Re: [TLS] DHE key derivation

Michael D'Errico <mike-list@pobox.com> Fri, 27 September 2013 15:22 UTC

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Date: Fri, 27 Sep 2013 08:21:54 -0700
From: Michael D'Errico <mike-list@pobox.com>
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To: Yaron Sheffer <yaronf.ietf@gmail.com>
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Cc: tls@ietf.org
Subject: Re: [TLS] DHE key derivation
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Yaron Sheffer wrote:
> While we're opening (maybe) the negotiation of DHE, I'd like to clarify 
> an issue that bothers me in the implementation of DHE in TLS:
> 
> With DHE, the premaster secret depends only on the DH shared secret. We 
> know that DHE is commonly used with 1024-bit parameters. So even if you 
> have a 2048-bit RSA certificate, the session strength will be 1024 bits.
> 
> What if we mixed *both* the DH secret and the regular encrypted nonce 
> that's used in RSA ciphersuites into the premaster secret? Wouldn't we 
> get forward secrecy, as well as crypto strength equivalent to the higher 
> of the two lengths?

In DHE_RSA, RSA is used in signature mode to sign the DH parameters.
It is not recommended to use the same RSA key for both signing and
key encipherment.

Mike