Internet-Draft COSE HPKE October 2021
Tschofenig, et al. Expires 28 April 2022 [Page]
Workgroup:
COSE
Internet-Draft:
draft-tschofenig-cose-hpke-00
Published:
Intended Status:
Standards Track
Expires:
Authors:
H. Tschofenig
Arm Limited
R. Housley
Vigil Security
B. Moran
Arm Limited

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

Abstract

This specification defines hybrid public-key encryption (HPKE) for use with CBOR Object Signing and Encryption (COSE).

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 28 April 2022.

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

3.1. Overview

The CDDL for the COSE_Encrypt structure, as used with this specification, is shown in Figure 1. The structures referenced below are found in the CDDL.

HPKE, when used with COSE, follows a three layer structure:

  • Layer 0 (corresponding to the COSE_Encrypt structure) contains content encrypted with the CEK. This ciphertext may be detached. If not detached, then it is included in the COSE_Encrypt structure.
  • Layer 1 (see COSE_recipient_outer structure) includes the encrypted CEK.
  • Layer 2 (in the COSE_recipient_inner structure) contains parameters needed for HPKE to generate a shared secret used to encrypt the CEK from layer 1.
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,
}
Figure 1: CDDL for HPKE-based COSE_Encrypt Structure

The COSE_recipient_outer structure shown in Figure 1 includes the encrypted CEK (in the encCEK structure) and the COSE_recipient_inner structure, also shown in Figure 1, contains the ephemeral public key (in the unprotected structure).

3.2. HPKE Encryption with SealBase

The SealBase(pkR, info, aad, pt) function is used to encrypt a plaintext pt to a recipient's public key (pkR). For use in this specification, the plaintext "pt" passed into the SealBase is the CEK. The CEK is a random byte sequence of length appropriate for the encryption algorithm selected in layer 0. For example, AES-128-GCM requires a 16 byte key and the CEK would therefore be 16 bytes long.

The "info" parameter can be used to influence the generation of keys and the "aad" parameter provides additional authenticated data to the AEAD algorithm in use. If successful, SealBase() will output a ciphertext "ct" and an encapsulated key "enc". The content of enc is the ephemeral public key.

The content of the info parameter is based on the 'COSE_KDF_Context' structure, which is detailed in Figure 2.

3.3. HPKE Decryption with Open

The recipient will use the OpenBase(enc, skR, info, aad, ct) function with the enc and ct parameters received from the sender. The "aad" and the "info" parameters are obtained via the context of the usage.

The OpenBase function will, if successful, decrypt "ct". When decrypted, the result will be the CEK. The CK is the symmetric key used to decrypt the ciphertext in the COSE_Encrypt structure.

3.4. Info Structure

This specification re-uses the context information structure defined in [RFC8152] for use with the HPKE algorithm. This payload becomes the content of the info parameter for the HPKE functions. For better readability of this specification the COSE_KDF_Context structure is repeated in Figure 2.

   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
   ]
Figure 2: COSE_KDF_Context Data Structure for info parameter

Since this specification may be used in a number of different deployment environments flexibility for populating the fields in the COSE_KDF_Context structure is provided.

For better interoperability, the following recommended settings are provided:

  • PartyUInfo.identity corresponds to the kid found in the COSE_Sign_Tagged or COSE_Sign1_Tagged structure (when a digital signature is used). When utilizing a MAC, then the kid is found in the COSE_Mac_Tagged or COSE_Mac0_Tagged structure.
  • PartyVInfo.identity corresponds to the kid used for the respective recipient from the inner-most recipients array.
  • The value in the AlgorithmID field corresponds to the alg parameter in the protected structure in the inner-most recipients array.
  • keyDataLength is set to the number of bits of the desired output value.
  • protected refers to the protected structure of the inner-most array.

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''
             ]
            ]
        ]
     ]
)
Figure 3: COSE_Encrypt Example for HPKE

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      |
+-------------+-------+---------+------------+--------+---------------+

7. References

7.1. Normative References

[I-D.irtf-cfrg-hpke]
Barnes, R. L., Bhargavan, K., Lipp, B., and C. A. Wood, "Hybrid Public Key Encryption", Work in Progress, Internet-Draft, draft-irtf-cfrg-hpke-12, , <https://www.ietf.org/archive/id/draft-irtf-cfrg-hpke-12.txt>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
[RFC8152]
Schaad, J., "CBOR Object Signing and Encryption (COSE)", RFC 8152, DOI 10.17487/RFC8152, , <https://www.rfc-editor.org/info/rfc8152>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.

7.2. Informative References

[RFC2630]
Housley, R., "Cryptographic Message Syntax", RFC 2630, DOI 10.17487/RFC2630, , <https://www.rfc-editor.org/info/rfc2630>.
[RFC8937]
Cremers, C., Garratt, L., Smyshlyaev, S., Sullivan, N., and C. Wood, "Randomness Improvements for Security Protocols", RFC 8937, DOI 10.17487/RFC8937, , <https://www.rfc-editor.org/info/rfc8937>.

Appendix A. Acknowledgements

TBD: Add your name here.

Authors' Addresses

Hannes Tschofenig
Arm Limited
Russ Housley
Vigil Security, LLC
Brendan Moran
Arm Limited