1. Anonymity (Privacy) Suppose you are surfing the Web.
You don’t want Web site to know your IP address
And you don’t want your ISP to sniff your traffic.
Possible solution: use anonymizing proxy
anonymizing
proxy
SSL
clear
Introduction

2. Proxify.com
Just go to a website and enter URL.
No software to install.
Script on page causes browser to create SSL session between browser and proxify.
Anonymous browsing is free
To post, must pay fee
Problem: Proxify.com still knows which IP addresses are visiting which sites.
Introduction

3. Tor: Chaining proxies
Features:
Proxy servers are chained, making it more difficult to trace which IPs visited which servers
Anyone can contribute a proxy server (P2P)
Open-source, cannot have any backdoors
Uses SOCKS for proxy protocol: can be used with all application layer protocols.
Brief History:
Originally supported by the Navy, later also by Electronic Frontier Foundation.
In 2002 the code was given to Roger Dingledine and Nick Mathewson, two Boston-based programmers
Introduction

4. How it works Basic Principles:
List of servers is obtained from directory server
Data is sent through 3 randomly chosen servers
Encryption is applied in a layered manner, each of the servers peels off a layer (like in an onion).
The path changes every minute.
Introduction

5. Layered encryption: Overview
Suppose Alice wants to communicate with Bob (Web server), via two servers.
Establishing keys and circuit:
Alice has certificates for both servers.
Alice first does a D-H exchange with Server 1, establishing a session key KA1.
She then does a D-H exchange with Server 2, via Server 1, establishing a session key KA2.
“Circuit” is now established between Alice and Server 2.
Sending message m to Bob
Alice encrypts twice: KA1(KA2(m)), sends to Server 1.
Server 1 decrypts, obtains KA2(m), sends to Server 2
Server 2 decrypts, sends m to Bob.
Introduction

6. Diffie-Hellman exchange between Alice and first server
RSA, hashing and handshaking is used to prevent man-in-middle attack and provide perfect “forward secrecy”
So Alice knows she’s talking with server and not with Trudy
So if someone obtains K1- in the future, will not get gxy x
c = K1+(gx)
gy, H( gxy ) y
First server
in chain with
keys K1+ and K1-
They now both have KA1 = gxy
Alice
Introduction

7. What can woman-in-middle do?
Can Trudy pretend to Alice that she is first server?
If Trudy records all messages and in future obtains K1-, will she be able to decrypt?
Introduction

8. Tor protocol: Two onion-ring proxies
One layer of encryption has been removed
Note the double braces, encryption has been applied twice
Introduction

9. What do the servers know?
Server 1 knows:
AliceIP wants to communicate with some dest IP; but doesn’t learn the dest IP.
Server 1 never gets Alice’s certificate, so only knows of AliceIP and not Alice.
Doesn’t see m.
Server 2 knows:
Some IP address wants to communicate with Bob, but doesn’t see the IP address.
Sees m.
Since Bob doesn’t use encryption, last server necessarily sees m
Introduction

10. Anonymity: Summary
Proxy server + encryption can provide a degree of anonymity
But what if you can’t trust the proxy server?
Chaining proxy servers provide more protection.
As long as the server don’t collude
Introduction