Re: [TLS] STRAW POLL: Size of the Minimum FF DHE group

Manuel Pégourié-Gonnard <mpg@polarssl.org> Wed, 05 November 2014 12:17 UTC

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Date: Wed, 05 Nov 2014 13:17:17 +0100
From: Manuel Pégourié-Gonnard <mpg@polarssl.org>
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To: Bodo Moeller <bmoeller@acm.org>, "tls@ietf.org" <tls@ietf.org>
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Subject: Re: [TLS] STRAW POLL: Size of the Minimum FF DHE group
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On 05/11/2014 12:56, Bodo Moeller wrote:
> Manuel Pégourié-Gonnard <mpg@polarssl.org>:
>> multiplication is quadratic, and the length of the exponent, which is 
>> roughly the number of multiplications, is linear in the size of the group.
>> 
> No, because you can still use short exponents. If you're doing that,
> draft-ietf-tls-negotiated-ff-dhe-02 recommends (at least) 224 bits [*] for
> the exponent when using the 2432-bit modulus.
> 
Right, thanks for pointing that out. For some reason I assumed the short
exponent would be longer for the 2432-bit group than for the 2048-bit, but of
course this is stupid because both target the 112-bit security level, and the
question is which FF size corresponds to that level.

Manuel.