Internet-Draft hybrid-kems June 2025
Connolly & Barnes Expires 14 December 2025 [Page]
Workgroup:
Crypto Forum
Internet-Draft:
draft-irtf-cfrg-hybrid-kems-latest
Published:
Intended Status:
Informational
Expires:
Authors:
D. Connolly
SandboxAQ
R. Barnes
Cisco

Hybrid PQ/T Key Encapsulation Mechanisms

Abstract

Post-quantum (PQ) algorithms are designed to resist attack by a quantum computer, in contrast to "traditional" algorithms. However, given the novelty of PQ algorithms, there is some concern that PQ algorithms currently believed to be secure will be broken. Hybrid constructions that combine both PQ and traditional algorithms can help moderate this risk while still providing security against quantum attack. In this document, we define constructions for hybrid Key Encapsulation Mechanisms (KEMs) based on combining a traditional KEM and a PQ KEM. Hybrid KEMs using these constructions provide strong security properties as long as the undelying algorithms are secure.

Discussion Venues

This note is to be removed before publishing as an RFC.

Discussion of this document takes place on the Crypto Forum Research Group mailing list (cfrg@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/cfrg.

Source for this draft and an issue tracker can be found at https://github.com/cfrg/draft-irtf-cfrg-hybrid-kems.

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Table of Contents

1. Introduction

Post-quantum (PQ) cryptographic schemes offer new constructions based on problems conjectured as resistant to attacks possible on a quantum computer. Key Encapsulation Mechanisms (KEMs), are a standardized class of cryptographic scheme that can be used to build protocols in lieu of traditional, quantum-vulnerable variants such as finite field or elliptic curve Diffie-Hellman (DH) based protocols.

Given the novelty of PQ algorithms, however, there is some concern that PQ algorithms currently believed to be secure will be broken. Hybrid constructions that combine both PQ and traditional algorithms can help moderate this risk while still providing security against quantum attack. If construted properly, a hybrid KEM will retain certain security properties even if one of the two constituent KEMs is compromised. If the PQ KEM is broken, then the hybrid KEM should continue to provide security against non-quantum attackers by virtue of its traditional KEM component. If the traditional KEM is broken by a quantum computer, then the hybrid KEM should continue to resist quantum attack by virtue of its PQ KEM component.

In addition to guarding against algorithm weaknesses, this property also guards against flaws in implementations, such as timing attacks. Hybrid KEMs can also facilitate faster deployment of PQ security by allowing applications to incorporate PQ algorithms while still meeting compliance requirements based on traditional algorithms.

In this document, we define generic schemes for constructing hybrid KEMs from a traditional algorithm and a PQ KEM. The aim of this document is provide a small set of techniques to achieve specific security properties given conforming component algorithms, which should make these techniques suitable for a broad variety of use cases.

The remainder of this document is structured as follows: first, in Section 4 and Section 5, we define the abstractions on which the schemes are built, and then the schemes themselves. Then, in Section 6, we lay out the security analyses that support these constructions, including the security requirements for constituent components and the security notions satisfied by hybrid KEMS constructed according to the schemes in the document Section 6.3. Finally, we discuss some "path not taken", related topics that might be of interest to readers, but which are not treated in depth.

2. Requirements Notation

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. Notation

This document is consistent with all terminology defined in [I-D.ietf-pquip-pqt-hybrid-terminology].

The following terms are used throughout this document:

When x is a byte string, we use the notation x[..i] and x[i..] to denote the slice of bytes in x starting from the beginning of x and leading up to index i, including the i-th byte, and the slice the bytes in x starting from index i to the end of x, respectively. For example, if x = [0, 1, 2, 3, 4], then x[..2] = [0, 1] and x[2..] = [2, 3, 4].

4. Cryptographic Dependencies

The generic hybrid PQ/T KEM constructions we define depend on the the following cryptographic primitives:

In the remainder of this section, we describe functional aspects of these mechanisms. The security properties we require in order for the resulting hybrid KEM to be secure are discussed in Section 6.

