Use of Hybrid Public-Key Encryption (HPKE) with CBOR Object Signing and Encryption (COSE)
draft-ietf-cose-hpke-03
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| Last updated | 2023-02-27 | ||
| Replaces | draft-tschofenig-cose-hpke | ||
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draft-ietf-cose-hpke-03
COSE H. Tschofenig
Internet-Draft
Intended status: Standards Track B. Moran
Expires: 31 August 2023 Arm Limited
27 February 2023
Use of Hybrid Public-Key Encryption (HPKE) with CBOR Object Signing and
Encryption (COSE)
draft-ietf-cose-hpke-03
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.
This document defines the use of the HPKE base mode with COSE. Other
modes are supported by HPKE but not by this specification.
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
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This Internet-Draft will expire on 31 August 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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than English.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
3. HPKE for COSE . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1.1. Single Recipient / One Layer Structure . . . . . . . 5
3.1.2. Multiple Recipients / Two Layer Structure . . . . . . 6
3.2. HPKE Encryption with SealBase . . . . . . . . . . . . . . 7
3.3. HPKE Decryption with OpenBase . . . . . . . . . . . . . . 8
3.4. Info Structure . . . . . . . . . . . . . . . . . . . . . 8
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Single Recipient / One Layer Example . . . . . . . . . . 9
4.2. Multiple Recipients / Two Layer . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
6.1. COSE Algorithms Registry . . . . . . . . . . . . . . . . 12
6.2. COSE Header Algorithm Parameters . . . . . . . . . . . . 12
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.1. Normative References . . . . . . . . . . . . . . . . . . 13
7.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Contributors . . . . . . . . . . . . . . . . . . . . 13
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
Hybrid public-key encryption (HPKE) [RFC9180] is a scheme that
provides public key encryption of arbitrary-sized plaintexts given a
recipient's public key. HPKE utilizes a non-interactive ephemeral-
static Diffie-Hellman exchange to establish a shared secret. The
motivation for standardizing a public key encryption scheme is
explained in the introduction of [RFC9180].
The HPKE specification defines several features for use with public
key encryption and a subset of those features is applied to COSE
([RFC9052], [RFC9053]). Since COSE provides constructs for
authentication, those are not re-used from the HPKE specification.
This specification uses the "base" mode, as it is called in HPKE
specification language.
2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This specification uses the following abbreviations and terms: -
Content-encryption key (CEK), a term defined in CMS [RFC2630]. -
Hybrid Public Key Encryption (HPKE) is defined in [RFC9180]. - pkR is
the public key of the recipient, as defined in [RFC9180]. - skR is
the private key of the recipient, as defined in [RFC9180]. - Key
Encapsulation Mechanism (KEM), see [RFC9180]. - Key Derivation
Function (KDF), see [RFC9180]. - Authenticated Encryption with
Associated Data (AEAD), 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 cases uses 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.
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HPKE in "base" mode requires little information to be exchanged
between a sender and a recipient, namely
* algorithm information (KEM, KDF, and AEAD identifiers),
* the encapsulated key structure, and
* an identifier of the static recipient key.
In the subsections below we explain how this information is carried
inside the COSE_Encrypt0 and the COSE_Encrypt for the one layer and
the two layer structure, respectively.
In both cases a new structure is used to convey information about the
HPKE sender, namely the HPKE encapsulated key structure
(encapsulated_key).
When the alg value is set to 'HPKE-v1-BASE', the encapsulated key
MUST be present in the unprotected header parameter and its value
MUST be of type encapsulated_key.
The CDDL grammar describing the encapsulated_key structure is:
encapsulated_key = [
kem_id : uint, ; kem identifier
kdf_id : uint, ; kdf identifier
aead_id : uint, ; aead identifier
enc : bstr, ; encapsulated key
]
+---------+----------------+------------+-------------------+
| Name | CBOR Type | Value | Description |
| | | Registry | |
+---------+----------------+------------+-------------------+
| kem_id | uint | HPKE | Identifier for |
| | | KEM IDs | the KEM |
| | | Registry | |
| | | | |
| kdf_id | uint | HPKE KDF | Identifier for |
| | | IDs | the KDF ID |
| | | | |
| aead_id | uint | HPKE AEAD | Identifier for |
| | | IDs | the AEAD ID |
| | | | |
| enc | bstr | | Encapsulated key |
| | | | defined by HPKE |
+---------+----------------+------------+-------------------+
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Figure 1: encapsulated_key structure
kem_id: This parameter is used to identify the KEM. The registry for
KEM ids has been established with RFC 9180.
kdf_id: This parameter contains the KDF identifier. The registry
containing the KDF ids has been established with RFC 9180.
aead_id: This parameter contains the AEAD identifier. The registry
containing the AEAD ids has been established with RFC 9180.
enc: This parameter contains the encapsulated key, which is output of
the HPKE KEM.
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. The resulting
ciphertext may be included in the COSE_Encrypt0 or may be detached.
