Programming Perl, 4th Edition

crypt

crypt PLAINTEXT, SALT

This function computes a one-way hash of a string exactly in the manner of crypt(3). This is somewhat useful for checking the password file for lousy passwords,^[236] although what you really want to do is prevent people from adding the bad passwords in the first place.

crypt is intended to be a one-way function, much like breaking eggs to make an omelette. There is no (known) way to decrypt an encrypted password apart from exhaustive, brute-force guessing.

When verifying an existing encrypted string, you should use the encrypted text as the SALT (like crypt($plain, $crypted) eq $crypted). This lets your code work with the standard crypt (and with more exotic implementations, too).

When choosing a new SALT, you minimally need to create a random two-character string whose characters come from the set [./0–9A–Za–z] (like join "", (".", "/", 0..9, "A".."Z", "a".."z")[rand 64, rand 64]). Older implementations of crypt needed only the first two characters of the SALT, but code that gives only the first two characters is now considered nonportable. See your local crypt(3) manpage for details.

Here’s an example that makes sure that whoever runs this program knows his own password:

$pwd = (getpwuid ($<))[1]; # Assumes we're on Unix system "stty –echo"; # or look into Term::ReadKey on CPAN print "Password: "; chomp($word = <STDIN>); print "\n"; system "stty echo"; if ...