4.1. Key encapsulation mechanisms

GenerateKeyPair or DeriveKeyPair ek dk ct Encaps Decaps ss == ss

A Key Encapsulation Mechanism (KEMs) comprises the following algorithms:

  • GenerateKeyPair() -> (ek, dk): A randomized algorithm that generates a public encapsulation key ek and a secret decapsulation key dk, each of which are byte strings.

  • DeriveKeyPair(seed) -> (ek, dk): A deterministic algorithm that takes as input a seed seed and generates a public encapsulation key ek and a secret decapsulation key dk, each of which are byte strings.

  • Encaps(ek) -> (ct, ss): A probabilistic encapsulation algorithm, which takes as input a public encapsulation key ek and outputs a ciphertext ct and shared secret ss.

  • Decaps(dk, ct) -> ss: A decapsulation algorithm, which takes as input a secret decapsulation key dk and ciphertext ct and outputs a shared secret ss.

A KEM may also provide a deterministic version of Encaps (e.g., for purposes of testing):

  • EncapsDerand(ek, randomness) -> (ct, shared_secret): A deterministic encapsulation algorithm, which takes as input a public encapsulation key ek and randomness randomness, and outputs a ciphertext ct and shared secret shared_secret.

We assume that the values produced and consumed by the above functions are all byte strings, with fixed lengths:

  • Nseed: The length in bytes of a key seed (input to DeriveKeyPair)

  • Nek: The length in bytes of a public encapsulation key

  • Ndk: The length in bytes of a secret decapsulation key

  • Nct: The length in bytes of a ciphertext produced by Encaps

  • Nss: The length in bytes of a shared secret produced by Encaps or Decaps

4.2. Nominal Groups

g Exp Exp pkA pkB skA skB Exp Exp pkAB pkBA

Nominal groups are an abstract model of elliptic curve groups, over which we instantiate Diffie-Hellman key agreement [ABH_21]. A nominal group comprises a set G together with a distinguished basis element g, an "exponentiation" map, and some auxiliary functions:

  • Exp(p, x) -> q: An algorithm that produces an element q of G from an element p and an integer x.

    • The integers x are called "scalars" to distinguish them from group elements.

    • Exp must respect multiplication in its scalar argument x, so that Exp(Exp(p, x), y) = Exp(p, x * y).

  • RandomScalar(seed) -> k: Produce a uniform pseudo-random scalar from the byte string seed.

  • ElementToSharedSecret(P) -> ss: Extract a shared secret from an element of the group (e.g., by taking the X coordinate of an ellpitic curve point).

We assume that scalars and group elements are represented by byte strings with fixed lengths:

  • Nseed: The length in bytes of a seed (input to RandomScalar)

  • Nscalar: The length in bytes of a scalar

  • Nelem: The length in bytes of a serialized group element

  • Nss: The length in bytes of a shared secret produced by ElementToSharedSecret

The security requirements for groups used with the schemes in this document are laid out in Section 6.3.2.

4.3. Key Derivation Functions

A Key Derivation Function (KDF) is a function that a function that produces keying material based on an input secret and other information.

While KDFs in the literature can typically consume and produce byte strings of arbitrary length, the KDFs used in this document have a simpler form, with a fixed output lengths:

  • Nin - The length in bytes of an input to this KDF.

  • Nout - The length in bytes of an output from this KDF.

  • KDF(input) -> output: Produce a byte string of length Nout from an input byte string.

The fixed sizes are for both security and simplicity.

For instances of the Extract()/Expand() KDF paradigm such as HKDF, we fix the salt and sizes to fit this form.

The security requirements for KDFs used with the schemes in this document are laid out in Section 6.3.3.

