Transport Layer Security M. U. Sardar Internet-Draft TU Dresden, Germany Intended status: Informational 11 June 2026 Expires: 13 December 2026 Analysis of Hybrid Key Exchange and Standalone ML-KEM in TLS 1.3 draft-usama-tls-risks-of-mlkem-03 Abstract The draft presents _symbolic_ and _computational_ analysis of hybrid key exchange and standalone ML-KEM. In our observation, we believe that hybrid key exchange is preferable over standalone ML-KEM until a powerful CRQC exists which breaks *all* the bits of pre-quantum. This draft also offers opinions of the IETF participants and some preliminary discussion to help the developers and policymakers make informed choices. Finally, it offers minimal implementation guidance for hybrids. About This Document This note is to be removed before publishing as an RFC. The latest revision of this draft can be found at https://muhammad- usama-sardar.github.io/risks-of-mlkem/draft-usama-tls-risks-of- mlkem.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-usama-tls-risks-of-mlkem/. Discussion of this document takes place on the Transport Layer Security Working Group mailing list (mailto:tls@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/tls/. Subscribe at https://www.ietf.org/mailman/listinfo/tls/. Source for this draft and an issue tracker can be found at https://github.com/muhammad-usama-sardar/risks-of-mlkem. Status of This Memo 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/. Sardar Expires 13 December 2026 [Page 1] Internet-Draft Analysis of Hybrid Key Exchange and Stan June 2026 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 13 December 2026. Copyright Notice Copyright (c) 2026 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Objectives . . . . . . . . . . . . . . . . . . . . . . . 3 1.1.1. Summary of Formal Methods Works . . . . . . . . . . . 3 1.1.2. Capturing the Opinions of IETF . . . . . . . . . . . 4 1.1.3. Minimal Implementation Guidance for Hybrids . . . . . 4 1.2. Motivation . . . . . . . . . . . . . . . . . . . . . . . 4 1.3. Intuition . . . . . . . . . . . . . . . . . . . . . . . . 4 1.3.1. Expected Learning . . . . . . . . . . . . . . . . . . 5 2. Conventions and Definitions . . . . . . . . . . . . . . . . . 5 2.1. Key Exchange and Key Encapsulation . . . . . . . . . . . 6 3. Computational Analysis . . . . . . . . . . . . . . . . . . . 6 3.1. Hybrids . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.2. Standalone ML-KEM . . . . . . . . . . . . . . . . . . . . 6 4. Symbolic Analysis . . . . . . . . . . . . . . . . . . . . . . 7 4.1. Minimum Viable Modeling . . . . . . . . . . . . . . . . . 7 4.2. Hybrid Key Exchange . . . . . . . . . . . . . . . . . . . 8 4.3. Standalone ML-KEM . . . . . . . . . . . . . . . . . . . . 8 4.4. Results . . . . . . . . . . . . . . . . . . . . . . . . . 8 5. Implementation-Facing Negative Cases . . . . . . . . . . . . 9 5.1. Argument Matrix for Implementation Review . . . . . . . . 10 6. Issues That Formal Methods Probably Cannot Solve . . . . . . 12 6.1. Which breaks first: ML-KEM-768 or X25519? . . . . . . . . 12 6.2. Does CRQC Break All Bits of Pre-quantum? . . . . . . . . 12 6.3. Policy/Regulations . . . . . . . . . . . . . . . . . . . 12 6.4. Recommendation of Designers . . . . . . . . . . . . . . . 12 Sardar Expires 13 December 2026 [Page 2] Internet-Draft Analysis of Hybrid Key Exchange and Stan June 2026 6.5. Thorough Review . . . . . . . . . . . . . . . . . . . . . 13 6.6. 'Significantly Harder' Argument . . . . . . . . . . . . . 13 6.7. Urgency . . . . . . . . . . . . . . . . . . . . . . . . . 14 6.8. "Cost" . . . . . . . . . . . . . . . . . . . . . . . . . 14 6.9. Is Publication Necessary? . . . . . . . . . . . . . . . . 15 6.10. Shiny New Crypto . . . . . . . . . . . . . . . . . . . . 15 6.11. Formal Mapping of FIPS to IETF BCP14 . . . . . . . . . . 15 6.12. Outstanding NIST Comments . . . . . . . . . . . . . . . . 16 6.13. Too Early . . . . . . . . . . . . . . . . . . . . . . . . 16 6.14. Patents . . . . . . . . . . . . . . . . . . . . . . . . . 16 6.15. Implementation Bugs . . . . . . . . . . . . . . . . . . . 16 6.16. Depth of Hybrids? . . . . . . . . . . . . . . . . . . . . 16 7. Security Considerations . . . . . . . . . . . . . . . . . . . 16 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 9.1. Normative References . . . . . . . . . . . . . . . . . . 17 9.2. Informative References . . . . . . . . . . . . . . . . . 