Re: [TLS] Data volume limits

Simon Josefsson <simon@josefsson.org> Wed, 16 December 2015 08:44 UTC

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From: Simon Josefsson <simon@josefsson.org>
To: Eric Rescorla <ekr@rtfm.com>
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Date: Wed, 16 Dec 2015 09:44:18 +0100
In-Reply-To: <CABcZeBNR76DqPo0Mukf5L2G-WBSC+RCZKhVGqBZq=tJYfEHLUg@mail.gmail.com> (Eric Rescorla's message of "Tue, 15 Dec 2015 13:14:30 -0800")
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Subject: Re: [TLS] Data volume limits
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Eric Rescorla <ekr@rtfm.com> writes:

> Watson kindly prepared some text that described the limits on what's safe
> for AES-GCM and restricting all algorithms with TLS 1.3 to that lower
> limit (2^{36} bytes), even though ChaCha doesn't have the same
> restriction.

Can we see a brief writeup explaining the 2^36 number?

I don't like re-keying.  It is usually a sign that your primitives are
too weak and you are attempting to hide that fact.  To me, it is similar
to discard the first X byte of RC4 output.

If AES-GCM cannot provide confidentiality beyond 64GB (which would
surprise me somewhat), I believe we ought to be careful about
recommending it.

Of course, the devil is in the details: if the risk is that the secret
key is leaked, that's fatal; if the risk is that the attacker can tell
whether two particular plaintext 128 byte blocks are the same or not in
the entire file, that can be a risk we can live with (similar to the
discard X bytes of RC4 fix).

I believe 64GB is within the range that people download in a web browser
these days.  More data intensive longer-running protocols often transfer
significantly more.

/Simon