4.4. PRG

A pseudorandom generator (PRG) is a deterministic function G whose outputs are longer than its inputs. When the input to G is chosen uniformly at random, it induces a certain distribution over the possible output. The output distribution is pseudorandom if it is indistinguishable from the uniform distribution.

The PRGs used in this document have a simpler form, with a fixed output lengths:

  • Nin - The length in bytes of an input to this PRG.

  • Nout - The length in bytes of an output from this PRG which is longer than Nin.

  • PRG(seed) -> output: Produce a byte string of length Nout from an input byte string seed.

The fixed sizes are for both security and simplicity.

MUST provide the bit-security required to source input randomness for PQ/T components from a seed that is expanded to a output length, of which a subset is passed to the component key generation algorithms.

The security requirements for PRGs used with the schemes in this document are laid out in Section 6.3.4.

5. Hybrid KEM Schemes

In this section, we define three generic schemes for building for hybrid KEMs:

These schemes share a common overall structure, differing mainly in how they compute the final shared secret and the security requirements of their components.

5.1. GHP

The GHP hybrid KEM depends on the following constituent components:

  • KEM_T: A traditional KEM

  • KEM_PQ: A post-quantum KEM

  • PRG: A PRG producing byte strings of length KEM_T.Nseed + KEM_PQ.Nseed (PRG.Nout == KEM_T.Nseed + KEM_PQ.Nseed)

  • KDF: A KDF producing byte strings of length GHP.Nss (KDF.Nout == GHP.Nss)

  • Label - A byte string used to label the specific combination of the above constituents being used.

The KEMs, groups, KDFs, and PRGs MUST meet the security requirements in Section 6.3.

The constants associated with the hybrid KEM are mostly derived from the concatenation of keys and ciphertexts:

Nek = KEM_T.Nek + KEM_PQ.Nek
Ndk = KEM_T.Ndk + KEM_PQ.Ndk
Nct = KEM_T.Nct + KEM_PQ.Nct

The Nseed and Nss constants should reflect the overall security level of the combined KEM, with the following recommended values:

Nseed = max(KEM_T.Nseed, KEM_PQ.Nseed)
Nss = min(KEM_T.Nss, KEM_PQ.Nss)

Given these constituent parts, the GHP hybrid KEM is defined as follows:

def GenerateKeyPair():
    seed = random(Nseed)
    return DeriveKeyPair(seed)

def DeriveKeyPair(seed):
    seed_full = PRG(seed)
    (seed_T, seed_PQ) = split(KEM_T.Nseed, KEM_PQ.Nseed, seed)
    (ek_T, dk_T) = KEM_T.DeriveKeyPair(seed_T)
    (ek_PQ, dk_PQ) = KEM_PQ.DeriveKeyPair(seed_PQ)
    ek_H = concat(ek_T, ek_PQ)
    dk_H = concat(dk_T, dk_PQ)
    return (ek_H, dk_H)

def Encaps(ek):
    (ek_T, ek_PQ) = split(KEM_T.Nek, KEM_PQ.Nek, ek)
    (ss_T, ct_T) = KEM_T.Encap(pk_T)
    (ss_PQ, ct_PQ) = KEM_PQ.Encap(pk_PQ)
    ss_H = KDF(concat(ss_PQ, ss_T, ct_PQ, ct_T, ek_PQ, ek_T, label))
    ct_H = concat(ct_T, ct_PQ)
    return (ss_H, ct_H)

def Decaps(dk, ct):
    (dk_T, dk_PQ) = split(KEM_T.Ndk, KEM_PQ.Ndk, dk)
    ek_T = KEM_T.ToEncaps(dk_T)
    ek_PQ = KEM_PQ.ToEncaps(dk_PQ)

    (ct_T, ct_PQ) = split(KEM_T.Nct, KEM_PQ.Nct, ct)
    ss_T = KEM_T.Decap(dk_T, ct_T)
    ss_PQ = KEM_PQ.Decap(dk_PQ, ct_PQ)

    ss_H = KDF(concat(ss_PQ, ss_T, ct_PQ, ct_T, ek_PQ, ek_T, label))
    return ss_H

5.2. PRE

The PRE hybrid KEM is a performance optimization of the GHP KEM, optimized for the case where encapsulation keys are large and frequently reused. In such cases, hashing the entire encapsulation key is expensive, and the same value is hashed repeatedly. The PRE KEM thus computes an intermediate hash of the hybrid encapsulation key, so that the hash value can be computed once and used across many encapsulation or decapsulation operations.

