Internet DRAFT - draft-tschofenig-cose-hpke
draft-tschofenig-cose-hpke
COSE H. Tschofenig
Internet-Draft Arm Limited
Intended status: Standards Track R. Housley
Expires: April 28, 2022 Vigil Security
B. Moran
Arm Limited
October 25, 2021
Use of Hybrid Public-Key Encryption (HPKE) with CBOR Object Signing and
Encryption (COSE)
draft-tschofenig-cose-hpke-00
Abstract
This specification defines hybrid public-key encryption (HPKE) for
use with CBOR Object Signing and Encryption (COSE).
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
3. HPKE for COSE . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 3
3.2. HPKE Encryption with SealBase . . . . . . . . . . . . . . 4
3.3. HPKE Decryption with Open . . . . . . . . . . . . . . . . 5
3.4. Info Structure . . . . . . . . . . . . . . . . . . . . . 5
4. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
Hybrid public-key encryption (HPKE) [I-D.irtf-cfrg-hpke] 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, which is then used to encrypt plaintext.
The HPKE specification defines several features for use with public
key encryption and a subset of those features is applied to COSE
[RFC8152]. Since COSE provides constructs for authenticcation, those
are not re-used from the HPKE specification. This specification uses
the "base" mode (as it is called in HPKE specification language).
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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 RFC 2630 [RFC2630].
- Hybrid Public Key Encryption (HPKE) is defined in
[I-D.irtf-cfrg-hpke]. - pkR is the public key of the recipient, as
defined in [I-D.irtf-cfrg-hpke]. - skR is the private key of the
recipient, as defined in [I-D.irtf-cfrg-hpke].
3. HPKE for COSE
3.1. Overview
The CDDL for the COSE_Encrypt structure, as used with this
specification, is shown in Figure 1. The structures referenced below
are found in the CDDL.
HPKE, when used with COSE, follows a three layer structure:
- Layer 0 (corresponding to the COSE_Encrypt structure) contains
content encrypted with the CEK. This ciphertext may be detached.
If not detached, then it is included in the COSE_Encrypt
structure.
- Layer 1 (see COSE_recipient_outer structure) includes the
encrypted CEK.
- Layer 2 (in the COSE_recipient_inner structure) contains
parameters needed for HPKE to generate a shared secret used to
encrypt the CEK from layer 1.
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COSE_Encrypt_Tagged = #6.96(COSE_Encrypt)
SUIT_Encryption_Info = COSE_Encrypt_Tagged
; Layer 0
COSE_Encrypt = [
Headers,
ciphertext : bstr / nil,
recipients : [+COSE_recipient_outer]
]
; Layer 1
COSE_recipient_outer = [
protected : bstr .size 0,
unprotected : header_map, ; must contain alg
encCEK : bstr, ; CEK encrypted with HPKE-derived shared secret
recipients : [ + COSE_recipient_inner ]
]
; Layer 2
COSE_recipient_inner = [
protected : bstr .cbor header_map, ; must contain HPKE alg
unprotected : header_map, ; must contain kid and ephemeral public key
empty : null,
empty : null
]
header_map = {
Generic_Headers,
* label =values,
}
Figure 1: CDDL for HPKE-based COSE_Encrypt Structure
The COSE_recipient_outer structure shown in Figure 1 includes the
encrypted CEK (in the encCEK structure) and the COSE_recipient_inner
structure, also shown in Figure 1, contains the ephemeral public key
(in the unprotected structure).
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). For use in this
specification, 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.
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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. If successful, SealBase() will output a
ciphertext "ct" and an encapsulated key "enc". The content of enc is
the ephemeral public key.
The content of the info parameter is based on the 'COSE_KDF_Context'
structure, which is detailed in Figure 2.
3.3. HPKE Decryption with Open
The recipient will use the OpenBase(enc, skR, info, aad, ct) function
with the enc and ct parameters received from the sender. The "aad"
and the "info" parameters are obtained via the context of the usage.
The OpenBase function will, if successful, decrypt "ct". When
decrypted, the result will be the CEK. The CK is the symmetric key
used to decrypt the ciphertext in the COSE_Encrypt structure.
3.4. Info Structure
This specification re-uses the context information structure defined
in [RFC8152] for use with the HPKE algorithm. This payload becomes
the content of the info parameter for the HPKE functions. For better
readability of this specification the COSE_KDF_Context structure is
repeated in Figure 2.
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 2: COSE_KDF_Context Data Structure for info parameter
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Since this specification may be used in a number of different
deployment environments flexibility for populating the fields in the
COSE_KDF_Context structure is provided.
For better interoperability, the following recommended settings are
provided:
- PartyUInfo.identity corresponds to the kid found in the
COSE_Sign_Tagged or COSE_Sign1_Tagged structure (when a digital
signature is used). When utilizing a MAC, then the kid is found
in the COSE_Mac_Tagged or COSE_Mac0_Tagged structure.
- PartyVInfo.identity corresponds to the kid used for the respective
recipient from the inner-most recipients array.
- The value in the AlgorithmID field corresponds to the alg
parameter in the protected structure in the inner-most recipients
array.
- keyDataLength is set to the number of bits of the desired output
value.
