Security Strategies in Linux Platforms and Applications, 3rd Edition

The Shadow Password Suite

For more than 20 years, Unix systems used the /etc/passwd file as a way to store both users and their associated passwords. To that point, in the early days of Linux, most users on multiuser systems were trusted users, so it didn’t matter so much that the passwd file was available to all users. It had to be, given the number of utilities that needed read access to the passwd file to acquire information about the system’s users.

NOTE

Passwords stored in the /etc/shadow file are not encrypted. Instead, they are hashed. A cryptographic hash is a fixed-length value that is derived using something called a one-way function. ...