[Operators] Please enable Forward Secrecy for your servers!

Jonathan Schleifer js-xmpp-operators at webkeks.org
Mon Jul 27 18:58:10 UTC 2015

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!

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


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