Re: [TLS] Data volume limits

Florian Weimer <fweimer@redhat.com> Mon, 04 January 2016 12:13 UTC

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To: "Salz, Rich" <rsalz@akamai.com>, "Blumenthal, Uri - 0553 - MITLL" <uri@ll.mit.edu>, Eric Rescorla <ekr@rtfm.com>
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From: Florian Weimer <fweimer@redhat.com>
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Subject: Re: [TLS] Data volume limits
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On 12/28/2015 10:09 PM, Salz, Rich wrote:
>> When the key is changed, the change procedure should involve new randomness. 
> 
> I don't think this is necessary, and I don't think the common crypto expertise agrees with you, either. But I am not a cryptographer, maybe one of the ones on this list can chime in.
> 
> "Crank the KDF" suffices.

The attacks against GCM are at the stage where even “periodically
increment the key by one“ would thwart them, right?

The risk is that without real re-key (introducing additional
randomness), someone might come up with a better attack that reduces the
security level below the design target, and which requires similar
effort as the existing GCM attack (four years of traffic at terabit
speed, it seems).

Real re-key is difficult to introduce as an afterthought (see my recent
response to Hubert), and I'd rather see such issues fixed at the cipher
level if at all possible.  The current update-key mechanism doesn't have
the complexity issue of real re-key, but it's ambiguous if it's a design
goal to paper over cipher deficiencies in the rest of the protocol.

Florian