In addition to arXiv as already mentioned, see too this **[directory of mathematics preprint servers][2]**:

> 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][1]**.

Regarding viXra, **[we discussed it over on Math SE][3]** awhile ago. It is distinct from the **[snarXiv][4]**.


  [1]: http://eprint.iacr.org/
  [2]: http://www.mathontheweb.org/mathweb/mi-preprints.html
  [3]: https://math.stackexchange.com/questions/401974/submit-papers-arxiv-or-vixra
  [4]: http://snarxiv.org/