The PRE KEM is identical to the GHP KEM except for the shared secret computation. One additional KDF is required:

  • KeyHash: A KDF producing byte strings of length GHP.Nss (KeyHash.Nout == GHP.Nss)

The GenerateKeyPair and DeriveKeyPair algorithms for PRE are identical to those of the GHP KEM. The Encaps and Decaps method use a modified shared secret computation:

def Encaps(ek):
    (ek_T, ek_PQ) = split(KEM_T.Nek, KEM_PQ.Nek, ek)
    (ss_T, ct_T) = KEM_T.Encap(pk_T)
    (ss_PQ, ct_PQ) = KEM_PQ.Encap(pk_PQ)

    ekh = KeyHash(concat(ek_T, ek_PQ))
    ss_H = KDF(concat(ss_PQ, ss_T, ct_PQ, ct_T, ekh, label))

    ct_H = concat(ct_T, ct_PQ)
    return (ss_H, ct_H)

def Decaps(dk, ct):
    (dk_T, dk_PQ) = split(KEM_T.Ndk, KEM_PQ.Ndk, dk)
    ek_T = KEM_T.ToEncaps(dk_T)
    ek_PQ = KEM_PQ.ToEncaps(dk_PQ)

    (ct_T, ct_PQ) = split(KEM_T.Nct, KEM_PQ.Nct, ct)
    ss_T = KEM_T.Decap(dk_T, ct_T)
    ss_PQ = KEM_PQ.Decap(dk_PQ, ct_PQ)

    ekh = KeyHash(concat(ek_T, ek_PQ))
    ss_H = KDF(concat(ss_PQ, ss_T, ct_PQ, ct_T, ekh, label))
    return ss_H

5.3. QSF

The QSF hybrid KEM (QSF below) depends on the following constituent components:

  • Group_T: A nominal group

  • KEM_PQ: A post-quantum KEM

  • PRG: A PRG producing byte strings of length Group_T.Nseed + KEM_PQ.Nseed (Expand.Nout == Group_T.Nseed + KEM_PQ.Nseed)

  • KDF: A KDF producing byte strings of length QSF.Nss (KDF.Nout == KDF.Nss)

  • Label - A byte string used to label the specific combination of the above constituents being used.

We presume that Group_T, KEM_PQ, and the KDFs meet the interfaces described in Section 4 and MUST meet the security requirements described in Section 6.3.

The constants associated with the hybrid KEM are mostly derived from the concatenation of keys and ciphertexts:

Nek = Group_T.Nelem + KEM_PQ.Nek
Ndk = Group_T.Nscalar + KEM_PQ.Ndk
Nct = Group_T.Nelem + KEM_PQ.Nct

The Nseed and Nss constants should reflect the overall security level of the combined KEM, with the following recommended values:

Nseed = max(Group_T.Nseed, KEM_PQ.Nseed)
Nss = min(Group_T.Nss, KEM_PQ.Nss)

