In cryptography, the **security parameter** is a variable that measures the input size of the problem. Both the resource requirements of the cryptographic algorithm or protocol as well as the adversary's probability of breaking security are expressed in terms of the security parameter.

The security parameter size *n* is usually expressed in unary representation (i.e. a *n*-long string of bits 1) so that the cryptographic algorithm execution efficiency can be polynomial-time in the length of the input size.

Typical security parameters:

- The length, in bits, of the key used in a cryptographic scheme.