Internet DRAFT - draft-connolly-cfrg-hpke-mlkem
draft-connolly-cfrg-hpke-mlkem
Crypto Forum D. Connolly
Internet-Draft SandboxAQ
Intended status: Informational 28 February 2024
Expires: 31 August 2024
ML-KEM for HPKE
draft-connolly-cfrg-hpke-mlkem-00
Abstract
This memo defines the ML-KEM-768-based and ML-KEM-1024-based
ciphersuites for HPKE (RFC9180). ML-KEM is believed to be secure
even against adversaries who possess a quantum computer.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://dconnolly.github.io/draft-connolly-cfrg-xwing-kem/draft-
connolly-cfrg-xwing-kem.html. Status information for this document
may be found at https://datatracker.ietf.org/doc/draft-connolly-cfrg-
hpke-mlkem/.
Discussion of this document takes place on the Crypto Forum Research
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Source for this draft and an issue tracker can be found at
https://github.com/dconnolly/draft-connolly-cfrg-xwing-kem.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . 2
1.2. Not an authenticated KEM . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Construction . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Encap and Decap . . . . . . . . . . . . . . . . . . . . . 3
3.2. AuthEncap and AuthDecap . . . . . . . . . . . . . . . . . 3
4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Normative References . . . . . . . . . . . . . . . . . . 5
6.2. Informative References . . . . . . . . . . . . . . . . . 6
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 6
Appendix B. Change log . . . . . . . . . . . . . . . . . . . . . 6
B.1. Since draft-connolly-cfrg-hpke-mlkem-00 . . . . . . . . . 6
Appendix C. Test Vectors . . . . . . . . . . . . . . . . . . . . 6
C.1. ML-KEM-768, HKDF-SHA256, AES-128-GCM . . . . . . . . . . 6
C.2. ML-KEM-1024, HKDF-SHA512, AES-256-GCM . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
1.1. Motivation
The final draft for ML-KEM is expected in 2024. For parties that
must move to exclusively post-quantum algorithms, having a pure PQ
choice for public-key hybrid encryption is desireable. HPKE is the
leading modern protocol for public-key encryption, and ML-KEM as a
post-quantum IND-CCA2-secure KEM fits nicely into HPKE's design.
Supporting multiple security levels for ML-KEM allow a spectrum of
use cases including settings where NIST PQ security category 5 is
required.
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1.2. Not an authenticated KEM
ML-KEM is a plain KEM that does not support the static-ephemeral key
exchange that allows HPKE based on Diffie-Hellman based KEMs its
(optional) authenticated modes.
2. Conventions and Definitions
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.
3. Construction
We construct 'wrapper' KEMs based on ML-KEM to bind the encapsulated
shared secret ciphertext into the shared secret value, such that the
final KEM has similar binding security properties as the original
DHKEM HPKE was designed around.
The encapsulation and decapsulation keys are computed, serialized,
and deserialized the same as in [FIPS203].
We use HKDF-SHA256 and HKDF-SHA512 as the HPKE KDFs and AES-128-GCM
and AES-256-GCM as the AEADs for ML-KEM-768 and ML-KEM-1024
respectively.
3.1. Encap and Decap
~~ def Encap(pkR): ss, ct = MLKEM.Encaps(pkR)
shared_secret = ExtractAndExpand(ss, ct)
return shared_secret, ct ~~
~~ def Decap(enc, skR): ss, ct = MLKEM.Decaps(enc, skR)
shared_secret = ExtractAndExpand(ss, ct)
return shared_secret, ct ~~
3.2. AuthEncap and AuthDecap
HPKE-ML-KEM is not an authenticeted KEM and does not support
AuthEncap() or AuthDecap(), see Section 1.2.
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4. Security Considerations
HPKE's IND-CCA2 security relies upon the IND-CCA2 security of the
underlying KEM and AEAD schemes. ML-KEM is believed to be IND-CCA
secure via multiple analyses.
