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) [RFC9180] 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 [RFC9180].¶

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 [RFC9180]. - pkR is the public key of the recipient, as defined in [RFC9180]. - skR is the private key of the recipient, as defined in [RFC9180].¶

3. HPKE for COSE

COSE_Encrypt0_Tagged = #6.16(COSE_Encrypt0)

; Layer 0
COSE_Encrypt0 = [
    Headers,
    ciphertext : bstr / nil,
]

   Enc_structure = [
       context : "Encrypt0",
       protected : empty_or_serialized_map,
       external_aad : bstr
   ]

96(
    [
        / algorithm id TBD1 for COSE_ALG_HPKE_AES_128_GCM /
        << {1: TBD1} >>,
        {
            / ephemeral public key structure /
            -1: << {
                / kty set to EC2  /
                1: 2,
                                / crv set to P-256 /
                -1: 1,
                                / x-coordinate /
                -2: h'985E2FDE3E67E1F7146AB305AA98FE89
                      B1CFE545965B6CFB066C0BB19DE7E489',
                                / y-coordinate /
                -3: h'4AC5E777A7C96CB5D70B8A40E2951562
                      F20C21DB021AAD12E54A8DBE7EF9DF10'
                } >>,
             4: 'kid-2'
        },
        / encrypted plaintext /
        h'4123E7C3CD992723F0FA1CD3A903A588
          42B1161E02D8E7FD842C4DA3B984B9CF'
    ]
)

COSE_Encrypt_Tagged = #6.96(COSE_Encrypt)

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

/ Layer 1 /
COSE_recipient = [
  protected   : bstr .cbor header_map,
  unprotected : header_map,
  encCEK      : bstr,
]

header_map = {
  Generic_Headers,
  * label => values,
}

96_0([
    / protected header /
    << {
             / alg set to AES-GCM-128 /
             1: 1
           }
        >>,
    / unprotected header /
    {
           / nonce value /
           5: h'938b528516193cc7123ff037809f4c2a'
        },
    / detached ciphertext /
    null,
    / recipient structure /
    [
        / algorithm id TBD4 for COSE_ALG_HPKE_AES_128_GCM /
        << {1: TBD4} >>,
        / unprotected header /
        {
            / ephemeral public key structure /
            -1: << {
                            / kty set to EC2  /
                1: 2,
                                / crv set to P-256 /
                -1: 1,
                                / x-coordinate /
                -2: h'985E2FDE3E67E1F7146AB305AA98FE89
                      B1CFE545965B6CFB066C0BB19DE7E489',
                                / y-coordinate /
                -3: h'4AC5E777A7C96CB5D70B8A40E2951562
                      F20C21DB021AAD12E54A8DBE7EF9DF10'
                } >>,
             4: 'kid-2'
        },
        / encrypted CEK /
        h'9aba6fa44e9b2cef9d646614dcda670dbdb31a3b9d37c7a
          65b099a8152533062',
    ],
])

18(
  [
    / protected / h'a10126' / {
            \ alg \ 1:-7 \ ECDSA 256 \
          } / ,
    / unprotected / {
          / kid / 4:'sender@example.com'
        },
    / payload /     h'AA19...B80C',
    / signature /   h'E3B8...25B8'
  ]
)

4. Security Considerations

This specification is based on HPKE and the security considerations of HPKE [RFC9180] 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. Hence, some form of public key distribution mechanism is assumed to exist.¶

HPKE relies on a source of randomness to be available on the device. Additionally, with the two layer structure the CEK is randomly generated and the it MUST be ensured that the guidelines for random number generations are followed.¶

The COSE_Encrypt structure MUST be authenticated using COSE constructs like COSE_Sign, COSE_Sign1, COSE_MAC, or COSE_MAC0.¶

When COSE_Encrypt or COSE_Encrypt0 is used with a detached ciphertext then the subsequently applied integrity protection via COSE_Sign, COSE_Sign1, COSE_MAC, or COSE_MAC0 does not cover this detached ciphertext. Implementers MUST ensure that the detached ciphertext also experiences integrity protection. This is, for example, the case when an AEAD cipher is used to produce the detached ciphertext but may not be guaranteed by non-AEAD ciphers.¶

5. IANA Considerations

This document requests IANA to add new values to the COSE Algorithms registry and to the COSE Elliptic Curves 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