| Internet-Draft | ML-KEM IKEv2 | November 2024 |
| Kampanakis & Ravago | Expires 8 May 2025 | [Page] |
NIST recently standardized ML-KEM, a new key encapsulation mechanism, which can be used for quantum-resistant key establishment. This draft specifies how to use ML-KEM as an additional key exchange in IKEv2 along with traditional key exchanges. This Post-Quantum Traditional Hybrid Key Encapsulation Mechanism approach allows for negotiating IKE and Child SA keys which are safe against cryptanalytically-relevant quantum computers and theoretical weaknesses in ML-KEM.¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶
This Internet-Draft will expire on 8 May 2025.¶
Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
A Cryptanalytically-relevant Quantum Computer (CRQC), if it became a reality, could threaten today's public key establishment algorithms. Someone storing encrypted communications that use (Elliptic Curve) Diffie-Hellman ((EC)DH) to establish keys could decrypt these communications in the future after a CRQC became available to them. Such communications include Internet Key Exchange Protocol Version 2 (IKEv2).¶
To address this concern, the Mixing Preshared Keys in IKEv2 specification [RFC8784] introduced Post-quantum Preshared Keys as a temporary option for stirring a pre-shared key of adequate entropy in the derived Child SA encryption keys in order to provide quantum-resistance. This specification can be used in conjunction with PPK as defined in [RFC8784].¶
Since then, NIST has been working on a public project [NIST-PQ] for standardizing quantum-resistant algorithms which include key encapsulation and signatures. At the end of Round 3, they picked Kyber as the first Key Encapsulation Mechanism (KEM) for standardization [I-D.draft-cfrg-schwabe-kyber-04]. Kyber was then standardized as Module-Lattice-based Key-Encapsulation Mechanism (ML-KEM) in 2024 [FIPS203].¶
As post-quantum public keys and ciphertexts may make UDP packet sizes larger than common network Maximum Transport Units (MTU), the Intermediate Exchange in IKEv2 document [RFC9242] defined how to do additional large message exchanges by using new IKE_INTERMEDIATE or IKE_FOLLOWUP_KE messages which can be fragmented at the IKEv2 layer before causing IP fragmentation [RFC7383]. Because [RFC9242] messages can only be used after IKE_SA_INIT, if a PQ KEM does not fit inside IKE_SA_INIT without causing IP fragmentation, then it should be used after IKE_SA_INIT as an additional key establishment algorithm. The Multiple Key Exchanges in IKEv2 specification [RFC9370] defined how to do up to seven additional key exchanges by using IKE_INTERMEDIATE or IKE_FOLLOWUP_KE messages and by deriving new SKEYSEED and KEYMAT key materials. This specification was created to enable new post-quantum key exchanges to be used in the derived IKE and Child SA keys and provide quantum resistance.¶
This document describes how ML-KEM can be used as a quantum-resistant KEM in IKEv2 in an IKE_SA_INIT key exchange, or in one additional IKE_INTERMEDIATE or IKE_FOLLOWUP_KE key exchange after an initial IKE_SA_INIT or CREATE_CHILD_SA respectively. This approach of combining a quantum-resistant with a traditional algorithm, is commonly called Post-Quantum Traditional (PQ/T) Hybrid [I-D.ietf-pquip-pqt-hybrid-terminology-04] key exchange and combines the security of a well-established algorithm with relatively new quantum-resistant algorithms. The result is a new Child SA key or an IKE or Child SA rekey with keying material which is safe against a CRQC. Another use of a PQ/T Hybrid key exchange in IKEv2 is for someone that wants to exchange keys using the high security parameter of ML-KEM. As these may not fit in common network packet payload sizes, they will need to be sent in a IKE_FOLLOWUP_KE or CREATE_CHILD_SA key exchange which can be fragmented. This specification is a profile of the Multiple Key Exchanges in IKEv2 specification [RFC9370] and registers new algorithm identifiers for ML-KEM key exchanges in IKEv2.¶
In the context of the NIST Post-Quantum Cryptography Standardization Project [NIST-PQ], key exchange algorithms are formulated as KEMs, which consist of three steps:¶
'KeyGen() -> (pk, sk)': A probabilistic key generation algorithm, which generates a public / encapsulation key 'pk' and a private / decapsulation key 'sk'. The resulting pk is sent to the responder in the KEi payload.¶
'Encaps(pk) -> (ct, ss)': A probabilistic encapsulation algorithm, which takes as input a public key 'pk' (from the KEi) and outputs a ciphertext 'ct' and shared secret 'ss'. The 'ct' is sent back to intiator in the KEr payload.¶
