Libcrypt is a copy prevention mechanism developed by Sony for use in their PlayStation systems. Note that not all PlayStation titles actually use Libcrypt -- some are simply unprotected. The protection consists of a two-stage verification algorithm: the first stage attempts to validate the game disc as an original PlayStation disc; following that, a checksum is then calculated from the data on disc and is passed as a magic number to subroutines.
The protection code is written in the PlayStation's native assembly language, which is enough to thwart some older modchips and Action Replay-like devices, as they work by intercepting library calls rather than tracing CPU registers. However, newer circumvention devices don't have much of a problem faking the response codes. The protection scanning software ClonyXXL rates Libcrypt as 3/10 in terms of difficulty, implying that it is relatively easy to circumvent.