Re: [TLS] Last Call: draft-hoffman-tls-additional-random-ext (Additional Random

Simon Josefsson <simon@josefsson.org> Fri, 23 April 2010 15:36 UTC

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From: Simon Josefsson <simon@josefsson.org>
To: Paul Hoffman <paul.hoffman@vpnc.org>
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Date: Fri, 23 Apr 2010 17:36:02 +0200
In-Reply-To: <p06240803c7f60d8cde2c@[10.20.30.249]> (Paul Hoffman's message of "Thu\, 22 Apr 2010 07\:29\:46 -0700")
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Cc: ietf@ietf.org, tls@ietf.org
Subject: Re: [TLS] Last Call: draft-hoffman-tls-additional-random-ext (Additional Random
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Paul Hoffman <paul.hoffman@vpnc.org>; writes:

> At 12:51 AM +0200 4/22/10, Simon Josefsson wrote:
>>In which environments is the extension useful?
>>
>>The only motivation in the document that I can find is this:
>>
>>  In some application environments, it is desirable to have the client
>>  and/or the server be able to input more random material in the master
>>  key calculation than is allowed by the fixed-length Random value.
>>
>>I believe more justification than that is required for Proposed
>>Standard.
>>
>>In particular, what I'd like to see is references to some application
>>environments where the extension is desirable, and the rationale why it
>>is desirable in that environment.
>>
>>Without a rationale for when the extension is useful, it is impossible
>>for implementers to know when use of this extension is warranted or not.
>
> The environment I described in the earlier thread is TLS with
> Diffie-Hellman. I thought that saying that was sufficient, but I guess
> it wasn't.

People shouldn't have to read the mailing list to understand the
applicability, so please describe some environments in the document.

> In Diffie-Hellman key establishment with static keys, even if the PRNG
> of one side is bad, the resulting pre-master secret is still sound.
> Neither side knows whether or not the PRNG of the other side is bad, so
> each side wants to supply sufficient randomness for the master secret
> even if the other side's PRNG is bad. If a side with a bad PRNG adds its
> own input, it doesn't hurt the randomness of the result, but a side with
> a good PRNG can bring up the amount of randomness.

Are you saying that the 28 bytes of randomness provided in the client
and server hello is not sufficient?

> I did not want to list this as the justification because there may be
> other reasons to use the extension, and I don't want readers to think
> that this is the only one. For example, future types of TLS key
> establishment might have similar properties as static-static
> Diffie-Hellman.
>
> Does that help?

This information certainly helps, but it belongs in the document.

/Simon