Salt (cryptography)

In cryptography, a salt is random data that is used as an additional input to a one-way function that "hashes" a password or passphrase. Salts are closely related to the concept of nonce. The primary function of salts is to defend against dictionary attacks or against its hashed equivalent, a pre-computed rainbow table attack.

Salts are used to safeguard passwords in storage. Historically a password was stored in plaintext on a system, but over time additional safeguards developed to protect a user's password against being read from the system. A salt is one of those methods.

A new salt is randomly generated for each password. In a typical setting, the salt and the password (or its version after Key stretching) are concatenated and processed with a cryptographic hash function, and the resulting output (but not the original password) is stored with the salt in a database. Hashing allows for later authentication without keeping and therefore risking the plaintext password in the event that the authentication data store is compromised.

Since salts do not have to be memorized by humans they can make the size of the rainbow table required for a successful attack prohibitively large without placing a burden on the users. Since salts are different in each case, they also protect commonly used passwords, or those who use the same password on several sites, by making all salted hash instances for the same password different from each other.

Cryptographic salts are broadly used in many modern computer systems, from Unix system credentials to Internet security.

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 8 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.

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