17 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 19 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20 1. Introduction Readers are assumed to be familiar with [NistFips203], [I-D.ietf-tls-rfc8446bis], and [I-D.ietf-tls-mlkem]. Please note that the draft has currently several hyperlinks. 1.1. Objectives The draft serves three objectives: * Summary of formal methods (symbolic and computational) works for hybrid key exchange and standalone ML-KEM * Capturing the opinions of the IETF participants to avoid repetition * Minimal implementation guidance for hybrids 1.1.1. Summary of Formal Methods Works The draft covers the formal methods for the security considerations of [I-D.ietf-tls-mlkem]. This includes _symbolic_ and _computational_ analysis (to be interpreted as in SoK (https://eprint.iacr.org/2019/1393.pdf)) of integration of standalone ML-KEM in the context of TLS. Specifically, it covers the formal analysis [FATT-CHANCE] in ProVerif on the potential issue of Sardar Expires 13 December 2026 [Page 3] Internet-Draft Analysis of Hybrid Key Exchange and Stan June 2026 asymmetry. The analysis confirms that asymmetry is not a problem. 1.1.2. Capturing the Opinions of IETF The draft also aims to reduce the endless repetition of arguments from both sides presented on several lists by documenting these arguments so they can simply be referred to. This draft captures what _we_ understand them to be saying. The goal is that people can be a bit more fair and balanced to acknowledge others' opinions, rather than stating their opinion as universal truth, or else present substantial scientific evidence if they claim their opinion to be universal truth. In our understanding, the question largely boils down to: whether the traditional or post-quantum cryptographic primitive breaks first? 1.1.3. Minimal Implementation Guidance for Hybrids The implementation concern is not only whether ML-KEM is secure as a primitive, but whether a TLS deployment can show that both hybrid components were validated, transcript-bound, and fail-closed under the negotiated group. 1.2. Motivation [rfc3552] requires to document the risks in the security considerations. To support those requirements for [I-D.ietf-tls-mlkem], this draft aims to formally study the security of standalone ML-KEM in TLS 1.3. 1.3. Intuition Leaking out the ECDHE key from hybrid key exchange should downgrade the security to the level of a standalone ML-KEM. Therefore, hybrid key exchange is _in general_ more secure, unless: * ECDHE is fully broken, in which case it still falls equivalent to standalone ML-KEM, * in the _hypothetical_ scenario that there is an implementation bug in the ECDHE part which is triggered only in composition. We have not yet seen any concrete evidence of such a scenario on the list. We believe that _in general_: 1. Migration from ECDHE to hybrid key exchange is security improvement. 2. Migration from hybrid key exchange to standalone ML-KEM is security regression, unless CRQC exists to break all ECC keys. Sardar Expires 13 December 2026 [Page 4] Internet-Draft Analysis of Hybrid Key Exchange and Stan June 2026 1.3.1. Expected Learning We believe formal methods can provide additional value for security considerations of this draft in order to maintain the high cryptographic assurance of TLS. Since we have no guarantee on whether ECDHE will break before ML-KEM, it seems appropriate to do thorough cryptographic analysis. We believe the Harvest Now, Decrypt Later (HNDL) attack applies equally well to standalone ML-KEM. Adversary can record all traffic and decrypt it when ML-KEM is broken. The opinions of WG participants here vary from "ML-KEM is secure" to "ML-KEM is probably already secrectly broken." Formal methods can operate under the assumption that ML-KEM is secure, and focus on the integration of ML-KEM in TLS under this assumption. * As an example, formal methods can help justify design choices, such as the preference for hybrid key exchanges. It can also help identify all the assumptions under which the properties hold. * As a relevant data point in the context of standardization, LAKE WG has done formal analysis for EDHOC-PSK with KEM (ref (https://mailarchive.ietf.org/arch/msg/ lake/2XGOI9OCwylJUfSCasvvwM2FXmw/)). * _Computational_ analysis (cf. SoK (https://eprint.iacr.org/2019/1393.pdf)) -- using tools such as CryptoVerif -- seems like a reasonable approach to ensure security of ML-KEM in TLS, such as binding shared secret ss to the TLS transcript hash. We believe that the focus of symbolic analysis ought to be on the *integration* details (transcript binding, key schedule, agreement) for standalone ML-KEM in the context of TLS, rather than the *primitive* itself. 