draft-ietf-cose-hpke.txt

COSE                                                       H. Tschofenig
Internet-Draft                                                     H-BRS
Intended status: Standards Track                          O. Steele, Ed.
Expires: 10 January 2025                                       Transmute
                                                              D. Ajitomi
                                                                 bibital
                                                            L. Lundblade
                                                     Security Theory LLC
                                                             9 July 2024


Use of Hybrid Public-Key Encryption (HPKE) with CBOR Object Signing and
                           Encryption (COSE)
                        draft-ietf-cose-hpke-09

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.

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   This Internet-Draft will expire on 10 January 2025.





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Copyright Notice

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   3
   3.  HPKE for COSE . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   4
       3.1.1.  HPKE Direct Encryption Mode . . . . . . . . . . . . .   4
       3.1.2.  HPKE Key Encryption Mode  . . . . . . . . . . . . . .   5
     3.2.  Key Representation  . . . . . . . . . . . . . . . . . . .   9
   4.  Ciphersuite Registration  . . . . . . . . . . . . . . . . . .   9
     4.1.  COSE_Keys for COSE-HPKE Ciphersuites  . . . . . . . . . .  11
   5.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     5.1.  HPKE Direct Encryption Mode . . . . . . . . . . . . . . .  12
     5.2.  HPKE Key Encryption Mode  . . . . . . . . . . . . . . . .  13
       5.2.1.  COSE_Encrypt  . . . . . . . . . . . . . . . . . . . .  13
       5.2.2.  COSE_MAC  . . . . . . . . . . . . . . . . . . . . . .  15
     5.3.  Key Representation  . . . . . . . . . . . . . . . . . . .  18
       5.3.1.  KEM Public Key for HPKE-Base-P256-SHA256-A128GCM  . .  18
       5.3.2.  KEM Private Key for HPKE-Base-P256-SHA256-A128GCM . .  18
       5.3.3.  KEM Public Key for
               HPKE-Base-X25519-SHA256-CHACHA20POLY1305  . . . . . .  19
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  19



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   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
     7.1.  COSE Algorithms Registry  . . . . . . . . . . . . . . . .  20
     7.2.  COSE Header Parameters  . . . . . . . . . . . . . . . . .  22
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  22
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  23
   Appendix A.  Contributors . . . . . . . . . . . . . . . . . . . .  23
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

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.

   This document defines the use of the HPKE with 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].

3.  HPKE for COSE




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3.1.  Overview

   This specification supports two modes of HPKE in COSE, namely

   *  HPKE Direct Encryption mode, where HPKE is used to encrypt the
      plaintext.  This mode can only be used with a single recipient.
      Section 3.1.1 provides the details.

   *  HPKE Key Encryption mode, where HPKE is used to encrypt a content
      encryption key (CEK) and the CEK is subsequently used to encrypt
      the plaintext.  This mode supports multiple recipients.
      Section 3.1.2 provides the details.

   In both cases a new COSE header parameter, called 'ek', 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 'ek' 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.  HPKE Direct Encryption Mode

   With the HPKE Direct Encryption mode 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 'ek' (encapsulated key) parameter into the
   unprotected header.  Although the use of the 'kid' parameter in
   COSE_Encrypt0 is discouraged by RFC 9052, this documents RECOMMENDS
   the use of the 'kid' parameter (or other parameters) to explicitly
   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.

   The HPKE specification describes 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.

   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 used for the HPKE Direct Encryption Mode

   The COSE_Encrypt0 MAY be tagged or untagged.

   An example is shown in Section 5.1.

3.1.2.  HPKE Key Encryption Mode

   With the HPKE Key Encryption mode 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 the
      content (plaintext) encrypted with the CEK.  This ciphertext may
      be detached, and 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



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      parameter and the unprotected header MUST contain the 'ek'
      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.

3.1.2.1.  Recipient Encryption

   This describes the Recipient_structure.  It serves instead of
   COSE_KDF_Context for COSE-HPKE recipients (and possibly other COSE
   algorithms defined outside this document).  It MUST be used for COSE-
   HPKE recipients as it provides the protection for recipient protected
   headers.  It is patterned after the Enc_structure in [RFC9052], but
   is specifically for a COSE_recipient, never a COSE_Encrypt.  The
   COSE_KDF_Context MUST NOT be used in COSE-HPKE.

