[Script Info]
Title: 
[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:04.95,0:00:08.16,Default,,0000,0000,0000,,A strong cipher is one which disguises your fingerprint.
Dialogue: 0,0:00:08.96,0:00:10.84,Default,,0000,0000,0000,,To make a lighter fingerprint
Dialogue: 0,0:00:10.84,0:00:14.27,Default,,0000,0000,0000,,is to flatten this distribution of letter frequencies.
Dialogue: 0,0:00:17.60,0:00:18.94,Default,,0000,0000,0000,,By the mid 15th century,
Dialogue: 0,0:00:18.94,0:00:21.27,Default,,0000,0000,0000,,we had advanced the polyalphabetic ciphers
Dialogue: 0,0:00:21.27,0:00:23.36,Default,,0000,0000,0000,,to accomplish this.
Dialogue: 0,0:00:23.36,0:00:28.17,Default,,0000,0000,0000,,Imagine Alice and Bob shared a secret shift word.
Dialogue: 0,0:00:28.17,0:00:30.77,Default,,0000,0000,0000,,First, Alice converts the word into numbers
Dialogue: 0,0:00:30.77,0:00:33.22,Default,,0000,0000,0000,,according of the letter position in the alphabet.
Dialogue: 0,0:00:33.22,0:00:37.83,Default,,0000,0000,0000,,Next, this sequence of numbers is repeated along the message.
Dialogue: 0,0:00:39.48,0:00:41.98,Default,,0000,0000,0000,,Then each letter in the message is encrypted
Dialogue: 0,0:00:41.98,0:00:44.16,Default,,0000,0000,0000,,by shifting according to the number below it.
Dialogue: 0,0:00:45.13,0:00:48.81,Default,,0000,0000,0000,,Now, she's using multiple shifts instead of a single shift
Dialogue: 0,0:00:48.81,0:00:53.64,Default,,0000,0000,0000,,across the message as Caesar had done before.
Dialogue: 0,0:00:53.64,0:00:56.63,Default,,0000,0000,0000,,Then, the encrypted message is sent openly to Bob.
Dialogue: 0,0:00:58.49,0:01:01.63,Default,,0000,0000,0000,,Bob decrypts the message by subtracting the shifts
Dialogue: 0,0:01:01.63,0:01:04.86,Default,,0000,0000,0000,,according to the secret word he also has a copy of.
Dialogue: 0,0:01:06.12,0:01:08.04,Default,,0000,0000,0000,,Now imagine a codebreaker, Eve,
Dialogue: 0,0:01:08.04,0:01:10.17,Default,,0000,0000,0000,,intercepts a series of messages
Dialogue: 0,0:01:10.17,0:01:12.64,Default,,0000,0000,0000,,and calculates the letter frequencies,
Dialogue: 0,0:01:13.67,0:01:18.04,Default,,0000,0000,0000,,she'll find a flatter distribution or a lighter fingerprint,
Dialogue: 0,0:01:18.04,0:01:20.59,Default,,0000,0000,0000,,so how could she break this?
Dialogue: 0,0:01:22.55,0:01:26.17,Default,,0000,0000,0000,,Remember, codebreakers look for information leaked,
Dialogue: 0,0:01:26.17,0:01:29.38,Default,,0000,0000,0000,,the same as finding a partial fingerprint.
Dialogue: 0,0:01:29.38,0:01:32.41,Default,,0000,0000,0000,,Any time there's a differential in letter frequencies,
Dialogue: 0,0:01:32.41,0:01:34.93,Default,,0000,0000,0000,,a leak of information occurs.
Dialogue: 0,0:01:36.13,0:01:39.83,Default,,0000,0000,0000,,This difference is caused by repetition in the encrypted message.
Dialogue: 0,0:01:41.70,0:01:45.91,Default,,0000,0000,0000,,In this case, Alice's cipher contains a repeating code word.
Dialogue: 0,0:01:47.22,0:01:51.10,Default,,0000,0000,0000,,To break the encryption, Eve would first need to determine
Dialogue: 0,0:01:51.10,0:01:55.57,Default,,0000,0000,0000,,the length of the shift word used, not the word itself.
Dialogue: 0,0:01:55.57,0:01:56.92,Default,,0000,0000,0000,,She'll need to go through
Dialogue: 0,0:01:56.92,0:02:00.49,Default,,0000,0000,0000,,and check the frequency distribution of different intervals.
Dialogue: 0,0:02:00.49,0:02:04.43,Default,,0000,0000,0000,,When she checks the frequency distribution of every fifth letter,
Dialogue: 0,0:02:04.43,0:02:07.65,Default,,0000,0000,0000,,the fingerprint will reveal itself.
Dialogue: 0,0:02:07.65,0:02:10.14,Default,,0000,0000,0000,,The problem now, is to break five Caesar ciphers
Dialogue: 0,0:02:10.14,0:02:12.82,Default,,0000,0000,0000,,in a repeating sequence.
Dialogue: 0,0:02:12.82,0:02:14.73,Default,,0000,0000,0000,,Individually, this is a trivial task
Dialogue: 0,0:02:14.73,0:02:17.14,Default,,0000,0000,0000,,as we have seen before,
Dialogue: 0,0:02:17.14,0:02:19.74,Default,,0000,0000,0000,,the added strength of the cipher is the time taken
Dialogue: 0,0:02:19.74,0:02:22.91,Default,,0000,0000,0000,,to determine the length of shift word used.
Dialogue: 0,0:02:22.91,0:02:26.91,Default,,0000,0000,0000,,The longer the shift word, the stronger the cipher.