Salt (cryptography)
Here is an incomplete example of a salt value for storing passwords. This first table has two username and password combinations. The password is not stored.
| Username | Password |
|---|---|
| user1 | password123 |
| user2 | password123 |
The salt value is generated at random and can be any length, in this case the salt value is 8 bytes long. The salt value is appended to the plaintext password and then the result is hashed, this is referred to as the hashed value. Both the salt value and hashed value are stored.
| Username | Salt value | String to be hashed | Hashed value = SHA256 (Password + Salt value) |
|---|---|---|---|
| user1 | E1F53135E559C253 | password123E1F53135E559C253 | 72AE25495A7981C40622D49F9A52E4F1565C90F048F59027BD9C8C8900D5C3D8 |
| user2 | 84B03D034B409D4E | password12384B03D034B409D4E | B4B6603ABC670967E99C7E7F1389E40CD16E78AD38EB1468EC2AA1E62B8BED3A |
As the table above illustrates, different salt values will create completely different hashed values, even when the plaintext passwords are exactly the same. Additionally, dictionary attacks are mitigated to a degree as an attacker cannot practically precompute the hashes. However, a salt cannot protect common or easily guessed passwords.
In cryptography, a salt is random data that is used as an additional input to a one-way function that hashes data, a password or passphrase. 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 exposure of the plaintext password in the event that the authentication data store is compromised.
Salts defend against a pre-computed hash attack, e.g. rainbow tables.[1] Since salts do not have to be memorized by humans they can make the size of the hash 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 users 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.
Salts are closely related to the concept of a cryptographic nonce.