COSE                                                       H. Tschofenig
Internet-Draft                                               Arm Limited
Intended status: Standards Track                              R. Housley
Expires: 12 January 2023                                  Vigil Security
                                                                B. Moran
                                                             Arm Limited
                                                            11 July 2022


Use of Hybrid Public-Key Encryption (HPKE) with CBOR Object Signing and
                           Encryption (COSE)
                        draft-ietf-cose-hpke-02

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

   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
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   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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 12 January 2023.

Copyright Notice

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




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   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.

   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   3
   3.  HPKE for COSE . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   3
       3.1.1.  One Layer Structure . . . . . . . . . . . . . . . . .   4
     3.2.  One Layer Example . . . . . . . . . . . . . . . . . . . .   5
       3.2.1.  Two Layer Structure . . . . . . . . . . . . . . . . .   6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  COSE Algorithms Registry  . . . . . . . . . . . . . . . .  11
       5.1.1.  COSE_ALG_HPKE_AES_128_GCM . . . . . . . . . . . . . .  11
       5.1.2.  COSE_ALG_HPKE_AES_256_GCM . . . . . . . . . . . . . .  11
       5.1.3.  COSE_ALG_HPKE_CHACHA20_POLY1305 . . . . . . . . . . .  11
     5.2.  COSE Elliptic Curves Registry . . . . . . . . . . . . . .  12
       5.2.1.  COSE_CRV_HPKE_P256_SHA256 . . . . . . . . . . . . . .  12
       5.2.2.  COSE_CRV_HPKE_P384_SHA384 . . . . . . . . . . . . . .  12
       5.2.3.  COSE_CRV_HPKE_P521_SHA512 . . . . . . . . . . . . . .  12
       5.2.4.  COSE_CRV_HPKE_X25519_SHA256 . . . . . . . . . . . . .  13
       5.2.5.  COSE_CRV_HPKE_X448_SHA512 . . . . . . . . . . . . . .  13
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14




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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

3.1.  Overview

   This specification supports two uses of HPKE in COSE, namely

   *  HPKE in a single sender - single recipient setup.  This use cases
      uses a one layer structure for efficiency.  Section 3.1.1 provides
      the details.

   *  HPKE in a single sender - multiple recipient setup.  This use case
      requires a two layer structure.  Section 3.2.1 provides the
      details.

   HPKE in "base" mode requires little information to be exchanged
   between a sender and a recipient, namely

   *  algorithm information,




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   *  the ephemeral public key, and

   *  an identifier of the static recipient key.

   In the subsections below we explain how this information is carried
   inside the COSE_Encrypt0 and the COSE_Encrypt1 for the one layer and
   the two layer structure, respectively.

3.1.1.  One Layer Structure

   With the one layer structure the information carried inside the
   COSE_recipient structure is embedded inside the COSE_Encrypt0.

   HPKE is used to directly encrypt the plaintext.  The resulting
   ciphertext may be included in the COSE_Encrypt0 or may be detached.

   A sender MUST set the alg parameter in the protected header, which
   indicates the use of HPKE.  The values for the alg parameter MUST be
   taken from Section 5.1, or values registered in the future with the
   COSE_ALG_HPKE_* prefix.

   The sender MUST place the kid and ephemeral public key into the
   unprotected header.

   Figure 1 shows the COSE_Encrypt0 CDDL structure.

   COSE_Encrypt0_Tagged = #6.16(COSE_Encrypt0)

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

           Figure 1: CDDL for HPKE-based COSE_Encrypt0 Structure

   The COSE_Encrypt0 MAY be tagged or untagged.

   An example is shown in Section 3.2.

3.1.1.1.  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 COSE_Encrypt0, the plaintext "pt" passed into the SealBase
   is the raw plaintext.




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   In the absence of an application profile standard specifying
   otherwise a COSE-HPKE-compliant application MUST use an empty "info"
   parameter.  The Enc_structure, defined in Section 5.3 of [RFC8152],
   is used as input to the "aad" parameter.

   The CDDL fragment is defined as:

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

   The "external_aad" is empty, unless an application profile standard
   specifies otherwise.

   If SealBase() is successful, it will output a ciphertext "ct" and an
   encapsulated key "enc".  The content of enc is the ephemeral public
   key.

3.1.1.2.  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.

   In the absence of an application profile standard specifying
   otherwise a COSE-HPKE-compliant application MUST use an empty "info"
   parameter.  The Enc_structure, defined in Section 5.3 of [RFC8152],
   is used as input to the "aad" parameter.  The CDDL fragment is shown
   in the previous section.

   The OpenBase function will, if successful, decrypt "ct".  When
   decrypted, the result is the raw plaintext.

3.2.  One Layer Example

   This example shows a COSE_Encrypt0 structure.  HPKE was used to
   encrypt plaintext with AES-128-GCM.  The ephemeral NIST P-256 key key
   generated by the HPKE SealBase().












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   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'
       ]
   )

                  Figure 2: COSE_Encrypt0 Example for HPKE

3.2.1.  Two Layer Structure

   With the two layer structure the HPKE information is conveyed in the
   COSE_recipient structure, i.e. one COSE_recipient structure per
   recipient.

   In this approach the following layers are involved:

   *  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 conveys the encrypted CEK in the
      encCEK structure.  The protected header MUST contain the algorithm
      information and the unprotected header MUST contain the ephemeral
      public key and the key id (kid) of the static recipient public
      key.



