
[Narrator] The answer is 1011001,

and the way to get that is to look at the 2 possible messages.

We had for Y 1011001.

For N, we had 1001110, and what we want to do is XOR them to figure out the difference between those 2.

The XOR of Y and N is equal to 0010111,

and so this is the value that will flip a Y answer into an N answer.

Now we want to XOR that with the actual message that was sent.

We don't know the key, but we know that

that message is the result of XORing Alice's message with the key,

and so by XORing those, we get this value 1011001,

and if that's what Mallory sends, when Bob XORs that with the key,

what Bob will get is the XOR of Alice's message.

Alice's message XOR'd with this value,

which will flip Alice's message to be the opposite.

The point of this question is to illustrate the malleability of the onetime pad.

Someone who intercepts the transmission can alter it and change the meaning in predictable ways.

This is a very dangerous property for a cipher to have.