ML-KEM Post-Quantum Key Agreement for TLS 1.3
draft-connolly-tls-mlkem-key-agreement-01
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Author | Deirdre Connolly | ||
| Last updated | 2024-03-21 | ||
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| Intended RFC status | (None) | ||
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| Stream | Stream state | (No stream defined) | |
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draft-connolly-tls-mlkem-key-agreement-01
Transport Layer Security D. Connolly
Internet-Draft SandboxAQ
Intended status: Informational 22 March 2024
Expires: 23 September 2024
ML-KEM Post-Quantum Key Agreement for TLS 1.3
draft-connolly-tls-mlkem-key-agreement-01
Abstract
This memo defines ML-KEM-768 and ML-KEM-1024 as a standalone
NamedGroup for use in TLS 1.3 to achieve post-quantum key agreement.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-connolly-tls-mlkem-key-
agreement/.
Discussion of this document takes place on the Transport Layer
Security Working Group mailing list (mailto:tls@ietf.org), which is
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at https://www.ietf.org/mailman/listinfo/tls/.
Source for this draft and an issue tracker can be found at
https://github.com/dconnolly/draft-connolly-tls-mlkem-key-agreement.
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 23 September 2024.
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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
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Key encapsulation mechanisms . . . . . . . . . . . . . . . . 3
4. Construction . . . . . . . . . . . . . . . . . . . . . . . . 3
4.1. Negotiation . . . . . . . . . . . . . . . . . . . . . . . 3
4.2. Transmitting encapsulation keys and ciphertexts . . . . . 4
4.3. Shared secret calculation . . . . . . . . . . . . . . . . 5
5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . 8
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
1.1. Motivation
FIPS 203 standard (ML-KEM) is a new FIPS standard for post-quantum
key agreement via lattice-based key establishment mechanism (KEM).
Having a fully post-quantum (not hybrid) key agreement option for TLS
1.3 is necessary for migrating beyond hybrids and for users that need
to be fully post-quantum.
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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. Key encapsulation mechanisms
This document models key agreement as key encapsulation mechanisms
(KEMs), which consist of three algorithms:
* KeyGen() -> (pk, sk): A probabilistic key generation algorithm,
which generates a public encapsulation key pk and a secret
decapsulation key sk.
* Encaps(pk) -> (ct, shared_secret): A probabilistic encapsulation
algorithm, which takes as input a public encapsulation key pk and
outputs a ciphertext ct and shared secret shared_secret.
* Decaps(sk, ct) -> shared_secret: A decapsulation algorithm, which
takes as input a secret decapsulation key sk and ciphertext ct and
outputs a shared secret shared_secret.
ML-KEM-768 and ML-KEM-1024 conform to this API:
* ML-KEM-768 has encapsulation keys of size 1184 bytes,
decapsulation keys of 2400 bytes, ciphertext size of 1088 bytes,
and shared secrets of size 32 bytes
* ML-KEM-1024 has encapsulation keys of size 1568 bytes,
decapsulation keys of 3168 bytes, ciphertext size of 1568 bytes,
and shared secrets of size 32 bytes
4. Construction
We define the KEMs as NamedGroups and sent in the supported_groups
extension.
4.1. Negotiation
Each method is its own solely post-quantum key agreement method,
which are assigned their own identifiers, registered by IANA in the
TLS Supported Groups registry:
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enum {
...,
/* ML-KEM Key Agreement Methods */
mlkem768(0x0768),
mlkem1024(0x1024)
...,
} NamedGroup;
4.2. Transmitting encapsulation keys and ciphertexts
The encapsulation key and ciphertext values are directly encoded with
fixed lengths as in [FIPS203]; the representation and length of
elements MUST be fixed once the algorithm is fixed.
