Re: [TLS] Also, on client-random and server-random

Ravi Ganesan <ravi@findravi.com> Wed, 03 February 2010 12:15 UTC

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Date: Wed, 3 Feb 2010 04:16:00 -0800
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From: Ravi Ganesan <ravi@findravi.com>
To: Adam Langley <agl@google.com>
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Subject: Re: [TLS] Also, on client-random and server-random
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Sorry my comments were
i) only meant to apply to RSA based ciphersuites (all we support as
yet in MashSSL).
ii) And, I said it exactly backwards.

In the abbreviated handshake as S does not know who C is upon getting
a client-hello, if you did not have the server-random, then a
malicious client could (if there were no other checks), get the server
to provide a large number of server-finisheds to carefully chosen
client-randoms, w/o completing the protocol.

Conversely, in the full handshake as C does not know who S is when
computing certificate-verify (assuming it is requested), a malicious
server could (if there were no other checks), get the client to
provide a large number of certificate-verifies on carefully chosen
server-randoms w/o completing the protocol.

Again, there was a time in SSL when these two random numbers were NOT
in protocol, (see this description of SSL 2.0
http://www.mozilla.org/projects/security/pki/nss/ssl/draft02.html for
instance) and then they appear. I was trying to understand the
motivation for why they were added in, and the above two reasons were
all I could find.


On Wed, Feb 3, 2010 at 2:16 AM, Adam Langley <agl@google.com> wrote:
> On Tue, Feb 2, 2010 at 11:22 PM, Ravi Ganesan <ravi@findravi.com> wrote:
>> FWIW, in my deconstruction of why these values exist, it seemed to me
>> that if you ran the full handshake with only the server-random [...] you were ok
>
> Consider a TLS connection using DH that I just sniffed from the network.
>
> If there's no client random then what stops me from intercepting
> future connections from the same client, to the same server and
> replaying the server's side to the client? The server random would be
> the same, as would the DH value and signature. I think, from a quick
> thought experiment, that the master secret would end up the same and
> the client would accept it. There's no reason that the client is
> required to use a different DH public value.
>
>
> AGL
>