Shadow password
On a unix system, the file /etc/passwd generally is world-readable but writeable only by root. The passwd file contains information about valid user accounts for a particular system. This information includes the username, the user ID, the default group ID for the user, the user's full name, the home directory path, the login shell, the user's crypted password, and password expiration infromation. Shadow passwords refers to the process of storing the crypted password information in a separate file named /etc/shadow that only root can read. When shadow passwords are used, an asterisk appears in /etc/passwd in place of the crypted password string. The shadow password system raises the bar for crackers wishing to steal a password using a dictionary attack, because root access is required to get the crypted passwords.
