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In <9210080452.AA12560@deepthought.cs.utexas.edu> STDN%MARIST@VM.MARIST.EDU (Dan Newcombe) writes:

>Well , the subject really says it...

>  what is this DES crypt function, and why is it so important?

>Sorry for being ignorant.
>  -Dan

I have a description from Michael Glad in Denmark:


To represent binary values at encrypted password and the encryption salts,
the UNIX crypt algorithm uses at 6 bit character set, i.e. a mapping of the
integers 0..63 to ASCII characters. The mapping is

	./0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ

Salt values have to be in this set. The salt characters are converted one at a
time giving two 6 bit values. They are combined to a 12 bit entity where the
6 bit representing the first salt character gets shifted by 6. The 12 bit value
is then used to swap entries in the E selection table. The entries with
index 0..11 are permuted with the entries with index 12..33 according to bits
set in the salt value. The entries in the table specify a selection, the table
has 48 entries with values in the range 1..32 meaning that a 32 bit value is
expanded to 48 bit.

The 6 bit representation is also used to pack the output of the 25 DES round.
DES output is 64 bits. Two implicit zero bits are then added giving 66 bits.
These bits are then converted to ASCII 6 bits a time giving 11 characters.
Being prefixed with the two salt characters, we get 13 chars alltogether.

The password is used to give the DES key using 7 bits per character. Passwords
shorter than 8 bytes are padded with zero bytes.

The salting (and the conventions for representing output) are the only changes
made to DES. 

Fast implementations are available. Just to mention one random chosen :-),
UFC-crypt is a package which fits nicely into Crack (it is mentioned in the Crack
docs. It is availble for anon. ftp from many sites, also from 'ftp.daimi.aau.dk'
in 'pub/ufc-crypt.pl1e.tar.Z'. It may appear a little hard to understand unless
you've studied the DES standard closely.

The password encryption is the same in virtually all UNIX versions.
The 'Berkeley Crypt' supports 24 bits salts (simple extension) but this is not
in common use. Digital has a 'Enhanced security option' which used a 'crypt16'
function. UFC implements it too. It was recently made to mincemeat in a discussion
in alt.security.

I hope I've clarified things a little. You're welcome to contact me if you have
further questions.

Sincerely,

	-- Michael Glad, email: glad@daimi.aau.dk



Regards,
Jorgen Bo Madsen

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