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Use of Hybrid Public-Key Encryption (HPKE) with CBOR Object Signing and Encryption (COSE)
draft-ietf-cose-hpke-07

Document Type Active Internet-Draft (cose WG)
Authors Hannes Tschofenig , Orie Steele , Ajitomi, Daisuke , Laurence Lundblade
Last updated 2024-02-23 (Latest revision 2023-10-22)
Replaces draft-tschofenig-cose-hpke
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draft-ietf-cose-hpke-07
COSE                                                       H. Tschofenig
Internet-Draft                                                          
Intended status: Standards Track                          O. Steele, Ed.
Expires: 24 April 2024                                         Transmute
                                                              D. Ajitomi
                                                                        
                                                            L. Lundblade
                                                     Security Theory LLC
                                                         22 October 2023

Use of Hybrid Public-Key Encryption (HPKE) with CBOR Object Signing and
                           Encryption (COSE)
                        draft-ietf-cose-hpke-07

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) function.  Authentication for
   HPKE in COSE is provided by COSE-native security mechanisms or by one
   of the authenticated variants of HPKE.

   This document defines the use of the HPKE with 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 24 April 2024.

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Copyright Notice

   Copyright (c) 2023 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
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   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
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   Without obtaining an adequate license from the person(s) controlling
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   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 . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   4
       3.1.1.  Single Recipient / One Layer Structure  . . . . . . .   4
       3.1.2.  Multiple Recipients / Two Layer Structure . . . . . .   5
     3.2.  Info Parameter  . . . . . . . . . . . . . . . . . . . . .   7
   4.  Ciphersuite Registration  . . . . . . . . . . . . . . . . . .   7
   5.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Single Recipient / One Layer Example  . . . . . . . . . .  10
     5.2.  Multiple Recipients / Two Layer . . . . . . . . . . . . .  10
       5.2.1.  COSE_Encrypt  . . . . . . . . . . . . . . . . . . . .  11
       5.2.2.  COSE_MAC  . . . . . . . . . . . . . . . . . . . . . .  13
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
     7.1.  COSE Algorithms Registry  . . . . . . . . . . . . . . . .  16
     7.2.  COSE Header Parameters  . . . . . . . . . . . . . . . . .  22
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  22
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  22

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   Appendix A.  Contributors . . . . . . . . . . . . . . . . . . . .  23
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

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 provides a variant of public key encryption of
   arbitrary-sized plaintexts for a recipient public key.  It also
   includes three authenticated variants, including one that
   authenticates possession of a pre-shared key, one that authenticates
   possession of a key encapsulation mechanism (KEM) private key, and
   one that authenticates possession of both a pre-shared key and a KEM
   private key.

   This specification utilizes HPKE as a foundational building block and
   carries the output to COSE ([RFC9052], [RFC9053]).

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].

   *  Key Encapsulation Mechanism (KEM), see [RFC9180].

   *  Key Derivation Function (KDF), see [RFC9180].

   *  Authenticated Encryption with Associated Data (AEAD), see
      [RFC9180].

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   *  Additional Authenticated Data (AAD), see [RFC9180].

3.  HPKE for COSE

3.1.  Overview

   This specification supports two uses of HPKE in COSE, namely

   *  HPKE in a single recipient setup.  This use case utilizes a one
      layer COSE structure.  Section 3.1.1 provides the details.

   *  HPKE in a multiple recipient setup.  This use case requires a two
      layer COSE structure.  Section 3.1.2 provides the details.  While
      it is possible to support the single recipient use case with a two
      layer structure, the single layer setup is more efficient.

   In both cases a new COSE header parameter, called 'encapsulated_key',
   is used to convey the content of the enc structure defined in the
   HPKE specification.  "Enc" represents the serialized public key.

   For use with HPKE the 'encapsulated_key' header parameter MUST be
   present in the unprotected header parameter and MUST contain the
   encapsulated key, which is output of the HPKE KEM, and it is a bstr.

3.1.1.  Single Recipient / 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 and the resulting
   ciphertext is either included in the COSE_Encrypt0 or is detached.
   If a payload is transported separately then it is called "detached
   content".  A nil CBOR object is placed in the location of the
   ciphertext.  See Section 5 of [RFC9052] for a description of detached
   payloads.

