Salt (cryptography)

Common mistakes

Salt re-use

Using the same salt for all passwords is dangerous because a precomputed table which simply accounts for the salt will render the salt useless. However, see Pepper.

Generation of precomputed tables for databases with unique salts for every password is not viable because of the computational cost of doing so. But, if a common salt is used for all the entries, creating such a table (that accounts for the salt) then becomes a viable and possibly successful attack.

Because salt re-use can cause users with the same password to have the same hash, cracking a single hash can result in other passwords being compromised too.

Salt length

If a salt is too short, an attacker may precompute a table of every possible salt appended to every likely password. Using a long salt ensures such a table would be prohibitively large. 16 bytes (128 bits) or more is generally sufficient to provide a large enough space of possible values, minimizing the risk of collisions (i.e., two different passwords ending up with the same salt).

Benefits

To understand the difference between cracking a single password and a set of them, consider a file with users and their hashed passwords. Say the file is unsalted. Then an attacker could pick a string, call it attempt[0], and then compute hash(attempt[0]). A user whose hash stored in the file is hash(attempt[0]) may or may not have password attempt[0]. However, even if attempt[0] is not the user's actual password, it will be accepted as if it were, because the system can only check passwords by computing the hash of the password entered and comparing it to the hash stored in the file. Thus, each match cracks a user password, and the chance of a match rises with the number of passwords in the file. In contrast, if salts are used, the attacker would have to compute hash(attempt[0] || salt[a]), compare against entry A, then hash(attempt[0] || salt[b]), compare against entry B, and so on. This prevents any one attempt from cracking multiple passwords, given that salt re-use is avoided.

Salts also combat the use of precomputed tables for cracking passwords. Such a table might simply map common passwords to their hashes, or it might do something more complex, like store the start and end points of a set of precomputed hash chains. In either case, salting can defend against the use of precomputed tables by lengthening hashes and having them draw from larger character sets, making it less likely that the table covers the resulting hashes. In particular, a precomputed table would need to cover the string [salt + hash] rather than simply [hash].

The modern shadow password system, in which password hashes and other security data are stored in a non-public file, somewhat mitigates these concerns. However, they remain relevant in multi-server installations which use centralized password management systems to push passwords or password hashes to multiple systems. In such installations, the root account on each individual system may be treated as less trusted than the administrators of the centralized password system, so it remains worthwhile to ensure that the security of the password hashing algorithm, including the generation of unique salt values, is adequate.

Another (lesser) benefit of a salt is as follows: two users might choose the same string as their password. Without a salt, this password would be stored as the same hash string in the password file. This would disclose the fact that the two accounts have the same password, allowing anyone who knows one of the account's passwords to access the other account. By salting the passwords with two random characters, even if two accounts use the same password, no one can discover this just by reading hashes. Salting also makes it extremely difficult to determine if a person has used the same password for multiple systems.

Unix implementations

1970s–1980s

Earlier versions of Unix used a password file /etc/passwd to store the hashes of salted passwords (passwords prefixed with two-character random salts). In these older versions of Unix, the salt was also stored in the passwd file (as cleartext) together with the hash of the salted password. The password file was publicly readable for all users of the system. This was necessary so that user-privileged software tools could find user names and other information. The security of passwords is therefore protected only by the one-way functions (enciphering or hashing) used for the purpose. Early Unix implementations limited passwords to eight characters and used a 12-bit salt, which allowed for 4,096 possible salt values. This was an appropriate balance for 1970s computational and storage costs.

Since the 1980s

The shadow password system is used to limit access to hashes and salt. The salt is eight characters, the hash is 86 characters, and the password length is effectively unlimited, barring stack overflow errors.

Web-application implementations

It is common for a web application to store in a database the hash value of a user's password. Without a salt, a successful SQL injection attack may yield easily crackable passwords. Because many users re-use passwords for multiple sites, the use of a salt is an important component of overall web application security. Some additional references for using a salt to secure password hashes in specific languages or libraries (PHP, the .NET libraries, etc.) can be found in the external links section below.