'Decaps(sk, ct) -> ss': A decapsulation algorithm, which takes as input a secret key 'sk' and ciphertext 'ct' (from the KEr) and outputs a shared secret 'ss', or in some rare cases a distinguished error value.¶
ML-KEM is a standardized lattice-based key encapsulation mechanism [FIPS203]. It uses Module Learning with Errors as its underlying primitive which is a structured lattices variant that offers good performance and relatively small and balanced key and ciphertext sizes. ML-KEM was standardized with three parameters, ML-KEM-512, ML-KEM-768, and ML-KEM-1024. These were mapped by NIST to the three security levels defined in the NIST PQC Project, Level 1, 3, and 5. These levels correspond to the hardness of breaking AES-128, AES-192 and AES-256 respectively.¶
This specification introduces ML-KEM-512, ML-KEM-768 and ML-KEM-1024 to IKEv2 key exchanges as conservative security level parameters which will not have material performance impact on IKEv2/IPsec tunnels which usually stay up for long periods of time and transfer sizable amounts of data. Since the ML-KEM-768 and ML-KEM-1024 public key and ciphertext sizes can exceed the typical network MTU, these key exchanges could require two or three network IP packets from both the initiator and the responder.¶
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.¶
ML-KEM key exchanges can be negotiated in IKE_INTERMEDIATE or IKE_FOLLOWUP_KE messages as defined in the Multiple Key Exchanges in IKEv2 specification [RFC9370]. We summarize them here for completeness.¶
Section 2.2.2 of [RFC9370] specifies that KEi(0), KEr(0) are regular key exchange messages in the first IKE_SA_INIT exchange which end up generating a set of keying material, SK_d, SK_a[i/r], and SK_e[i/r]. The peers then perform an IKE_INTERMEDIATE exchange, carrying new Key Exchange payloads. These are protected with the SK_e[i/r] and SK_a[i/r] keys which were derived from the IKE_SA_INIT as per Section 3.3.1 of the Intermediate Exchange in IKEv2 document [RFC9242]. The initiator generates an ML-KEM keypair (sk, pk) using KeyGen(), and sends the public key (pk) to the responder inside a KEi(1) payload. The responder will encapsulate a shared secret ss using Encaps(pk) and the resulting ciphertext (ct) is sent to initiator using the KEr(1). After the initiator receives KEr(1), it will decapsulate it using Decaps(sk, ct). Both Encaps and Decaps return the shared secret (ss) and both peers have a common shared secret SK(1) at the end of this KE(1) exchange. The ML-KEM shared secret is stirred into new keying material SK_d, SK_a[i/r], and SK_e[i/r] as defined in Section 2.2.2 of the Multiple Key Exchanges in IKEv2 document [RFC9370]. Afterwards the peers continue to the IKE_AUTH exchange phase as defined in Section 3.3.2 of the Intermediate Exchange in IKEv2 specification [RFC9242].¶
ML-KEM can also be used to create or rekey a Child SA or rekey the IKE SA by using a IKE_FOLLOWUP_KE message after a CREATE_CHILD_SA message. After the ML-KEM additional key exchange KE(1) has taken place using and IKE_FOLLOWUP_KE exchange, the IKE or Child SA are rekeyed by stirring the new ML-KEM shared secret SK(1) in SKEYSEED and KEYMAT as specified in Section 2.2.4 of [RFC9370].¶
ML-KEM-768 and ML-KEM-1024 public keys and ciphertexts may make UDP packet sizes larger typical network MTUs (1500 bytes). Thus, IKE_INTERMEDIATE or IKE_FOLLOWUP_KE messages carrying ML-KEM public keys and ciphertexts may be IKEv2 fragmented as per the IKEv2 Message Fragmentation specification [RFC7383].¶
Although, this document focuses on using ML-KEM as the second key exchange in a PQ/T Hybrid KEM [I-D.ietf-pquip-pqt-hybrid-terminology-04] scenario, ML-KEM-512 and ML-KEM-768 Key Exchange Method identifiers TBD35 and TBD36 respectively MAY be used in IKE_SA_INIT as a quantum-resistant-only key exchange. The encapsulation key and ciphertext sizes for these ML-KEM parameters may not push the UDP packet to size larger than typical network MTUs of 1500 bytes. ML-KEM-1024 Key Exchange Method identifier TBD37 SHOULD NOT be used in IKE_SA_INIT messages which could exceed typical network MTUs and cannot be IKEv2 fragmented.¶
The KE payload is shown below and the fields inside it has meaning as defined in Section 3.4 of the IKEv2 standard [RFC7296]:¶
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload |C| RESERVED | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key Exchange Method Num | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Key Exchange Data ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
