Libcrypt is a copy protection mechanism developed by Sony for use in their PlayStation systems. 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 can emulate the response codes. The protection scanning software ClonyXXL rates Libcrypt as 2/5 skulls in terms of difficulty, implying that it is relatively easy to circumvent.
Only some PlayStation titles actually use Libcrypt -- most are simply unprotected.