Seti doesn't make the code or exact algorithm it uses public, I don't know about other projects. They say this is for security reasons, mostly at Seti's end. Personally I think this is Security by Ignorance, and isn't much security at all.

They have explained in general how they search, there are several links on the Seti site in the sience section that cover how the algorithm works.

The new system, Boinc, had better have some security built in. Not so much because of local or server side hacking but because I could see some clever hacker hijack the autoupdate system to distribute their own code and do some distributed key breaking.

Jay