Given these constituent parts, we define the QSF hybrid KEM as follows:

def GenerateKeyPair():
    seed = random(Nseed)
    return DeriveKeyPair(seed)

def DeriveKeyPair(seed):
    seed_full = PRG(seed)
    (seed_T, seed_PQ) = split(Group_T.Nseed, KEM_PQ.Nseed, seed)

    dk_T = Group_T.RandomScalar(seed_T))
    ek_T = Group_T.Exp(Group_T.g, dk_T)
    (ek_PQ, dk_PQ) = KEM_PQ.DeriveKeyPair(seed_PQ)

    ek_H = concat(ek_T, ek_PQ)
    dk_H = concat(dk_T, dk_PQ)
    return (ek_H, dk_H)

def Encaps(ek):
    (ek_T, ek_PQ) = split(Group_T.Nek, KEM_PQ.Nek, ek)

    sk_E = Group_T.RandomScalar(random(GroupT.nseed))
    ct_T = Group_T.Exp(GroupT.g, sk_E)
    ss_T = Group_T.ElementToSharedSecret(Group_T.Exp(ek_T, sk_E))
    (ss_PQ, ct_PQ) = KEM_PQ.Encap(ek_PQ)

    ss_H = KDF(concat(ss_PQ, ss_T, ct_T, ek_T, Label))
    ct_H = concat(ct_T, ct_PQ)
    return (ss_H, ct_H)

def Decaps(dk, ct):
    (dk_T, dk_PQ) = split(Group_T.Ndk, KEM_PQ.Ndk, dk)
    (ct_T, ct_PQ) = split(Group_T.Nct, KEM_PQ.Nct, ct)

    ek_T = Group_T.ToEncaps(dk_T)
    ek_PQ = KEM_PQ.ToEncaps(dk_PQ)

    ss_T = Group_T.ElementToSharedSecret(Group_T.Exp(ct_T, dk_T))
    ss_PQ = KEM_PQ.Decap(dk_PQ, ct_PQ)

    ss_H = KDF(concat(ss_PQ, ss_T, ct_T, ek_T, Label))
    return ss_H

6. Security Considerations

Hybrid KEM constructions aim to provide security by combining two or more schemes so that security is preserved if all but one schemes are replaced by an arbitrarily bad scheme. Informally, these hybrid KEMs are secure if the KDF is secure, and either the traditional component is secure, or the post-quantum KEM is secure: this is the 'hybrid' property.

6.1. Security Properties

6.1.1. IND-CCA Security

Also known as IND-CCA2 security for general public key encryption, for KEMs that encapsulate a new random 'message' each time.

The notion of INDistinguishability against Chosen-Ciphertext Attacks (IND-CCA) [RS92] is now widely accepted as the standard security notion for asymmetric encryption schemes. IND-CCA security requires that no efficient adversary can recognize which of two messages is encrypted in a given ciphertext, even if the two candidate messages are chosen by the adversary himself.

6.1.2. Ciphertext Second Preimage Resistant (C2PRI) Security

Also known in the literature as ciphertext collision resistance (CCR).

The notion where, even if a KEM has broken IND-CCA security (either due to construction, implementation, or other), its internal structure, based on the Fujisaki-Okamoto transform, guarantees that it is impossible to find a second ciphertext that decapsulates to the same shared secret K: this notion is known as ciphertext second preimage resistance (C2SPI) for KEMs [XWING]. The same notion has also been described as chosen ciphertext resistance elsewhere [CDM23].

6.2. Binding Properties

It is often useful for a KEM to have certain "binding properties", by which certain parameters determine certain others [CDM23]. These properties are referred to with labels of the form X-BIND-P-Q. For example, LEAK-BIND-K-PK means that for a given shared secret (K), there is a unique encapsulation key (PK) that could have produced it, even if all of the secrets involved are given to the adversary after the encapsulation operation is completed (LEAK).

The property LEAK-BIND-K,PK-CT is related to the C2PRI property discussed above. Related to the ciphertext collision-freeness of the underlying PKE scheme of a FO-transform KEM. Also called ciphertext collision resistance.

6.3. Security Requirements for Constituent Components

  • TODO: We need to provide more thorough description, and verify that these requirements align with the requirements of the security proofs in the literature, especially [GHP2018] and [XWING].

