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title: "Key Encapsulation Mechanisms for TLS 1.3" abbrev: connolly-tls-kems category: info

docname: draft-connolly-tls-kems-latest submissiontype: IETF number: date: consensus: true v: 3 keyword:

  • kems
  • tls

area: "Security" workgroup: "Transport Layer Security" venue: group: "Transport Layer Security" type: "Working Group" mail: "tls@ietf.org" arch: "https://mailarchive.ietf.org/arch/browse/tls/" github: "dconnolly/draft-connolly-tls-mlkem-key-agreement"

author:

fullname: Deirdre Connolly
organization: SandboxAQ
email: durumcrustulum@gmail.com

normative: RFC9180: FIPS203: DOI.10.6028/NIST.FIPS.203

informative: AVIRAM: target: https://mailarchive.ietf.org/arch/msg/tls/F4SVeL2xbGPaPB2GW_GkBbD_a5M/ title: "[TLS] Combining Secrets in Hybrid Key Exchange in TLS 1.3" date: 2021-09-01 author: - ins: Nimrod Aviram - ins: Benjamin Dowling - ins: Ilan Komargodski - ins: Kenny Paterson - ins: Eyal Ronen - ins: Eylon Yogev 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 LUCKY13: target: https://ieeexplore.ieee.org/iel7/6547086/6547088/06547131.pdf title: "Lucky Thirteen: Breaking the TLS and DTLS record protocols" author: - ins: N. J. Al Fardan - ins: K. G. Paterson RACCOON: target: https://raccoon-attack.com/ title: "Raccoon Attack: Finding and Exploiting Most-Significant-Bit-Oracles in TLS-DH(E)" author: - ins: R. Merget - ins: M. Brinkmann - ins: N. Aviram - ins: J. Somorovsky - ins: J. Mittmann - ins: J. Schwenk date: 2020-09 tlsiana: I-D.ietf-tls-rfc8447bis

--- abstract

This memo defines the use of Key Encapsulation Mechanisms (KEMs) as NamedGroups for use in TLS 1.3 key establishment.

--- middle

Introduction

Motivation

New standards for key agreement via key establishment mechanisms (KEMs) are being introduced, especially around post-quantum security properties. Having a key establishment option for TLS 1.3 based on KEMs alone is necessary for eventual movement beyond hybrids and for users that need to be fully post-quantum sooner than later.

Conventions and Definitions

{::boilerplate bcp14-tagged}

Key encapsulation mechanisms {#kems}

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.

Construction {#construction}

We define the KEMs as NamedGroups and sent in the supported_groups extension.

Negotiation {#negotiation}

Each method is its own KEM method, which are assigned their own identifiers, registered by IANA in the TLS Supported Groups registry; an example of the NamedGroup instances of a MyKEM primitive with at least two parameter sets:

    enum {

         ...,

          /* MyKEM Key Establishment Methods */
          mykem123(0x0123),
          mykem789(0x0789)

         ...,

    } NamedGroup;

Transmitting encapsulation keys and ciphertexts {#construction-transmitting}

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.

Shared secret calculation {#construction-shared-secret}

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"}

Discussion {#discussion}

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.

Security Considerations

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.

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 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.

Because of the inclusion of the 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}}.

[TODO: extrapolate on Kemmy Schmidt implications; in the mlkem document, strongly encourage implementers to use the seed variant of FIPS 203 to achieve strong binding properties]

IANA Considerations

None.

--- back

Acknowledgments

{:numbered="false"}

Thanks to Douglas Stebila for consultation on the draft-ietf-tls-hybrid-design design.