¶
The Key Exchange Data from the initiator to the responder contains the public key (pk) from the KeyGen() operation encoded as a raw byte array (i.e., output of ByteEncode) as defined in Section 7.1 of Module-Lattice-Based KEM standard [FIPS203].¶
The Key Exchange Data from the responder to the initiator contains the ciphertext (ct) from the Encaps operation encoded as a raw byte array.¶
Table 1 shows the Payload Length, Key Exchange Method Num identifier and the Key Exchange Data Size in octets for Key Exchange Payloads from the initiator and the responder for the ML-KEM variants specified in this document.¶
| KEM | Payload Length (initiator / responder) | Key Exchange Method Num | Data Size in Octets (initiator / responder) |
|---|---|---|---|
| ML-KEM-512 | 808 / 776 | TBD35 | 800 / 768 |
| ML-KEM-768 | 1192 / 1096 | TBD36 | 1184 / 1088 |
| ML-KEM-1024 | 1576 / 1576 | TBD37 | 1568 / 1568 |
Receiving and handling of malformed ML-KEM public keys or ciphertexts SHOULD follow the input validation described in the Module-Lattice-Based KEM standard [FIPS203].¶
Responders SHOULD perform the checks specified in section 7.2 of the Module-Lattice-Based KEM standard [FIPS203] before the Encaps(pk) operation. If the checks fail, the responder SHOULD send a Notify payload of type INVALID_SYNTAX as a response to the request from initiator.¶
Initiators SHOULD perform the Ciphertext type check specified in section 7.3 of the Module-Lattice-Based KEM standard [FIPS203] before the Decaps(sk, ct) operation. If the check fails, the initiator MUST reject the ciphertext and MUST fail the exchange. In this case, the initiator MAY send a Notify payload of type INVALID_SYNTAX to the responder as a separate INFORMATIONAL exchange, usually with no other payloads. This is an exception for the general rule of not starting new exchanges based on errors in responses.¶
Note that during decapsulation, ML-KEM uses implicit rejection which leads the decapsulating entity to implicitly reject the decapsulated shared secret by setting it to a hash of the ciphertext together with a random value stored in the ML-KEM secret when the re-encrypted shared secret does not match the original one.¶
All security considerations from [RFC9242] and [RFC9370] apply to the ML-KEM exchanges described in this specification.¶
The main security property for KEMs standardized by NIST is indistinguishability under adaptive chosen ciphertext attacks (IND-CCA2), which means that shared secret values should be indistinguishable from random strings even given the ability to have 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 weaker security notion is indistinguishability under chosen plaintext attacks (IND-CPA), which means that the shared secret values should be indistinguishable from random strings given a copy of the public key. IND-CPA roughly corresponds to security against a passive attacker, and sometimes corresponds to one-time key exchange. As with (EC)DH keys today, generating an ephemeral key exchange keypair for ECDH and ML-KEM is still REQUIRED per connection by this specification (IND-CPA security).¶
The ML-KEM public key generated by the initiator and the ciphertext generated by the responder use randomness (usually a seed) which MUST be independent of any other random seed used in the IKEv2 negotiation. For example, at the initiator, the ML-KEM and (EC)DH keypairs used in a PQ/T Hybrid key exchange should not be generated from the same seed.¶
SKEYSEED and KEYMAT in this specification are generated from PQ/T Hybrid key exchanges by using shared secrets, nonces, and SPIs with a pseudorandom function as defined in [RFC9370]. As discussed in [PQ-PROOF2], such PQ/T Hybrid key derivations are IND-CPA, but not proven to be IND-CCA2 secure although the keys could be reused if the nonces are never reused.¶
IANA is requested to assign three values for the names "mlkem-512", "mlkem-768", and "mlkem-1024" in the IKEv2 "Transform Type 4 - Key Exchange Method Transform IDs" and has listed this document as the reference. The Recipient Tests field should also point to this document:¶
| Number | Name | Status | Recipient Tests | Reference |
|---|---|---|---|---|
| TBD35 | ml-kem-512 | [TBD, this draft, Section 2.3], | [TBD, this draft] | |
| TBD36 | ml-kem-768 | [TBD, this draft, Section 2.3], | [TBD, this draft] | |
| TBD37 | ml-kem-1024 | [TBD, this draft, Section 2.3], | [TBD, this draft] | |
| 38-1023 | Unassigned |
The authors would like to thank Valery Smyslov, Graham Bartlett, Scott Fluhrer, Ben S, and Leonie Bruckert for their valuable feedback.¶