2. Conventions and Definitions 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. * Symbolic analysis: see SoK (https://eprint.iacr.org/2019/1393.pdf) Sardar Expires 13 December 2026 [Page 5] Internet-Draft Analysis of Hybrid Key Exchange and Stan June 2026 * Computational analysis: see SoK (https://eprint.iacr.org/2019/1393.pdf) * Standalone ML-KEM refers to [I-D.ietf-tls-mlkem]. * Hybrid key exchange refers to [I-D.ietf-tls-ecdhe-mlkem] and [I-D.ietf-tls-hybrid-design]. We believe that symbolic and computational models are complementary and not a substitute of each other. 2.1. Key Exchange and Key Encapsulation In traditional key exchange (DHKE), both endpoints send their public key shares g^x and g^y to derive a shared secret g^xy. In key encapsulation, there is essentially only one endpoint (say client) which generates the key pair (dk,ek) where dk represents the _secret decapsulation key_ and ek represents the _public encapsulation key_. In a KEM, only one of the endpoints (client in above example) sends the public encapsulation key ek and the peer (server) sends a ciphertext ct. 3. Computational Analysis 3.1. Hybrids Technically, a proof of [I-D.ietf-tls-hybrid-design-09] is done in the computational model using CryptoVerif (cf. ref (https://bblanche.gitlabpages.inria.fr/publications/ BlanchetJacommeCSF24.pdf)). As per list discussion, it appears that the proof applies to the latest version of the spec [I-D.ietf-tls-hybrid-design], as there seem to be no substantive changes from the perspective of formal proof. 3.2. Standalone ML-KEM Some existing computational analysis for standalone ML-KEM in TLS include this (https://eprint.iacr.org/2021/844) and this (https://eprint.iacr.org/2024/1360). Both are based on pen-and-paper (computational) proofs. At the symbolic level, some analysis -- such as this (https://eprint.iacr.org/2022/1111.pdf) for KEMTLS in Tamarin -- exists. In our understanding, both client and server encapsulate, which may bring the symmetry. Sardar Expires 13 December 2026 [Page 6] Internet-Draft Analysis of Hybrid Key Exchange and Stan June 2026 4. Symbolic Analysis For brevity, we omit other assumptions in the properties below and focus on the difference. This assumes hybrid constructor to be secure. 4.1. Minimum Viable Modeling Based on the discussion on list, simply replacing ideal DHKE by ideal ML-KEM in the formal model is not very useful. We ought to focus on the more security-critical questions about integration of ML-KEM in TLS. We present a few high-level observations to consider for security considerations of [I-D.ietf-tls-mlkem]: * The model ought to consider that any agent could have initiated the TLS, rather than assigning the agents with static roles of client and server in the model. When agents are assigned non- static roles, it would be interesting to see whether the asymmetry issue becomes visible in some property. We consider it very critical for security considerations of [I-D.ietf-tls-mlkem] and this is the key point of this draft. * Different failure modes proposed on list can be modeled. * A large part of the problem is the careful investigation of what to model, under what threat model, under what system model, under what implementation scenarios etc. We believe some of this is important for security considerations of [I-D.ietf-tls-mlkem]. * It will be interesting to see some analysis about any subtle cases where hybrid key exchange in TLS is _not_ at least as good as standalone ML-KEM in TLS. Our understanding is that some participants would like to see some statement on the comparison since hybrid key exchange is the de facto industry standard. * We believe brainstorming about some robustness (vs. security) properties would also be useful. Even if the security properties hold, does standalone ML-KEM make side-channel leakage easier? This might be a valuable consideration for the implementers. * Analysis may be helpful to ensure that the changes -- such as the removal of hash function (cf. Appendix C.1, bullet 3 in [NistFips203]) -- from Kyber to ML-KEM preserve the security proofs of Kyber. Any analysis on these or related security and robustness matters is very