WEBVTT 0 00:01.550 --> 00:06.680 In this lecture we’ll dive deeper into how Onion Routing and Tor work. 1 00:07.500 --> 00:13.940 In the last lecture, we've learned that in order to remain anonymous, the Tor client will bounce the data 2 00:13.940 --> 00:19.620 through three different servers or Tor relays and the last relay will connect to the Internet 3 00:19.620 --> 00:22.740 server, lets say to a Web site on your behalf. 4 00:23.250 --> 00:30.660 It's like a proxy. When building a Tor circuit the first relay is called the “Entry Guard” and 5 00:30.660 --> 00:32.270 the last relay in the circuit 6 00:32.490 --> 00:33.960 is the “exit relay” 7 00:34.230 --> 00:37.680 that sends the traffic out onto the public Internet. 8 00:38.790 --> 00:42.180 Let's open the Tor browser and connect to Google. 9 00:42.720 --> 00:48.900 Don't worry for the moment about how to get the Tor browser, because I'll show you how to install 10 00:48.920 --> 00:51.090 and use it in the next lectures. 11 00:51.990 --> 00:55.110 So I'm connecting to Google.com. 12 00:58.400 --> 01:05.710 This is Google and I'm connecting to it through the Tor Network. To see the circuit that Tor Browser 13 01:05.780 --> 01:11.510 is using for the current tab go to the site information menu, in the URL bar. 14 01:12.020 --> 01:13.520 This is the Tor circuit! 15 01:14.640 --> 01:16.640 Pay special attention to the Tor 16 01:16.680 --> 01:17.370 relays. 17 01:17.760 --> 01:19.650 Note that they are three. 18 01:21.650 --> 01:26.360 My browser has connected to the Guard relay that is located in Ukraine. 19 01:26.750 --> 01:29.570 The middle relay is in the United Kingdom. 20 01:30.020 --> 01:37.790 And the exit relay in France; and the exit relay has connected on my behalf to Google.com. 21 01:38.820 --> 01:41.860 Let's take a deeper dive into how Tor works. 22 01:42.410 --> 01:49.870 The first step in building a Tor connection is the Tor browser or client to download a list of Tor 23 01:49.880 --> 01:54.320 notes, servers or relays from unknown directory server. 24 01:55.160 --> 02:03.980 Each tor client selects a few relays, at random, to use as entry points and uses only those relays for 25 02:03.980 --> 02:06.650 the first hop, which is normal Tor behavior. 26 02:07.040 --> 02:15.260 So the first relay in your circuit is called an "entry guard" and it’s a fast and stable relay that remains 27 02:15.290 --> 02:22.370 the first one in your circuit for 2-3 months in order to protect against a known anonymity-breaking 28 02:22.370 --> 02:22.880 attack. 29 02:23.420 --> 02:27.890 The rest of your circuit changes with every new Web site you visit 30 02:28.250 --> 02:34.250 and together, all of these relays will provide the full privacy protection of Tor. 31 02:35.240 --> 02:42.020 Now, instead of taking a direct route from source to destination, like in the case of a normal Internet 32 02:42.020 --> 02:49.250 routing, data packets on the Tor network take a random pathway through several relays and no observer, 33 02:49.310 --> 02:54.650 at any single point, can tell where the data came from, or where it is going to. 34 02:55.860 --> 03:01.890 To create a private network pathway within the Tor network, the Tor client builds a secret 35 03:02.130 --> 03:05.610 of encrypted connections through three different relays. 36 03:06.090 --> 03:11.040 The client will negotiate a symmetric encryption key with each 37 03:11.040 --> 03:11.420 relay 38 03:11.700 --> 03:13.470 so there are three encryption keys. 39 03:14.040 --> 03:21.240 The client will encrypt the message three times, in layers, with these three encryption keys. 40 03:22.310 --> 03:29.000 It will first encrypt the message with K3, the resulting encrypted message will be encrypted again with 41 03:29.000 --> 03:29.590 K2 42 03:29.960 --> 03:37.370 and finally again with K1. It's like the layers of an onion and the message resides at the heart 43 03:37.580 --> 03:39.360 of the onion. Let's 44 03:39.650 --> 03:41.090 see what that means! 45 03:42.050 --> 03:49.160 When the message leaves the Tor client, it has been encrypted three times; the outer layer being 46 03:49.160 --> 03:57.140 encrypted with K1, and that means that the first relay, the guard relay can unlock the first layer of 47 03:57.140 --> 03:59.510 encryption because it has the key. 48 04:00.440 --> 04:03.250 The guard relay has the key named K1. 49 04:05.470 --> 04:13.510 Note that the guard relay cannot read the message because it’s still encrypted twice (with k2 and 50 04:13.510 --> 04:16.570 k3). It’s complete gibberish for it. 51 04:17.500 --> 04:24.460 What can it do is forward the message to the middle relay, which has K2, and can decrypt the second 52 04:24.460 --> 04:24.840 layer. 53 04:26.110 --> 04:34.150 The middle relay cannot look at the message because it’s still encrypted with k3 and it doesn’t have 54 04:34.150 --> 04:34.660 the key. 55 04:35.620 --> 04:37.750 The middle relay will forward 56 04:37.810 --> 04:45.850 the message to the last relay, called the exit relay, which in turn will decrypt the last layer with 57 04:45.850 --> 04:48.430 K3 and get the original message. 