[Script Info]
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Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:00.00,0:00:03.00,Default,,0000,0000,0000,,[Narrator] The answer is 1011001,
Dialogue: 0,0:00:03.00,0:00:07.00,Default,,0000,0000,0000,,and the way to get that is to look at the 2 possible messages.
Dialogue: 0,0:00:07.00,0:00:12.00,Default,,0000,0000,0000,,We had for Y 1011001.
Dialogue: 0,0:00:12.00,0:00:22.00,Default,,0000,0000,0000,,For N, we had 1001110, and what we want to do is XOR them to figure out the difference between those 2.
Dialogue: 0,0:00:22.00,0:00:30.00,Default,,0000,0000,0000,,The XOR of Y and N is equal to 0010111,
Dialogue: 0,0:00:30.00,0:00:36.00,Default,,0000,0000,0000,,and so this is the value that will flip a Y answer into an N answer.
Dialogue: 0,0:00:36.00,0:00:39.00,Default,,0000,0000,0000,,Now we want to XOR that with the actual message that was sent.
Dialogue: 0,0:00:39.00,0:00:41.00,Default,,0000,0000,0000,,We don't know the key, but we know that
Dialogue: 0,0:00:41.00,0:00:46.00,Default,,0000,0000,0000,,that message is the result of XORing Alice's message with the key,
Dialogue: 0,0:00:46.00,0:00:53.00,Default,,0000,0000,0000,,and so by XORing those, we get this value 1011001,
Dialogue: 0,0:00:53.00,0:00:57.00,Default,,0000,0000,0000,,and if that's what Mallory sends, when Bob XORs that with the key,
Dialogue: 0,0:00:57.00,0:01:01.00,Default,,0000,0000,0000,,what Bob will get is the XOR of Alice's message.
Dialogue: 0,0:01:01.00,0:01:04.00,Default,,0000,0000,0000,,Alice's message XOR'd with this value,
Dialogue: 0,0:01:04.00,0:01:07.00,Default,,0000,0000,0000,,which will flip Alice's message to be the opposite.
Dialogue: 0,0:01:07.00,0:01:11.00,Default,,0000,0000,0000,,The point of this question is to illustrate the malleability of the one-time pad.
Dialogue: 0,0:01:11.00,0:01:16.00,Default,,0000,0000,0000,,Someone who intercepts the transmission can alter it and change the meaning in predictable ways.
Dialogue: 0,0:01:16.00,9:59:59.99,Default,,0000,0000,0000,,This is a very dangerous property for a cipher to have.