   Recipient_structure = [ 
       context: "Recipient",
       next_layer_alg: int/tstr,
       recipient_protected_header: empty_or_serialize_map,
       recipient_aad: bstr
   ]

   *  "next_layer_alg" is the algorithm ID of the COSE layer for which
      the COSE_recipient is encrypting a key.  It is the algorithm that
      the key MUST be used with.  This value MUST match the alg
      parameter in the next lower COSE layer.  (This serves the same
      purpose as the alg ID in the COSE_KDF_Context.  It also mitigates
      attacks where a person-in-the-middle changes the following layer
      algorithm from an AEAD algorithm to one that is not foiling the
      protection of the following layer headers).

   *  "recipient_protected_header" contains the protected headers from
      the COSE_recipient CBOR-encoded deterministically with the "Core
      Deterministic Encoding Requirements", specified in Section 4.2.1
      of RFC 8949 [STD94].

   *  "recipient_aad" contains any additional context the application
      wishes to protect.  If none, it is a zero-length string.  This is
      distinct from the external_aad for the whole COSE encrypt.  It is
      per-recipient.  Since it is not a header, it may be secret data
      that is not transmitted.  It provides a means to convey many of
      the fields in COSE_KDF_Context.




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3.1.2.2.  COSE-HPKE Recipient Construction

   This is the procedure for creating a COSE_recipient for COSE-HPKE.

   When a COSE_recipeint is constructed for a COSE-HPKE recipient, this
   is given as the "aad" parameter to the HPKE Seal() API.  The "info"
   parameter to HPKE_Seal is not used with COSE_HPKE.

   The creation of the COSE_recipient is as follows:

   1.  Prepare a Recipient_structure

   2.  Obtain the key To used use by the next lowest layer

   3.  Pass in the following parameters to HPKE Seal API

       1.  Public key of recipient for "pKR"

       2.  Empty string for "info"

       3.  CBOR-encoded Recipient_structure for "aad"

       4.  The key for next lowest COSE layer for "pt"

   4.  The following are returned from the HPKE Seal API

       1.  The "enc" is placed in the "ek" header of the COSE_recipient

       2.  The "ct" is placed in the "ciphertext" field of the
           COSE_recipient

   The decoding and decryption of a COSE_recipient is as follows:

   1.  Prepare a Recipient_structure

   2.  Pass in the following parameters to HPKE Open API

       1.  The "ek" header for "enc"

       2.  Secret key for recipient for "sKR"

       3.  Empty string for "info"

       4.  CBOR-encoded Recipient_structure for "aad"

       5.  The cipher text from the COSE_recipient as "ct"





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   3.  What is returned from HPKE Open API is the key for the next
       lowest COSE layer

   It is not necessary to fill in recipient_aad as HPKE itself covers
   the attacks that recipient_aad (and COSE_KDF_Context (and SP800-56A))
   are used to mitigate.  COSE-HPKE use cases may use it for any purpose
   they wish, but it should generally be for small identifiers, context
   or secrets, not to protect bulk external data.  Bulk external data
   should be protected at layer 0 with external_aad.

   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 used for the HPKE Key Encryption Mode

   The COSE_Encrypt MAY be tagged or untagged.

   When encrypting the content at layer 0 then the instructions in
   Section 5.3 of [RFC9052] MUST to be followed, which includes the
   calculation of the authenticated data strcture.

   An example is shown in Section 5.2.








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3.2.  Key Representation

   The COSE_Key with the existing key types can be used to represent KEM
   private or public keys.  When using a COSE_Key for COSE-HPKE, the
   following checks are made:

   *  The "kty" field MUST be present, and it MUST be one of the key
      types for HPKE KEM.

   *  If the "kty" field is "OKP" or "EC2", the "crv" field MUST be
      present and it MUST be a curve for HPKE KEM.

   *  If the "alg" field is present, it MUST be one of the supported
      COSE-HPKE "alg" values and the key type of its KEM MUST match the
      "kty" field.  If the "kty" field is "OKP" or "EC2", the curve of
      the KEM MUST match the "crv" field.  The valid combinations of the
      "alg", "kty" and "crv" are shown in Figure 3.

   *  If the "key_ops" field is present, it MUST include only "derive
      bits" for the private key and MUST be empty for the public key.

   Examples of the COSE_Key for COSE-HPKE are shown in Section 5.3.

