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Re: passwd hashing algorithm
From: pcl () foo oucs ox ac uk (Paul C Leyland)
Date: Wed, 19 Apr 1995 12:34:22 +0100


David Wagner, dawagner () princeton edu, wrote:

Just one trivial elaboration on an informative message from
Steve Bellovin:

                         There's only one facet of triple DES that's
at all useful here:  it provides an easy way to accept longer passwords.
But as I've noted, there are other ways to do that.  (Double DES is
most likely quite sufficient if you want to pursue that route, though;
few people are going to use passwords longer than 16 characters, and
the attacks on double DES described in the cryptographic literature
require O(2^55) storage, if I recall correctly -- I may be off by a
factor or so of 2.)


If anyone actually plans to use double DES (or triple DES)
for hashing passwords (which I don't recommend), be aware
that there's a huge difference between:

1. 25 iterations of DES with the first 8 bytes of the
   password as key, followed by 25 iterations of DES
   with the second 8 bytes of password as key.

This is essentially how DEC's C2-security in OSF/1 works for passwords
9-16 characters long.  A further weakness is that the output of
bigcrypt() gives the length of the plaintext password in units of 8
characters!  So if ever you can get hold of the password file, you can
concentrate on the easy cases.  In particular, it might be worth while
making a complete table of all common suffices and then searching the
9-16 character portions for matches.

The old crypt16(), from Ultrix and supported as a migration tool in
OSF/1 was even worse.  It used 20 rounds on one block and 5 on the
second, making password searching even faster than traditional
crypt().  I wrote the mods. for Crack to use crypt16().  The suffix
attack works like a dream -- 5 times faster than regular Crack -- and
concentrating on the <9 char. passwords goes in 80% of the time.  DEC
screwed up badly there.

2. repeat 25 times:
     an iteration of DES with the first 8 bytes of the
     password as key, followed by an iteration of DES
     with the second 8 bytes of password as key.

(1) can be broken on a workstation with ~ 2^32 steps (and
very little in the way of memory); (2) is probably very
strong.  The same comment goes for triple DES.

But still not strong enough.  Password guessing will work in many
cases because people will still choose guessable passwords, unless you
have a fascist password program.  If you have one of those,
traditional crypt is still adequate, IMO.

The moral of the story?  If you wanna hash a long string,
use a hash function (i.e. MD5), not a block cipher; or
else be very careful. :-)

IMO, you should do both.


Paul



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