6.3.1. Security Requirements for KEMs

Component KEMs MUST be IND-CCA-secure [GHP2018] [XWING].

For instances of QSF, the component KEM MUST also be ciphertext second preimage resistant (C2PRI) [XWING]: this allows the component KEM encapsulation key and ciphertext to be left out from the KDF input.

6.3.2. Security Requirements for Groups

The groups MUST be modelable as nominal groups in which the strong Diffie-Hellman problem holds [ABH_21] [XWING].

The Montgomery curves Curve25519 and Curve448 have been shown to be modelable as nominal groups in [ABH_21] as well as showing the X25519() and X448() functions respectively pertain to the nominal group exp(X, y) function, specifically clamping secret keys when they are generated, instead of clamping secret keys together with exponentiation.

6.3.3. Security Requirements for KDFs

KDFs MUST be secure pseudorandom functions (PRFs) when keyed with the shared secret output from the post-quantum IND-CCA-secure KEM component algorithm in QSF [XWING] or any of the component IND-CCA-secure KEMs when used in KitchenSink [GHP2018] or PreHash.

KDFs must be secure instances of random oracles in the ROM and QROM [GHP2018] [XWING]. Proofs of indifferentiability from random oracles [MRH03] give good confidence here, as any function proven indifferentiable from a random oracle is resistant against collision, first, and second preimage attacks . An indifferentiability bound guarantees security against specific attacks. Although indifferentiability does not capture all properties of a random oracle [RSS11], indifferentiability still remains the best way to rule out structural attacks.

Sponge-based constructions such as SHA-3 have been shown to be indifferentiable against classical [BDP_08] as well as quantum adversaries [ACM_25].

HKDF has been shown to be indifferentiable from a random oracle under specific constraints [LBB20]:

  • that HMAC is indifferentiable from a random oracle, which for HMAC-SHA-256 has been shown in [DRS_13], assuming the compression function underlying SHA-256 is a random oracle, which it is indifferentiably when used prefix-free.

  • the values of HKDF's IKM input do not collide with values of info || 0x01. This MUST be enforced by the concrete instantiations that use HKDF as its KDF.

The choice of the KDF security level SHOULD be made based on the security level provided by the constituent KEMs. The KDF SHOULD at least have the security level of the strongest constituent KEM.

6.3.4. Security Requirements for PRGs

The functions used to expand a key seed to multiple key seeds is closer to a pseudorandom generator (PRG) in its security requirements [AOB_24]. A secure PRG is an algorithm PRG : {0, 1}^n → {0, 1}^m, such that no polynomial-time adversary can distinguish between PRG(r) (for r $← {0, 1}^n) and a random z $← {0, 1}^m [Rosulek]. The uniform string r ∈ {0, 1}^n is called the seed of the PRG.

A PRG is not to be confused with a random (or pseudorandom) number generator (RNG): a PRG requires the seed randomness to be chosen uniformly and extend it; an RNG takes sources of noisy data and transforms them into uniform outputs.

A PRG is a particular mode of use of a random oracle [BDP_11]. Examples used in such a manner include SHAKE256.

6.4. Security Properties of Hybrid KEMs

All generic constructions in this document produce IND-CCA-secure KEMs when correctly instantiated concretely with cryptographic components that meet the respective security requirements. Any changes to the routines, including key generation/derivation, are not guaranteed to produce secure results.

6.5. Other Considerations

6.5.1. Domain Separation

ASCII-encoded bytes provide oracle cloning [BDG2020] in the security game via domain separation. The IND-CCA security of hybrid KEMs often relies on the KDF function KDF to behave as an independent random oracle, which the inclusion of the label achieves via domain separation [GHP2018].

By design, the calls to KDF in these constructions and usage anywhere else in higher level protoocl use separate input domains unless intentionally duplicating the 'label' per concrete instance with fixed paramters. This justifies modeling them as independent functions even if instantiated by the same KDF. This domain separation is achieved by using prefix-free sets of label values. Recall that a set is prefix-free if no element is a prefix of another within the set.

