Dear all,
We are enthusiastic about Nick Sullivan's announcement of his RFC draft
for a TLS 1.3 key schedule based on Keccak and happy with the many
reactions on the mailing list, so we thought it would be good to give
you our 2 cents.
# Including a Keccak-based AEAD option
In Table 1, the draft proposes AES-GCM and ChaCha20-Poly1305 as AEAD
schemes, but no Keccak-based scheme. As suggested by other participants,
it would be nice to also offer the option of a Keccak-based AEAD scheme.
This would allow one to potentially reduce the code size (or area) and
trust surface even further.
We did the exercise recently in our paper "Shaking up authenticated
encryption" presented at EuroS&P (https://eprint.iacr.org/2024/1618). It
defines two fully committing AEAD schemes, both with security provably
reducible to (Turbo)SHAKE128/256.
# Instantiating the key derivation
The EuroS&P paper also defines a duplex object and a deck function, both
also reducing to the security of (Turbo)SHAKE128/256. Thanks to this
reduction, the former could be used as primitives in the key derivation,
solving much of the domain separation. The use of "trailer" bytes that
accumulate all domain separation bits the final functions are very
simple to implement. Moreover, by overwriting input blocks (instead of
XORing them in), they have a nice property that each call to the
underlying permutation can be a ratchet: the only requirement is that at
least 128/256 bits of the output shall not be returned.
# Kravatte vs (Turbo)SHAKE
Kravatte is a very fast primitive that could be used for AEAD. However,
it needs a secret key upon initialization and is therefore not suited
for key derivation.
Kravatte is a deck function obtained by applying the Farfalle
construction with Keccak-p[6 rounds] and two rolling functions. It is
*not* built on top of Keccak and therefore it security cannot be reduced
to that of (Turbo)SHAKE. Actually, its security cannot be reduced to a
simpler primitive, so the security of Kravatte must be established by
the cryptanalysis of Kravatte itself.
# On the number of rounds
MarsupilamiFourteen (M14) was given as an example of a function calling
Keccak-p with a number of rounds that is not a multiple of 6.
M14 dates back from 2018 as a 256-bit version of (now called) KT128. The
reasoning for adding two rounds was to allow for extra safety margin
while giving more budget to the adversary. Since then, we have seen the
number of rounds that can be attacked under cryptanalysis slow down, and
now we think that 12 rounds provides a comfortable safety margin for
Keccak, even when targeting 256-bit security with a capacity of 512
bits. So, RFC 9861 proposes TurboSHAKE256 and KT256 on top of
Keccak-p[12 rounds] and not 14 rounds.
Note by Joan: I answered Markku indeed that I could not think of any
proposal where the round count would not be a multiple of 6, thereby
dismissing MarsipulamiFourteen but also our CAESAR AEAD candidate Ketje
that does single-round calls in the encryption phase. My mindset was
that both were proposals for which we think we have more interesting
alternatives.
Kind regards,
The Keccak team
Guido Bertoni, Joan Daemen, Seth Hoffert, Silvia Mella, Michaël Peeters,
Gilles Van Assche and Ronny Van Keer
Op 08-07-2026 om 18:01 schreef Nick Sullivan:
Hi Hannes,
As Thom noted below in the chain, the motivation is to modernize the
key schedule, which has two main advantages:
1. Efficiency gains: As the analysis on-list spells out, it’s a
dramatic improvement to the number of hashes/permutations needed. But
as you noted, is not the hot path at all.
2. Removing a hard dependency on SHA-2 from future designs, as Thom
noted. This gain isn’t immediate, but it clears the way for future
configurations that don’t rely on SHA-2 for the CertificateVerify to
drop SHA-2 completely from the code base.
Nick
On Wed, Jul 8, 2026 at 3:58 PM Thom Wiggers <[email protected]> wrote:
Hi Hannes,
I don’t think runtime performance is an issue, but rather code
size (or area), by getting rid of SHA2. (Of course, this is long
into the future). The sponge-based constructions also have
theoretical benefits.
Cheers,
Thom
Op 8 jul 2026, om 13:47 heeft Hannes Tschofenig
<[email protected]> het volgende geschreven:
Hi Markku, Hi Nick!
I will certainly look closer into the details but it appears that
you are optimizing TLS in the wrong place. The key derivation is
the least expensive part in TLS and spending time optimizing it
will bring little benefit. I am saying this because I have for
years been looking at optimizing different parts of the TLS
protocol with constrained IoT in mind.
This brings me to the core question: What is the problem you are
trying to solve in the first place? I do not recall that anyone
has voiced performance problems with the key derivation in TLS
before this draft was published.
Ciao
Hannes
Am 08.07.2026 um 12:16 schrieb Markku-Juhani O. Saarinen:
Hi,
Thanks for this. I quickly put together an implementation of
draft-sullivan-tls-xof-ciphers-00.txt around Rustls to do some
measurements:
https://github.com/mjosaarinen/altkdf-rs
( Editorial comments in
https://github.com/mjosaarinen/altkdf-rs/blob/main/FINDINGS.md )
The theoretical side of the design seems very defensible --
clean proof target. In terms of concrete security, the Keccak
variants have a much larger security margin than the SHA-2 family.
