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A strong cipher is one which disguises your fingerprint.

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To make a lighter fingerprint

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is to flatten this distribution of letter frequencies.

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By the mid 15th century,

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we had advanced the polyalphabetic ciphers

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to accomplish this.

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Imagine Alice and Bob shared a secret shift word.

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First, Alice converts the word into numbers

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according of the letter position in the alphabet.

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Next, this sequence of numbers is repeated along the message.

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Then each letter in the message is encrypted

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by shifting according to the number below it.

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Now, she's using multiple shifts instead of a single shift

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across the message as Caesar had done before.

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Then, the encrypted message is sent openly to Bob.

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Bob decrypts the message by subtracting the shifts

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according to the secret word he also has a copy of.

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Now imagine a codebreaker, Eve,

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intercepts a series of messages

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and calculates the letter frequencies,

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she'll find a flatter distribution or a lighter fingerprint,

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so how could she break this?

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Remember, codebreakers look for information leaked,

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the same as finding a partial fingerprint.

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Any time there's a differential in letter frequencies,

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a leak of information occurs.

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This difference is caused by repetition in the encrypted message.

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In this case, Alice's cipher contains a repeating code word.

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To break the encryption, Eve would first need to determine

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the length of the shift word used, not the word itself.

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She'll need to go through

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and check the frequency distribution of different intervals.

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When she checks the frequency distribution of every fifth letter,

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the fingerprint will reveal itself.

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The problem now, is to break five Caesar ciphers

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in a repeating sequence.

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Individually, this is a trivial task

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as we have seen before,

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the added strength of the cipher is the time taken

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to determine the length of shift word used.

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The longer the shift word, the stronger the cipher.