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A strong cipher 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 to 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 decrypts 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, code breakers
look for information leak, the same as finding a
partial fingerprint. Any time 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 code word. To break the encryption,
Even would first need to determine the
length of this shift word 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 Cesar Ciphers in 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.