Evaluating True Cryptographic Key Space Size

Cybersecurity professionals have relied on the key space of a cipher to compare encryption algorithms and select the best encryptions for transmitted data. Peer reviewed strong ciphers have been assumed to maintain strength for all messages. It is thought that only brute force attacks can break these ciphers, so the key space calculation for these algorithms uses the maximum key space to determine the unicity distance. Unfortunately, the key space is heavily dependent on the user and habits of the user, as well as the content of the message. In this paper, we present factors that affect the key space size and show that the effects of these factors can seriously decrease the security of a cipher for a particular message. By considering these factors, a cybersecurity practitioner can properly assess vulnerability and choose the best security for that message.