welcome. Sardar Expires 13 December 2026 [Page 7] Internet-Draft Analysis of Hybrid Key Exchange and Stan June 2026 4.2. Hybrid Key Exchange Hybrid key exchange still maintains the DHKE part. From formal (symbolic) analysis perspective, g^x and g^y are still sent in hybrid key exchange, g^xy is still computed and we believe the commutativity property is applicable for that part as-is. From formal (symbolic) analysis perspective, ML-KEM is complementary to that. Specifically, from Section 4 of [I-D.ietf-tls-ecdhe-mlkem], for the symbolic analysis, X25519MLKEM768 in TLS may be viewed as: client's key_exchange value = ek || gx server's key_exchange value = ct || gy shared secret = ss || gxy Formally, the property hybrid key exchange provides is: Security properties of TLS hold unless *both* `gxy` and `ss` are available to the adversary. As presented in Section 3.1, hybrid key exchange preserves ECDHE component gxy, and concatenates ML-KEM component ss as an additional factor. So as long as _at least_ one of these two secrets is not available to the adversary, all security properties should hold. In particular, even if ML-KEM is completely broken, i.e., ss is available to the adversary, the protocol retains the security level of ECDHE. 4.3. Standalone ML-KEM On the other hand, the formal property standalone ML-KEM provides is: Security properties of TLS hold unless `ss` is available to the adversary. 4.4. Results For the FATT process [TLS-FATT], the symbolic analysis [FATT-CHANCE] was done in ProVerif by Nadim Kobeissi, who concludes: Sardar Expires 13 December 2026 [Page 8] Internet-Draft Analysis of Hybrid Key Exchange and Stan June 2026 The hybrid neutralizes every weakness standalone ML-KEM carries, both the confidentiality single point of failure and the authentication (key-confirmation) failure that rides along with it, by demanding that both of its components fail at once; and moving from today’s classical (EC)DHE to a hybrid never gives up ground, because the classical guarantee survives intact inside the combination. That is the precise sense in which the hybrid strictly dominates standalone. None of this says standalone ML-KEM is broken. Under its stated assumption, that ML-KEM holds, it is secure under our models. The argument against it is one of robustness, not of any present-day attack: it stakes everything on a single, relatively young assumption, where the hybrid keeps a mature fallback in reserve. For a deployer who cannot afford to be wrong about lattice cryptography for the lifetime of the data being protected, that distinction is the whole game. The two caveats bound the picture honestly: reuse is a real and quantifiable cost rather than a catastrophe, and the role-asymmetry worry, while genuinely the draft’s headline concern, did not surface as a distinct vulnerability in the server-authenticated, one-initiator-per-session setting analyzed here. Results confirm integration of KEM in TLS is secure as long as the primitive itself is secure. In our understanding, results also imply a clear preference for hybrids under the Dolev-Yao model, in the sense that if the shared secret from ML-KEM becomes available to the adversary (for example, due to implementation bug), both confidentiality and authentication are broken in standalone ML-KEM, whereas under same condition, both confidentiality and authentication still hold as long as (EC)DHE is still not available to the adversary. We believe this applies until a powerful CRQC exists which breaks *all* the bits of pre-quantum, where the condition of (EC)DHE being available to the adversary is violated. The artifacts are available here (https://github.com/symbolicsoft/ reftls) for independent review. 5. Implementation-Facing Negative Cases The formal analysis above is not an implementation test suite and does not replace protocol conformance testing. However, a short set of negative cases can help implementers check that the intended hybrid binding property is reflected in their APIs, transcript handling, and key schedule integration. In particular, implementations of hybrid key exchange ought to reject, or fail safely on, cases such as the following: Sardar Expires 13 December 2026 [Page 9] Internet-Draft Analysis of Hybrid Key Exchange and Stan June 2026 * *malformed or missing shares*: The negotiated group is a hybrid group, but one component of the peer key share is missing, malformed, or associated with a