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Re: [TLS] DHE key derivation

Michael D'Errico <mike-list@pobox.com> Fri, 27 September 2013 16:18 UTC

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Date: Fri, 27 Sep 2013 09:18:51 -0700
From: Michael D'Errico <mike-list@pobox.com>
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Subject: Re: [TLS] DHE key derivation
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Daniel Kahn Gillmor wrote:
>> It is not recommended to use the same RSA key for both signing and
>> key encipherment.
> 
> I hear this recommendation a lot, and I think it makes sense, but i'm
> wondering if there is a "canonical" reference for it.  It seems to be
> violated far more often than it is honored.

One contemporary reference is Cryptography Engineering, page 201.

The recommendation is if you want to reuse N for both operations, then
choose different public exponents such as e=3 for signing and e=5 for
encryption.  I'm pretty sure you can't do this using the same X.509
certificate.

Mike

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