   The sender MUST set the alg parameter in the protected header, which
   indicates the use of HPKE.

   The sender MUST place the 'encapsulated_key' parameter into the
   unprotected header.  Although the use of the 'kid' parameter in
   COSE_Encrypt0 is discouraged by RFC 9052, this profile allows the use
   of the 'kid' parameter (or other parameters) to identify the static
   recipient public key used by the sender.  If the COSE_Encrypt0
   contains the 'kid' then the recipient may use it to select the
   appropriate private key.

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   HPKE defines an API and this API uses an "aad" parameter as input.
   When COSE_Encrypt0 is used then there is no AEAD function executed by
   COSE natively and HPKE offers this functionality.

   The "aad" parameter provided to the HPKE API is constructed as
   follows (and the design has been re-used from [RFC9052]):

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

   empty_or_serialized_map = bstr .cbor header_map / bstr .size 0

   The protected field in the Enc_structure contains the protected
   attributes from the COSE_Encrypt0 structure at layer 0, encoded in a
   bstr type.

   The external_aad field in the Enc_structure contains the Externally
   Supplied Data described in Section 4.3 and Section 5.3 in RFC 9052.
   If this field is not supplied, it defaults to a zero-length byte
   string.

   The HPKE APIs also use an "info" parameter as input and the details
   are provided in Section 3.2.

   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 5.1.

3.1.2.  Multiple Recipients / 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.

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   In this approach the following layers are involved:

   *  Layer 0 (corresponding to the COSE_Encrypt structure) contains the
      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 HPKE alg
      parameter and the unprotected header MUST contain the
      'encapsulated_key' parameter.  The unprotected header MAY contain
      the kid parameter to identify the static recipient public key the
      sender has been using with HPKE.

   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.

   The COSE_recipient structure, shown in Figure 2, 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 2: CDDL for HPKE-based COSE_Encrypt Structure

   The COSE_Encrypt MAY be tagged or untagged.

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   An example is shown in Section 5.2.

3.2.  Info Parameter

   The HPKE specification defines the "info" parameter as a context
   information structure that is used to ensure that the derived keying
   material is bound to the context of the transaction.

   This section provides a suggestion for constructing the info
   structure.  HPKE leaves the info parameter for these two functions as
   optional.  Application profiles of this specification MAY populate
   the fields of the COSE_KDF_Context structure or MAY use a different
   structure as input to the "info" parameter.  If no content for the
   "info" parameter is not supplied, it defaults to a zero-length byte
   string.

   This specification re-uses the context information structure defined
   in [RFC9053] 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 3.

      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 3: COSE_KDF_Context Data Structure as 'info' Parameter for
                                    HPKE

4.  Ciphersuite Registration

   This specification registers a number of ciphersuites for use with
   HPKE.  A ciphersuite is thereby a combination of several algorithm
   configurations:

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   *  HPKE Mode

   *  KEM algorithm

   *  KDF algorithm

   *  AEAD algorithm

   The "KEM", "KDF", and "AEAD" values are conceptually taken from the
   HPKE IANA registry [HPKE-IANA].  Hence, COSE-HPKE cannot use a
   algorithm combination that is not already available with HPKE.

   For better readability of the algorithm combination ciphersuites
   labels are build according to the following scheme:

   HPKE-<Version>-<Mode>-<KEM>-<KDF>-<AEAD>

   The "Mode" indicator may be populated with the following values from
   Table 1 of [RFC9180]:

   *  "Base" refers to "mode_base" described in Section 5.1.1 of
      [RFC9180], which only enables encryption to the holder of a given
      KEM private key.

   *  "PSK" refers to "mode_psk", described in Section 5.1.2 of
      [RFC9180], which authenticates using a pre-shared key.

   *  "Auth" refers to "mode_auth", described in Section 5.1.3 of
      [RFC9180], which authenticates using an asymmetric key.

   *  "Auth_Psk" refers to "mode_auth_psk", described in Section 5.1.4
      of [RFC9180], which authenticates using both a PSK and an
      asymmetric key.

   For a list of ciphersuite registrations, please see Section 7.  The
   following table summarizes the relationship between the ciphersuites
   registered in this document and the values registered in the HPKE
   IANA registry [HPKE-IANA].

