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a strong cypher is one which disguises your fingerprint to make a lighter fingerprint is to flatten this distribution of letter frequencies by the mid 15th century we had advanced the polyalphabetic ciphers to accomplish this imagine Alice and Bob shared a secret Shift word first Alice converts the word into numbers according to the letter position in the alphabet next this sequence of numbers is repeated along the message then each letter in the message is encrypted by shifting according to the number below it now she is using multiple shifts instead of a single shift across the message as Caesar had done before then the encrypted message is sent openly to Bob Bob decrypt the message by subtracting the shifts according to the secret word he also has a copy of now imagine a Code Breaker Eve intercepts a series of messages and calculates the letter frequencies she will find a flatter distribution or a lighter fingerprint so how could she break this remember codebreakers look for information leaked the same is finding a partial fingerprint anytime there is a differential in letter frequencies a leak of information occurs this difference is caused by repetition in the encrypted message in this case alice's cipher contains a repeating codeword to break the encryption Eve would first need to determine the length of this shift board used not the word itself she will need to go through and check the frequency distribution of different intervals when she checks the frequency distribution of every fifth letter the fingerprint will reveal itself the problem now is to break five Caesar ciphers and a repeating sequence individually this is a trivial task as we have seen before the added strength of this cipher is the time taken to determine the length of the shift word used the longer the shift word the stronger the cipher