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Re: [IPsec] Stephen Farrell's Yes on draft-ietf-ipsecme-safecurves-05: (with COMMENT)

Yoav Nir <ynir.ietf@gmail.com> Thu, 13 October 2016 12:27 UTC

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From: Yoav Nir <ynir.ietf@gmail.com>
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Date: Thu, 13 Oct 2016 15:27:39 +0300
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To: Stephen Farrell <stephen.farrell@cs.tcd.ie>
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Cc: ipsec@ietf.org, ipsecme-chairs@ietf.org, The IESG <iesg@ietf.org>, Tero Kivinen <kivinen@iki.fi>, draft-ietf-ipsecme-safecurves@ietf.org
Subject: Re: [IPsec] Stephen Farrell's Yes on draft-ietf-ipsecme-safecurves-05: (with COMMENT)
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Hi, Stephen

> 
> - Wouldn't it be good to encourage minimising re-use of
> public values for multiple key exchanges? As-is, the text
> sort-of encourages use for "many key exchanges" in
> section 4.

I don’t think so. Re-use reduces the computation cost of an IKE Responder (or TLS server) without sacrificing security.  There was some discussion of this in CFRG, but I see that it didn’t make it into RFC 7748, so all I can find is some StackExchange question ([1]).

It does make the static keypair valuable. It is definitely not a good idea to store the private key on-disk and keep it forever, but generating a new key once in a while and discarding the old key is usually a good compromise there.

Anyway key-pair reuse is established practice. Using constant-time implementations is essential to making this practice safe, and the Security Considerations sections says just that.

Yoav

[1] http://crypto.stackexchange.com/questions/11012/reuse-of-a-dh-ecdh-public-key

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