Anonymity on the Web: A Brief Overview By: Nipun Arora uni-na2271

What is Anonymity? “Anonymity is the state of being not identifiable within a set of subjects.” ◦ There is no such thing as absolute anonymity First suggested by David Chaum in his seminal paper[1] on anonymous r er systems Anonymity deals with hiding identity of the user ◦ Relationship between users is hidden ◦ Perfect Forward secrecy:  defending against statistical analysis attacks ◦ Confidentiality/Availability/Integrity is preserved  Services available even though hidden and integrity of the connection is maintained

Why we require anonymity? Defends against a common form of Internet surveillance known as "traffic analysis." Internet traffic analysis allows adversaries to model user behavior and interests Confidentiality (Encryption) does not prevent adversaries from determining the identity of the user and his/her peers.

Overview Goal of Anonymity ◦ Inititator Anonymity  responder(server) cannot determine the identity of the client(initiator) ◦ Responder Anonymity  attacker cannot determine who the intended receiver of the particular package is ◦ Unlikability:  attacker may determine senders and receivers but not the associations between them (attacker doesn’t know who communicates with whom)

Types of adversaries against anonymity ◦ Outsiders  Global Passive Adversary: Attacker which can have a Global view of the internet and monitor internet activity ◦ Insiders  Local eavesdropper : Attacker monitoring activity on some restricted domain  Compromised router, or malicious server Threat Model

Taxonomy of Anonymity Systems Recent anonymyzing systems can be divided in broadly two categories ◦ Low Latency Anonymizers: Low latency anonymity systems are used for interactive applications. Such systems guarantee low response time essential to interactive applications such as web browsing.  TOR: The Onion Router is one of the most commonly used Low Latency Anonymizers  JAP- Java Anonymous Proxy  An.on/Anonymizer.com

Latency Tolerant Anonymizers: Delay Tolerant anonymizers are used for applications which do not require the low response time essential to interactive applications. Eg. E=mail  Mixminion: Type III anonymous r er system

Mix nets & Mix Cascades David Chaum. “Untraceable electronic mail, return addresses, and digital pseudonyms”. Communications of the ACM, February Set of anonymizing relays/proxies to evade an eavesdropper from linking initiator and the responder. Mixes- Each relay in the network is a ‘mix’

Mix Cascade’s String A, B and C are passed as packets to Mix1 Mix 1 randomly forwards it to Mix 2, Mix 3, Mix 4 A cascade is several such mixes put in a relay It is extremely difficult for an adversary to make an end to end connection between sender and reveiver in case of mix cascade

Layered Encryption: ‘Onion’ Several layers of data The data being sent is encapsulated in something similar to an ‘onion’. At each hop in the mixed cascade the onion is peeled to find the next relay point. This concept forms the basis paradigm of the onion routing project. Address of first relay Address of the 2 nd relay Data

Overview of Tor Architecture What is Tor ? - The Onion Routing Project (currently in second gen.) Key Features: - Network of proxies - Uses “3-hop” relays (ORs) by default - – The three relay points are called Entry Node, Middleman and Exit Node. * Although can be extended to as many hops as possible. - Many connections can be multiplexed over the same Tor Circuit.

Key Features(continued...) - Encrypted connections to connect the Entry Node, Middle Man and Exit Node(uses SSL encryption) - All the information is not stored in a single Onion Router (makes it more secure) - Provides hidden services: - Services not accessible for an outsider - Tor creates a new url for the server (a string, NOT DNS NAME) within the.onion domain.

The Tor circuit Client (Alice) fetches the directory listing of ORs from the directory service (Dave) Here Dave contains all the addresses of the complete Tor network. Alice creates the onion with complete addresses of all the relay points it has to pass through. : 9002

Circut made completely...

Sources of Vulnerabilities for Anonymizers Attacks strategies are mostly based on monitoring internet activity Statistics being used are ◦ Round Trip Time ◦ Throughput ◦ Latency ◦ Clock Skew: TCP timestamp clock drift error, helps in identifying hosts which have a similar drift An attack by a truly ‘Global Passive Adversary’ cannot be defended.

Conclusion Anonymity is Necessary!!! Used in places to maintain secrecy in blogspots or for journalists making comments on contentious issues. Hiding personal information mantained in histories of several servers Secure Banking passwords Avoiding Spams

References 1. D. L. Chaum. Untraceable Electronic Mail, Return Addresses, and Digital Pseudonyms. Communincations of the ACM, 24(2):84–90, February R. Dingledine, N. Mathewson, and P. Syverson Tor: The Second-Generation Onion Router. In Proceedings of the 13th USENIX Security Symposium, pages 303–319, August 2004.

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