| Internet-Draft | connolly-tls-mlkem-key-agreement | November 2024 |
| Connolly | Expires 10 May 2025 | [Page] |
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.¶
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/.¶
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Source for this draft and an issue tracker can be found at https://github.com/dconnolly/draft-connolly-tls-mlkem-key-agreement.¶
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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.¶
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 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¶
We define the KEMs as NamedGroups, sent in the supported_groups
extension.¶
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;
¶
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 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.¶
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.¶
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.¶
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.¶
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.¶
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].¶
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].¶
0x0200¶
MLKEM512¶
Y¶
N¶
This document¶
FIPS 203 version of ML-KEM-512¶
0x0201¶
MLKEM768¶
Y¶
N¶
This document¶
FIPS 203 version of ML-KEM-768¶
0x0202¶
MLKEM1024¶
Y¶
N¶
This document¶
FIPS 203 version of ML-KEM-1024¶
Thanks to Douglas Stebila for consultation on the draft-ietf-tls-hybrid-design design.¶