I think a judge would have absolutely no problem with feds exploiting Tor or GPG to get to their targets. As far as backdoors go, that is why things should be open source. You can simply have a hash value of the code and then everyone who audits it can confirm they audited the files that hashed to so and so value, and people who use it can hash it to verify its integrity prior to using it. The closest thing to bulletproof way to prevent exploits would be formal verification I imagine, but not even the bitcoin clients themselves have such a high degree of security. I think you over estimate the amount of code complexity required for a blind mix, particularly if you consider so much of it is already done in crypto libraries. here is one blind mint protocol (I tried to fix up the formatting) A lot of these pieces are done, just need hooked together. All the code would consist of, would be these math formulas for the client and server (many parts of which are in crypto libraries), a tiny bit of networking code so the client and server can communicate with each other, and maybe a wrapper for a bitcoin client (although that isn't even required).