1 00:00:00,000 --> 00:00:03,000 [Narrator] The answer is 1011001, 2 00:00:03,000 --> 00:00:07,000 and the way to get that is to look at the 2 possible messages. 3 00:00:07,000 --> 00:00:12,000 We had for Y 1011001. 4 00:00:12,000 --> 00:00:22,000 For N, we had 1001110, and what we want to do is XOR them to figure out the difference between those 2. 5 00:00:22,000 --> 00:00:30,000 The XOR of Y and N is equal to 0010111, 6 00:00:30,000 --> 00:00:36,000 and so this is the value that will flip a Y answer into an N answer. 7 00:00:36,000 --> 00:00:39,000 Now we want to XOR that with the actual message that was sent. 8 00:00:39,000 --> 00:00:41,000 We don't know the key, but we know that 9 00:00:41,000 --> 00:00:46,000 that message is the result of XORing Alice's message with the key, 10 00:00:46,000 --> 00:00:53,000 and so by XORing those, we get this value 1011001, 11 00:00:53,000 --> 00:00:57,000 and if that's what Mallory sends, when Bob XORs that with the key, 12 00:00:57,000 --> 00:01:01,000 what Bob will get is the XOR of Alice's message. 13 00:01:01,000 --> 00:01:04,000 Alice's message XOR'd with this value, 14 00:01:04,000 --> 00:01:07,000 which will flip Alice's message to be the opposite. 15 00:01:07,000 --> 00:01:11,000 The point of this question is to illustrate the malleability of the one-time pad. 16 00:01:11,000 --> 00:01:16,000 Someone who intercepts the transmission can alter it and change the meaning in predictable ways. 17 00:01:16,000 --> 99:59:59,999 This is a very dangerous property for a cipher to have.