Decrypt Unix Password File

How can I decrypt Unix password file?

Jerome Tan <jti@i-manila.com.ph> writes:

> How can I decrypt Unix password file?

If the /etc/passwd file does not use shadow passwords, then the second field
of each line contains the 'salt' and a value dependent on both the salt and
the secret password.

One can try to compute the function of all reasonable dictionary words with
the salts in the /etc/passwd file, and hope that some of them match the
values listed in the file.

There are many programs that do this, e.g., look for 'crack'.

This attack can be made more difficult if you force your users not to use
easy-to-guess passwords, and if you use something like NIS and shadowing to
make the public part of the passwords harder to get.

---

Dr.Dimitri Vulis KOTM
Brighton Beach Boardwalk BBS, Forest Hills, N.Y.: +1-718-261-2013, 14.4Kbps

On Tue, 23 Jul 1996, Dr.Dimitri Vulis KOTM wrote:
> Jerome Tan <jti@i-manila.com.ph> writes:
> > How can I decrypt Unix password file?
> There are many programs that do this, e.g., look for 'crack'.
> This attack can be made more difficult if you force your users not to use
> easy-to-guess passwords, and if you use something like NIS and shadowing to
> make the public part of the passwords harder to get.

     From my conversations with Mr. Tan, he seems to be a high school 
bent of mischeif. He is the one who asked about penetating firewalls, 
and now wants to know how to hack a unix passwd file. 

     Now, I am not philosophically opposed to hacking, unless you are doing
it to a machine that I am responsible for, (in which case you'd better hope
the FBI finds you before I do) but I don't think that it would be a good 
idea to just give him the information. He would wind up getting caught all 
too easily, and might point to this list as a source of information on 
cracking techniques. 

     I don't know if this should go to the whole list, so you can 
bounce it there if you think it proper.

Petro, Christopher C.
petro@suba.com <prefered for any non-list stuff>
snow@smoke.suba.com

> 
> On Tue, 23 Jul 1996, Dr.Dimitri Vulis KOTM wrote:
> > Jerome Tan <jti@i-manila.com.ph> writes:
> > > How can I decrypt Unix password file?
> > There are many programs that do this, e.g., look for 'crack'.
> > This attack can be made more difficult if you force your users not to use
> > easy-to-guess passwords, and if you use something like NIS and shadowing to
> > make the public part of the passwords harder to get.
> 
>      From my conversations with Mr. Tan, he seems to be a high school 
> bent of mischeif. He is the one who asked about penetating firewalls, 
> and now wants to know how to hack a unix passwd file. 
> 
>      Now, I am not philosophically opposed to hacking, unless you are doing
> it to a machine that I am responsible for, (in which case you'd better hope
> the FBI finds you before I do) but I don't think that it would be a good 
> idea to just give him the information. He would wind up getting caught all 
> too easily, and might point to this list as a source of information on 
> cracking techniques. 

Just what they want, anyway -- make cypherpunks look like villins.

> 
>      I don't know if this should go to the whole list, so you can 
> bounce it there if you think it proper.

I think you used your judgement well in this case.

There are many places to start learning about firewalls and UNIX security.
I recommend the _Building Internet Firewalls_ O'reilly book, as well as
_Practical UNIX & Internet Security_ as well.  I don't feel right about
spoon feeding cracking info to someone like this.

(PS:  The animal on the _Building Internet Firewalls_ book is hidden
behind the gates.  It is a Trojan Horse.  I heard this secondhand.)

> 
> Petro, Christopher C.
> petro@suba.com <prefered for any non-list stuff>
> snow@smoke.suba.com
>

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On Tue, 23 Jul 1996, Jerome Tan wrote:

> How can I decrypt Unix password file?

You can't decrypt a password file.  The password is hashed by using the
password as a DES key, and encrypting a string of 8 NULs 25 times.  The
E-tables of the DES algorithm are permutated according to the twelve-bit salt
which is encoded in the first two characters of the hashed password field.
The E-tables are permutated by swapping the entries N and N+24 if the Nth bit
of the salt value is 1.  A salt value of 0 will result in straight DES being
used 25 times.  This is the only salt value that can't be used in the UNIX
password file.

A program like Crack will use a dictionary attack to crack a password file.
It's available at ftp://ftp.funet.fi/pub/security.

- -- Mark

PGP encrypted mail prefered
Key fingerprint = d61734f2800486ae6f79bfeb70f95348
http://www.voicenet.com/~markm/  


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