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 +--------------------------------------------------+------------------+
 | COSE-HPKE                                        |      HPKE        |
 | Cipher Suite Label                               | KEM | KDF | AEAD |
 +--------------------------------------------------+-----+-----+------+
 | HPKE-Base-P256-SHA256-AES128GCM                  |0x10 | 0x1 | 0x1  |
 | HPKE-Base-P256-SHA256-ChaCha20Poly1305           |0x10 | 0x1 | 0x3  |
 | HPKE-Base-P384-SHA384-AES256GCM                  |0x11 | 0x2 | 0x2  |
 | HPKE-Base-P384-SHA384-ChaCha20Poly1305           |0x11 | 0x2 | 0x3  |
 | HPKE-Base-P521-SHA512-AES256GCM                  |0x12 | 0x3 | 0x2  |
 | HPKE-Base-P521-SHA512-ChaCha20Poly1305           |0x12 | 0x3 | 0x3  |
 | HPKE-Base-X25519-SHA256-AES128GCM                |0x20 | 0x1 | 0x1  |
 | HPKE-Base-X25519-SHA256-ChaCha20Poly1305         |0x20 | 0x1 | 0x3  |
 | HPKE-Base-X448-SHA512-AES256GCM                  |0x21 | 0x3 | 0x2  |
 | HPKE-Base-X448-SHA512-ChaCha20Poly1305           |0x21 | 0x3 | 0x3  |
 | HPKE-Base-X25519Kyber768-SHA256-AES256GCM        |0x30 | 0x1 | 0x2  |
 | HPKE-Base-X25519Kyber768-SHA256-ChaCha20Poly1305 |0x30 | 0x1 | 0x3  |
 | HPKE-Base-CP256-SHA256-ChaCha20Poly1305          |0x13 | 0x1 | 0x3  |
 | HPKE-Base-CP256-SHA256-AES128GCM                 |0x13 | 0x1 | 0x1  |
 | HPKE-Base-CP521-SHA512-ChaCha20Poly1305          |0x15 | 0x3 | 0x3  |
 | HPKE-Base-CP521-SHA512-AES256GCM                 |0x15 | 0x3 | 0x2  |
 +--------------------------------------------------+-----+-----+------+

   Note that the last four entries in the table refer to the compact
   encoding of the public keys defined in [I-D.irtf-cfrg-dnhpke].

   As the list indicates, the ciphersuite labels have been abbreviated
   at least to some extend to maintain the tradeoff between readability
   and length.

5.  Examples

   This section provides a set of examples that shows all COSE message
   types (COSE_Encrypt0, COSE_Encrypt and COSE_MAC) to which the COSE-
   HPKE can be applied.  Each example includes the following information
   that can be used to check the interoperability of COSE-HPKE
   implementations:

   *  plaintext: Original data of the encrypted payload.

   *  external_aad: Externally supplied AAD.

   *  skR: A recipient private key.

   *  skE: An ephemeral sender private key paired with the
      encapsulated_key.

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5.1.  Single Recipient / One Layer Example

   This example assumes that a sender wants to communicate an encrypted
   payload to a single recipient in the most efficient way.

   An example of the COSE_Encrypt0 structure using the HPKE scheme is
   shown in Figure 4.  Line breaks and comments have been inserted for
   better readability.

   This example uses the following:

   *  alg: HPKE-Base-P256-SHA256-AES128GCM

   *  plaintext: "This is the content."

   *  external_aad: "COSE-HPKE app"

   *  skR: h'57c92077664146e876760c9520d054aa93c3afb04e306705db609030850
      7b4d3'

   *  skE: h'42dd125eefc409c3b57366e721a40043fb5a58e346d51c133128a772371
      60218'

   16([
       / alg = HPKE-Base-P256-SHA256-AES128GCM (Assumed: 35) /
       h'a1011823',
       {
           / kid /
           4: h'3031',
           / encapsulated_key /
           -4: h'045df24272faf43849530db6be01f42708b3c3a9
                 df8e268513f0a996ed09ba7840894a3fb946cb28
                 23f609c59463093d8815a7400233b75ca8ecb177
                 54d241973e',
       },
       / encrypted plaintext /
       h'35aa3d98739289b83751125abe44e3b977e4b9abbf2c8cfaade
         b15f7681eef76df88f096',
   ])

                  Figure 4: COSE_Encrypt0 Example for HPKE

5.2.  Multiple Recipients / Two Layer

   In this example we assume that a sender wants to transmit a payload
   to two recipients using the two-layer structure.  Note that it is
   possible to send two single-layer payloads, although it will be less
   efficient.

