[Script Info]
Title: 
[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:00.00,0:00:04.71,Default,,0000,0000,0000,,
Dialogue: 0,0:00:04.71,0:00:07.65,Default,,0000,0000,0000,,For over 400 years,\Nthe problem remained.
Dialogue: 0,0:00:07.65,0:00:11.76,Default,,0000,0000,0000,,How could Alice design a cipher\Nthat hides her fingerprint,
Dialogue: 0,0:00:11.76,0:00:14.58,Default,,0000,0000,0000,,thus stopping the\Nleak of information?
Dialogue: 0,0:00:14.58,0:00:18.15,Default,,0000,0000,0000,,The answer is randomness.
Dialogue: 0,0:00:18.15,0:00:20.89,Default,,0000,0000,0000,,Imagine Alice rolled\Na 26 sided die
Dialogue: 0,0:00:20.89,0:00:23.36,Default,,0000,0000,0000,,to generate a long\Nlist of random shifts,
Dialogue: 0,0:00:23.36,0:00:26.81,Default,,0000,0000,0000,,and shared this with Bob\Ninstead of a code word.
Dialogue: 0,0:00:26.81,0:00:28.86,Default,,0000,0000,0000,,Now, to encrypt\Nher message, Alice
Dialogue: 0,0:00:28.86,0:00:31.97,Default,,0000,0000,0000,,uses the list of\Nrandom shifts instead.
Dialogue: 0,0:00:31.97,0:00:34.01,Default,,0000,0000,0000,,It is important that\Nthis list of shifts
Dialogue: 0,0:00:34.01,0:00:38.44,Default,,0000,0000,0000,,be as long as the message,\Nas to avoid any repetition.
Dialogue: 0,0:00:38.44,0:00:41.25,Default,,0000,0000,0000,,Then she sends it to Bob, who\Ndecrypts the message using
Dialogue: 0,0:00:41.25,0:00:44.08,Default,,0000,0000,0000,,the same list of random\Nshifts she had given him.
Dialogue: 0,0:00:44.08,0:00:46.87,Default,,0000,0000,0000,,
Dialogue: 0,0:00:46.87,0:00:49.46,Default,,0000,0000,0000,,Now Eve will have a problem,\Nbecause the resulting
Dialogue: 0,0:00:49.46,0:00:53.21,Default,,0000,0000,0000,,encrypted message will have\Ntwo powerful properties.
Dialogue: 0,0:00:53.21,0:00:56.76,Default,,0000,0000,0000,,One, the shifts never fall\Ninto a repetitive pattern.
Dialogue: 0,0:00:56.76,0:00:59.35,Default,,0000,0000,0000,,
Dialogue: 0,0:00:59.35,0:01:02.86,Default,,0000,0000,0000,,And two, the encrypted message\Nwill have a uniform frequency
Dialogue: 0,0:01:02.86,0:01:04.23,Default,,0000,0000,0000,,distribution.
Dialogue: 0,0:01:04.23,0:01:07.05,Default,,0000,0000,0000,,Because there is no frequency\Ndifferential and therefore
Dialogue: 0,0:01:07.05,0:01:09.74,Default,,0000,0000,0000,,no leak, it is now\Nimpossible for Eve
Dialogue: 0,0:01:09.74,0:01:10.74,Default,,0000,0000,0000,,to break the encryption.
Dialogue: 0,0:01:10.74,0:01:14.09,Default,,0000,0000,0000,,
Dialogue: 0,0:01:14.09,0:01:18.08,Default,,0000,0000,0000,,This is the strongest\Npossible method of encryption,
Dialogue: 0,0:01:18.08,0:01:21.52,Default,,0000,0000,0000,,and it emerged towards the\Nend of the 19th century.
Dialogue: 0,0:01:21.52,0:01:25.86,Default,,0000,0000,0000,,It is now known as\Nthe one-time pad.
