Re: [TLS] Static DH timing attack

Hugo Krawczyk <> Thu, 10 September 2020 16:01 UTC

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From: Hugo Krawczyk <>
Date: Thu, 10 Sep 2020 12:00:40 -0400
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To: Dan Brown <>
Cc: "Salz, Rich" <>, "" <>
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Subject: Re: [TLS] Static DH timing attack
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What you suggest, namely,  DH for both static and ephemeral keys is what
OPTLS was about and this approach is now specified in

I was never too happy with the name semi-static for such protocol, and
people may think that if static is bad then semi-static is semi-bad :-)
So maybe it should be replaced with something else

It is essentially a DH-KEM, namely a KEM implemented via DH. It has the
advantage over generic KEMs that  g^x sent by the client acts as an
ephemeral KEM public key (producing g^xy) and also as an encapsulation
under the server's public key (producing g^xs).  This shaves one full round
trip relative to a generic KEM-based protocol (this applies to the
protocols with and without client authentication).


On Thu, Sep 10, 2020 at 11:19 AM Dan Brown <> wrote:

> *From:* TLS <> *On Behalf Of *Salz, Rich
> > Do we need a short RFC saying “do not use static DH” ?
> Don’t TLS 0-RTT and ESNI/ECH via HPKE use a type of (semi)static ECDH? If
> so, then an RFC to ban static (EC)DH in TLS would need to be very clear
> about not referring to these use cases of static ECDH.
> My 2c. What about combining static ECDH (instead of signatures) with
> ephemeral ECDH, e.g. for more fully deniable authentication?  (ECMQV does
> this.)  (Perhaps this is also similar to the KEMTLS proposal for PQC,
> - still need to study that.)
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