Slide title

In CAPITALS

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Muhammad Rizwan Asghar

August 21, 2020

TOR AND ATTACKS ON TOR

Lecture 12b

COMPSCI 316

Cyber Security

Source of most slides: Northeastern University, USA

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FOCUS OF THIS LECTURE

Understand TOR

Learn how TOR works

Discuss attacks on TOR

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TOR

TOR stands for The Onion Router

It enables anonymous communication

It is free and open-source

Basic design: A mix network with

improvements

Mixes in TOR are known as Relays

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IMPROVEMENTS IN TOR

Perfect forward secrecy

Introduces guards to improve source anonymity

Takes bandwidth into account when selecting relays

Offers hidden services

– Servers that are only accessible via TOR

– Hidden services provide 2-way anonymity

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DEPLOYMENT AND STATS

Largest anonymity preserving service

– Publicly available since 2002

– Continues to be developed and improved

Currently, over 6000 TOR relays

– Source: https://metrics.torproject.org/networksize.html

All relays are run by volunteers

It is suspected that some are controlled by intelligence

agencies

Roughly, 500K-6M users

– Source: https://metrics.torproject.org/userstats-relay-country.html

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TOR IN ACTION

Relays form an anonymous circuit

Traffic is protected with layers of encryption

Relay

<KP, KS>

<KP, KS>

<KP, KS>

<KP, KS>

<KP, KS>

<KP, KS>

<KP, KS>

<KP, KS>[KP , KP , KP]

Encrypted

Tunnels

Non-encrypted

data

E(KP , E(KP , E(KP , M))) = C

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TOR CIRCUIT

TOR users can use any number of relays

Default configuration is 3

Entry/

Guard

Middle Exit

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TOR: MALICIOUS ENTRY

Source known

Destination unknown

Entry/

Guard

Middle Exit

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TOR: MALICIOUS RELAY

Source unknown

Destination unknown

Entry/

Guard

Middle Exit

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TOR: MALICIOUS EXIT

Source unknown

Destination known

Entry/

Guard

Middle Exit

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TOR: MALICIOUS ENTRY & EXIT

Source known

Destination known

Entry/

Guard

Middle Exit

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PREDECESSOR ATTACK

Assumptions

– N total relays

– M of them are controlled by an attacker

Attacker goal: control the first and last relays

– M/N chance for first relay

– (M-1)/(N-1) chance for the last relay

– Roughly, (M/N)2 chance overall for a single circuit

However, clients periodically build new circuits

– Over time, the chances for the attacker to be in the correct

positions improves!

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CIRCUIT LIFETIME

To mitigate the predecessor attack, one possibility is to

increase the circuit lifetime

Issues

– If you happen to choose the attacker as guard and exit, you

are screwed

– A single attacker in the circuit (as guard or exit) can still

perform statistical inference attacks

Bottom line: long lived circuits are not a solution

– TOR’s default circuit lifetime is 10 minutes

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SELECTING RELAYS

How do clients locate TOR relays?

TOR Consensus File

– Hosted by trusted directory servers

– Lists all known relays

IP address, uptime, measured bandwidth, etc.

Not all relays are created equal

– Entry/guard and exit relays are specially labelled

TOR does not select relays randomly

– Chance of selection is proportional to bandwidth

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SAMPLE QUESTION

Let us assume that we have a TOR circuit of

length 3. Which one of the following least affects

anonymity?

a) A malicious guard

b) A malicious middle node

c) A malicious exit node

d) Both a and c

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SAMPLE QUESTION: ANSWER

Let us assume that we have a TOR circuit of

length 3. Which one of the following least affects

anonymity?

a) A malicious guard

b) A malicious middle node

c) A malicious exit node

d) Both a and c

Answer) b

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TO BE CONTINUED

See the next lecture

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Questions?

Thanks for your attention!