0:00:00.000,0:00:03.000 Here's how this works as a protocol--we have two participants. 0:00:03.000,0:00:08.000 We'll call Alice the generator--that means she is going to make the circuit, 0:00:08.000,0:00:11.000 and Bob the evaluator--that means he's going to evaluate the circuit. 0:00:11.000,0:00:15.000 At the beginning of the protocol, they've agreed on some circuit they want to evaluate. 0:00:15.000,0:00:17.000 For this to be interesting would be much larger than that. 0:00:17.000,0:00:21.000 It takes inputs from both Bob and from Alice. 0:00:21.000,0:00:25.000 Alice is going to generate a garble table for each one of these logic gates in the circuit, 0:00:25.000,0:00:28.000 and send the garbled circuit to Bob. 0:00:28.000,0:00:31.000 She is also going to send her input values, but because they're 0:00:31.000,0:00:33.000 random nonces Bob can't tell what they mean. 0:00:33.000,0:00:37.000 Bob's going to evaluate the circuit using the garbled circuit protocol 0:00:37.000,0:00:39.000 decrypting one entry from each of these. 0:00:39.000,0:00:42.000 At the end of this, Bob's going to get some output values. 0:00:42.000,0:00:46.000 Then they're going to do something to turn that into the semantic value, 0:00:46.000,0:00:49.000 which possibly Bob receives, possibly Alice receives. 0:00:49.000,0:00:51.000 We can design the protocol either way. 0:00:51.000,0:00:56.000 I am not going to talk about that final step of how you turn the encrypted values 0:00:56.000,0:00:58.000 at the end of the circuit into meaningful values. 0:00:58.000,0:01:01.000 We'll leave that as a question for your exam. 0:01:01.000,0:01:04.000 But there is one more question that I do need to talk about. 0:01:04.000,0:01:08.000 That's the question of how does Bob obtain his inputs to the circuit. 0:01:08.000,0:01:13.000 To evaluate these tables, Bob needs to have both Alice's inputs and his inputs. 0:01:13.000,0:01:17.000 Here are the choices. Bob could generate them himself. 0:01:17.000,0:01:19.000 He could ask Alice to provide his inputs. 0:01:19.000,0:01:23.000 He could ask Alice for both possible values for all these wire labels, 0:01:23.000,0:01:26.000 and then select the right one corresponding to his inputs. 0:01:26.000,9:59:59.000 Or none of these possibilities actually work.