The sender MUST set the alg parameter in the protected header, which
indicates the use of HPKE.
The sender MUST place the kid parameter and the encapsulated_key
structure into the unprotected header. The kid identifies the static
recipient public key used by the sender. The recipient uses the kid
to determine the appropriate private key.
Figure 2 shows the COSE_Encrypt0 CDDL structure.
COSE_Encrypt0_Tagged = #6.16(COSE_Encrypt0)
; Layer 0
COSE_Encrypt0 = [
Headers,
ciphertext : bstr / nil,
]
Figure 2: CDDL for HPKE-based COSE_Encrypt0 Structure
The COSE_Encrypt0 MAY be tagged or untagged.
An example is shown in Section 4.1.
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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:
* 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 structure as well as 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. For example, the content
encrypted at layer 0 may be a firmware image. The same encrypted
firmware image may need to be sent to many recipients; however, each
recipient uses their own private key to obtain the CEK.
The COSE_recipient structure, shown in Figure 3, is repeated for each
recipient.
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COSE_Encrypt_Tagged = #6.96(COSE_Encrypt)
/ Layer 0 /
COSE_Encrypt = [
Headers,
ciphertext : bstr / nil,
recipients : + COSE_recipient
]
/ Layer 1 /
COSE_recipient = [
protected : bstr .cbor header_map,
unprotected : header_map,
encCEK : bstr,
]
header_map = {
Generic_Headers,
* label => values,
}
Figure 3: CDDL for HPKE-based COSE_Encrypt Structure
The COSE_Encrypt MAY be tagged or untagged.
An example is shown in Section 4.2.
3.2. HPKE Encryption with SealBase
The SealBase(pkR, info, aad, pt) function is used to encrypt a
plaintext pt to a recipient's public key (pkR).
IMPORTANT: For use in COSE_Encrypt, the plaintext "pt" passed into
the SealBase is the CEK. The CEK is a random byte sequence of length
appropriate for the encryption algorithm selected in layer 0. For
example, AES-128-GCM requires a 16 byte key and the CEK would
therefore be 16 bytes long. In case of COSE_Encrypt0, the plaintext
"pt" passed into the SealBase is the raw plaintext.
The "info" parameter can be used to influence the generation of keys
and the "aad" parameter provides additional authenticated data to the
AEAD algorithm in use. This specification does not mandate the use
of the info and the aad parameters. Application-specific profiles of
this specification MAY mandate the use of the info and the aad
parameters.
If SealBase() is successful, it will output a ciphertext "ct" and an
encapsulated key "enc".
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The content of the info parameter is based on the 'COSE_KDF_Context'
structure, which is detailed in Figure 4.
3.3. HPKE Decryption with OpenBase
The recipient will use the OpenBase(enc, skR, info, aad, ct) function
with the enc and ct parameters received from the sender. The "aad"
and the "info" parameters are used as mandated by an application-
specific profile of this specification.
The OpenBase function will, if successful, decrypt "ct". When
decrypted, the result will be either the CEK (if using COSE_Encrypt),
or the raw plaintext (if using COSE_Encrypt0). The CEK is the
symmetric key used to decrypt the ciphertext in layer 0.
3.4. Info Structure
This section provides a suggestion for constructing the info
structure, when used with SealBase() and OpenBase(). Note that the
use of the aad and the info structures for these two functions is
optional. Profiles of this specification MAY require their use and
may define different info structure.
This specification re-uses the context information structure defined
in [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 4.
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 4: COSE_KDF_Context Data Structure for info parameter
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4. Examples
4.1. 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 5. Line breaks and comments have been inserted for
better readability.
It uses the following algorithm combination: - KEM: DHKEM(P-256,
HKDF-SHA256) - KDF: HKDF-SHA256 - AEAD: AES-128-GCM
// payload: "This is the content", aad: ""
//
16([
h'a10120', // alg = HPKE-v1-BASE
{
4: h'3031', // kid
-4: [ // encapsulated_key
16, // kem = DHKEM(P-256, HKDF-SHA256)
1, // kdf = HKDF-SHA256
1, // aead = AES-128-GCM
h'048c6f75e463a773082f3cb0d3a701348a578c67
80aba658646682a9af7291dfc277ec93c3d58707
818286c1097825457338dc3dcaff367e2951342e
9db30dc0e7', // enc
],
},
/ encrypted plaintext /
h'ee22206308e478c279b94bb071f3a5fbbac412a6effe34195f7
c4169d7d8e81666d8be13',
])
Figure 5: COSE_Encrypt0 Example for HPKE
4.2. 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 6. Line breaks and comments have been inserted for
better readability.
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It uses the following algorithm combination:
* At layer 0 AES-128-GCM is used for encryption of the detached
plaintext "This is the content.".
* At the recipient structure at layer 1, DHKEM(P-256, HKDF-SHA256)
(as the KEM), with AES-128-GCM (as the AEAD) and HKDF-SHA256 (as
the KDF) is used.
