Theoretical Security

Perfect secrecy

The notion of a cryptosystem being ‘unbreakable’ is modeled by the concept of perfect secrecy.

Motivation for perfect secrecy

An exhaustive key search can always be launched against any cryptosystem. However, there is an even more basic attack that can also always be conducted against a cryptosystem that doesn’t even involve trying to obtain the decryption key—an attacker can simply try to guess the plaintext.

Guessing the plaintext is an attack that can never be prevented. Of course, for long and complicated plaintexts it’s very unlikely that an interceptor will be able to guess the plaintext correctly, but there will always be a chance (ideally a very small one) that they could.

Note: Guessing the plaintext becomes a much more plausible scenario when the number of possible plaintexts is small, such as when the plaintext is a four-digit PIN or a short password.