If Eve knows that the same substitution is used for both encryption and decryption,does it make her job of?

Question

If Eve knows that the same substitution is used for both encryption and decryption,

does it make her job of breaking the cipher any easier? Why?

Answer 1

I assume you are referring to a simple substitution cipher where letters of the alphabet are consistently changed to other letters.. If the same substitution is used for both en- and decryption it implies that if a letter is enciphered twice it goes to itself. This then means that plain and enciphered letters are in pairs. So if A goes to T, then T must go to A. This makes breaking the cipher much easier as when you work out a pair one way you immediately know it also works the other way around. (Of course a letter might go to itself, but presumably there won't be very many of these as the cipher is weaker.)

Answer 2

It depends! Since Eve is not the only persona exist. Satan also exist, with all of his followers but the Great Creator, nothing, no one beats HIM. But i am sure she can do something with the approval of the Great Creator!