- protected refers to the protected structure of the inner-most
array.
4. Example
An example of the COSE_Encrypt structure using the HPKE scheme is
shown in Figure 3. It uses the following algorithm combination:
- AES-GCM-128 for encryption of detached ciphertext.
- AES-GCM-128 for encryption of the CEK.
- Key Encapsulation Mechanism (KEM): NIST P-256
- Key Derivation Function (KDF): HKDF-SHA256
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96(
[
// protected field with alg=AES-GCM-128
h'A10101',
{ // unprotected field with iv
5: h'26682306D4FB28CA01B43B80'
},
// null because of detached ciphertext
null,
[ // COSE_recipient_outer
h'', // empty protected field
{ // unprotected field with ...
1: 1 // alg=A128GCM
},
// Encrypted CEK
h'FA55A50CF110908DA6443149F2C2062011A7D8333A72721A',
/ recipients / [ // COSE_recipient_inner
[
/ protected / h'a1013818' / {
\ alg \ 1:TBD1 \ HPKE/P-256+HKDF-256 \
} / ,
/ unprotected / {
// HPKE encapsulated key
/ ephemeral / -1:{
/ kty / 1:2,
/ crv / -1:1,
/ x / -2:h'98f50a4ff6c05861c8...90bbf91d6280',
/ y / -3:true
},
// kid for recipient static ECDH public key
/ kid / 4:'meriadoc.brandybuck@buckland.example'
},
// empty ciphertext
/ ciphertext / h''
]
]
]
]
)
Figure 3: COSE_Encrypt Example for HPKE
5. Security Considerations
This specification is based on HPKE and the security considerations
of HPKE [I-D.irtf-cfrg-hpke] are therefore applicable also to this
specification.
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HPKE assumes that the sender is in possession of the public key of
the recipient. A system using HPKE COSE has to assume the same
assumptions and public key distribution mechanism is assumed to
exist.
Since the CEK is randomly generated it must be ensured that the
guidelines for random number generations are followed, see [RFC8937].
The SUIT_Encryption_Info structure shown in this document does not
provide authentication. Hence, the SUIT_Encryption_Info structure
has to be used in combination with other COSE constructs, such as the
COSE_Sign or COSE_Sign1.
6. IANA Considerations
This document requests IANA to create new entries in the COSE
Algorithms registry established with [RFC8152].
+-------------+-------+---------+------------+--------+---------------+
| Name | Value | KDF | Ephemeral- | Key | Description |
| | | | Static | Wrap | |
+-------------+-------+---------+------------+--------+---------------+
| HPKE/P-256+ | TBD1 | HKDF - | yes | none | HPKE with |
| HKDF-256 | | SHA-256 | | | ECDH-ES |
| | | | | | (P-256) + |
| | | | | | HKDF-256 |
+-------------+-------+---------+------------+--------+---------------+
| HPKE/P-384+ | TBD2 | HKDF - | yes | none | HPKE with |
| HKDF-SHA384 | | SHA-384 | | | ECDH-ES |
| | | | | | (P-384) + |
| | | | | | HKDF-384 |
+-------------+-------+---------+------------+--------+---------------+
| HPKE/P-521+ | TBD3 | HKDF - | yes | none | HPKE with |
| HKDF-SHA521 | | SHA-521 | | | ECDH-ES |
| | | | | | (P-521) + |
| | | | | | HKDF-521 |
+-------------+-------+---------+------------+--------+---------------+
| HPKE | TBD4 | HKDF - | yes | none | HPKE with |
| X25519 + | | SHA-256 | | | ECDH-ES |
| HKDF-SHA256 | | | | | (X25519) + |
| | | | | | HKDF-256 |
+-------------+-------+---------+------------+--------+---------------+
| HPKE | TBD4 | HKDF - | yes | none | HPKE with |
| X448 + | | SHA-512 | | | ECDH-ES |
| HKDF-SHA512 | | | | | (X448) + |
| | | | | | HKDF-512 |
+-------------+-------+---------+------------+--------+---------------+
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7. References
7.1. Normative References
[I-D.irtf-cfrg-hpke]
Cisco, Inria, Inria, and Cloudflare, "Hybrid Public Key
Encryption", draft-irtf-cfrg-hpke-12 (work in progress),
September 2021.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
7.2. Informative References
[RFC2630] Housley, R., "Cryptographic Message Syntax", RFC 2630,
DOI 10.17487/RFC2630, June 1999,
<https://www.rfc-editor.org/info/rfc2630>.
[RFC8937] Cremers, C., Garratt, L., Smyshlyaev, S., Sullivan, N.,
and C. Wood, "Randomness Improvements for Security
Protocols", RFC 8937, DOI 10.17487/RFC8937, October 2020,
<https://www.rfc-editor.org/info/rfc8937>.
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Appendix A. Acknowledgements
TBD: Add your name here.
Authors' Addresses
Hannes Tschofenig
Arm Limited
EMail: hannes.tschofenig@arm.com
Russ Housley
Vigil Security, LLC
EMail: housley@vigilsec.com
Brendan Moran
Arm Limited
EMail: Brendan.Moran@arm.com
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