different group. * *mixed transcript context*: The ECDHE and KEM values are individually well-formed, but assembled from different handshakes, transcript contexts, or negotiated groups. * *fallback after hybrid negotiation*: A peer attempts to continue the handshake as standalone ECDHE or standalone ML-KEM after a hybrid group was negotiated. * *premature secret derivation*: Application traffic secrets are derived before both hybrid components have been validated and accepted under the negotiated group. * *API/logging ambiguity*: Exported state, logs, traces, or implementation APIs make a hybrid exchange appear as if only one component was used or accepted. These cases are not intended to create a new formal proof obligation. They are implementation-facing checks that help bridge the formal "both components are bound and accepted" property to concrete failure modes that implementations can accidentally mishandle. 5.1. Argument Matrix for Implementation Review The discussion above can be read as an argument matrix for implementers and reviewers. The point is not to resolve every policy preference in this document, but to make each recurring argument map to a concrete review question. +=======================+=======================+===================+ | Argument | Implementation-facing | Why it matters | | | review question | | +=======================+=======================+===================+ | Hybrid key exchange | Does the | Otherwise a | | retains two | implementation make | successful | | components. | it clear that both | handshake may | | | the ECDHE and ML-KEM | not actually | | | components were | reflect the | | | present, validated, | formal "both | | | and bound to the same | components are | | | negotiated group and | bound and | | | transcript? | accepted" | | | | property. | +-----------------------+-----------------------+-------------------+ | Standalone ML-KEM has | Are failures in | A standalone | Sardar Expires 13 December 2026 [Page 10] Internet-Draft Analysis of Hybrid Key Exchange and Stan June 2026 | a single KEM shared | encapsulation, | construction has | | secret. | decapsulation, | no second key- | | | transcript binding, | exchange | | | and key-schedule | component to | | | input handled as | preserve | | | fail-closed errors | confidentiality | | | rather than as retry | if the ML-KEM | | | or fallback paths? | path is | | | | mishandled. | +-----------------------+-----------------------+-------------------+ | Hybrid fallback is | After a hybrid group | Silent | | useful only when it | is negotiated, can | continuation | | is explicit. | either endpoint | changes the | | | silently continue as | negotiated | | | standalone ECDHE or | security | | | standalone ML-KEM? | property and | | | | makes interop | | | | failures hard to | | | | distinguish from | | | | downgrades. | +-----------------------+-----------------------+-------------------+ | Cost claims are | Are claimed savings | Treating "cost" | | deployment-dependent. | measured separately | as a single | | | for wire bytes, CPU, | value can hide | | | memory, latency, code | whether a | | | complexity, and | deployment is | | | operational rollout? | trading away | | | | cryptographic | | | | robustness for a | | | | negligible or | | | | unmeasured gain. | +-----------------------+-----------------------+-------------------+ | Formal models do not | Do APIs, logs, | Reviewers need | | cover every | exported state, and | observable | | implementation | test harnesses expose | evidence that | | interface. | enough detail to show | the | | | which component | implementation | | | failed or succeeded? | behavior matches | | | | the protocol- | | | | level claim. | +-----------------------+-----------------------+-------------------+ Table 1 This matrix is deliberately small. It is intended to help a reviewer decide whether a concrete implementation or deployment argument belongs in the formal-methods discussion, in implementation guidance, or in a separate operational cost analysis. Sardar Expires 13 December 2026 [Page 11] Internet-Draft Analysis of Hybrid Key Exchange and Stan June 2026 6. Issues That Formal Methods Probably Cannot Solve The answers to the following issues are largely dependent on several factors, and the opinions vary largely. It is necessary to mention that even several respectable cryptographers in the community are not aligned on the issue -- for example see the long bet (https://github.com/FiloSottile/ecc-vs- lattices-long-bet). Hence, our personal opinion is probably not that important. Probably the best we can do is to capture _our_ understanding of the views of WG participants. Disclaimer: This is not meant to be an exhaustive list. This is also not meant to pritoritize any concerns over others. This is a sincere attempt to slowly capture the opinions to avoid endless repetitions from both sides. Some substantive concerns may be missing. If your substantive concern is missing, it is unintentional. Please simply submit a *precise* and *concise* PR, preferably with a reference. 