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5.2.1.  COSE_Encrypt

   An example of the COSE_Encrypt structure using the HPKE scheme is
   shown in Figure 5.  Line breaks and comments have been inserted for
   better readability.

   This example uses the following:

   *  Encryption alg: AES-128-GCM

   *  plaintext: "This is the content."

   *  detatched ciphertext: h'cc168c4e148c52a83010a75250935a47ccb8682dee
      bcef8fce5d60c161e849f53a2dc664'

   *  kid:"01"

      -  alg: HPKE-Base-P256-SHA256-AES128GCM

      -  external_aad: "COSE-HPKE app"

      -  skR: h'57c92077664146e876760c9520d054aa93c3afb04e306705db609030
         8507b4d3'

      -  skE: h'97ad883f949f4cdcb1301b9446950efd4eb519e16c4a3d78304eec83
         2692f9f6'

   *  kid:"02"

      -  alg: HPKE-Base-X25519-SHA256-CHACHA20POLY1305

      -  external_aad: "COSE-HPKE app"

      -  skR: h'bec275a17e4d362d0819dc0695d89a73be6bf94b66ab726ae0b1afe3
         c43f41ce'

      -  skE: h'b8ed3f4df56c230e36fa6620a47f24d08856d242ea547c5521ff7bd6
         9af8fd6f'

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96_0([
    / alg = AES-128-GCM (1) /
    h'a10101',
    {
        / iv /
        5: h'b3fb95dde18c6f90a9f0ae55',
    },
    / detached ciphertext /
    null,
    [
        [
            / alg = HPKE-Base-P256-SHA256-AES128GCM (Assumed: 35) /
            h'a1011823',
            {
                / kid /
                4: h'3031',
                / encapsulated_key /
                -4: h'04d97b79486fe2e7b98fb1bd43
                      c4faee316ff38d28609a1cf568
                      40a809298a91e601f1cc0c2ba4
                      6cb67b41f4651b769cafd9df78
                      e58aa7f5771291bd4f0f420ba6',
            },
            / ciphertext containing encrypted CEK /
            h'24450f54ae93375351467d17aa7a795cfede2
              c03eced1ad21fcb7e7c2fe64397',
        ],
        [
            / alg = HPKE-Base-X25519-SHA256-CHACHA20POLY1305 (Assumed: 42) /
            h'a101182a',
            {
                / kid /
                4: h'3032',
                / encapsulated_key /
                -4: h'd1afbdc95b0e735676f6bca34f
                      be50f2822259ac09bfc3c500f1
                      4a05de9b2833',
            },
            / ciphertext containing encrypted CEK /
            h'079b443ec6dfcda6a5f8748aff3875146a8ed
              40359e1279b545166385d8d9b59',
        ],
    ],
])

               Figure 5: COSE_Encrypt Example for HPKE

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   To offer authentication of the sender the payload in Figure 5 is
   signed with a COSE_Sign1 wrapper, which is outlined in Figure 6.  The
   payload in Figure 6 is meant to contain the content of Figure 5.

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

                  Figure 6: COSE_Encrypt Example for HPKE

5.2.2.  COSE_MAC

   An example of the COSE_MAC structure using the HPKE scheme is shown
   in Figure 7.

   This example uses the following:

   *  MAC alg: HMAC 256/256

   *  payload: "This is the content."

   *  kid:"01"

      -  alg: HPKE-Base-P256-SHA256-AES128GCM

      -  external_aad: "COSE-HPKE app"

      -  skR: h'57c92077664146e876760c9520d054aa93c3afb04e306705db609030
         8507b4d3'

      -  skE: h'e5dd9472b5807636c95be0ba2575020ba91cbb3561b52be141da8967
         8c664307'

   *  kid:"02"

      -  alg: HPKE-Base-X25519-SHA256-CHACHA20POLY1305

      -  external_aad: "COSE-HPKE app"

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      -  skR: h'bec275a17e4d362d0819dc0695d89a73be6bf94b66ab726ae0b1afe3
         c43f41ce'

      -  skE: h'78a49d7af71b5244498e943f361aa0250184afc48b8098a68ae97ccd
         2cd7e56f'