In TLS 1.3 a KEM encapsulation key or KEM ciphertext is represented
as a KeyShareEntry:
struct {
NamedGroup group;
opaque key_exchange<1..2^16-1>;
} KeyShareEntry;
These are transmitted in the extension_data fields of
KeyShareClientHello and KeyShareServerHello extensions:
struct {
KeyShareEntry client_shares<0..2^16-1>;
} KeyShareClientHello;
struct {
KeyShareEntry server_share;
} KeyShareServerHello;
The client's shares are listed in descending order of client
preference; the server selects one algorithm and sends its
corresponding share.
For the client's share, the key_exchange value contains the pk output
of the corresponding ML-KEM NamedGroup's KeyGen algorithm.
For the server's share, the key_exchange value contains the ct output
of the corresponding ML-KEM NamedGroup's Encaps algorithm.
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4.3. Shared secret calculation
The shared secret output from the ML-KEM Encaps and Decaps algorithms
over the appropriate keypair and ciphertext results in the same
shared secret shared_secret, which is inserted into the TLS 1.3 key
schedule in place of the (EC)DHE shared secret, as shown in Figure 1.
0
|
v
PSK -> HKDF-Extract = Early Secret
|
+-----> Derive-Secret(...)
+-----> Derive-Secret(...)
+-----> Derive-Secret(...)
|
v
Derive-Secret(., "derived", "")
|
v
shared_secret -> HKDF-Extract = Handshake Secret
^^^^^^^^^^^^^ |
+-----> Derive-Secret(...)
+-----> Derive-Secret(...)
|
v
Derive-Secret(., "derived", "")
|
v
0 -> HKDF-Extract = Master Secret
|
+-----> Derive-Secret(...)
+-----> Derive-Secret(...)
+-----> Derive-Secret(...)
+-----> Derive-Secret(...)
Figure 1: Key schedule for key agreement
5. Discussion
*Larger encapsulation keys and/or ciphertexts* The HybridKeyExchange
struct in Section 4.2 limits public keys and ciphertexts to 2^16-1
bytes; this is bounded by the same (2^16-1)-byte limit on the
key_exchange field in the KeyShareEntry struct. All defined
parameter sets for ML-KEM have encapsulation keys and ciphertexts
that fall within the TLS constraints.
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*Failures* Some post-quantum key exchange algorithms, including ML-
KEM, have non-zero probability of failure, meaning two honest parties
may derive different shared secrets. This would cause a handshake
failure. ML-KEM has a cryptographically small failure rate;
implementers should be aware of the potential of handshake failure.
Clients can retry if a failure is encountered.
6. Security Considerations
*IND-CCA* The main security property for KEMs is indistinguishability
under adaptive chosen ciphertext attack (IND-CCA2), which means that
shared secret values should be indistinguishable from random strings
even given the ability to have other arbitrary ciphertexts
decapsulated. IND-CCA2 corresponds to security against an active
attacker, and the public key / secret key pair can be treated as a
long-term key or reused. A common design pattern for obtaining
security under key reuse is to apply the Fujisaki-Okamoto (FO)
transform [FO] or a variant thereof [HHK].
Key exchange in TLS 1.3 is phrased in terms of Diffie-Hellman key
exchange in a group. DH key exchange can be modeled as a KEM, with
KeyGen corresponding to selecting an exponent x as the secret key and
computing the public key g^x; encapsulation corresponding to
selecting an exponent y, computing the ciphertext g^y and the shared
secret g^(xy), and decapsulation as computing the shared secret
g^(xy). See [HPKE] for more details of such Diffie-Hellman-based key
encapsulation mechanisms. Diffie-Hellman key exchange, when viewed
as a KEM, does not formally satisfy IND-CCA2 security, but is still
safe to use for ephemeral key exchange in TLS 1.3, see e.g.
[DOWLING].
