crypt

crypt-PLAINTEXT,SALT

Encrypts a string exactly like the crypt(3) function in the C library (assuming that you actually have a version there that has not been extirpated as a potential munition). This can prove useful for checking the password file for lousy passwords, amongst other things. Only the guys wearing white hats should do this.

Note that crypt is intended to be a one-way function, much like breaking eggs to make an omelette. There is no (known) corresponding decrypt function (in other words, the crypt() is a one-way hash function). As a result, this function isn't all that useful for cryptography. (For that, see your nearby CPAN mirror.)

When verifying an existing encrypted string you should use the encrypted text as the salt (like crypt($plain, $crypted) eq $crypted). This allows your code to work with the standard crypt and with more exotic implementations. In other words, do not assume anything about the returned string itself, or how many bytes in the encrypted string matter.

Traditionally the result is a string of 13 bytes: two first bytes of the salt, followed by 11 bytes from the set [./0-9A-Za-z], and only the first eight bytes of the encrypted string mattered, but alternative hashing schemes (like MD5), higher level security schemes (like C2), and implementations on non-UNIX platforms may produce different strings.

When choosing a new salt 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]).

Here's an example that makes sure that whoever runs this program knows their own password:

    $pwd = (getpwuid($<))[1];

    system "stty -echo";
    print "Password: ";
    chomp($word = <STDIN>);
    print "\n";
    system "stty echo";

    if (crypt($word, $pwd) ne $pwd) {
	die "Sorry...\n";
    } else {
	print "ok\n";
    }  

Of course, typing in your own password to whoever asks you for it is unwise.

The crypt function is unsuitable for encrypting large quantities of data, not least of all because you can't get the information back. Look at the by-module/Crypt and by-module/PGP directories on your favorite CPAN mirror for a slew of potentially useful modules.

If using crypt() on a Unicode string (which potentially has characters with codepoints above 255), Perl tries to make sense of the situation by trying to downgrade (a copy of the string) the string back to an eight-bit byte string before calling crypt() (on that copy). If that works, good. If not, crypt() dies with Wide character in crypt.