Password Cracking - Prevention

Prevention

The best method of preventing password cracking is to ensure that attackers cannot get access even to the hashed password. For example, on the Unix operating system, hashed passwords were originally stored in a publicly accessible file /etc/passwd. On modern Unix (and similar) systems, on the other hand, they are stored in the file /etc/shadow, which is accessible only to programs running with enhanced privileges (i.e., "system" privileges). This makes it harder for a malicious user to obtain the hashed passwords in the first instance. Unfortunately, many common network protocols transmit passwords in cleartext or use weak challenge/response schemes.

Modern Unix systems have replaced traditional DES-based password hashing with stronger methods such as bcrypt and scrypt. Other systems have also begun to adopt these methods. For instance, the Cisco IOS originally used a reversible Vigenère cipher to encrypt passwords, but now uses md5-crypt with a 24-bit salt when the "enable secret" command is used. These newer methods use large salt values which prevent attackers from efficiently mounting offline attacks against multiple user accounts simultaneously. The algorithms are also much slower to execute which drastically increases the time required to mount a successful offline attack.

Many hashes used for storing passwords, such as MD5 and the SHA family, are designed for fast computation and efficient implementation in hardware. Using key stretching algorithms, such as PBKDF2, to form password hashes can significantly reduce the rate at which passwords can be tested.

Solutions like a security token give a formal proof answer by constantly shifting password. Those solutions abruptly reduce the timeframe for brute forcing (attacker needs to break and use the password within a single shift) and they reduce the value of the stolen passwords because of its short time validity.