58 04:48.580 --> 04:56.320 So the final relay knows the destination like say twitter.com and forwards the packet to a that server. 59 04:57.290 --> 05:01.110 Note that the data in the packet could still be encrypted 60 05:01.220 --> 05:07.550 if the protocol was https or unencrypted, if the protocol was http. 61 05:08.810 --> 05:12.620 The server receives the message and responds back. 62 05:13.130 --> 05:17.270 Now, on the way back, the exact reverse process happens. 63 05:18.620 --> 05:25.700 The first relay, which was the exit relay, gets the normal message, encrypts it with K3 and then 64 05:25.700 --> 05:33.860 forwards it to the middle relay, which encrypts it with K-2 and forwards it to the last relay, which 65 05:33.920 --> 05:39.290 in turn encrypts the message with K1 and sands it to the client. 66 05:40.850 --> 05:47.900 This one has all three keys and can unlook all three layers of encryption and get the original 67 05:47.900 --> 05:48.440 message, 68 05:48.770 --> 05:50.480 the response from the server. 69 05:51.720 --> 05:54.160 And now comes something important! 70 05:54.550 --> 06:00.820 The Tor circuit is extended one hop at a time, and each relay along the way 71 06:00.850 --> 06:04.660 knows only which relay gave it data and which relay it is giving data to 72 06:04.870 --> 06:10.200 So a relay knows only its direct neighbors. 73 06:10.840 --> 06:19.450 No individual relay ever knows the complete path that a data packet has taken because each relay sees 74 06:19.690 --> 06:21.910 no more than one hop in the circuit, 75 06:22.360 --> 06:30.430 neither an eavesdropper nor a compromised relay can use traffic analysis to link the connection's source 76 06:30.490 --> 06:31.450 and destination. 77 06:32.350 --> 06:39.310 In this example, if for example the middle relay gets compromised, it doesn’t know the identity 78 06:39.400 --> 06:42.010 of the source nor the destination server. 79 06:42.820 --> 06:50.390 It only knows that it has received the message from the entry guard and that it has to forward 80 06:50.400 --> 06:52.570 the message to the exit relay. 81 06:53.570 --> 07:02.180 If we continue with the analogy, the Guardia Node only knows that the client is using Tor and the Exit 82 07:02.180 --> 07:07.100 Node only knows that someone which is anonymous is visiting 83 07:07.190 --> 07:12.440 twitter.com. The exit node doesn't knows the identity of the source. 84 07:13.130 --> 07:16.760 It only knows the next hop to the Tor client. 85 07:18.150 --> 07:21.950 This is the Tor browser; if you connect to different websites 86 07:22.140 --> 07:25.710 then a new Tor Circuit for each website will be created. 87 07:26.400 --> 07:33.300 But connections to a single website address will be made over the same Tor circuit, meaning you can 88 07:33.300 --> 07:41.550 browse different pages of a single website, in separate tabs or windows, without any loss of functionality. 89 07:42.400 --> 07:44.100 Let's take a look at that! 90 07:45.120 --> 07:49.040 Using the Tor browser, I'm connecting to Twitter.com. 91 07:56.040 --> 07:57.090 This is the circuit: 92 07:58.230 --> 08:01.500 Ukraine, Netherlands, Germany, and twitter.com. 93 08:03.520 --> 08:06.590 And I'll click on a link on this Web page. 94 08:06.770 --> 08:10.310 So, in fact, I am opening a new Web page from the same domain. 95 08:16.990 --> 08:18.520 It's the same circuit. 96 08:22.030 --> 08:26.500 This browser: Ukraine, Netherlands, Germany, and twitter.com. 97 08:27.610 --> 08:29.200 And here the same. 98 08:30.190 --> 08:34.870 But if I'm connecting to another server, let's say Facebook.com, 99 08:35.270 --> 08:37.120 a new circuit will be created. 100 08:40.020 --> 08:42.000 you see, it's another circuit! 101 08:44.000 --> 08:50.540 And for efficiency, the Tor client uses the same circuit for connections that happen within the same 102 08:50.540 --> 08:52.200 10 minutes period of time. 103 08:53.360 --> 09:00.900 Later, requests are given a new circuit to keep people from linking someone's earlier actions to the 104 09:00.900 --> 09:01.510 new ones. 105 09:02.660 --> 09:11.960 Also, note that Tor only works for TCP streams and can be used by any application with SOCKS support. 106 09:12.650 --> 09:20.810 One drawback of using Tor is that because of these relays, it's much slower than a normal Internet 107 09:20.810 --> 09:22.730 connection or even a VPN. 108 09:23.240 --> 09:25.190 Just consider that the Tor 109 09:25.190 --> 09:30.500 notes are run by volunteers and might be serving lots of clients at once. 110 09:30.770 --> 09:35.420 And also consider that they may be not be geographically located nearby. 111 09:35.970 --> 09:42.560 Maybe a Tor client from Germany is connected to a server located in Romania, going through a relay from South 112 09:42.560 --> 09:43.100 Korea, 113 09:43.550 --> 09:45.680 and another one located in Canada. 114 09:46.160 --> 09:49.490 So the packets will make large global hops. 115 09:51.090 --> 09:52.250 OK, that's all! 116 09:52.560 --> 09:59.880 Now you have a deep understanding of how onion routing and the particular implementation called Tor 117 10:00.090 --> 10:00.600 work.