In unconditional security, the system (algorithm) is secure even if attacker has unbounded computing power since the ciphertext provides insufficient information to uniquely determine the corresponding plaintext.
- Security measured using information theory.
- With exception of one-time pad, there’s no unconditionally secure encryption algorithm.
- Hence, strive for algorithm that meets one or both of:
- Cost of breaking cipher exceeds value of encrypted information.
- Time required to break cipher exceeds useful lifetime of information. Algorithm is computationally secure if either of these two criteria met.