfo : [ PartyInfo ],
       PartyVInfo : [ PartyInfo ],
       SuppPubInfo : [
           keyDataLength : uint,
           protected : empty_or_serialized_map,
           ? other : bstr
       ],
       ? SuppPrivInfo : bstr
   ]

4. Example

An example of the COSE_Encrypt structure using the HPKE scheme is shown in Figure 3. Line breaks and comments have been inserted for better readability. It uses the following algorithm combination:¶

• AES-GCM-128 for encryption of detached ciphertext in layer 0.¶
• AES-GCM-128 for encryption of the CEK in layer 1 as well as ECDH with NIST P-256 and HKDF-SHA256 as a Key Encapsulation Mechanism (KEM).¶

The algorithm selection is based on the registry of the values offered by the alg parameters.¶

96_0([
    / protected header with alg=AES-GCM-128 /
    h'a10101',
    / unprotected header with nonce /
    {5: h'938b528516193cc7123ff037809f4c2a'},
    / detached ciphertext /
    null,
    / recipient structure /
    [
        / protected field with alg for HPKE /
        h'a1013863',
        / unprotected header /
        {
            / ephemeral public key with x / y coodinate /
            -1: h'a401022001215820a596f2ca8d159c04942308ca90
                  cfbfca65b108ca127df8fe191a063d00d7c5172258
                  20aef47a45d6d6c572e7bd1b9f3e69b50ad3875c68
                  f6da0caaa90c675df4162c39',
             /  kid for recipient static ECDH public key /
             4: h'6b69642d32',
        },
        / encrypted CEK /
        h'9aba6fa44e9b2cef9d646614dcda670dbdb31a3b9d37c7a
          65b099a8152533062',
    ],
])

Note that the COSE_Sign1 wrapper outside the COSE_Encrypt structure is not shown in the example above.¶

5. Security Considerations

This specification is based on HPKE and the security considerations of HPKE [I-D.irtf-cfrg-hpke] are therefore applicable also to this specification.¶

HPKE assumes the sender is in possession of the public key of the recipient and HPKE COSE makes the same assumptions. Some form of public key distribution mechanism is assumed to exist.¶

Since the CEK is randomly generated it must be ensured that the guidelines for random number generations are followed, see [RFC8937].¶

The COSE_Encrypt structure must be authenticated using COSE constructs like COSE_Sign, or COSE_Sign1.¶

6. IANA Considerations

This document requests IANA to add new values to the COSE Algorithms registry defined in [RFC8152] (in the Standards Action With Expert Review category):¶

Appendix A. Acknowledgements

We would like to thank Goeran Selander, John Mattsson and Ilari Liusvaara for their review feedback.¶

Authors' Addresses