RFC 208 (v2) crypt() default salt

This and other RFCs are available on the web at
  http://dev.perl.org/rfc/

=head1 TITLE

crypt() default salt

=head1 VERSION

  Maintainer: Mark Dominus <[email protected]>
  Date: 11 Sep 2000
  Last Modified: 13 Sep 2000
  Mailing List: [email protected]
  Number: 208
  Version: 2
  Status: Developing

=head1 ABSTRACT

A frequently-asked question is how to generate an appropaite random
salt for password encryption.  I propose that Perl generate the salt
automatically if the salt argument is omitted in the call to crypt().

=head1 DESCRIPTION

At present, crypt() requires two arguments:

         crypt PLAINTEXT,SALT

It then passes these arguments directly to the C library crypt()
function.

When encrypting a new password, the programmer is required to generate
a salt at random:

        @letters = ('A' .. 'Z', 'a' .. 'z', '0' .. '9', '/', '.');
        $salt = $letters[rand@letters] . $letters[rand@letters];
        $passwd = crypt($passwd, $salt);

This is inconvenient and nonportable.  It's also nonobvious: people
frequently ask in the newsgroups how to do it.  I propose that if the
SALT argument is omitted, Perl should generate an appropriate salt
internally and use that.

        $passwd = crypt($passwd);    # Same as above

On systems where the password format is different, Perl can do the
appropriate thing.

=head1 IMPLEMENTATION

For the standard DES-based crypt, the implementation is
straightforward trivial. Perl already has many functions that take an
optional argument, and the C internals of the random-salt generator
are well-known.

Details will vary for systems using alternative password hashing
schemes.  On some systems, no salt need be generated.  These can be
taken care of with a suitably ifdef'ed section of code if necessary.

If the random number generator has not yet been seeded, Perl should
seed it.  

Michael Schwern has developed a partial demonstration implementation
in pure Perl.  It is available from

         http://www.pobox.com/~schwern/src/RFC-Prototype-0.01.tar.gz


=head1 MIGRATION

C<crypt()> with only one argument is presently a compile-time error,
so there are probably few translation issues.  The meaning of this
program will change:

        $" = ', ';
        $code = "crypt(@ARGV)";
        eval $code;
        die $@ if $@;

But I don't think this is anything to worry about---it should fall
into the "other 5%" category.

=head1 REFERENCES

perlfunc manpage for discussion of crypt()

crypt(3)