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97_0([
    / alg = HMAC 256/256 (5) /
    h'a10105',
    {},
    / payload = 'This is the content.' /
    h'546869732069732074686520636f6e74656e742e',
    / tag /
    h'5cdcf6055fcbdb53b4001d8fb88b2a46b200ed28e1ed77e16ddf43fb3cac3a98',
    [
        [
            / alg = HPKE-Base-P256-SHA256-AES128GCM (Assumed: 35) /
            h'a1011823',
            {
                / kid = '01' /
                4: h'3031',
                / encapsulated_key /
                -4: h'043ac21632e45e1fbd733f002a
                      621aa4f3d94737adc395d5a7cb
                      6e9554bd1ad273aec991493786
                      d72616d9759bf8526e6e20c1ed
                      c41ba5739f2b2e441781aa0eb4',
            },
            / ciphertext containing encrypted MAC key /
            h'5cee2b4235a7ff695164f7a8d1e79ccf3ca3d
              e8b22f3592626020a95b2a8d3fb4d7aa7fe37
              432426ee70073a368f29d1',
        ],
        [
            / alg = HPKE-Base-X25519-SHA256-CHACHA20POLY1305 (Assumed: 42) /
            h'a101182a',
            {
                / kid = '02' /
                4: h'3032',
                / encapsulated_key /
                -4: h'02cffacc60def3bb3d0a1c3661
                      227c9de8dc2b1d3939dd2c07d4
                      49ebb0bba324',
            },
            / ciphertext containing encrypted MAC key /
            h'3f5b8b60271d5234dbea554dc1461d0239e9f
              4589f6415e8563b061dbcb37795a616111b78
              2b4c589b534309327ffadc',
        ],
    ],
])

                 Figure 7: COSE_MAC Example for HPKE

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6.  Security Considerations

   This specification is based on HPKE and the security considerations
   of [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 but outside
   the scope of this document.

   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 it MUST be ensured that the guidelines in [RFC8937] for
   random number generations are followed.

   HPKE in Base mode does not offer authentication as part of the HPKE
   KEM.  In this case COSE constructs like COSE_Sign, COSE_Sign1,
   COSE_MAC, or COSE_MAC0 can be used to add authentication.  HPKE also
   offers modes that offer authentication.

   If 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.

7.  IANA Considerations

   This document requests IANA to add new values to the 'COSE
   Algorithms' and to the 'COSE Header Parameters' registries.

7.1.  COSE Algorithms Registry

   *  Name: HPKE-Base-P256-SHA256-AES128GCM

   *  Value: TBD1 (Assumed: 35)

   *  Description: Cipher suite for COSE-HPKE in Base Mode that uses the
      DHKEM(P-256, HKDF-SHA256) KEM, the HKDF-SHA256 KDF and the AES-
      128-GCM AEAD.

   *  Capabilities: [kty]

   *  Change Controller: IESG

   *  Reference: [[TBD: This RFC]]

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   *  Recommended: Yes

   *  Name: HPKE-Base-P256-SHA256-ChaCha20Poly1305

   *  Value: TBD2 (Assumed: 36)

   *  Description: Cipher suite for COSE-HPKE in Base Mode that uses the
      DHKEM(P-256, HKDF-SHA256) KEM, the HKDF-SHA256 KDF and the
      ChaCha20Poly1305 AEAD.

   *  Capabilities: [kty]

   *  Change Controller: IESG

   *  Reference: [[TBD: This RFC]]

   *  Recommended: Yes

   *  Name: HPKE-Base-P384-SHA384-AES256GCM

   *  Value: TBD3 (Assumed: 37)

   *  Description: Cipher suite for COSE-HPKE in Base Mode that uses the
      DHKEM(P-384, HKDF-SHA384) KEM, the HKDF-SHA384 KDF, and the AES-
      256-GCM AEAD.

   *  Capabilities: [kty]

   *  Change Controller: IESG

   *  Reference: [[TBD: This RFC]]

   *  Recommended: Yes

   *  Name: HPKE-Base-P384-SHA384-ChaCha20Poly1305

   *  Value: TBD4 (Assumed: 38)

   *  Description: Cipher suite for COSE-HPKE in Base Mode that uses the
      DHKEM(P-384, HKDF-SHA384) KEM, the HKDF-SHA384 KDF, and the
      ChaCha20Poly1305 AEAD.

