In addition to arXiv as already mentioned, see too this directory of mathematics preprint servers:
It makes available the current homepage URLs and email contacts of mathematical preprint and e-print servers throughout the world. It is a tool to help find these servers in order to browse the articles posted on them or to post an article to the server itself. The servers are divided into three categories: umbrella servers which cover all areas of mathematics such as the Mathematics ArXiv and the MPRESS/MathNet.preprints server, special subject servers and servers administered by mathematics departments and institutes. There is an additional link to retired preprint services.
Insofar as mathematics and cryptography intersect, I would suggest the IACR cryptography eprint service.
Regarding viXra, we discussed it over on Math SE awhile ago. It is distinct from the snarXiv.
