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 the polyalphabetic ciphers
to accomplish this.
Imagine Alice and Bob shared a secret shift word.
First, Alice converts the word into numbers
according of 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's 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 codebreaker, Eve,
intercepts a series of messages
and calculates the letter frequencies,
she'll find a flatter distribution or a lighter fingerprint,
so how could she break this?
Remember, codebreakers look for information leaked,
the same as finding a partial fingerprint.
Any time there's 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, Eve would first need to determine
the length of the shift word used, not the word itself.
She'll 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
in a repeating sequence.
Individually, this is a trivial task
as we have seen before,
the added strength of the cipher is the time taken
to determine the length of shift word used.
The longer the shift word, the stronger the cipher.