Computational Security

The number of possible keys is finite.

• And much less than the number of possible messages.

• $K << M$

Thus, security ultimately depends on the computing power of cryptanalysts to go through all keys.

• Computations per time period.

• Storage capacity.

• Resistance time is mainly given by key length.

Provable security.

• The computational security can be demonstrated by comparing it with known hard problems.