Decrypt Unix Password File

[Mark M.]

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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

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