[TLS] Hybrid key exchange in TLS 1.3 and variable-length secrets

[Nimrod Aviram <[e-mail]>]

Dear all,

We are writing to ask about the possible security impact of
variable-length secrets on the "Hybrid key exchange in TLS 1.3" RFC
[1].

As you probably know, when using key material of variable length
and processing this material using hash functions, a timing side
channel may arise. 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. Lucky Thirteen [2]
and the newly-disclosed Raccoon Attack [3]. To be clear, we are not
aware of any attack on the proposed standard. Rather, we view this as
an opportunity to defend-in-depth against such attacks, while work on
the standard is in progress.

Our proposal is to add language to the RFC explaining that variable-length
secrets may enable such attacks, and should therefore be avoided when
possible.  Currently, the language seems to allow for variable-length
secrets, should the need arise:
"Variable-length shared secrets ... if it is envisioned that this
specification
be used with algorithms which do not have fixed-length shared secrets ..."

We also note that a related RFC exists, "Hybrid Post-Quantum Key
Encapsulation Methods (PQ KEM) for Transport Layer Security 1.2"
[4]. However, that RFC apparently only uses BIKE, Kyber or SIKE as the
PQ KEM. To our knowledge, all three KEMs have fixed-length secrets. It
may be prudent to add cautionary language to that document as well,
in case other KEMs are used in the future.

Kind regards,
The Raccoon Team

[1]
https://www.ietf.org/id/draft-ietf-tls-hybrid-design-00.html#name-open-questions
[2] https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6547131
[3] https://raccoon-attack.com/
[4]
https://tools.ietf.org/id/draft-campagna-tls-bike-sike-hybrid-05.html#name-key-exchange-algorithms