title: "ML-KEM Post-Quantum Key Agreement for TLS 1.3" abbrev: connolly-tls-mlkem category: info
docname: draft-connolly-tls-mlkem-latest submissiontype: IETF number: date: consensus: true v: 3 keyword:
- mlkem
- tls
- post-quantum
area: "Security" workgroup: "Transport Layer Security" venue: group: "Transport Layer Security" type: "Working Group" mail: "[email protected]" arch: "https://mailarchive.ietf.org/arch/browse/tls/" github: "dconnolly/draft-tls-mlkem-key-agreement"
fullname: Deirdre Connolly
organization: SandboxAQ
email: [email protected]
normative: RFC9180: FIPS203: DOI.10.6028/NIST.FIPS.203
informative: CDM23: title: "Keeping Up with the KEMs: Stronger Security Notions for KEMs and automated analysis of KEM-based protocols" target: https://eprint.iacr.org/2023/1933.pdf date: 2023 author: - ins: C. Cremers name: Cas Cremers org: CISPA Helmholtz Center for Information Security - ins: A. Dax name: Alexander Dax org: CISPA Helmholtz Center for Information Security - ins: N. Medinger name: Niklas Medinger org: CISPA Helmholtz Center for Information Security
DOWLING: DOI.10.1007/s00145-021-09384-1 FO: DOI.10.1007/s00145-011-9114-1 HHK: DOI.10.1007/978-3-319-70500-2_12 HPKE: RFC9180 hybrid: I-D.ietf-tls-hybrid-design tlsiana: I-D.ietf-tls-rfc8447bis
--- 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.
--- middle
FIPS 203 standard (ML-KEM) is a new FIPS / CNSA 2.0 standard for post-quantum key agreement via lattice-based key establishment mechanism (KEM). Having a fully post-quantum (not hybrid) FIPS-compliant key agreement option for TLS 1.3 is necessary for eventual movement beyond hybrids and for users that need to be fully post-quantum sooner than later.
{::boilerplate bcp14-tagged}
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 keypkand a secret decapsulation keysk.Encaps(pk) -> (ct, shared_secret): A probabilistic encapsulation algorithm, which takes as input a public encapsulation keypkand outputs a ciphertextctand shared secretshared_secret.Decaps(sk, ct) -> shared_secret: A decapsulation algorithm, which takes as input a secret decapsulation keyskand ciphertextctand outputs a shared secretshared_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
We align with {{hybrid}} except that instead of joining ECDH options
with a KEM, we just have the KEM as a NamedGroup.
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 */
mlkem768(0x0768),
mlkem1024(0x1024)
...,
} NamedGroup;
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.
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
{{fig-key-schedule}}.
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(...)
{: #fig-key-schedule title="Key schedule for key agreement"}
Larger encapsulation keys and/or ciphertexts The HybridKeyExchange
struct in {{construction-transmitting}} 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.
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.
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.
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}}.
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: : 0x0512 (please)
Description: : MLKEM512
DTLS-OK: : Y
Recommended: : N
Reference: : This document
Comment: : FIPS 203 version of ML-KEM-512
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
--- back
{:numbered="false"}
Thanks to Douglas Stebila for consultation on the draft-ietf-tls-hybrid-design design.