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, which is then used to encrypt plaintext.¶

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 authenticcation, 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 RFC 2630 [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)

SUIT_Encryption_Info = COSE_Encrypt_Tagged

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

; Layer 1
COSE_recipient_outer = [
  protected   : bstr .size 0,
  unprotected : header_map, ; must contain alg
  encCEK      : bstr, ; CEK encrypted with HPKE-derived shared secret
  recipients  : [ + COSE_recipient_inner ]
]

; Layer 2
COSE_recipient_inner = [
  protected   : bstr .cbor header_map, ; must contain HPKE alg
  unprotected : header_map, ; must contain kid and ephemeral public key
  empty       : null,
  empty       : null
]

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. It uses the following algorithm combination:¶

• AES-GCM-128 for encryption of detached ciphertext.¶
• AES-GCM-128 for encryption of the CEK.¶
• Key Encapsulation Mechanism (KEM): NIST P-256¶
• Key Derivation Function (KDF): HKDF-SHA256¶

96(
    [
        // protected field with alg=AES-GCM-128
        h'A10101',
        {    // unprotected field with iv
             5: h'26682306D4FB28CA01B43B80'
        },
        // null because of detached ciphertext
        null,
        [  // COSE_recipient_outer
            h'',          // empty protected field
            {             // unprotected field with ...
                 1: 1     //     alg=A128GCM
            },
            // Encrypted CEK
            h'FA55A50CF110908DA6443149F2C2062011A7D8333A72721A',
            / recipients / [  // COSE_recipient_inner
             [
               / protected / h'a1013818' / {
                   \ alg \ 1:TBD1 \ HPKE/P-256+HKDF-256 \
                 } / ,
               / unprotected / {
                 // HPKE encapsulated key
                 / ephemeral / -1:{
                   / kty / 1:2,
                   / crv / -1:1,
                   / x / -2:h'98f50a4ff6c05861c8...90bbf91d6280',
                   / y / -3:true
                 },
                 // kid for recipient static ECDH public key
                 / kid / 4:'meriadoc.brandybuck@buckland.example'
               },
               // empty ciphertext
               / ciphertext / h''
             ]
            ]
        ]
     ]
)

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 that the sender is in possession of the public key of the recipient. A system using HPKE COSE has to assume the same assumptions and 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 SUIT_Encryption_Info structure shown in this document does not provide authentication. Hence, the SUIT_Encryption_Info structure has to be used in combination with other COSE constructs, such as the COSE_Sign or COSE_Sign1.¶

6. IANA Considerations

This document requests IANA to create new entries in the COSE Algorithms registry established with [RFC8152].¶

+-------------+-------+---------+------------+--------+---------------+
| Name        | Value | KDF     | Ephemeral- | Key    | Description   |
|             |       |         | Static     | Wrap   |               |
+-------------+-------+---------+------------+--------+---------------+
| HPKE/P-256+ | TBD1  | HKDF -  | yes        | none   | HPKE with     |
| HKDF-256    |       | SHA-256 |            |        | ECDH-ES       |
|             |       |         |            |        | (P-256) +     |
|             |       |         |            |        | HKDF-256      |
+-------------+-------+---------+------------+--------+---------------+
| HPKE/P-384+ | TBD2  | HKDF -  | yes        | none   | HPKE with     |
| HKDF-SHA384 |       | SHA-384 |            |        | ECDH-ES       |
|             |       |         |            |        | (P-384) +     |
|             |       |         |            |        | HKDF-384      |
+-------------+-------+---------+------------+--------+---------------+
| HPKE/P-521+ | TBD3  | HKDF -  | yes        | none   | HPKE with     |
| HKDF-SHA521 |       | SHA-521 |            |        | ECDH-ES       |
|             |       |         |            |        | (P-521) +     |
|             |       |         |            |        | HKDF-521      |
+-------------+-------+---------+------------+--------+---------------+
| HPKE        | TBD4  | HKDF -  | yes        | none   | HPKE with     |
| X25519 +    |       | SHA-256 |            |        | ECDH-ES       |
| HKDF-SHA256 |       |         |            |        | (X25519) +    |
|             |       |         |            |        | HKDF-256      |
+-------------+-------+---------+------------+--------+---------------+
| HPKE        | TBD4  | HKDF -  | yes        | none   | HPKE with     |
| X448 +      |       | SHA-512 |            |        | ECDH-ES       |
| HKDF-SHA512 |       |         |            |        | (X448) +      |
|             |       |         |            |        | HKDF-512      |
+-------------+-------+---------+------------+--------+---------------+

Appendix A. Acknowledgements

TBD: Add your name here.¶

Authors' Addresses