Hacking Local Passwords







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A number of different authentication schemes are available to Linux, but local passwords are by far the most common methodology. Local passwords in modern Linux systems are stored in the /etc/shadow directory using a DES, MD5, or Blowfish oneway hash and some form of password salt.

Of these three, DES is often the default password encryption methodology used (particularly in older systems) and should be changed to Blowfish, since DES only allows 8-byte passwords and MD5 is now considered questionable due to collisions (where multiple inputs are found to have the same MD5 hash value), published MD5 Rainbow tables, and ever-increasing processor speeds. Also, shortcuts have recently been identified in MD5 that aid brute forcing. Attackers have also discovered that it is quicker to shave off and recompute the first 4 bytes than to try to brute-force them.

The password salt induces further disorder in password hashing by adding a certain number of bits of perturbation, depending on the hashing algorithm, beyond what the hashing algorithm already provides. Essentially, it is included to increase password complexity, providing increased protection against brute-force attacks.

Additionally, it provides protection against users recognizing other accounts with the same password, if they happen to have access to the /etc/shadow file. Specifically, if two user accounts have the same password, their password hashes will still be different because the salt used on the password is random, rendering the password hashes completely different.

The salt can also be used to identify the one-way hashing algorithm used on passwords. For instance, passwords encrypted using MD5 have a "$1" at the beginning.

Likewise, those with "$2" are encrypted using Blowfish. Hashes that begin with an underscore, "_", are encrypted using DES and a user-specified number of perturbations. Hashes beginning with any other characters utilize a fixed number of perturbations.

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