Getting Passwords from Random Data (portably!)

Over at Mark Kolich’s blog, he wrote several months ago about using a source of randomness (/dev/urandom) to generate passwords. The idea is simple enough: take the random data, strip out only the printable characters, and then print the desired length of characters for a password.

Shortly thereafter, he described how to use a simple shell script to generate many passwords – such as for setting up many different accounts.

Working with HP-UX and OpenVMS as I do, I immediately thought: how could I do this in Perl, making the idea portable and making a program that will work on both UNIX and OpenVMS? It was easy – and easy to make it flexible as well. Here is the program that I came up with:


# code released by David Douthitt into the public domain

use Getopt::Long;

GetOptions( 'l=i' => \$opt_l,
            'p=s' => \$opt_p,
            'm=i' => \$opt_m );

$pat{"ext"} = "[[:alnum:][:punct:]]";
$pat{"alnum"} = "[[:alnum:]]";
$pat{"alpha"} = "[[:alpha:]]";
$pat{"simple"} = "[a-km-z2-9]";
$pat{"normal"} = "[a-km-z2-9A-HJ-NPR-Z]";

if (defined($opt_p)) {
   if (defined($pat{$opt_p})) {
      $pat = $pat{$opt_p};
   } else {
      print "undefined pattern!\n";
} else {
   $pat = $pat{"normal"};

$max = (defined($opt_m) ? $opt_m : 1000);
$len = (defined($opt_l) ? $opt_l : 6);

$x = $len;

for $i (0..$max) {
   $c = chr(int(rand(255)));
   if ($c =~ /$pat/o) {
      $s .= $c;
      if (--$x == 0) {
         print "$s\n";
         $x = $len;
         $s = "";

Note that since OpenVMS does not use the “#!” notation, that this line will be ignored as a comment and the program needs to be invoked via direct invocation of perl itself.

As an aside, Mark says how he prefers random passwords. Me, I prefer “pronouncable” passwords – still random, but using phoenemes which makes the generation process just that more complicated – and complicates internationalization. Apple’s MacOS X comes with a password generator that can generate random and pronouncable passwords.

However, with the proper password storage system a fully randomized password is good – or is it? A completely random password of eight characters could be zzzzzzzz as much as anything else. Perhaps a password with a random distribution of characters (rather than a random selection of characters) would be better. I’m not aware of any password generators that guarantee a random distribution instead of a random collection.

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5 thoughts on “Getting Passwords from Random Data (portably!)”

    1. Actually, for MacOS X you can utilize the built-in password generator tool. There’s at least one free software package that makes this capability available at a click.

  1. David, I’m glad you enjoyed my blog post on using /dev/urandom to generate random passwords. It’s always nice to see someone out there found my blog useful. Would you mind if I added a link to this blog post on

    BTW, I see that you’re the author of “GNU Screen: A Comprehensive Manual”. I cannot live without “screen”; it’s an absolute must have for any developer or system administrator.

    -Mark Kolich

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