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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 may be 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.

   The COSE_recipient structure shown in Figure 3 is repeated for each
   recipient.

   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,
   }

            Figure 3: CDDL for HPKE-based COSE_Encrypt Structure

   The COSE_Encrypt MAY be tagged or untagged.

   HPKE algorithms take an info parameter that can be used to influence
   the generation of keys (e.g., to fold in identity information) and an
   aad parameter that provides additional authenticated data to the AEAD
   algorithm in use.

   An example is shown in Section 3.2.1.3.







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3.2.1.1.  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 COSE_Encrypt, 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.

   In the absence of an application profile standard specifying
   otherwise, a COSE-HPKE-compliant implementation MUST leave the info
   and the aad parameters empty when used with the two layer structure.

   If SealBase() is successful, it will output a ciphertext "ct" and an
   encapsulated key "enc".  The content of enc is the ephemeral public
   key.

3.2.1.2.  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.

   In the absence of an application profile standard specifying
   otherwise, a COSE-HPKE-compliant implementation MUST leave the info
   and the aad parameters empty when used with the two layer structure.

   The OpenBase function will, if successful, decrypt "ct".  When
   decrypted, the result will be the CEK.  The CEK is the symmetric key
   used to decrypt the ciphertext in layer 0 of the COSE_Encrypt
   structure.

3.2.1.3.  Two Layer Examples

   An example of the COSE_Encrypt structure using the HPKE scheme is
   shown in Figure 4.  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.

   *  Encryption of the CEK in layer 1 utilizing HPKE 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 (see Section 5).




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   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',
       ],
   ])

                  Figure 4: COSE_Encrypt Example for HPKE

   To offer authentication of the sender the payload in Figure 4 is
   signed with a COSE_Sign1 wrapper, which is shown in Figure 5.  The
   payload in Figure 5 corresponds to the content shown in Figure 4.





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

                  Figure 5: COSE_Encrypt Example for HPKE

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).





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5.1.  COSE Algorithms Registry

5.1.1.  COSE_ALG_HPKE_AES_128_GCM

   *  Name: COSE_ALG_HPKE_AES_128_GCM

   *  Value: TBD1

   *  Description: HPKE with AES-128-GCM

   *  Capabilities: [kty]

   *  Change Controller: IESG

   *  Reference: [[TBD: This RFC]]

   *  Recommended: Yes

5.1.2.  COSE_ALG_HPKE_AES_256_GCM

   *  Name: COSE_ALG_HPKE_AES_256_GCM

   *  Value: TBD2

   *  Description: HPKE with AES-256-GCM

   *  Capabilities: [kty]

   *  Change Controller: IESG

   *  Reference: [[TBD: This RFC]]

   *  Recommended: Yes

5.1.3.  COSE_ALG_HPKE_CHACHA20_POLY1305

   *  Name: COSE_ALG_HPKE_CHACHA20_POLY1305

   *  Value: TBD3

   *  Description: HPKE with CHACHA20-POLY1305

   *  Capabilities: [kty]

   *  Change Controller: IESG

   *  Reference: [[TBD: This RFC]]




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   *  Recommended: Yes

5.2.  COSE Elliptic Curves Registry

5.2.1.  COSE_CRV_HPKE_P256_SHA256

   *  Name: COSE_CRV_HPKE_P256_SHA256

   *  Value: TBD4

   *  Key Type:

   *  Description: NIST P256 and SHA256 for use with HPKE

   *  Change Controller: IESG

   *  Reference: [[TBD: This RFC]]

   *  Recommended: Yes

5.2.2.  COSE_CRV_HPKE_P384_SHA384

   *  Name: COSE_CRV_HPKE_P384_SHA384

   *  Value: TBD5

   *  Key Type:

   *  Description: NIST P384 and SHA384 for use with HPKE

   *  Change Controller: IESG

   *  Reference: [[TBD: This RFC]]

   *  Recommended: Yes

5.2.3.  COSE_CRV_HPKE_P521_SHA512

   *  Name: COSE_CRV_HPKE_P521_SHA512

   *  Value: TBD6

   *  Key Type:

   *  Description: NIST P521 and SHA512 for use with HPKE

   *  Change Controller: IESG




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   *  Reference: [[TBD: This RFC]]

   *  Recommended: Yes

5.2.4.  COSE_CRV_HPKE_X25519_SHA256

   *  Name: COSE_CRV_HPKE_X25519_SHA256

   *  Value: TBD7

   *  Key Type:

   *  Description: X25519 and SHA256 for use with HPKE

   *  Change Controller: IESG

   *  Reference: [[TBD: This RFC]]

   *  Recommended: Yes

5.2.5.  COSE_CRV_HPKE_X448_SHA512

   *  Name: COSE_CRV_HPKE_X448_SHA512

   *  Value: TBD8

   *  Key Type:

   *  Description: X448 and SHA512 for use with HPKE

   *  Change Controller: IESG

   *  Reference: [[TBD: This RFC]]

   *  Recommended: Yes

6.  References

6.1.  Normative References

   [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>.



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   [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>.

   [RFC9180]  Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid
              Public Key Encryption", RFC 9180, DOI 10.17487/RFC9180,
              February 2022, <https://www.rfc-editor.org/info/rfc9180>.

6.2.  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


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












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