Tor

TOR Browser
“THE ONION ROUTER”
By G. AbboudSeptember 2016

TOR BROWSER | SEPT 2016 2
I. INTRODUCTION
II. THE ONION ROUTING
III. BEHIND THE SCENES
I. HOW IT’S BUILT
II. HOW IT WORKS
III. TOR DESIGN
I. CELLS
II. CIRCUITS AND STREAMS
III. OPENING AND CLOSING STREAMS
IV. METRICS
V. STAYING ANONYMOUS
VI. DISADVANTAGES
VII. FUTURE DIRECTIONS
VIII.REFERENCES

I. INTRODUCTION

INTRODUCTION
What is Tor?
Tor is free software and an open network that helps you defend against traffic
analysis.
It prevents somebody watching your Internet connection from learning what sites
you visit, it prevents the sites you visit from learning your physical location, and it
lets you access sites which are blocked.

INTRODUCTION
The Tor network is a group of volunteer-operated servers that allows people to
improve their privacy and security on the Internet. (more than 7000 relays)
Tor's users employ this network by connecting through a series of virtual tunnels
rather than making a direct connection.

INTRODUCTION
Developer(s) The Tor Project, Inc
Initial release 20 September 2002
Stable release 0.2.8.8(23 September 2016)
Development status Active
Written in C,Python
Operating system • Microsoft Windows
• Unix-like (Android,Linux, OS X)
Size 2–4 MB
Type Onion routing, Anonymity
License BSD
Website torproject.org
Repository gitweb.torproject.org/tor.git
Godfather Paul Syverson http://www.syverson.org/

II. THE ONION ROUTING

THE ONION ROUTING
Onion routing is a technique for anonymous communication over a computer
network.
In an onion network, messages are encapsulated in layers of encryption,
analogous to layers of an onion.
Onion routing is implemented by encryption in the application layer of a
communication protocol stack.

THE ONION ROUTING
In this example onion, the source of the data
sends the onion to Router A, which removes a
layer of encryption to learn only where to send
it next and where it came from (though it does
not know if the sender is the origin or just
another node).
Router A sends it to Router B, which decrypts
another layer to learn its next destination.
Router B sends it to Router C, which removes
the final layer of encryption and transmits the
original message to its destination.

III. BEHIND THE SCENES

BEHIND THE SCENES >> HOW IT’S BUILT
The Tor Browser is based on Mozilla's Extended Support Release (ESR) Firefox
branch
It includes HTTPS-Everywhere

BEHIND THE SCENES >> HOW IT WORKS
Tor distributes your transactions over several places on the Internet, so no single
point can link you to your destination

Instead of taking a direct route from source to destination, data packets on the Tor
network take a random pathway through several relays that cover your tracks.
No individual relay ever knows
the complete path that a data
packet has taken.

The circuit is extended one hop at a time, and each relay along the way knows only
which relay gave it data and which relay it is giving data to.
Tor client incrementally builds
a circuit of encrypted connections
through relays on the network.
The client negotiates a separate
set of encryption keys for each hop
to ensure that each hop can't
trace these connections as they
pass through.

BEHIND THE SCENES >> TOR DESIGN
The Tor network is an overlay network; each onion router (OR) runs as a normal
user-level process without any special privileges.
Each onion router maintains a TLS connection to every other onion router.
Each user runs local software called an onion proxy (OP) to fetch directories,
establish circuits across the network, and handle connections from user
applications.
These onion proxies accept TCP streams and multiplex them across the circuits. The
onion router on the other side of the circuit connects to the requested destinations
and relays data.

Each onion router maintains a long-term identity key and a short-term onion key.
• The identity key is used to sign TLS certiﬁcates, to sign the OR’s router descriptor
(a summary of its keys, address, bandwidth, exit policy, and so on), and (by
directory servers) to sign directories.
• The onion key is used to decrypt requests from users to set up a circuit and
negotiate ephemeral keys*.
*A cryptographic key is called ephemeral if it is generated for each execution of a key establishment process
The TLS protocol also establishes a short-term link key when communicating
between ORs.
Short-term keys are rotated periodically and independently, to limit the impact of
key compromise.

BEHIND THE SCENES >> TOR DESIGN >> CELLS
Onion routers communicate with one another, and with users’ OPs, via TLS
connections with ephemeral keys.
Traffic passes along these connections in fixed-size cells. Each cell is 512 bytes, and
consists of a header and a payload.
The header includes a circuit identifier (circID) that specifies which circuit the cell
refers to (many circuits can be multiplexed over the single TLS connection), and a
command to describe what to do with the cell’s payload.

2 types of cells (Control cells and Relay cells)
Control cells: always interpreted by the node that receives them
Commands:
• Padding (currently used for keepalive, but also usable for link padding)
• Create or Created (used to set up a new circuit);
• Destroy (to tear down a circuit).

Relay cells: carry end-to-end stream data.
Commands:
• Relay data (for data ﬂowing down the stream)
• Relay begin (to open a stream)
• Relay end (to close a stream cleanly)
• Relay teardown (to close a broken stream),
• Relay connected (to notify the OP that a relay begin has succeeded)
• Relay extend and relay extended (to extend the circuit by a hop, and to acknowledge)
• Relay truncate and relay truncated (to tear down only part of the circuit, and to
acknowledge)
• Relay sendme (used for congestion control)
• Relay drop (used to implement long-range dummies).