4.  Ciphersuite Registration

   A ciphersuite is a group of algorithms, often sharing component
   algorithms such as hash functions, targeting a security level.  An
   HPKE ciphersuite, is composed of the following choices:

   *  HPKE Mode

   *  KEM Algorithm

   *  KDF Algorithm

   *  AEAD Algorithm

   The "KEM", "KDF", and "AEAD" values are chosen from the HPKE IANA
   registry [HPKE-IANA].

   For readability the algorithm ciphersuites labels are built 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]:




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   *  "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, which all use the "Base" mode and the
   values registered in the HPKE IANA registry [HPKE-IANA].

 +--------------------------------------------------+------------------+
 | COSE-HPKE                                        |      HPKE        |
 | Cipher Suite Label                               | KEM | KDF | AEAD |
 +--------------------------------------------------+-----+-----+------+
 | HPKE-Base-P256-SHA256-A128GCM                    |0x10 | 0x1 | 0x1  |
 | HPKE-Base-P384-SHA384-AS256GCM                   |0x11 | 0x2 | 0x2  |
 | HPKE-Base-P521-SHA512-AS256GCM                   |0x12 | 0x3 | 0x2  |
 | HPKE-Base-X25519-SHA256-A128GCM                  |0x20 | 0x1 | 0x1  |
 | HPKE-Base-X25519-SHA256-ChaCha20Poly1305         |0x20 | 0x1 | 0x3  |
 | HPKE-Base-X448-SHA512-AS256GCM                   |0x21 | 0x3 | 0x2  |
 | HPKE-Base-X448-SHA512-ChaCha20Poly1305           |0x21 | 0x3 | 0x3  |
 +--------------------------------------------------+-----+-----+------+

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

   The ciphersuite list above is a minimal starting point.  Additional
   ciphersuites can be registered into the already existing registry.
   For example, once post-quantum cryptographic algorithms have been
   standardized it might be beneficial to register ciphersuites for use
   with COSE-HPKE.  Additionally, ciphersuites utilizing the compact
   encoding of the public keys, as defined in [I-D.irtf-cfrg-dnhpke],
   may be standardized for use in constrained environments.








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   As a guideline for ciphersuite submissions to the IANA CoSE algorithm
   registry, the designated experts must only register combinations of
   (KEM, KDF, AEAD) triple that consitute valid combinations for use
   with HPKE, the KDF used should (if possible) match one internally
   used by the KEM, and components should not be mixed between global
   and national standards.

4.1.  COSE_Keys for COSE-HPKE Ciphersuites

   The COSE-HPKE ciphersuite uniquely determines the type of KEM for
   which a COSE_Key is used.  The following mapping table shows the
   valid combinations of the COSE-HPKE ciphersuite, COSE_Key type and
   its curve.

   +---------------------+--------------+
   | COSE-HPKE           | COSE_Key     |
   | Ciphersuite Label   | kty | crv    |
   +---------------------+-----+--------+
   | HPKE-Base-P256-\*   | EC2 | P-256  |
   | HPKE-Base-P384-\*   | EC2 | P-384  |
   | HPKE-Base-P521-\*   | EC2 | P-521  |
   | HPKE-Base-X25519-\* | OKP | X25519 |
   | HPKE-Base-X448-\*   | OKP | X448   |
   | HPKE-Base-CP256-\*  | EC2 | P-256  |
   | HPKE-Base-CP384-\*  | EC2 | P-384  |
   | HPKE-Base-CP521-\*  | EC2 | P-521  |
   +---------------------+-----+--------+

       Figure 3: COSE_Key Types and Curves for COSE-HPKE Ciphersuites

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, and also provides some examples of key
   representation for HPKE KEM.

   Each example of the COSE message 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.





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

5.1.  HPKE Direct Encryption Mode

   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 HPKE Direct Encryption Mode 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-A128GCM

   *  plaintext: "This is the content."

