[Operators] IM Observatory and Diffie-Hellman parameters

Thijs Alkemade thijs at xnyhps.nl
Wed Nov 13 22:58:25 UTC 2013

On 13 nov. 2013, at 23:46, Dave Cridland <dave at cridland.net> wrote:

> On Wed, Nov 13, 2013 at 10:41 PM, Thijs Alkemade <thijs at xnyhps.nl> wrote:
> On 13 nov. 2013, at 19:21, Dave Cridland <dave at cridland.net> wrote:
>> To decrypt all communications using 1024-bit DH over a year is likely to be vastly bigger than for one conversation; the same isn't true for RSA, for example, where you could solve the private key once.
> This got me pondering, and I'm not quite convinced this is true. It's a bit
> late, so sorry if what I'm saying has some cryptographic errors.
> A naive brute-force attack on a DH key exchange would try g^1, g^2, g^3, ...
> to try to find either the exponent used by the server or the one used by the
> client. Assuming the DH group is the same, doing this for one key or for two
> or more keys at the same time should not take that much more time (I'd expect
> the multiplication by g to dominate the comparisons).
> Ah, so you're suggesting a brute-force attack against multiple parallel DH uses of the same key would be cost-effective?
> That's interesting, and if you're right - and you may well be - then I'm certainly quite wrong here.
> I've copied the security@ list on this one, where wiser minds than me hang out.

Not the same key - just multiple keys generated using the same DH group.
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