The crypt() function is required by the POSIX Standard (and is there so passwords can be encrypted). But that is a one-way operation. You can use crypt() to encrypt your password and you can use crypt() to encode a user-entered password guess and then use strcmp() to see if the two encrypted strings match.

There is also the encrypt() function (which is in the current standards) and the crypt utility (which is not in the current standards), which are capable of encoding and decoding text. But, US government regulations prohibit exporting that code to many other countries. So while a Solaris system shipped by Oracle to a US customer might contain the crypt utility and the encrypt() function, a corresponding Solaris system sent to China, Iran, or Russia (and maybe any other country outside the US borders) would get a an encrypt() function in libc that always fails and returns with errno set to ENOTSUP when asked to decode encrypted text (and maybe also when asked to encode text). I no idea whether systems Oracle ships with the reduced capability encrypt() are shipped with a non-functioning crypt utility or if it is dropped from the distribution completely.