Relay cells: carry end-to-end stream data.
Relay cells have an additional header (the relay header) at the front of the payload,
containing a streamID (stream identiﬁer: many streams can be multiplexed over a
circuit); an end-to-end checksum for integrity checking; the length of the relay
payload; and a relay command.
The entire contents of the relay header and the relay cell payload are encrypted or
decrypted together as the relay cell moves along the circuit, using the 128-bit AES
cipher in counter mode to generate a cipher stream.

BEHIND THE SCENES >> TOR DESIGN >> CIRCUITS AND STREAMS
Onion Routing originally built one circuit for each TCP stream.
In Tor, each circuit can be shared by many TCP streams.
To limit linkability among their streams, users’ OPs build a new circuit periodically if
the previous ones have been used, and expire old used circuits that no longer have
any open streams.
OPs consider rotating to a new circuit once a minute.
Also, because circuits are built in the background, OPs can recover from failed
circuit creation without harming user experience.

>> Constructing a circuit
To begin creating a new circuit,
the OP (Alice) sends a create cell
to the ﬁrst node in her chosen
path (Bob). (She chooses a new
circID CAB not currently used on
the connection from her to Bob.)
Alice builds a two-hop circuit and begins fetching a web page.

The create cell’s payload contains
the ﬁrst half of the Difﬁe-Hellman
handshake (gx), encrypted to the
onion key of the OR (Bob).
Bob responds with a created cell
containing gy along with a hash of
the negotiated key K = gxy.
Once the circuit has been established, Alice and Bob can send one another relay cells encrypted with the negotiated key.

To extend the circuit further,
Alice sends a relay extend cell to
Bob, specifying the address of
the next OR (call her Carol), and
an encrypted gx2 for her.
Bob copies the half-handshake
into a create cell, and passes it to
Carol to extend the circuit.
When Carol responds with a created cell, Bob wraps the payload into a relay extended cell
and passes it back to Alice.
Now the circuit is extended to Carol, and Alice and Carol share a common key K2 = gx2y2.

To extend the circuit to a third node or
beyond, Alice proceeds as above, always
telling the last node in the circuit to
extend one hop further.
This circuit-level handshake protocol
achieves unilateral entity authentication
and unilateral key authentication (Alice
and the OR agree on a key, and Alice
knows only the OR learns it).

BEHIND THE SCENES >> TOR DESIGN >> OPENING & CLOSING
STREAMS
Closing a Tor stream is analogous to closing a TCP stream: it uses a two-step handshake for
normal operation, or a onestep handshake for errors.
If the stream closes abnormally, the adjacent node simply sends a relay teardown cell.
If the stream closes normally, the node sends a relay end cell down the circuit, and the other
side responds with its own relay end cell.
Because all relay cells use layered encryption, only the destination OR knows that a given relay
cell is a request to close a stream.

IV. METRICS

METRICS
Under https://metrics.torproject.org/ you can find interesting facts about the Tor
network, the largest deployed anonymity network to date

V. STAYING ANONYMOUS

STAYING ANONYMOUS
Tor can't solve all anonymity problems.
It focuses only on protecting the transport of data. You need to use protocol-
specific support software if you don't want the sites you visit to see your identifying
information. For example, you can use Tor Browser while browsing the web to
withhold some information about your computer's configuration.
Also, to protect your anonymity, be smart. Don't provide your name or other
revealing information in web forms. Be aware that, like all anonymizing networks
that are fast enough for web browsing, Tor does not provide protection against
end-to-end timing attacks: If your attacker can watch the traffic coming out of your
computer, and also the traffic arriving at your chosen destination, he can use
statistical analysis to discover that they are part of the same circuit.

VI. DISADVANTAGES

DISADVANTAGES
By using TOR network, you will be able to get anonymity while browsing but some
obvious drawbacks are there which are like:
• Performance will suffer, it's one of the major disadvantage.
• Low latency anonymizers are prone to traffic analysis.
• Exit nodes see your traffic in plain
If you don't use a secure protocol on top of Tor, the exit node can sniff your
passwords etc. Using SSL or SSH you should be fine, but be sure to validate the
certificate/fingerprints.

VII. FUTURE DIRECTIONS

FUTURE DIRECTIONS
Providing a usable anonymizing network on the Internet today is an ongoing
challenge.
Security and usability don't have to be at odds: As Tor's usability increases, it will
attract more users, which will increase the possible sources and destinations of
each communication, thus increasing security for everyone.
TOR is making progress, but need your help…
Please consider running a relay or volunteering as a developer.

VIII. REFERENCES

REFERENCES
https://www.torproject.org/about/overview.html.en
http://www.onion-router.net/
http://ntrg.cs.tcd.ie/undergrad/4ba2.05/group10/index
http://en.wikipedia.org/wiki/Onion_routing
Tor: The Second-Generation Onion Router research paper by
• Roger Dingledine The Free Haven Project arma@freehaven.net
• Nick Mathewson The Free Haven Project nickm@freehaven.net
• Paul Syverson Naval Research Lab syverson@itd.nrl.navy.mil

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