Re: [TLS] On Curve25519 and other possibilities (e.g. ietf256p, ietf384p, ietf521p,

Alyssa Rowan <akr@akr.io> Sat, 28 June 2014 17:30 UTC

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Date: Sat, 28 Jun 2014 18:30:45 +0100
From: Alyssa Rowan <akr@akr.io>
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Subject: Re: [TLS] On Curve25519 and other possibilities (e.g. ietf256p, ietf384p, ietf521p,
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On 28/06/2014 18:01, Watson Ladd wrote:

>> See our recent side channel attacks on EC-DSA with the
>> Eurovision
> Simple solution: write constant time software. The principal of
> side channel attacks being established, one simply eliminates them
> all.

And, of course, it would be remiss of me not to point out that all the
good Curve25519 implementations are both fast and constant-time, the
curve having been specified with the constant-time Montgomery ladder
in mind in the first place. This is actually an area where 25519 wins big.

(It should also be pointed out that constant-time over Weierstrass
curves is possible, with enough care; see Langley/Möller/Kasper's
implementation contributed to OpenSSL. I have yet to see any
implementation capable of doing such a feat with arbitrary curves.)

As far as code quality and reuse is concerned, what we've learned from
things like Heartbleed I think is that "many eyes make bugs shallow"
is only true when the eyes actually _look_ at the code. I don't think
monoculture is to blame (we could just as easily have 10 bad
implementations as 1), just complacency and a lack of sufficient
auditing. I'd be much happier with a few implementations that everyone
looks at very closely than many which aren't as well-audited.

- -- 
/akr
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