6.1. Which breaks first: ML-KEM-768 or X25519? In our understanding, the key open question boils down to: Which of the two cryptographic mechanisms breaks first? How does that relate to the CRQC being developed? How many bits of pre-quantum cryptography can that CRQC actually break? Since all of three can be kept secret for some time, the opinions vary a lot on the different possible combinations. 6.2. Does CRQC Break All Bits of Pre-quantum? Some participants believe that CRQC will break all bits of pre- quantum cryptographic, while some others believe that it will break only a few bits (https://cr.yp.to/papers/mldsa- 20260601.pdf#breakable-keys). 6.3. Policy/Regulations Some countries have a regulatory preference for hybrid key exchange. 6.4. Recommendation of Designers The authors of Kyber/ML-KEM (see this (https://pq-crystals.org/kyber/ index.shtml)) say: Sardar Expires 13 December 2026 [Page 12] Internet-Draft Analysis of Hybrid Key Exchange and Stan June 2026 For users who are interested in using Kyber, we recommend the following: * Use Kyber in a so-called hybrid mode in combination with established "pre-quantum" security; for example in combination with elliptic-curve Diffie-Hellman. [...] A WG participant shares (https://mailarchive.ietf.org/arch/msg/tls/ NnGrdavTY6KGTVQo46xaPbSHQzw/) that: I recently asked one of the members of the CRYSTALS team whether this is still his view, and the response was: "Yes, of course." 6.5. Thorough Review Please see a very thorough review here (https://mailarchive.ietf.org/arch/msg/tls/jlsYHENwqMv- 4XPRvunqKsAL36k/), which is self-sufficient. 6.6. 'Significantly Harder' Argument Some participants believe in the 'significantly harder' argument, which assumes independence of breakage of ML-KEM and traditionals: If the probablity of one being broken over the next n years is p, and the probability of the other being broken over the next n years is q, then the probability of both being broken is pq. Please see this (https://github.com/FiloSottile/ecc-vs-lattices-long- bet#2a-what-counts-as-a-break) for what "broken" may mean here modulo some exclusions (https://github.com/FiloSottile/ecc-vs-lattices-long- bet#5-exclusions). Given the very different type of cryptographic constructions involved, independence might be a reasonable assumption. However, some participants disagree with 'significantly harder' argument with a reasonable counter-argument that in reality, cryptography is much more complicated than that (cf. this (https://mailarchive.ietf.org/arch/msg/tls/ AK7QUiiGX3ynsOhXeUuwn_IY7ik/)): Depending on the algorithms and the composition method, the probability can clearly be q, or smaller than pq. Sardar Expires 13 December 2026 [Page 13] Internet-Draft Analysis of Hybrid Key Exchange and Stan June 2026 In our understanding, most other counter-arguments seem to break the exclusions (https://github.com/FiloSottile/ecc-vs-lattices-long- bet#5-exclusions). Please note that this argument is based on the security of _primitives_, rather than the _composition_ of primitives in protocols. Hence, formal methods probably have nothing to help here. 6.7. Urgency It is unclear _whether_ and if applicable _when_ Cryptographically- Relevant Quantum Computer (CRQC) will eventually become practical. The opinions vary from never because of complicated physics (see this (https://eprint.iacr.org/2025/1237)) to be _prepared_ for it as early as 2029 (see Google 2029 (https://blog.google/innovation-and- ai/technology/safety-security/cryptography-migration-timeline/) and Cloudflare 2029 (https://blog.cloudflare.com/post-quantum-roadmap/)). Technically, please note that Google has not even released the *quantum circuit* underlying their recent claims -- apparently the reason for this urgency. So Google's claims may not yet be