Use of Hybrid Public-Key Encryption (HPKE) with CBOR Object Signing and Encryption (COSE)
draft-ietf-cose-hpke-06

COSE                                                       H. Tschofenig
Internet-Draft                                                          
Intended status: Standards Track                          O. Steele, Ed.
Expires: 9 April 2024                                          Transmute
                                                              D. Ajitomi
                                                                        
                                                            L. Lundblade
                                                     Security Theory LLC
                                                          7 October 2023

Use of Hybrid Public-Key Encryption (HPKE) with CBOR Object Signing and
                           Encryption (COSE)
                        draft-ietf-cose-hpke-06

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

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   This Internet-Draft will expire on 9 April 2024.

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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  . . . . . . . . . .   9
     5.2.  Multiple Recipients / Two Layer . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
     7.1.  COSE Algorithms Registry  . . . . . . . . . . . . . . . .  13
     7.2.  COSE Header Parameters  . . . . . . . . . . . . . . . . .  18
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  18
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  19
   Appendix A.  Contributors . . . . . . . . . . . . . . . . . . . .  19
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .  20

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   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

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].

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

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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.

   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.

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   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.

   In this approach the following layers are involved:

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   *  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.

   An example is shown in Section 5.2.

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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:

   *  HPKE Mode

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   *  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

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 HPKE-Base-P256-SHA256-AES128GCM, which corresponds
   to the following HPKE algorithm combination:

   *  KEM: DHKEM(P-256, HKDF-SHA256)

   *  KDF: HKDF-SHA256

   *  AEAD: AES-128-GCM

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   *  Mode: Base

   *  payload: "This is the content"

   *  aad: ""

   16([
       / alg = TBD1 (Assumed: 35) /
       h'a1011823',
       {
           / kid /
           4: h'3031',
           / encapsulated_key /
           -4: h'048c6f75e463a773082f3cb0d3a701348a578c67
                 80aba658646682a9af7291dfc277ec93c3d58707
                 818286c1097825457338dc3dcaff367e2951342e
                 9db30dc0e7',
       },
       / encrypted plaintext /
       h'ee22206308e478c279b94bb071f3a5fbbac412a6effe34195f7
         c4169d7d8e81666d8be13',
   ])

                  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.

   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 AES-128-GCM for encryption of the plaintext "This
   is the content." with aad="" at layer 0.  The ciphertext is detached.

   At the recipient structure at layer 1, this example uses HPKE-Base-
   P256-SHA256-AES128GCM as the algorithm, which correspond to the
   following HPKE algorithm combination:

   *  KEM: DHKEM(P-256, HKDF-SHA256)

   *  KDF: HKDF-SHA256

   *  AEAD: AES-128-GCM

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   *  Mode: Base

   96_0([
       / alg = AES-128-GCM (1) /
       h'a10101',
       {
         / iv /
         5: h'67303696a1cc2b6a64867096'
       },
       / detached ciphertext /
       h'',
       [
           [
               / alg = TBD1 (Assumed: 35) /
               h'a1011823',
               {
                   / kid /
                   4: h'3031',
                   / encapsulated_key /
                   36: h'0421ccd1b00dd958d77e10399c
                         97530fcbb91a1dc71cb3bf41d9
                         9fd39f22918505c973816ecbca
                         6de507c4073d05cceff73e0d35
                         f60e2373e09a9433be9e95e53c',
               },
               / ciphertext containing encrypted CEK /
               h'bb2f1433546c55fb38d6f23f5cd95e1d72eb4
                 c129b99a165cd5a28bd75859c10939b7e4d',
           ],
           [
               / alg = TBD1 (Assumed: 35) /
               h'a1011823',
               {
                   / kid /
                   4: h'313233', // kid
                   / encapsulated_key /
                   -4: h'6de507c4073d05cceff73e0d35
                         f60e2373e09a9433be9e95e53c
                         9fd39f22918505c973816ecbca
                         6de507c4073d05cceff73e0d35
                         f60e2373e09a9433be9e95e53c',
               },
               / ciphertext containing encrypted CEK /
               h'c4169d7d8e81666d8be13bb2f1433546c55fb
                 c129b99a165cd5a28bd75859c10939b7e4d',
           ]
       ],
   ])

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                  Figure 5: COSE_Encrypt Example for HPKE

   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

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.

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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]]

   *  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]

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   *  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]]

   *  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]

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   *  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]]

   *  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]

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   *  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]]

   *  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]

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   *  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]]

   *  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]

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   *  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]]

   *  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>.

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   [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>.

   [I-D.irtf-cfrg-dnhpke]
              Harkins, D., "Deterministic Nonce-less Hybrid Public Key
              Encryption", Work in Progress, Internet-Draft, draft-irtf-
              cfrg-dnhpke-02, 28 September 2023,
              <https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
              dnhpke-02>.

   [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.

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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

   Daisuke Ajitomi
   Japan
   Email: dajiaji@gmail.com

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

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