security: add ASCII based CAPTCHA mechanism

[messede-degod]

Add ASCII based CAPTCHA mechanism for new instance creation.

[theaog]

this is a good idea, could prevent users from abusing the service by using scripts that automatically create new sessions.

could you expand on this idea, how would you implement it?

[theaog]

this program might help: https://github.com/moul/captcha
captcha -engine=math

[SkyperTHC]

https://github.com/moul/captcha

trigger when system is under attack.
needs a timeout (when no entry for 30 seconds then exit(255) to stop scripts from hanging.
2nd cutoff to wait forever if system is under further attack.

[messede-degod]

ok, i added that feature and made a pull request here: moul/captcha#130.
mean while my fork should be usable : https://github.com/messede-degod/captcha