4096 bit SSL keys

[Georgi Guninski]

On Sat, Sep 03, 2016 at 05:29:07PM -0700, Riad S. Wahby wrote:
> John <jnn at synfin.org> wrote:
> > The reason I asked: updating a few certs at office recently I nuked
> > an older F5 LTM device by installing a 4096 bit key/cert pair - the
> > load on the appliance (Linux based) shot up from less than 1 to about
> > 30 and became so excruciatingly slow it was nearly impossible to back
> > the change out (web GUI and ssh were both nearly non-responsive)..
> 

I think this could be some crypto/SSL issue, consider asking them.


> Multiplying two n-bit numbers naively costs O(n^2) operations (one
> can do better with Karatsuba and related tricks), and exponentiation
> costs O(n) multiplications (so O(n^3) in total). So assuming the device
> is using something like Montgomery multiplication you'd expect about
> 8x increased load. 30x sounds like there *could* be some other issue
> with the implementation, but a significant slowdown is not unexpected.
>

This applies to straightforward implementation. openssl is
protecting against timing attacks, which additionally slows, but
unless there is bug, the load shouldn't jump so much.


A 2010 thread discusses timings with sign/verify with 100K bit RSA
keys, pycrypt (python + some libraries) is significantly faster than
openssl. The keys/certs are somewhere on the interwebs:
http://openssl-dev.openssl.narkive.com/Y60Bau1J/inconsistent-timings-for-rsa-sign-verify-with-100k-bit-rsa-keys
=====
inconsistent timings for rsa sign/verify with 100K bit rsa keys
sign      verify
key1 5min  <1sec
key2 48min 21min
(tested on patched openssl1.0.0a)
=====


Besides quantum computers, another threat is private factoring
algorithm, sufficiently faster than the public ones. Say one more
square root in the subexponential stuff or the constants
significantly lower.