Page "Cryptanalysis" Paragraph 5

Attacks can be classified based on what type of information the attacker has available.

As a basic starting point it is normally assumed that, for the purposes of analysis, the general algorithm is known ; this is Shannon's Maxim " the enemy knows the system " -- in its turn, equivalent to Kerckhoffs ' principle.

This is a reasonable assumption in practice — throughout history, there are countless examples of secret algorithms falling into wider knowledge, variously through espionage, betrayal and reverse engineering.

( And on occasion, ciphers have been reconstructed through pure deduction ; for example, the German Lorenz cipher and the Japanese Purple code, and a variety of classical schemes ).