Re: [TLS] PRF in 1.3

Henrik Grubbström <grubba@gmail.com> Wed, 06 August 2014 11:43 UTC

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References: <201407281624.10986.davemgarrett@gmail.com> <53D9147B.1080203@nthpermutation.com>
Date: Wed, 6 Aug 2014 13:43:15 +0200
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To: Michael StJohns <msj@nthpermutation.com>
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Subject: Re: [TLS] PRF in 1.3
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On Wed, Jul 30, 2014 at 5:51 PM, Michael StJohns <msj@nthpermutation.com> wrote:
> On 7/28/2014 4:24 PM, Dave Garrett wrote:
>>
>> https://github.com/tlswg/tls13-spec/issues/26
>>
[...]
> I don't believe this is the right place to start asking the questions.
> Instead, let's look at where the PRF is used and in what context, and then
> figure out whether or not what we have is good enough.

Agreed.

[...]
> So breaking it down:
>
> Is (1) secure enough to derive keys?   The construct used in TLS is similar
> to the section 5.2 construct of NIST SP800-108.  But the SP800 construct has
> one extra piece missing from this one - the L parameter which ensures that
> the key stream changes if the number of output bits change.  There's also
> expanded guidance on the context field - the equivalent values in TLS are
> currently just the client and server random.   I've got some issues with
> that - see
> http://tools.ietf.org/html/draft-stjohns-tls-tls13-crypto-infra-00, and I
> would add information about the types and lengths of the keys to be derived
> from the key stream to the context.  With those changes, a recursive based
> HMAC PRDG at the heart of the KDF is probably sufficiently secure for key
> derivation going forward.

I'd like to hear from the people that rejected P_SHA256 for the AES256
suites in RFC 5289 (which instead use P_SHA384). I suspect that it is
to avoid leaking the entire state of the PRF in the AES256 keys and/or
to avoid correlation between the client-side and server-side keys.

[...]
> The requirements for (2) (key not available until later) argues that the TLS
> PRF has to continue to be built on a hash function unless the data signed by
> (2) changes.  The discussion for (4) suggests that we can't just say that
> SHA256 will always be the value.

Agreed.

/grubba

-- 
Henrik Grubbström                                       grubba@grubba.org
Roxen Internet Software AB                              grubba@roxen.com