   *  Capabilities: [kty]

   *  Change Controller: IESG

   *  Reference: [[TBD: This RFC]]

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   *  Recommended: Yes

   *  Name: HPKE-Base-P521-SHA512-AES256GCM

   *  Value: TBD5 (Assumed: 39)

   *  Description: Cipher suite for COSE-HPKE in Base Mode that uses the
      DHKEM(P-521, HKDF-SHA512) KEM, the HKDF-SHA512 KDF, and the AES-
      256-GCM AEAD.

   *  Capabilities: [kty]

   *  Change Controller: IESG

   *  Reference: [[TBD: This RFC]]

   *  Recommended: Yes

   *  Name: HPKE-Base-P521-SHA512-ChaCha20Poly1305

   *  Value: TBD6 (Assumed: 40)

   *  Description: Cipher suite for COSE-HPKE in Base Mode that uses the
      DHKEM(P-521, HKDF-SHA512) KEM, the HKDF-SHA512 KDF, and the
      ChaCha20Poly1305 AEAD.

   *  Capabilities: [kty]

   *  Change Controller: IESG

   *  Reference: [[TBD: This RFC]]

   *  Recommended: Yes

   *  Name: HPKE-Base-X25519-SHA256-AES128GCM

   *  Value: TBD7 (Assumed: 41)

   *  Description: Cipher suite for COSE-HPKE in Base Mode that uses the
      DHKEM(X25519, HKDF-SHA256) KEM, the HKDF-SHA256 KDF, and the AES-
      128-GCM AEAD.

   *  Capabilities: [kty]

   *  Change Controller: IESG

   *  Reference: [[TBD: This RFC]]

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   *  Recommended: Yes

   *  Name: HPKE-Base-X25519-SHA256-ChaCha20Poly1305

   *  Value: TBD8 (Assumed: 42)

   *  Description: Cipher suite for COSE-HPKE in Base Mode that uses the
      DHKEM(X25519, HKDF-SHA256) KEM, the HKDF-SHA256 KDF, and the
      ChaCha20Poly1305 AEAD.

   *  Capabilities: [kty]

   *  Change Controller: IESG

   *  Reference: [[TBD: This RFC]]

   *  Recommended: Yes

   *  Name: HPKE-Base-X448-SHA512-AES256GCM

   *  Value: TBD9 (Assumed: 43)

   *  Description: Cipher suite for COSE-HPKE in Base Mode that uses the
      DHKEM(X448, HKDF-SHA512) KEM, the HKDF-SHA512 KDF, and the AES-
      256-GCM AEAD.

   *  Capabilities: [kty]

   *  Change Controller: IESG

   *  Reference: [[TBD: This RFC]]

   *  Recommended: Yes

   *  Name: HPKE-Base-X448-SHA512-ChaCha20Poly1305

   *  Value: TBD10 (Assumed: 44)

   *  Description: Cipher suite for COSE-HPKE in Base Mode that uses the
      DHKEM(X448, HKDF-SHA512) KEM, the HKDF-SHA512 KDF, and the
      ChaCha20Poly1305 AEAD.

   *  Capabilities: [kty]

   *  Change Controller: IESG

   *  Reference: [[TBD: This RFC]]

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   *  Recommended: Yes

   *  Name: HPKE-Base-X25519Kyber768-SHA256-AES256GCM

   *  Value: TBD11 (Assumed: 250)

   *  Description: Cipher suite for COSE-HPKE in Base Mode that uses the
      X25519Kyber768Draft00 KEM, the HKDF-SHA256 KDF, and the AES-
      256-GCM AEAD.

   *  Capabilities: [kty]

   *  Change Controller: IESG

   *  Reference: [[TBD: This RFC]]

   *  Recommended: No

   *  Name: HPKE-Base-X25519Kyber768-SHA256-ChaCha20Poly1305

   *  Value: TBD12 (Assumed: 251)

   *  Description: Cipher suite for COSE-HPKE in Base Mode that uses the
      X25519Kyber768Draft00 KEM, the HKDF-SHA256 KDF, and the
      ChaCha20Poly1305 AEAD.