Given how much work we put into reducing the number of
permutation calls with ML-KEM and Hybrid combiners -- carefully
debating and analyzing each permutation -- this one yields a
staggering reduction, making the key schedule much faster (and
the handshake probably too.)
For the representative full handshake: PSK + (EC)DHE + 0-RTT
leaves + NewSessionTicket + one KeyUpdate each direction + one
exporter, the per-endpoint counts over 24-round Keccak-f[1600] are:
41 * f1600: Deck implementation, measured stateful
46 * f1600: Deck implementation, measured recompute
52 * f1600: Section A.1 in draft-sullivan-tls-xof-ciphers-00
156 * f1600: HKDF-SHA3-256 / RFC 8446 baseline
117 * f1600: Appendix D "FIPS" KMAC256 schedule
So 41 vs 156 permutations by my count.
( Note: The draft slightly overcounts permutations in its
estimates. )
It's a quick prototype built with extensive AI assistance, but
it includes basic correctness measures: primitive KATs (RFC 9861
TurboSHAKE256, FIPS 202 SHAKE256, SP 800-185 KMAC256, including
multi-block and long-output), 73 self-generated Appendix C/D
vectors, and byte-for-byte reproduction of all of them by an
independent Python implementation written from the draft alone.
- Keccak-p[1600,nr] permutation and the rate-136/capacity-512 sponge
- Five framed deck operations (Init/Absorb/Fork/Squeeze/Ratchet)
- KMAC-layout MAC
- Three-stage E/H/T schedule with its two ratchets
- Section 5 derivations (record keys, Finished/PSK binders,
exporters, resumption and key-update, and the §10 external-PSK
importer with ImportedIdentityV2).
- All five cipher suites (0xFF01–0xFF05, both profiles, three AEADs)
Plus for comparisons:
- Appendix D FIPS-component schedule (RFC 8446 with KMAC256 as
the PRF)
- a permutation-count benchmark reproducing §A.1, live-secret
zeroization (§15.7.2.2)
Cheers,
-markku
Dr. Markku-Juhani O. Saarinen <[email protected]>
On Tue, Jul 7, 2026 at 2:34 AM Nick Sullivan
<[email protected]> wrote:
Dear TLS,
I'm sharing a draft for the group's consideration.
draft-sullivan-tls-xof-ciphers-00 runs the entire TLS 1.3
key schedule
on a single Keccak permutation, instead of HKDF built on
HMAC built on
the cipher suite's hash, which today is always SHA-2. This
is newly
practical because deployments using SHA-3, ML-KEM, or ML-DSA
already
carry a Keccak permutation, so the primitive is already in
the stack.
Each derived value comes out in one pass, so a full
handshake costs
about a third of the permutation calls an HKDF schedule over
the same
permutation would spend.
A cipher suite names an AEAD plus a schedule profile, and
nothing else
changes. There is no new extension, and the state machine,
record
layer, and wire format are untouched. Two profiles are
defined, one on
the standard SHA-3 function and one on a faster
reduced-round variant.
Test vectors are pinned to cipher-suite values, so the final
vectors
will follow the code point assignment.
https://datatracker.ietf.org/doc/draft-sullivan-tls-xof-ciphers/
This is a big change to the key schedule, and the draft is very
preliminary. Feedback on the approach, or interest in
implementing it,
would help a lot.
Best,
Nick
On Mon, Jul 6, 2026 at 7:03 PM <[email protected]> wrote:
>
> A new version of Internet-Draft
draft-sullivan-tls-xof-ciphers-00.txt has been
> successfully submitted by Nick Sullivan and posted to the
> IETF repository.
>
> Name: draft-sullivan-tls-xof-ciphers
> Revision: 00
> Title: TLS 1.3 Cipher Suites with Alternative
Key-Schedule Profiles
> Date: 2026-07-06
> Group: Individual Submission
> Pages: 46
> URL:
https://www.ietf.org/archive/id/draft-sullivan-tls-xof-ciphers-00.txt
> Status:
https://datatracker.ietf.org/doc/draft-sullivan-tls-xof-ciphers/
> HTML:
https://www.ietf.org/archive/id/draft-sullivan-tls-xof-ciphers-00.html
> HTMLized:
https://datatracker.ietf.org/doc/html/draft-sullivan-tls-xof-ciphers
>
>
> Abstract:
>
> TLS 1.3 builds its key schedule on HKDF over the cipher
suite's hash.
> This document defines TLS 1.3 cipher suites that build
it on a deck
> function over a single permutation instead, the one a
deployment
> already carries when it uses SHA-3, ML-KEM, or ML-DSA. One
> permutation then runs the whole schedule, and a full
handshake takes
> about a third of the permutation calls an HKDF schedule
over that
> permutation would. Such a cipher suite names an AEAD
algorithm
> together with a schedule profile that defines every
key-schedule
> function the connection uses. The profile follows from the
> negotiated cipher suite alone, so no new extension is
defined and the
> TLS 1.3 state machine and wire format are unchanged.
Two profiles
> are defined, one on the standard SHA-3 function and one
on a faster
> reduced-round variant of it.
>
>
>
> The IETF Secretariat
>
>
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