Dialogue: 0,0:01:25.86,0:01:28.99,Default,,0000,0000,0000,,In order to visualize the\Nstrength of the one-time pad,
Dialogue: 0,0:01:28.99,0:01:32.32,Default,,0000,0000,0000,,we must understand the\Ncombinatorial explosion
Dialogue: 0,0:01:32.32,0:01:34.60,Default,,0000,0000,0000,,which takes place.
Dialogue: 0,0:01:34.60,0:01:37.60,Default,,0000,0000,0000,,For example, the Caesar\NCipher shifted every letter
Dialogue: 0,0:01:37.60,0:01:42.97,Default,,0000,0000,0000,,by the same shift, which was\Nsome number between 1 and 26.
Dialogue: 0,0:01:42.97,0:01:44.97,Default,,0000,0000,0000,,So if Alice was to\Nencrypt her name,
Dialogue: 0,0:01:44.97,0:01:48.77,Default,,0000,0000,0000,,it would result in one of\N26 possible encryptions.
Dialogue: 0,0:01:48.77,0:01:52.29,Default,,0000,0000,0000,,A small number of possibilities,\Neasy to check them all,
Dialogue: 0,0:01:52.29,0:01:55.28,Default,,0000,0000,0000,,known as brute force search.
Dialogue: 0,0:01:55.28,0:01:58.06,Default,,0000,0000,0000,,Compare this to the one-time\Npad, where each letter would
Dialogue: 0,0:01:58.06,0:02:01.69,Default,,0000,0000,0000,,be shifted by a different\Nnumber between 1 and 26.
Dialogue: 0,0:02:01.69,0:02:04.00,Default,,0000,0000,0000,,Now think about the number\Nof possible encryptions.
Dialogue: 0,0:02:04.00,0:02:08.05,Default,,0000,0000,0000,,It's going to be 26 multiplied\Nby itself five times, which
Dialogue: 0,0:02:08.05,0:02:10.36,Default,,0000,0000,0000,,is almost 12 million.
Dialogue: 0,0:02:10.36,0:02:13.03,Default,,0000,0000,0000,,Sometimes it's\Nhard to visualize,
Dialogue: 0,0:02:13.03,0:02:15.85,Default,,0000,0000,0000,,so imagine she wrote her\Nname on a single page,
Dialogue: 0,0:02:15.85,0:02:20.90,Default,,0000,0000,0000,,and on top of it stacked\Nevery possible encryption.
Dialogue: 0,0:02:20.90,0:02:24.52,Default,,0000,0000,0000,,How high do you\Nthink this would be?
Dialogue: 0,0:02:24.52,0:02:28.75,Default,,0000,0000,0000,,With almost 12 million\Npossible five-letter sequences,
Dialogue: 0,0:02:28.75,0:02:32.11,Default,,0000,0000,0000,,this stack of paper\Nwould be enormous,
Dialogue: 0,0:02:32.11,0:02:35.13,Default,,0000,0000,0000,,over one kilometer high.
Dialogue: 0,0:02:35.13,0:02:38.24,Default,,0000,0000,0000,,When Alice encrypts her\Nname using the one-time pad,
Dialogue: 0,0:02:38.24,0:02:42.24,Default,,0000,0000,0000,,it is the same as picking\None of these pages at random.
Dialogue: 0,0:02:42.24,0:02:44.72,Default,,0000,0000,0000,,From the perspective of\NEve, the code breaker,
Dialogue: 0,0:02:44.72,0:02:46.91,Default,,0000,0000,0000,,every five letter\Nencrypted word she
Dialogue: 0,0:02:46.91,0:02:51.60,Default,,0000,0000,0000,,has is equally likely to\Nbe any word in this stack.
Dialogue: 0,0:02:51.60,0:02:55.24,Default,,0000,0000,0000,,So this is perfect\Nsecrecy in action.
Dialogue: 0,0:02:55.24,0:02:55.87,Default,,0000,0000,0000,,