[TLS] Re: WG Last Call: draft-ietf-tls-mlkem-08 (Ends 2026-07-08)

[Jacob Appelbaum <[e-mail]>]

Hi Henrick,

On 7/11/26 13:53, Henrick Hellström wrote:
> Hi Jacob,
> 
> On 2026-07-09 18:11, Jacob Appelbaum wrote:
>  > Obviously no one should use Dual_EC_DRBG. The problem is that users
>  > often do not know when their randomness source is sabotaged. Hashing
>  > does not solve all bad-RNG problems, but it does destroy the algebraic
>  > structure used in this known attack class.
> 
> Yes, I understand this might once have been brought in as a quick-fix 
> for Dual_EC-DRBG debacle, but at this time it ought to be replaced by a 
> more permanent fix.
> 
I agree that a permanent fix is better than an ad hoc fix.

Where I disagree is the implication that the issue has already been
permanently fixed in the TLS draft. If the permanent fix is "never hand
raw primary RBG output to ML-KEM.Encaps() as decapsulator-recoverable
`m`", then that should be stated clearly in the Security Considerations.

The Kyber hash was not merely nostalgia for Dual_EC_DRBG. It was a
cheap defense-in-depth step against exposing raw system RNG output to
the recipient. Prof. Dr. Peter Schwabe made that point explicitly during
the NIST discussion [0]. FIPS 203 removed that step and instead relies
on a requirement for approved randomness generation [1]. That dependency
is exactly what the TLS draft should surface.

 > Because who exactly do you mean by "user" in this context, and what
 > kind of conformance certification is required?

By "user" I mean the people who rely on TLS deployments: browser users,
citizens using government systems, operators, administrators, device
owners, library consumers, and people depending on remote services. They
are usually not in a position to audit whether a hardware RNG, firmware
RNG, HSM, cloud module, or library RNG is sabotaged.

That is why this belongs in protocol security guidance. A lab
conformance story is not the same thing as user protection.

 > If you would want the implementation to be FIPS 140 conformant, the
 > randomness source consumed by ML-KEM would have to be an approved
 > DRBG, but it can be a secondary DRBG, like I described, and it
 > should be one of the symmetric, more easily analysed DRBGs.

That may indeed be a good answer if it is mandatory, isolated, and
specified carefully.

A secondary symmetric DRBG seeded from a primary RBG can plausibly
destroy Dual_EC_DRBG-shaped (x-coordinate) structure, provided that raw
primary output is not exposed elsewhere and the secondary DRBG is
properly instantiated, reseeded, separated by use, and implemented. If
that is the intended FIPS-friendly permanent fix, then the draft should
say so.

What the draft should not do is silently inherit the assumption
"approved RBG" as if that alone answered the historical failure mode.
Dual_EC_DRBG was once standardized in the same general standards family.
The lesson should not be "trust the label"; it should be "do not expose
structured RBG output when cheap separation or whitening avoids it."

 > On the other hand, if you add a hash between the approved DRBG and
 > the ML-KEM implementation, it would become an algorithm that clearly
 > differs from the specifications and requirements of NIST SP 800-90,
 > so I doubt it would be FIPS 140 certifiable.

This may be true for a strict FIPS validation boundary. If so, it is not
a rebuttal; it is the problem stated plainly. Also, this isn't a draft 
about FIPS per se, though it is clearly very related to IETF finding 
consensus on FIPS 203.

If restoring Kyber's `m <- H(m)` step would make an implementation no
longer FIPS 140 certifiable as ML-KEM, then the TLS draft should say
what FIPS-constrained implementations are expected to do instead. For
example: use an approved, separate, symmetric DRBG or approved
derivation construction for ML-KEM encapsulation randomness, with domain
separation, so that raw primary RBG output is never used directly as
`m`.

For non-FIPS-constrained implementations, Kyber-style hashing or
context-bound derivation of `m` remains the simple defense-in-depth
answer. For FIPS-constrained implementations, the answer may be a
validated secondary DRBG construction. Either way, the security goal is
the same: do not give the decapsulating peer a clean sample of raw,
structured RNG output.

So I think we may be closer than it first appeared. If your position is:

   "Do not use raw primary RBG output directly as ML-KEM `m`; use a
   properly separated approved secondary DRBG instead if FIPS validation
   is required"

then I would appreciate seeing that written into the draft's Security
Considerations, along with the reason it matters to implementers.

