How do I Audit Unix Passwords?

To audit Unix passwords, you must compare each encrypted password in the Unix password file with a set of potential encrypted passwords. These potential encrypted passwords are created by encrypting every password in a list of plaintext passwords. This is an example of a dictionary attack.


The Unix passwd File
The traditional location for the Unix password file was /etc/passwd.

On a Unix system with either NIS/yp or password shadowing, much of the Unix password data may be stored elsewhere.

Unix password file format
An entry in the Unix password file consists of seven colon delimited fields:

Username
Encrypted Unix password (And optional password aging data)
User number
Group Number
GECOS Information
Home directory
Shell

Sample entry from /etc/passwd:

will:5fg63fhD3d5gh:9406:12:Will Spencer:/home/will:/bin/bash


Broken down, this passwd file line shows:

Username will
Encrypted Unix password 5fg63fhD3d5gh
User number 9406
Group Number 12
GECOS Information Will Spencer
Home directory /home/will
Shell /bin/bash

Auditing Unix passwords
Contrary to popular belief, Unix passwords cannot be decrypted. Unix passwords are encrypted with a one way function. The login program accepts the text you enter at the "Password:" prompt and then runs it through a cryptographic algorithm. The results of that algorithm are then compared against the encrypted form of your Unix password stored in the password file.

On a more technical level, the password that you enter is used as a key to encrypt a 64-bit block of NULLs. The first seven bits of each character are extracted to form a 56-bit key. This means that only eight characters are significant in a standard Unix password. The E-table is then modified using the salt, which is a 12-bit value, coerced into the first two chars of the stored password. The salt's purpose is to make precompiled password lists and DES hardware chips more time consuming to use. DES is then invoked for 25 iterations. The 64-bit output block and is then coerced into a 64-character alphabet (A-Z,a-z,".","/"). This involves translations in which several different values are represented by the same character, which is why Unix passwords cannot be decrypted.

Unix password auditing software uses wordlists to implement a dictionary attack. Each word in the wordlist is encrypted using the algorithm described above and the salts from the password file. The results are then compared to the encrypted form of the target password.

To audit Unix passwords under Unix or DOS/Windows, try John the Ripper. For the Macintosh, try Killer Cracker or Mac Krack.