Re: [TLS] draft-sheffer-tls-bcp: DH recommendations

Michael D'Errico <> Sun, 22 September 2013 04:47 UTC

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Date: Sat, 21 Sep 2013 21:46:52 -0700
From: Michael D'Errico <>
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Cc: TLS Mailing List <>
Subject: Re: [TLS] draft-sheffer-tls-bcp: DH recommendations
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Yoav Nir wrote:
> On Sep 22, 2013, at 2:44 AM, Patrick Pelletier <> wrote:
>> We *do* know how to move to 2048 bits without breaking anything: add an extension to negotiate DH size, just like how ECC curves are negotiated.  Although yes, this will take a while to deploy universally, it at least means early adopters can start using 2048 now, without breaking the stragglers like Java.
> Perhaps it would be better to have the client suggest the actual group rather than bit length in that case. Perhaps we could reuse the IKE numbering of groups, or else have the client specify it similar to ServerDHParams struct.

If the client chooses, then I'd prefer they choose from a list of
standardized groups similar to RFC 3526 rather than requiring the
server to (try to) verify that the parameters are usable.  We could
also sneak in a 256-bit q parameter to speed up the computations.