What is the primary purpose of the crypt(3) library on UNIX-like systems?

• A. Creates password representations for storage
• B. Encrypts file contents
• C. Manages user accounts
• D. Provides network authentication

Answer: (A)

The crypt(3) library is designed to produce password representations (hashes) for storage and later verification. It takes a plaintext password and a salt, runs it through a hashing algorithm, and returns a hashed string that can be stored in the system’s password database. When a user logs in, the system re-hashes the entered password with the same salt and compares it to the stored hash to verify the credentials. This is a one-way transformation, meaning you don’t recover the original password from the hash.

This differs from encrypting file contents (encryption is reversible with a key), from managing user accounts, or from providing network authentication by itself. crypt(3) supports multiple hash algorithms (DES-based, MD5-based, Blowfish, etc.) and encodes the chosen algorithm and salt into the stored representation, enabling proper verification across updates to hashing schemes.