Key derivation function

A Key derivation function is a cryptographic hash function which is designed to make a small key or password harder to attack using a dictionary attack or brute force attack.

It is normally expressed as $DK=(Key,Salt,Iterations)$ where $DK$ is the derived key, $Key$ is the original key, $Salt$ is a large random number (typically around $2^{6}4$), and $Iterations$ refers to the number of iterations of a sub-function (typically $1000$).

The values of $DK,Salt$ and $Iterations$ can then be stored insecurely.

When we have a large number of iterations it is practical time-wise for $DK$ to be computed for a single password, but not for a large number of passwords as a brute force attack would require. The use of $Salt$ prevents the attackers from precomputing a dictionary of derived keys.