Use of Hybrid Public-Key Encryption (HPKE) with CBOR Object Signing and Encryption (COSE)
draft-ietf-cose-hpke-01

COSE                                                       H. Tschofenig
Internet-Draft                                               Arm Limited
Intended status: Standards Track                              R. Housley
Expires: 8 September 2022                                 Vigil Security
                                                                B. Moran
                                                             Arm Limited
                                                            7 March 2022

Use of Hybrid Public-Key Encryption (HPKE) with CBOR Object Signing and
                           Encryption (COSE)
                        draft-ietf-cose-hpke-01

Abstract

   This specification defines hybrid public-key encryption (HPKE) for
   use with CBOR Object Signing and Encryption (COSE).  HPKE offers a
   variant of public-key encryption of arbitrary-sized plaintexts for a
   recipient public key.

   HPKE works for any combination of an asymmetric key encapsulation
   mechanism (KEM), key derivation function (KDF), and authenticated
   encryption with additional data (AEAD) encryption function.
   Authentication for HPKE in COSE is provided by COSE-native security
   mechanisms.

Status of This Memo

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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   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 OpenBase . . . . . . . . . . . . . .   5
     3.4.  Info Structure  . . . . . . . . . . . . . . . . . . . . .   5
   4.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  HPKE/P-256+HKDF-256 and AES-128-GCM . . . . . . . . . . .   8
     6.2.  HPKE/P-512+HKDF-512 and AES-256-GCM . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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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

3.1.  Overview

   The CDDL for the COSE_Encrypt structure, as used with this
   specification, is shown in Figure 1.

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

   *  Layer 0 (corresponding to the COSE_Encrypt structure) contains
      content (plaintext) encrypted with the CEK.  This ciphertext may
      be detached.  If not detached, then it is included in the
      COSE_Encrypt structure.

   *  Layer 1 (corresponding to a recipient structure) contains
      parameters needed for HPKE to generate a shared secret used to
      encrypt the CEK.  This layer includes the encrypted CEK.

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   This two-layer structure is used to encrypt content that can also be
   shared with multiple parties at the expense of a single additional
   encryption operation.  As stated above, the specification uses a CEK
   to encrypt the content at layer 0.

   For example, the content encrypted at layer 0 is a firmware image.
   The same ciphertext firmware image is processed by all of the
   recipients; however, each recipient uses their own private key to
   obtain the CEK.

 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,
 }

          Figure 1: CDDL for HPKE-based COSE_Encrypt Structure

   The COSE_recipient structure shown in Figure 1 is repeated for each
   recipient, and it includes the encrypted CEK as well as the sender-
   generated ephemeral public key in the unprotected header 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).

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   IMPORTANT: 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.  This specification does not mandate the use
   of the info and the aad parameters.

   If SealBase() is successful, it 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 OpenBase

   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 layer 0 of the COSE_Encrypt
   structure.

3.4.  Info Structure

   This section provides a suggestion for constructing the info
   structure, when used with SealBase() and OpenBase().  Note that the
   use of the aad and the info structures for these two functions is
   optional.  Profiles of this specification may require their use and
   may define different info structure.

   This specification re-uses the context information structure defined
   in [RFC8152] as a foundation for the info structure.  This payload
   becomes the content of the info parameter for the HPKE functions,
   when utilized.  For better readability of this specification the
   COSE_KDF_Context structure is repeated in Figure 2.

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      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

   The fields in Figure 2 are populated as follows:

   *  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 unprotected header structure of the recipient
      structure.

   *  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.  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.

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   *  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',
       ],
   ])

                  Figure 3: COSE_Encrypt Example for HPKE

   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.

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   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):

6.1.  HPKE/P-256+HKDF-256 and AES-128-GCM

   *  Name: HPKE_P256_HKDF256_AES128_GCM

   *  Value: TBD1

   *  Description: HPKE/P-256+HKDF-256 and AES-128-GCM

   *  Capabilities: [kty]

   *  Change Controller: IESG

   *  Reference: [[TBD: This RFC]]

   *  Recommended: Yes

6.2.  HPKE/P-512+HKDF-512 and AES-256-GCM

   *  Name: HPKE_P521_HKDF512_AES256_GCM

   *  Value: TBD2

   *  Description: HPKE/P-512+HKDF-512 and AES-256-GCM

   *  Capabilities: [kty]

   *  Change Controller: IESG

   *  Reference: [[TBD: This RFC]]

   *  Recommended: Yes

   TBD: More values to be added.

7.  References

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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

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

Authors' Addresses

   Hannes Tschofenig
   Arm Limited
   Email: hannes.tschofenig@arm.com

   Russ Housley
   Vigil Security, LLC
   Email: housley@vigilsec.com

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   Brendan Moran
   Arm Limited
   Email: Brendan.Moran@arm.com

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