Passwords serve to select a method from a class of methods, and keys especially to select encryption steps from an encryption system. It is wise to assume pessimistically that the enemy knows what method has been chosen — there are not too many of them, and most cryptographers are familiar with only a few. The ‘basic law of cryptology’, which Kerckhoffsl had formulated as “il faut qu’il puisse sans inconvénient tomber entre les mains de l’ennemi” was expressed more succinctly by Shannon in 1949: “the enemy knows the system being used.” It follows that one must be particularly careful in the choice of a key. It is a serious mistake to use obvious words. Porta gave the express warning: “the further removed the key words are from common knowledge, the greater the security they provide.” The use of keys had hardly become common practice before unauthorized persons succeeded in decrypting messages by guessing the key word.
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