   *  Capabilities: [kty]

   *  Change Controller: IESG

   *  Reference: [[TBD: This RFC]]

   *  Recommended: No

   *  Name: HPKE-Base-CP256-SHA256-ChaCha20Poly1305

   *  Value: TBD13 (Assumed: 45)

   *  Description: Cipher suite for COSE-HPKE in Base Mode that uses the
      DHKEM(CP-256, HKDF-SHA256) KEM, the HKDF-SHA256 KDF and the
      ChaCha20Poly1305 AEAD.

   *  Capabilities: [kty]

   *  Change Controller: IESG

   *  Reference: [[TBD: This RFC]]

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   *  Recommended: Yes

   *  Name: HPKE-Base-CP521-SHA512-ChaCha20Poly1305

   *  Value: TBD14 (Assumed: 46)

   *  Description: Cipher suite for COSE-HPKE in Base Mode that uses the
      DHKEM(CP-521, HKDF-SHA512) KEM, the HKDF-SHA512 KDF, and the
      ChaCha20Poly1305 AEAD.

   *  Capabilities: [kty]

   *  Change Controller: IESG

   *  Reference: [[TBD: This RFC]]

   *  Recommended: Yes

   *  Name: HPKE-Base-CP256-SHA256-AES128GCM

   *  Value: TBD15 (Assumed: 47)

   *  Description: Cipher suite for COSE-HPKE in Base Mode that uses the
      DHKEM(CP-256, HKDF-SHA256) KEM, the HKDF-SHA256 KDF and the
      AES128GCM AEAD.

   *  Capabilities: [kty]

   *  Change Controller: IESG

   *  Reference: [[TBD: This RFC]]

   *  Recommended: Yes

   *  Name: HPKE-Base-CP521-SHA512-AES256GCM

   *  Value: TBD16 (Assumed: 47)

   *  Description: Cipher suite for COSE-HPKE in Base Mode that uses the
      DHKEM(CP-521, HKDF-SHA512) KEM, the HKDF-SHA512 KDF, and the
      AES256GCM AEAD.

   *  Capabilities: [kty]

   *  Change Controller: IESG

   *  Reference: [[TBD: This RFC]]

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   *  Recommended: Yes

7.2.  COSE Header Parameters

   *  Name: encapsulated_key

   *  Label: TBDX (Assumed: -4)

   *  Value type: bstr

   *  Value Registry: N/A

   *  Description: HPKE encapsulated key

   *  Reference: [[This specification]]

8.  References

8.1.  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/rfc/rfc2119>.

   [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/rfc/rfc8174>.

   [RFC9052]  Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Structures and Process", STD 96, RFC 9052,
              DOI 10.17487/RFC9052, August 2022,
              <https://www.rfc-editor.org/rfc/rfc9052>.

   [RFC9053]  Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053,
              August 2022, <https://www.rfc-editor.org/rfc/rfc9053>.

   [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/rfc/rfc9180>.

8.2.  Informative References

   [HPKE-IANA]
              IANA, "Hybrid Public Key Encryption (HPKE) IANA Registry",
              October 2023,
              <https://www.iana.org/assignments/hpke/hpke.xhtml>.

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   [I-D.irtf-cfrg-dnhpke]
              Harkins, D., "Deterministic Nonce-less Hybrid Public Key
              Encryption", Work in Progress, Internet-Draft, draft-irtf-
              cfrg-dnhpke-03, 19 October 2023,
              <https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
              dnhpke-03>.

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

Appendix A.  Contributors

   We would like thank the following individuals for their contributions
   to the design of embedding the HPKE output into the COSE structure
   following a long and lively mailing list discussion:

   *  Richard Barnes

   *  Ilari Liusvaara

   Finally, we would like to thank Russ Housley and Brendan Moran for
   their contributions to the draft as co-authors of initial versions.

Appendix B.  Acknowledgements

   We would like to thank John Mattsson, Mike Prorock, Michael
   Richardson, and Goeran Selander for their review feedback.

Authors' Addresses

   Hannes Tschofenig
   Austria
   Email: hannes.tschofenig@gmx.net

   Orie Steele (editor)
   Transmute
   United States
   Email: orie@transmute.industries

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   Daisuke Ajitomi
   Japan
   Email: dajiaji@gmail.com

   Laurence Lundblade
   Security Theory LLC
   United States
   Email: lgl@securitytheory.com

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