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[Narrator] The answer is 1011001,
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and the way to get that is to look at the 2 possible messages.
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We had for Y 1011001.
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For N, we had 1001110, and what we want to do is XOR them to figure out the difference between those 2.
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The XOR of Y and N is equal to 0010111,
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and so this is the value that will flip a Y answer into an N answer.
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Now we want to XOR that with the actual message that was sent.
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We don't know the key, but we know that
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that message is the result of XORing Alice's message with the key,
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and so by XORing those, we get this value 1011001,
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and if that's what Mallory sends, when Bob XORs that with the key,
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what Bob will get is the XOR of Alice's message.
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Alice's message XOR'd with this value,
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which will flip Alice's message to be the opposite.
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The point of this question is to illustrate the malleability of the one-time pad.
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Someone who intercepts the transmission can alter it and change the meaning in predictable ways.
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This is a very dangerous property for a cipher to have.