   *  external_aad: "COSE-HPKE app"

   *  skR: h'57c92077664146e876760c9520d054aa93c3afb04e306705db609030850
      7b4d3'

   *  skE: h'42dd125eefc409c3b57366e721a40043fb5a58e346d51c133128a772371
      60218'

   16([
       / alg = HPKE-Base-P256-SHA256-A128GCM (Assumed: 35) /
       h'a1011823',
       {
           / kid /
           4: h'3031',
           / ek /
           -4: h'045df24272faf43849530db6be01f42708b3c3a9
                 df8e268513f0a996ed09ba7840894a3fb946cb28
                 23f609c59463093d8815a7400233b75ca8ecb177
                 54d241973e',
       },
       / encrypted plaintext /
       h'35aa3d98739289b83751125abe44e3b977e4b9abbf2c8cfaade
         b15f7681eef76df88f096',
   ])

                  Figure 4: COSE_Encrypt0 Example for HPKE








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5.2.  HPKE Key Encryption Mode

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

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:

   TODO: recompute this for Recipient_structure

   *  Encryption alg: AES-128-GCM

   *  plaintext: "This is the content."

   *  detatched ciphertext: h'cc168c4e148c52a83010a75250935a47ccb8682dee
      bcef8fce5d60c161e849f53a2dc664'

   *  kid:"01"

      -  alg: HPKE-Base-P256-SHA256-A128GCM

      -  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-A128GCM (Assumed: 35) /
            h'a1011823',
            {
                / kid /
                4: h'3031',
                / ek /
                -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',
                / ek /
                -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-A128GCM

      -  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-A128GCM (Assumed: 35) /
            h'a1011823',
            {
                / kid = '01' /
                4: h'3031',
                / ek /
                -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',
                / ek /
                -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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5.3.  Key Representation

   Examples of private and public KEM key representation are shown
   below.

5.3.1.  KEM Public Key for HPKE-Base-P256-SHA256-A128GCM

{
    / kty = 'EC2' /
    1: 2,
    / kid = '01' /
    2: h'3031',
    / alg = HPKE-Base-P256-SHA256-A128GCM (Assumed: 35) /
    3: 35,
    / crv = 'P-256' /
    -1: 1,
    / x /
    -2: h'65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de108de439c08551d',
    / y /
    -3: h'1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e9eecd0084d19c'
}

         Figure 8: Key Representation Example for HPKE-Base-
                         P256-SHA256-A128GCM

5.3.2.  KEM Private Key for HPKE-Base-P256-SHA256-A128GCM

{
    / kty = 'EC2' /
    1: 2,
    / kid = '01' /
    2: h'3031',
    / alg = HPKE-Base-P256-SHA256-A128GCM (Assumed: 35) /
    3: 35,
    / key_ops = ['derive_bits'] /
    4: [8],
    / crv = 'P-256' /
    -1: 1,
    / x /
    -2: h'bac5b11cad8f99f9c72b05cf4b9e26d244dc189f745228255a219a86d6a09eff',
    / y /
    -3: h'20138bf82dc1b6d562be0fa54ab7804a3a64b6d72ccfed6b6fb6ed28bbfc117e',
    / d /
    -4: h'57c92077664146e876760c9520d054aa93c3afb04e306705db6090308507b4d3',
}

         Figure 9: Key Representation Example for HPKE-Base-
                         P256-SHA256-A128GCM



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5.3.3.  KEM Public Key for HPKE-Base-X25519-SHA256-CHACHA20POLY1305

{
    / kty = 'OKP' /
    1: 1,
    / kid = '11' /
    2: h'3131',
    / alg = HPKE-Base-X25519-SHA256-CHACHA20POLY1305 (Assumed: 42) /
    3: 42,
    / crv = 'X25519' /
    -1: 4,
    / x /
    -2: h'cb7c09ab7b973c77a808ee05b9bbd373b55c06eaa9bd4ad2bd4e9931b1c34c22',
}

         Figure 10: Key Representation Example for HPKE-Base-
                    X25519-SHA256-CHACHA20POLY1305

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-A128GCM

   *  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-P384-SHA384-AS256GCM

   *  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-P521-SHA512-AS256GCM

   *  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]



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   *  Change Controller: IESG

   *  Reference: [[TBD: This RFC]]

   *  Recommended: Yes

   *  Name: HPKE-Base-X25519-SHA256-A128GCM

   *  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-AS256GCM

   *  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

7.2.  COSE Header Parameters

   *  Name: ek

   *  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>.

   [STD94]    Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949, December 2020.

              <https://www.rfc-editor.org/info/std94>

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-04, 5 February 2024,
              <https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
              dnhpke-04>.

   [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



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   *  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
   University of Applied Sciences Bonn-Rhein-Sieg
   Germany
   Email: hannes.tschofenig@gmx.net


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


   Daisuke Ajitomi
   bibital
   Japan
   Email: dajiaji@gmail.com


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

















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