[Operators] Please enable Forward Secrecy for your servers!

Mathias Ertl mati at fsinf.at
Mon Jul 27 20:27:36 UTC 2015


On 2015-07-27 20:58, Jonathan Schleifer wrote:
> Am 27.07.2015 um 20:09 schrieb Mathias Ertl <mati at fsinf.at>:
>>> On 2015-07-21 00:19, Jonathan Schleifer wrote: So, 4096 bit RSA
>>> just gives you an additional 16 bits for your AES, while doubling
>>> the number of RSA bits more than doubles the computational
>>> overhead…
>> I consider this argument invalid. It's not because "just additional
>> 16 bits" is wrong. Its because the "double the overhead" is
>> completely irrelevant. Even we have only two CPUs and still very
>> little CPU usage. So sure it's double. But double of next to
>> nothing is still nothing.
> Sorry, but this is plain wrong!
> Doubling the number of bits DOES NOT double the computational work -
> it's not growing linearly, but exponentially! For example, I have a
> smart card that can do RSA-2048 in little above one second. RSA-4096
> on the other hand takes 8 seconds. (The device in question is the
> FST-01 running Gnuk, in case you want to know to verify and not just
> take my word on it.)
> But it gets worse: Your statement that double of next to nothing is
> still nothing couldn't be more wrong. In fact, the SSL handshake is
> the most expensive part of most of the XMPP connections!

Sorry, but you missed the point _entirely_. I didn't claim that the TLS
handshake is the cheap part of an XMPP connection. I said that - based
on quite a bit of experience - running an XMPP server requires very
little CPU power, even with 4096 bit certificates. And with the same
experience that switching from 2048 to 4096 bit certs didn't bring us
*any* noticeable increase in CPU load.

> So, please get your facts straight, as this all can easily be
> researched using your favorite search engine.

I think I have my facts quite queer, thank you.

greetings, Mati

twitter: @mathiasertl | xing: Mathias Ertl | email: mati at er.tl
I only read plain-text mail!  I prefer signed/encrypted mail!

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