The algorithm selection is based on the registry of the values
offered by the alg parameters (see Section 6).
// plaintext: "This is the content.", aad: ""
96_0([
h'a10101', // alg = AES-128-GCM (1)
{5: h'67303696a1cc2b6a64867096'}, // iv
h'', // detached ciphertext
[
[
h'a10120', // alg = HPKE-v1-BASE (-1 #TBD)
{
4: h'3031', // kid
-4: [ // encapsulated_key
16, // kem = DHKEM(P-256, HKDF-SHA256)
1, // kdf = HKDF-SHA256
1, // aead = AES-128-GCM
/ enc output /
h'0421ccd1b00dd958d77e10399c
97530fcbb91a1dc71cb3bf41d9
9fd39f22918505c973816ecbca
6de507c4073d05cceff73e0d35
f60e2373e09a9433be9e95e53c',
],
},
// ciphertext containing encrypted CEK
h'bb2f1433546c55fb38d6f23f5cd95e1d72eb4
c129b99a165cd5a28bd75859c10939b7e4d',
],
[
h'a10120', // alg = HPKE-v1-BASE (-1 #TBD)
{
4: h'313233', // kid
-4: [ // encapsulated_key
16, // kem = DHKEM(P-256, HKDF-SHA256)
1, // kdf = HKDF-SHA256
1, // aead = AES-128-GCM
/ enc output /
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h'6de507c4073d05cceff73e0d35
f60e2373e09a9433be9e95e53c
9fd39f22918505c973816ecbca
6de507c4073d05cceff73e0d35
f60e2373e09a9433be9e95e53c',
],
},
// ciphertext containing encrypted CEK
h'c4169d7d8e81666d8be13bb2f1433546c55fb
c129b99a165cd5a28bd75859c10939b7e4d',
]
],
])
Figure 6: COSE_Encrypt Example for HPKE
To offer authentication of the sender the payload in Figure 6 is
signed with a COSE_Sign1 wrapper, which is shown in Figure 7. The
payload in Figure 7 corresponds to the content shown in Figure 6.
18(
[
/ protected / h'a10126' / {
\ alg \ 1:-7 \ ECDSA 256 \
} / ,
/ unprotected / {
/ kid / 4:'sender@example.com'
},
/ payload / h'AA19...B80C',
/ signature / h'E3B8...25B8'
]
)
Figure 7: COSE_Encrypt Example for HPKE
5. Security Considerations
This specification is based on HPKE and the security considerations
of HPKE [RFC9180] are therefore applicable also to this
specification.
HPKE assumes the sender is in possession of the public key of the
recipient and HPKE COSE makes the same assumptions. Hence, some form
of public key distribution mechanism is assumed to exist.
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HPKE relies on a source of randomness to be available on the device.
Additionally, with the two layer structure the CEK is randomly
generated and the it MUST be ensured that the guidelines for random
number generations are followed.
The COSE_Encrypt structure MUST be authenticated using COSE
constructs like COSE_Sign, COSE_Sign1, COSE_MAC, or COSE_MAC0.
When COSE_Encrypt or COSE_Encrypt0 is used with a detached ciphertext
then the subsequently applied integrity protection via COSE_Sign,
COSE_Sign1, COSE_MAC, or COSE_MAC0 does not cover this detached
ciphertext. Implementers MUST ensure that the detached ciphertext
also experiences integrity protection. This is, for example, the
case when an AEAD cipher is used to produce the detached ciphertext
but may not be guaranteed by non-AEAD ciphers.
6. IANA Considerations
This document requests IANA to add new values to the 'COSE
Algorithms' and to the 'COSE Header Algorithm Parameters' registries
in the 'Standards Action With Expert Review category.
6.1. COSE Algorithms Registry
* Name: HPKE-v1-BASE
* Value: TBD1 (Assumed: -1)
* Description: HPKE in version 1 in base mode for use with COSE
* Capabilities: [kty]
* Change Controller: IESG
* Reference: [[TBD: This RFC]]
* Recommended: Yes
6.2. COSE Header Algorithm Parameters
* Name: encapsulated_key
* Label: TBD2 (Assumed: -4)
* Value type: encapsulated_key
* Value Registry: N/A
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* Description: Encapsulated key for KEM-like algorithms
7. References
7.1. 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>.
7.2. Informative References
[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.
* Daisuke Ajitomi
* Richard Barnes
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* Ilari Liusvaara
Finally, we would like to thank Russ Housley for his contributions to
the draft as a co-author of initial versions.
Appendix B. Acknowledgements
We would like to thank John Mattsson, Mike Prorock, Michael
Richardson, Goeran Selander, Laurence Lundblade and Orie Steele for
their review feedback.
Authors' Addresses
Hannes Tschofenig
Email: hannes.tschofenig@gmx.net
Brendan Moran
Arm Limited
Email: Brendan.Moran@arm.com
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