COSE                                                       H. Tschofenig
Internet-Draft                                               Arm Limited
Intended status: Standards Track                              R. Housley
Expires: 14 July 2022                                     Vigil Security
                                                                B. Moran
                                                             Arm Limited
                                                         10 January 2022


Use of Hybrid Public-Key Encryption (HPKE) with CBOR Object Signing and
                           Encryption (COSE)
                        draft-ietf-cose-hpke-00

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 14 July 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.



Tschofenig, et al.        Expires 14 July 2022                  [Page 1]


Internet-Draft                  COSE HPKE                   January 2022


   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   3
   3.  HPKE for COSE . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.2.  HPKE Encryption with SealBase . . . . . . . . . . . . . .   4
     3.3.  HPKE Decryption with Open . . . . . . . . . . . . . . . .   5
     3.4.  Info Structure  . . . . . . . . . . . . . . . . . . . . .   5
   4.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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).








Tschofenig, et al.        Expires 14 July 2022                  [Page 2]


Internet-Draft                  COSE HPKE                   January 2022


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.















Tschofenig, et al.        Expires 14 July 2022                  [Page 3]


Internet-Draft                  COSE HPKE                   January 2022


 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.




Tschofenig, et al.        Expires 14 July 2022                  [Page 4]


Internet-Draft                  COSE HPKE                   January 2022


   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





Tschofenig, et al.        Expires 14 July 2022                  [Page 5]


Internet-Draft                  COSE HPKE                   January 2022


   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














Tschofenig, et al.        Expires 14 July 2022                  [Page 6]


Internet-Draft                  COSE HPKE                   January 2022


   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.




Tschofenig, et al.        Expires 14 July 2022                  [Page 7]


Internet-Draft                  COSE HPKE                   January 2022


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




Tschofenig, et al.        Expires 14 July 2022                  [Page 8]


Internet-Draft                  COSE HPKE                   January 2022


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, 2 September 2021,
              <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, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8152]  Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              RFC 8152, DOI 10.17487/RFC8152, July 2017,
              <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,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

7.2.  Informative References

   [RFC2630]  Housley, R., "Cryptographic Message Syntax", RFC 2630,
              DOI 10.17487/RFC2630, June 1999,
              <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, October 2020,
              <https://www.rfc-editor.org/info/rfc8937>.

Appendix A.  Acknowledgements

   TBD: Add your name here.

Authors' Addresses

   Hannes Tschofenig
   Arm Limited

   Email: hannes.tschofenig@arm.com






Tschofenig, et al.        Expires 14 July 2022                  [Page 9]


Internet-Draft                  COSE HPKE                   January 2022


   Russ Housley
   Vigil Security, LLC

   Email: housley@vigilsec.com


   Brendan Moran
   Arm Limited

   Email: Brendan.Moran@arm.com









































Tschofenig, et al.        Expires 14 July 2022                 [Page 10]