Re: [Cfrg] Constant-time implementations

Watson Ladd <watsonbladd@gmail.com> Tue, 14 October 2014 15:05 UTC

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Date: Tue, 14 Oct 2014 08:05:47 -0700
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From: Watson Ladd <watsonbladd@gmail.com>
To: Yoav Nir <ynir.ietf@gmail.com>
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Subject: Re: [Cfrg] Constant-time implementations
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On Tue, Oct 14, 2014 at 6:49 AM, Yoav Nir <ynir.ietf@gmail.com> wrote:
> Doesn’t it become better (or at least safer) at some point to set a timer before beginning the operation and then not use the results until the timer expires?
>
> Sure this has a cost in memory, but any fool can write an implementation with an upper bound on processing time, whereas true constant time is both hard and has a 20% overhead (according to DJB’s message upthread).

Any fool can download tweetnacl.c and get an implementation of
curve25519 that is constant-time. Likewise any fool can rip
implementations out of eBATS. We don't need custom implementations and
we certainly don't need people who can't write constant time code
doing crypto.

Furthermore, setting a timer doesn't deal with cache-based attacks. If
you want more speed than current proposals provide, let me interest
you in genus 2 Jacobians, endomorphisms, exotic uses of base change,
long before you get any benefit from removing timing-attack
resistance. Is Curve25519 really too slow?

Sincerely,
Watson Ladd
>
> Yoav
>
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