The HPKE key schedule is independent of the encapsulated KEM shared
secret ciphertext of the ciphersuite KEM, and dependent on the shared
secret produced by the KEM. If HPKE had committed to the
encapsulated shared secret ciphertext, we wouldn't have to worry
about the binding properties of the ciphersuite KEM's X-BIND-K-CT
properties. These computational binding properties for KEMs were
formalized in [CDM23].
ML-KEM, unlike DHKEM, is also an implicitly-rejecting instantiation
of the Fujisaki-Okamoto transform, meaning the ML-KEM output shared
secret may be computed differently in case of decryption failure,
that reults in different binding properties, such as the lack of X-
BIND-CT-PK and X-BIND-CT-K completely.
The DHKEM construction in HPKE provides MAL-BIND-K-PK and MAL-BIND-
K-CT security (the shared secret 'binds' or uniquely determines the
encapsulation key and the encapsualted shared secret ciphertext),
where the adversary is able to create the key pairs any way they like
in addition to the key generation. ML-KEM as specified provides
LEAK-BIND-PK,K-CT security, where the involved key pairs are output
by the key generation algorithm of the KEM and then leaked to the
adversary. LEAK-BIND-PK,K-CT is a weaker property than the DHKEM
properties as it is not resistant in the presence of an actively
malicious adversary, and requires both the shared secret _and_the
public key together to uniquely bind the ciphertext, so its shared
secret alone is insufficient.
This results in a wrapper construction around ML-KEM to bind to the
encapsulated shared secret ciphertext as the kem_context provided to
ExtractAndExpand(). This binds the final shared_secret (K) to the
encapsulated shared secret ciphertext (CT), achieving MAL-BIND-K-CT.
ML-KEM already is MAL-BIND-K-PK as the hash of the encapsulation key
(PK) is an input the computation of the shared secret (K). Together
this wrapper KEM matches the binding properties of HPKE's default KEM
construction DHKEM in being MAL-BIND-K-CT and MAL-BIND-K-PK.
5. IANA Considerations
This document requests/registers two new entries to the "HPKE KEM
Identifiers" registry.
Value: 0x0070 (please)
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KEM: ML-KEM-768
Nsecret: 32
Nenc: 1088
Npk: 1184
Nsk: 2400
Auth: no
Reference: This document
Value: 0x0080 (please)
KEM: ML-KEM-1024
Nsecret: 32
Nenc: 1568
Npk: 1568
Nsk: 3168
Auth: no
Reference: This document
6. References
6.1. Normative References
[FIPS203] "*** BROKEN REFERENCE ***".
[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>.
[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>.
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6.2. Informative References
[CDM23] Cremers, C., Dax, A., and N. Medinger, "Keeping Up with
the KEMs: Stronger Security Notions for KEMs and automated
analysis of KEM-based protocols", 2023,
<https://eprint.iacr.org/2023/1933.pdf>.
Appendix A. Acknowledgments
The authors would like to thank Cas Cremers for their input.
Appendix B. Change log
*RFC Editor's Note:* Please remove this section prior to
publication of a final version of this document.
TODO
B.1. Since draft-connolly-cfrg-hpke-mlkem-00
TODO
Appendix C. Test Vectors
This section contains test vectors formatted similary to that which
are found in [RFC9180], with two changes. First, we only provide
vectors for the non-authenticated modes of operation. Secondly, as
ML-KEM encapsulation does not involve an ephemeral keypair, we omit
the ikmE, skEm, pkEm entries and provide an ier entry instead. The
value of ier is the randomness used to encapsulate, so ier[0:32] is
the seed that is fed to H in the first step of ML-KEM encapsulation
in [FIPS203].
C.1. ML-KEM-768, HKDF-SHA256, AES-128-GCM
TODO
C.2. ML-KEM-1024, HKDF-SHA512, AES-256-GCM
TODO
Author's Address
Deirdre Connolly
SandboxAQ
Email: durumcrustulum@gmail.com
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