Length diffentiation is sometimes used to achieve domain separation but as a technique it is brittle and prone to misuse [BDG2020] in practice so we favor the use of an explicit post-fix label.

6.5.2. Fixed-length

Variable-length secrets are generally dangerous. In particular, using key material of variable length and processing it using hash functions may result in a timing side channel. 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.

Furthermore, [AVIRAM] identified a risk of using variable-length secrets when the hash function used in the key derivation function is no longer collision-resistant.

If concatenation were to be used with values that are not fixed-length, a length prefix or other unambiguous encoding would need to be used to ensure that the composition of the two values is injective and requires a mechanism different from that specified in this document.

Therefore, this specification MUST only be used with algorithms which have fixed-length shared secrets.

7. Out of Scope

Considerations that were considered and not included in these designs:

Anonymity [GMP22], Deniability, Obfuscation, other forms of key-robustness or binding [GMP22], [CDM23]

TODO: deniable KEM cite and Kemeleon paper cite

7.1. More than two component KEMs

Design team decided to restrict the space to only two components, a traditional and a post-quantum KEM.

7.2. Parameterized output length

Not analyzed as part of any security proofs in the literature, and a complicatation deemed unnecessary.

8. References

8.1. Normative References

[FIPS202]
"SHA-3 standard :: permutation-based hash and extendable-output functions", National Institute of Standards and Technology (U.S.), DOI 10.6028/nist.fips.202, , <https://doi.org/10.6028/nist.fips.202>.
[FIPS203]
"Module-lattice-based key-encapsulation mechanism standard", National Institute of Standards and Technology (U.S.), DOI 10.6028/nist.fips.203, , <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, , <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, , <https://www.rfc-editor.org/rfc/rfc8174>.

8.2. Informative References

[ABH_21]
Joël Alwen, Bruno Blanchet, Eduard Hauck, Eike Kiltz, Benjamin Lipp, and Doreen Riepel, "Analysing the HPKE standard.", .
[ABR01]
Michel Abdalla, Mihir Bellare, and Phillip Rogaway, "The Oracle Diffie-Hellman Assumptions and an Analysis of DHIES", .
[ACM_25]
"The Sponge is Quantum Indifferentiable", , <https://eprint.iacr.org/2025/731.pdf>.
[ANSIX9.62]
ANS, "Public Key Cryptography for the Financial Services Industry: the Elliptic Curve Digital Signature Algorithm (ECDSA)", ANS X9.62-2005, .
[AOB_24]
"Formally verifying Kyber Episode V: Machine-checked IND-CCA security and correctness of ML-KEM in EasyCrypt", , <https://eprint.iacr.org/2024/843.pdf>.
[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", , <https://mailarchive.ietf.org/arch/msg/tls/F4SVeL2xbGPaPB2GW_GkBbD_a5M/>.
[BDG2020]
"Separate Your Domains: NIST PQC KEMs, Oracle Cloning and Read-Only Indifferentiability", , <https://eprint.iacr.org/2020/241.pdf>.
[BDP_08]
"On the Indifferentiability of the Sponge Construction", , <https://www.iacr.org/archive/eurocrypt2008/49650180/49650180.pdf>.
[BDP_11]
"Cryptographic sponge functions", , <https://keccak.team/files/CSF-0.1.pdf>.
[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", , <https://eprint.iacr.org/2023/1933.pdf>.
[DRS_13]
"To Hash or Not to Hash Again? (In)differentiability Results for H^2 and HMAC", , <https://eprint.iacr.org/2013/382.pdf>.
[FIPS186]
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Acknowledgments

TODO acknowledge.

Authors' Addresses

Deirdre Connolly
SandboxAQ
Richard Barnes
Cisco