Kind regards,
Jacob

[0] 
https://groups.google.com/a/list.nist.gov/g/pqc-forum/c/WFRDl8DqYQ4/m/o2XJ2YvfAwAJ

[1] https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.203.pdf

[2] https://csrc.nist.gov/pubs/fips/140-3/final

>> On 7/9/26 10:39, Henrick Hellstrom wrote:
>>
>>> On 2026-07-09 10:10, Tanja Lange wrote:
>>>
>>>> The question by Benjamin Kaduk was if there are cases where the
>> server hellow would not leak the state of the PRNG (adding P for
>> clarity^*) and Botan implements that protection. There are other
>> libraries that have seperate PRNGs for public and private values.
>>
>> To further support Tanja's point: The sub-topic is not ordinary PRNG
>> quality. It is avoiding leakage of bytes that can form a covert channel
>> and carry algebraic structure from kleptographic RNGs such as
>> Dual_EC_DRBG. That kind of leakage can happen through many protocol
>> fields, including TLS fields, and it is a specification-level defense-
>> in-depth gap.
>>
>> ML-KEM as finalized in FIPS 203 removed Kyber's hash over `m`, even
>> though at least one Kyber designer explicitly argued that hashing `m`
>> avoids sending system RNG output to the recipient [0]. NIST later
>> announced that the hash would be removed [1]. I have not seen an
>> analysis from NIST showing that this removal remains safe against their
>> own historical example: Dual_EC_DRBG, a DRBG NIST described as "provided
>> by NSA," which NIST later said there were many reasons to reject or
>> modify, but "instead, we left it in" [2][3].
>>
>> Any such analysis would have had to address the central point: hashing
>> destroys the Dual_EC_DRBG algebraic structure needed for efficient state
>> recovery.
>>
>>> Yes, but just hashing the output from a DRBG is not the approved way
>> to do it. Instead you are supposed to spawn a secondary DRBG with a seed
>> generated from a primary DRBG.
>>
>> That may be an approved construction for ordinary DRBG engineering, but
>> it is not obviously a defense against a DRBG whose output is
>> intentionally structured to leak its state. If the primary DRBG is
>> Dual_EC_DRBG-shaped, spawning another DRBG from it may simply give the
>> defender two things to audit. Please point me to the specific approval
>> text you mean where the "do it" is defending against this class of
>> Dual_EC_DRBG kleptographic backdoor.
>>
>> Dual_EC_DRBG output can look secure by many metrics and still act as a
>> covert channel [4]. With the relevant trapdoor/secret key(s), an
>> Adversary who can sample enough output can recover DRBG state and
>> predict future outputs. The general attack is well established [5][6]
>> [7]. The RSA/Dual_EC_DRBG reporting and the BULLRUN reporting are the
>> relevant historical context here [8][9].
>>
>> Obviously no one should use Dual_EC_DRBG. The problem is that users
>> often do not know when their randomness source is sabotaged. Hashing
>> does not solve all bad-RNG problems, but it does destroy the algebraic
>> structure used in this known attack class.
>>
>>> Just hashing wouldn't do much good, if you are really concerned that
>> the DRBG output might reveal anything about the internal state.
>>
>> For this attack class, it does good.
>>
>> Hashing `m` during `ML-KEM.Encaps()`, for example with the ML-KEM public
>> key and/or transcript context, binds `m` to context before the ML-KEM
>> ciphertext is sent. In the normal non-ECH TLS 1.3 case, that closes the
>> Dual_EC_DRBG-shaped oracle against the server's RNG output. With ECH
>> using ML-KEM or a hybrid KEM, the same concern can arise in both 
>> directions.
>>
>> A few objections have come up repeatedly:
>>
>> 0. Ad-hominem attacks
>>
>> 1. TLS already has random fields written to the wire, so we should not
>> worry about ML-KEM
>>
>> 2. NIST already issued FIPS 203, so the IETF should not revisit the issue
>>
>> 3. Updating drafts, RFCs, or implementations would be work
>>
>> 4. The issue also affects hybrids, so the hybrid recommendation should
>> be weakened
>>
>> 5. Hashing may not increase entropy
>>
>> 6. Hashing DRBG output merely creates a new DRBG, DRBG'
>>
>> Briefly:
>>
>> 0. The IETF mission is to make the Internet work better for the people
>> who use and manage it [10], and the IETF considers pervasive monitoring
>> an attack [11]. Cryptographic sabotage is part of how pervasive
>> monitoring becomes actionable intelligence. Attacking the person raising
>> the issue is irrelevant to the technical question.
>>
>> 1. It is true that TLS has a broader problem with raw random bytes in
>> protocol-visible fields. That is a reason to fix the broader issue, not
>> a reason to preserve the same problem in ML-KEM. Tanja pointed out that
>> Botan already has a TLS protection strategy. Other libraries use
>> different ad-hoc strategies, or none. That is exactly why IETF guidance
>> would help.
>>
>> 2. FIPS 203 being final is not the end of the story. NIST can publish
>> errata, revisions, or clarifications. NIST withdrew Dual_EC_DRBG after
