ML-KEM Post-Quantum Key Agreement for TLS 1.3
draft-connolly-tls-mlkem-key-agreement-05
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Author | Deirdre Connolly | ||
| Last updated | 2024-11-06 | ||
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draft-connolly-tls-mlkem-key-agreement-05
Transport Layer Security D. Connolly
Internet-Draft SandboxAQ
Intended status: Informational 6 November 2024
Expires: 10 May 2025
ML-KEM Post-Quantum Key Agreement for TLS 1.3
draft-connolly-tls-mlkem-key-agreement-05
Abstract
This memo defines ML-KEM-512, ML-KEM-768, and ML-KEM-1024 as a
standalone NamedGroups 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
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This Internet-Draft will expire on 10 May 2025.
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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 . . . . . . . . . . . . . . . . . . . . . . . 4
4.2. Transmitting encapsulation keys and ciphertexts . . . . . 4
4.3. Shared secret calculation . . . . . . . . . . . . . . . . 5
5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Larger encapsulation keys and/or ciphertexts . . . . . . 6
5.2. Failures . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6.1. Fixed lengths . . . . . . . . . . . . . . . . . . . . . . 7
6.2. IND-CCA . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.3. Binding properties . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
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-512, ML-KEM-768 and ML-KEM-1024 conform to this API:
* ML-KEM-512 has encapsulation keys of size 800 bytes, expanded
decapsulation keys of 1632 bytes, decapsulation key seeds of size
64 bytes, ciphertext size of 768 bytes, and shared secrets of size
32 bytes
* ML-KEM-768 has encapsulation keys of size 1184 bytes, expanded
decapsulation keys of 2400 bytes, decapsulation key seeds of size
64 bytes, ciphertext size of 1088 bytes, and shared secrets of
size 32 bytes
* ML-KEM-1024 has encapsulation keys of size 1568 bytes, expanded
decapsulation keys of 3168 bytes, decapsulation key seeds of size
64 bytes, ciphertext size of 1568 bytes, and shared secrets of
size 32 bytes
4. Construction
We define the KEMs as NamedGroups, sent in the supported_groups
extension.
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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:
enum {
...,
/* ML-KEM Key Agreement Methods */
mlkem512(0x0200),
mlkem768(0x0201),
mlkem1024(0x0202)
...,
} 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.
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For the client's share, the key_exchange value contains the pk output
of the corresponding KEM NamedGroup's KeyGen algorithm.
For the server's share, the key_exchange value contains the ct output
of the corresponding KEM NamedGroup's Encaps algorithm.
For all parameter sets, the server MUST perform the encapsulation key
check described in Section 7.2 of [FIPS203] on the client's
encapsulation key, and abort with an illegal_parameter alert if it
fails.
For all parameter sets, the client MUST check if the ciphertext
length matches the selected parameter set, and abort with an
illegal_parameter alert if it fails.
If ML-KEM decapsulation fails for any other reason, the connection
MUST be aborted with an internal_error alert.
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.
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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
5.1. Larger encapsulation keys and/or ciphertexts
The KeyShareEntry struct limits public keys and ciphertexts to 2^16-1
bytes; this is the (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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5.2. 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 less than
2^-138; implementers should be aware of the potential of handshake
failure. Clients can retry if a failure is encountered.
6. Security Considerations
6.1. Fixed lengths
For each NameGroup, the lengths are fixed (that is, constant) for
encapsulation keys, the ciphertexts, and the shared secrets.
Variable-length secrets are, generally speaking, dangerous. In
particular, when using key material of variable length and processing
it using hash functions, a timing side channel may arise. In broad
terms, when the secret is longer, the hash function may need to
process more blocks internally. In some unfortunate circumstances,
this has led to timing attacks, e.g. the Lucky Thirteen [LUCKY13] and
Raccoon [RACCOON] attacks.
[AVIRAM] identified a risk of using variable-length secrets when the
hash function used in the key derivation function is no longer
collision-resistant.
6.2. 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
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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.
6.3. Binding properties
TLS 1.3's key schedule commits to the the ML-KEM encapsulation key
and the 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.
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, nor of just honestly-generated but leaked key pairs
(LEAK adversaries). The same is true of 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 three new entries to the TLS Named
Group (or Supported Group) registry, according to the procedures in
Section 6 of [tlsiana].
Value: 0x0200
Description: MLKEM512
DTLS-OK: Y
Recommended: N
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Reference: This document
Comment: FIPS 203 version of ML-KEM-512
Value: 0x0201
Description: MLKEM768
DTLS-OK: Y
Recommended: N
Reference: This document
Comment: FIPS 203 version of ML-KEM-768
Value: 0x0202
Description: MLKEM1024
DTLS-OK: Y
Recommended: N
Reference: This document
Comment: FIPS 203 version of ML-KEM-1024
8. References
8.1. Normative References
[FIPS203] "Module-Lattice-Based Key-Encapsulation Mechanism
Standard", National Institute of Standards and Technology,
DOI 10.6028/nist.fips.203, August 2024,
<https://doi.org/10.6028/nist.fips.203>.
[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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[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
[AVIRAM] Nimrod Aviram, Benjamin Dowling, Ilan Komargodski, Kenny
Paterson, Eyal Ronen, and Eylon Yogev, "[TLS] Combining
Secrets in Hybrid Key Exchange in TLS 1.3", 1 September
2021, <https://mailarchive.ietf.org/arch/msg/tls/
F4SVeL2xbGPaPB2GW_GkBbD_a5M/>.
[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, Lecture Notes in Computer
Science 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>.
[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-11, 7 October 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
hybrid-design-11>.
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[LUCKY13] Al Fardan, N. J. and K. G. Paterson, "Lucky Thirteen:
Breaking the TLS and DTLS record protocols", n.d.,
<https://ieeexplore.ieee.org/
iel7/6547086/6547088/06547131.pdf>.
[RACCOON] Merget, R., Brinkmann, M., Aviram, N., Somorovsky, J.,
Mittmann, J., and J. Schwenk, "Raccoon Attack: Finding and
Exploiting Most-Significant-Bit-Oracles in TLS-DH(E)",
September 2020, <https://raccoon-attack.com/>.
[tlsiana] Salowey, J. A. and S. Turner, "IANA Registry Updates for
TLS and DTLS", Work in Progress, Internet-Draft, draft-
ietf-tls-rfc8447bis-10, 3 November 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
rfc8447bis-10>.
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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