justified. Moreover, in our understanding, these deadlines are for PQ-based protection in general regardless of hybrid key exchange or standalone KEMs in TLS. Since hybrid key exchange is wildly in use, these deadlines are mainly for quantum-safe authentication. In any case, some participants see no reason to create panic for publication of [I-D.ietf-tls-mlkem] based on this because many implementations -- such as OpenSSL -- have already implemented standalone ML-KEM, and it is just a matter of enabling it. And frankly, nobody needs permission from the IETF to enable it. 6.8. "Cost" "Cost" has been presented on the list as the motivation for standalone ML-KEM in TLS but we have not seen any supporting analysis. Our observation from Section 4 of [I-D.ietf-tls-ecdhe-mlkem] is that -- for example -- for X25519MLKEM768, the traditional part seems negligible compared to ML- KEM part in key_exchange: Sardar Expires 13 December 2026 [Page 14] Internet-Draft Analysis of Hybrid Key Exchange and Stan June 2026 +================+==================+===========================+ | Bytes in field | PQ part (ML-KEM) | Traditional part (X25519) | +================+==================+===========================+ | Client share | 1184 | 32 | +----------------+------------------+---------------------------+ | Server share | 1088 | 32 | +----------------+------------------+---------------------------+ Table 2 We believe other "costs" will depend on several factors -- including but not limited to implementation details and deployment scenario -- and it is quite *subjective*. There seems to be a need for a thorough study to understand the "cost." We invite the WG participants to perform cost analysis and share the results with the WG. 6.9. Is Publication Necessary? Code Points for ML-KEM have already been assigned. [I-D.barnes-tls-this-could-have-been-an-email] provides detailed rationale as to why publication of such documents and the debates around that may be unnecessary. In our understanding, [I-D.pwouters-crypto-current-practices] makes similar arguments. 6.10. Shiny New Crypto ML-KEM is quite new in the IETF and even in the IRTF. Some WG participants have shown concern over premature publication of [I-D.ietf-tls-mlkem] until a detailed analysis has been done by CFRG. CFRG is starting some efforts for analysis. The extended deadline for submission is 22.06. Please see the latest CFRG chairs email (https://mailarchive.ietf.org/arch/msg/ cfrg/6K43Ycr062Ym1G0q4WHxZQ2HW8M/) for further details. 6.11. Formal Mapping of FIPS to IETF BCP14 As discussed on the TLS list, we are not aware of any formal mapping of the FIPS recommendations to the IETF BCP14 terminology, such as SHOULD vs. MUST. In general, we believe re-using FIPS recommendations is ambiguous for IETF readers. Sardar Expires 13 December 2026 [Page 15] Internet-Draft Analysis of Hybrid Key Exchange and Stan June 2026 6.12. Outstanding NIST Comments Some participants believe that NIST has rushed through the process and not addressed all the comments that were submitted during the open review. Please see comments here (https://csrc.nist.gov/files/pubs/fips/203/ipd/docs/fips-203-initial- public-comments-2023.pdf). 6.13. Too Early Some participants simply believe that publication of [I-D.ietf-tls-mlkem] and related discussions are just too early and unnecessary. 6.14. Patents Some WG participants have raised some concerns related to patents. See some relevant patents here (https://datatracker.ietf.org/ipr/ search/?submit=draft&id=draft-ietf-tls-mlkem). 6.15. Implementation Bugs Some participants are worried about the implementations bugs. Some use it as advocacy for the use of hybrids that if one could exploit one of the two primitives, the other one can save. 6.16. Depth of Hybrids? Some participants have questioned the ML-KEM + ECC hybrids rather than, say, Module Lattices + McEliece + hash-based three-way composites. 7. Security Considerations The whole document is about improving security considerations. Like all security proofs, formal analysis is only as strong as its assumptions and model. The scope is typically limited, and the model does not necessarily capture real-world deployment complexity, implementation details, operational constraints, or misuse scenarios. Technically, formal proof only guarantees anything if all the assumptions hold, which is unlikely in practice. Formal methods should be used as complementary and not as substitute of other analysis methods. Sardar Expires 13 December 2026 [Page 16] Internet-Draft Analysis of Hybrid Key Exchange and Stan June 2026 8. IANA Considerations This document has no IANA actions. 