>> public reporting on BULLRUN and related cryptographic sabotage. A NIST
>> participant who described himself as a main author of FIPS 203 is on
>> this list, and I have asked for clarification on both the hashing
>> decision and the IPR implications.
>>
>> 3. Yes, updates take work. That is not a security argument. If the
>> Security Considerations of the draft omit an important assumption and a
>> cheap mitigation, that should be fixed. This is a constructive
>> suggestion and an easy win to build consensus.
>>
>> 4. The issue also applies to hybrid X25519MLKEM* constructions. The
>> answer is not to weaken hybrids; it is to hash `m` in the ML-KEM
>> component. Failure to hash `m` can contribute to compromising later
>> values, including later X25519 keypairs, if the same long-running RNG
>> state is used and no strong reseed or unknown additional input 
>> intervenes.
>>
>> 5. Hashing Dual_EC_DRBG output does not magically add entropy, and may
>> reduce it in some constructions. That is not the point. The point is
>> destroying hidden algebraic structure. We should not shift the
>> discussion from structure recovery to entropy accounting and then
>> conclude that no mitigation is useful.
>>
>> 6. Yes, hashing DRBG output can be described as constructing DRBG'. That
>> is fine. DRBG' should still be treated as suspect, but it no longer
>> preserves the Dual_EC_DRBG x-coordinate structure needed by the
>> published attacks. Unkeyed hashing appears sufficient to block the known
>> Dual_EC_DRBG recovery attacks; a keyed hash may be useful for stronger
>> designs, but it is not needed to make this point.
>>
>> My conclusion is simple: when history and user-centered harm reduction
>> are the concern, the arguments favor hashing. We know this class of
>> kleptographic attack is real. We know standards influence is part of the
>> game. We should not leak raw pre-whitened system entropy to the network,
>> and we should not hand raw RNG-derived `m` to a potentially adversarial
>> decapsulator.
>>
>> To restate the completely obvious: this isn't an accusation of a
>> backdoor in ML-KEM. It is a statement of fact that a defense-in-depth
>> protection was knowingly removed by NIST. NIST did so over the objection
>> of several people participating in the NIST PQC process, and without
>> analysis that accounts for the extremely serious failures of NIST in the
>> very recent past with regard to Dual_EC_DRBG. The change by NIST serves
>> as an example where surprise: no one is responsible for the user's
>> security even though the difference in exploitability for the
>> Dual_EC_DRBG scenario is roughly a single hash function call.
>>
>> The IETF should give implementers clear guidance and it should be in the
>> draft's Security Consideration at the very least. Hash `m`. Relatedly,
>> do not write raw RNG output into protocol-visible fields when cheap
>> whitening destroys known hidden structure.
>>
>> Kind regards,
>> Jacob Appelbaum
>>
>> P.S.
>>
>> Consider also the scale of the budgets for only a single agency [12]. It
>> may also be worth your time looking at the cryptographic libraries
>> worked on by former NSA people once they were officially out of
>> government and firmly on the defending side of the fence. It is also
>> informative for spotting the folks who have retained their first job
>> while taking on another, another thing we learned about PROJECT BULLRUN.
>> Remember: there is no need for accusations of a conspiracy as the
>> business plan as the documented [12] attack budget appears to exceed the
>> IETF's operating budget by a very large margin. The larger issue isn't
>> even really about the NSA as the NSA's track record serves as a fairly
>> clear example of what to look for from many other large-scale 
>> adversaries.
>>
>> [0] https://groups.google.com/a/list.nist.gov/g/pqc-forum/c/ 
>> WFRDl8DqYQ4/ m/o2XJ2YvfAwAJ
>>
>> [1] https://groups.google.com/a/list.nist.gov/g/pqc-forum/c/ 
>> WFRDl8DqYQ4/ m/MRa5O0CvAAAJ
>>
>> [2] https://rwc.iacr.org/2015/Slides/RWC-2015-Kelsey-final.pdf
>>
>> [3] https://csrc.nist.gov/csrc/media/projects/crypto-standards- 
>> development-process/documents/dualec_in_x982_and_sp800-90.pdf
>>
>> [4] https://csrc.nist.gov/glossary/term/covert_channel
>>
>> [5] https://rump2007.cr.yp.to/15-shumow.pdf
>>
>> [6] https://eprint.iacr.org/2015/767
>>
>> [7] https://www.usenix.org/system/files/conference/usenixsecurity14/ 
>> sec14-paper-checkoway.pdf
>>
>> [8] https://www.reuters.com/article/world/exclusive-secret-contract- 
>> tied-nsa-and-security-industry-pioneer-idUSBRE9BJ1C5/
>>
>> [9] https://www.theguardian.com/world/2013/sep/05/nsa-gchq-encryption- 
>> codes-security
>>
>> [10] https://datatracker.ietf.org/doc/html/rfc3935
>>
>> [11] https://datatracker.ietf.org/doc/html/rfc7258
>>
>> [12] https://archive.nytimes.com/www.nytimes.com/ 
>> interactive/2013/09/05/ us/documents-reveal-nsa-campaign-against- 
>> encryption.html
>