The friendly crypt() function that you're likely to find in any *nix operating system, applies a (hopefully) cryptographically strong hashing function to the supplied password and salt. The general idea behind this, is converting the cleartext password you gave it into a hash (some people uses the term signature).

With that hash, it is computationaly infeasible to find a strong-enough password. What this means in lay man terms, is that it is very hard to learn the original (cleartext) password out of the hash and salt that lives in /etc/passwd.

I know of two common implementations of the crypt() functions: The DES based and the MD5 based. Newer systems tend to use the MD5 based crypt(), for a number of reasons.

Note that the MD5-based crypt() is not the same as obtaining the hash of your password with Digest::MD5 or similar. The algorythm used internally by the MD5-based crypt() uses a number of transformations in which the MD5 algorythm is used, but is very different.

Crypt::PasswdMD5 implements this algorythm in Perl, allowing you to reproduce the result of said crypt() functions in non-*nix systems or systems without a compatible crypt() implementation.

Regards.