9. References 9.1. Normative References [I-D.ietf-tls-mlkem] Connolly, D., "ML-KEM Post-Quantum Key Agreement for TLS 1.3", Work in Progress, Internet-Draft, draft-ietf-tls- mlkem-07, 12 February 2026, . [I-D.ietf-tls-rfc8446bis] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", Work in Progress, Internet-Draft, draft- ietf-tls-rfc8446bis-14, 13 September 2025, . [NistFips203] "Module-lattice-based key-encapsulation mechanism standard", National Institute of Standards and Technology (U.S.), DOI 10.6028/nist.fips.203, August 2024, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [TLS-FATT] IETF TLS WG, "TLS FATT Process", June 2025, . 9.2. Informative References [FATT-CHANCE] Kobeissi, N., "FATT Chance: On the Robustness of Standalone and Hybrid ML-KEM Key Exchange in TLS 1.3", Cryptology ePrint Archive, Report 2026/1147 , 2026, . Sardar Expires 13 December 2026 [Page 17] Internet-Draft Analysis of Hybrid Key Exchange and Stan June 2026 [I-D.barnes-tls-this-could-have-been-an-email] Barnes, R., "Stop Doing Cryptographic Algorithm Drafts when Email to IANA is All You Need", Work in Progress, Internet-Draft, draft-barnes-tls-this-could-have-been-an- email-00, 23 February 2026, . [I-D.ietf-tls-ecdhe-mlkem] Kwiatkowski, K., Kampanakis, P., Westerbaan, B., and D. Stebila, "Post-quantum hybrid ECDHE-MLKEM Key Agreement for TLSv1.3", Work in Progress, Internet-Draft, draft- ietf-tls-ecdhe-mlkem-05, 26 May 2026, . [I-D.ietf-tls-hybrid-design] Stebila, D., Fluhrer, S., and S. Gueron, "Hybrid key exchange in TLS 1.3", Work in Progress, Internet-Draft, draft-ietf-tls-hybrid-design-16, 7 September 2025, . [I-D.ietf-tls-hybrid-design-09] Stebila, D., Fluhrer, S., and S. Gueron, "Hybrid key exchange in TLS 1.3", Work in Progress, Internet-Draft, draft-ietf-tls-hybrid-design-09, 7 September 2023, . [I-D.pwouters-crypto-current-practices] Wouters, P., "Current practices for new cryptography at the IETF", Work in Progress, Internet-Draft, draft- pwouters-crypto-current-practices-00, 3 November 2024, . [I-D.usama-tls-fatt-extension] Sardar, M. U., "Extensions to TLS FATT Process", Work in Progress, Internet-Draft, draft-usama-tls-fatt-extension- 07, 2 May 2026, . [rfc3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on Security Considerations", BCP 72, RFC 3552, DOI 10.17487/RFC3552, July 2003, . Sardar Expires 13 December 2026 [Page 18] Internet-Draft Analysis of Hybrid Key Exchange and Stan June 2026 Contributors Nadim Kobeissi performed a thorough formal analysis Section 4.4 at high priority based on our call for analysis in previous versions of the draft to get a confirmation. Text in Section 5 was proposed by Songbo Bu. Text in Section 7 is based on the proposal by John Preuß Mattsson. Section 6 is largely based on the opinions of many IETF participants. We gratefully thank Yaakov Stein and Ilari Liusvaara for their substantial technical guidance, valuable feedback, and contributions. Acknowledgments We thank Eric Rescorla, Brian E. Carpenter, and Tibor Jager for their valuable feedback. We acknowledge several IETF participants who have contributed to this draft with their insights. The research work is funded by German Research Foundation ("Deutsche Forschungsgemeinschaft.") History -00 * On popular demand, moved from [I-D.usama-tls-fatt-extension] to an independent I-D * Major change: added Section 2.1 * Some minor clarifications -01 * Added justification based on FATT process * Reorganization, specially in motivation * Added some common arguments: Section 6 * Comparison with hybrid key exchange -02 Sardar Expires 13 December 2026 [Page 19] Internet-Draft Analysis of Hybrid Key Exchange and Stan June 2026 * Added gap analysis * What to model and analyze? Section 4.1 * Added FATT review is harmless * Extended comparison with hybrid key exchange * Opinion of designers Section 6.4 -03 * Completely restructured and reframed after confirmation by formal analysis * Added implementation-facing negative cases and argument matrix * Some new arguments: implementation bugs, depth of hybrids, policy, all bits, which primitive breaks first? Author's Address Muhammad Usama Sardar TU Dresden, Germany Email: muhammad_usama.sardar@tu-dresden.de Sardar Expires 13 December 2026 [Page 20]