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

Martin Rex <mrex@sap.com> Thu, 22 April 2010 16:27 UTC

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From: Martin Rex <mrex@sap.com>
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To: paul.hoffman@vpnc.org (Paul Hoffman)
Date: Thu, 22 Apr 2010 18:24:17 +0200 (MEST)
In-Reply-To: <p06240803c7f60d8cde2c@[10.20.30.249]> from "Paul Hoffman" at Apr 22, 10 07:29:46 am
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Cc: simon@josefsson.org, ietf@ietf.org, tls@ietf.org
Subject: Re: [TLS] Last Call: draft-hoffman-tls-additional-random-ext
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Paul Hoffman wrote:
> 
> 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.

TLS needs _more_ than the secrecy of the pre-master secret to be secure.

Snippets from rfc-5246 (TLS v1.2):

http://tools.ietf.org/html/rfc5246#section-6.2.3.2

   6.2.3.2. CBC Block Cipher

   [...]

      The Initialization Vector (IV) SHOULD be chosen at random, and
      MUST be unpredictable.


http://tools.ietf.org/html/rfc5246#appendix-F.1.1.3

   F.1.1.3.  Diffie-Hellman Key Exchange with Authentication

   [...]

   If the client has a certificate containing fixed Diffie-Hellman
   parameters, its certificate contains the information required to
   complete the key exchange.  Note that in this case the client and
   server will generate the same Diffie-Hellman result (i.e.,
   pre_master_secret) every time they communicate.

   [...]

   If the same DH keypair is to be used for multiple handshakes, either
   because the client or server has a certificate containing a fixed DH
   keypair or because the server is reusing DH keys, care must be taken
   to prevent small subgroup attacks.  Implementations SHOULD follow the
   guidelines found in [SUBGROUP].


-Martin