TLS 1.3 does not require that ephemeral public keys be used only in a
single key exchange session; some implementations may reuse them, at
the cost of limited forward secrecy. As a result, any KEM used in
the manner described in this document MUST explicitly be designed to
be secure in the event that the public key is reused. Finite-field
and elliptic-curve Diffie-Hellman key exchange methods used in TLS
1.3 satisfy this criteria. For generic KEMs, this means satisfying
IND-CCA2 security or having a transform like the Fujisaki-Okamoto
transform [FO] [HHK] applied. While it is recommended that
implementations avoid reuse of KEM public keys, implementations that
do reuse KEM public keys MUST ensure that the number of reuses of a
KEM public key abides by any bounds in the specification of the KEM
or subsequent security analyses. Implementations MUST NOT reuse
randomness in the generation of KEM ciphertexts.
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*Binding properties* TLS 1.3's key schedule commits to the the ML-KEM
encapsulation key and the encapsulated shared secret ciphertext as
the key_exchange field as part of the key_share extension are
populated with those values are included as part of the handshake
messages, providing resilience against re-encapsulation attacks
against KEMs used for key agreement.
ML-KEM is MAL-BIND-K-PK-secure but only LEAK-BIND-K-CT and LEAK-BIND-
K,PK-CT-secure, but because of the inclusion of the ML-KEM ciphertext
in the TLS 1.3 key schedule there is no concern of malicious
tampering (MAL) adversaries, not just honestly-generated but leaked
key pairs (LEAK adversaries). The same is true of other KEMs with
weaker binding properties, even if they were to have more constraints
for secure use in contexts outside of TLS 1.3 handshake key
agreement.These computational binding properties for KEMs were
formalized in [CDM23].
7. IANA Considerations
This document requests/registers two new entries to the TLS Supported
Groups registry, according to the procedures in Section 6 of
[tlsiana].
Value: 0x0768 (please)
Description: MLKEM768
DTLS-OK: Y
Recommended: N
Reference: This document
Comment: FIPS 203 version of ML-KEM-768
Value: 0x1024 (please)
Description: MLKEM1024
DTLS-OK: Y
Recommended: N
Reference: This document
Comment: FIPS 203 version of ML-KEM-1024
8. References
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8.1. 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>.
8.2. 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>.
[DOWLING] Dowling, B., Fischlin, M., Günther, F., and D. Stebila, "A
Cryptographic Analysis of the TLS 1.3 Handshake Protocol",
Springer Science and Business Media LLC, Journal of
Cryptology vol. 34, no. 4, DOI 10.1007/s00145-021-09384-1,
July 2021, <https://doi.org/10.1007/s00145-021-09384-1>.
[FO] Fujisaki, E. and T. Okamoto, "Secure Integration of
Asymmetric and Symmetric Encryption Schemes", Springer
Science and Business Media LLC, Journal of Cryptology vol.
26, no. 1, pp. 80-101, DOI 10.1007/s00145-011-9114-1,
December 2011,
<https://doi.org/10.1007/s00145-011-9114-1>.
[HHK] Hofheinz, D., Hövelmanns, K., and E. Kiltz, "A Modular
Analysis of the Fujisaki-Okamoto Transformation", Springer
International Publishing, Theory of Cryptography pp.
341-371, DOI 10.1007/978-3-319-70500-2_12,
ISBN ["9783319704999", "9783319705002"], 2017,
<https://doi.org/10.1007/978-3-319-70500-2_12>.
[HPKE] 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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[hybrid] Stebila, D., Fluhrer, S., and S. Gueron, "Hybrid key
exchange in TLS 1.3", Work in Progress, Internet-Draft,
draft-ietf-tls-hybrid-design-09, 7 September 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
hybrid-design-09>.
[tlsiana] Salowey, J. A. and S. Turner, "IANA Registry Updates for
TLS and DTLS", Work in Progress, Internet-Draft, draft-
ietf-tls-rfc8447bis-08, 23 January 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
rfc8447bis-08>.
Acknowledgments
Thanks to Douglas Stebila for consultation on the draft-ietf-tls-
hybrid-design design.
Author's Address
Deirdre Connolly
SandboxAQ
Email: durumcrustulum@gmail.com
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