Crypt() Hashing Function

Crypt is a function developed for storing passwords (mainly on UNIX systems) which is based on hash functions. It takes as a parameter a character string (the password) and a salt (in order to complicate brute-force decryption attempts).

Originally the function used the DES standard with a 2 character salt, then the function was adapted to handle other algorithms like extended DES, MD5, Blowfish (known as bCrypt), SHA256 or SHA512.

To encode a password with crypt(), the user provides the plaintext password and a salt. The crypt() function combines these two inputs using the chosen hashing algorithm and applies the chosen parameters (such as a number of iterations/rounds 🔁) in order to generate a unique identifier string. The result is therefore a character string containing the password hash and the parameters used, which can be stored securely.

When a user wants to test a password with, the algorithm will have to regenerate the string again with the same parameters, if the 2 strings are identical, then the password is also identical.

In order to further secure the passwords, it is recommended to use a fairly long password, a unique salt for each password, and to apply algorithms with significant calculation costs (ie. must be long and memory intensive, which is generally done by increasing the number of iterations of the algorithm).

The only way to find a password is to try all the possibilities. This type of attack is potentially possible but very difficult in practice because it can take days or years of calculations.

If the password has no salt, then it is possible to use the rainbow tables, storing the most commonly used passwords.

If the password is short (low length), and the salt is known then trying all possible strings represents 255^(length in bytes) calculations.

If the algorithm used is weak, fast to compute and parallelizable, then it is possible to compute more crypt() per second and speed up the search.

A hash from the Crypt function usually begins with the symbol $ followed by a number. There is a variant where the string begins with _ (underscore) and another without an immediately recognizable signature.

The string then contains any parameters and the salt used. The calculated hashes are encoded in a base 64 using the alphabet ./0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz

Hashcat is a password recovery and hash cracking tool that can be used to attempt to recover passwords from hashes generated by the Crypt function and other hashing algorithms. It uses brute force and dictionary techniques to try to find the original value corresponding to a given hash.

The shadow file is a key component in Unix operating systems for secure password management. Usually placed in the /etc/ directory, the shadow file stores sensitive user account information, including password hashes.

No hashing system is invulnerable to brute force attacks. However, using a strong hashing algorithm, many iterations and random salt, attempts will be much more difficult and time-consuming for attackers.