The document provides an overview of Tor and flash proxies. It defines key terms like anonymity and circumvention. It explains that flash proxies allow circumvention of censored networks by connecting to Tor bridges through volunteer web servers instead of directly. The technical implementation involves a client proxy plugin and server proxy that implement a new transport protocol to connect to Tor in a censored environment. The document provides instructions for installing Tor and flashproxy software and configuring them to work together.

1. Tor – Glossary (1/3)
● Glossary:
– Anonymity:
● It isn't cryptography, cryptography just protects content, but
not the privacy of what are you doing:
– Anonymity protects the privacy of what are you doing on Internet:
probably they know that you are doing something, but they don't
know what are you doing (with whom and which application)
● The only way of having effective anonymity is in a multitude
(it's not possible to get anonymity on your own)
– Circumvention:
● There is somebody filtering the traffic, only it is possible to
reach the official allowed protocols, sites and information:
– Circumvention provides the mediums in order to reach the not
allowed protocols, sites and information using the allowed protocols,
sites and information (i.e. evading the filters using the filters itself)

2. Tor – Glossary (2/3)
● Glossary:
– Onion routing:
● A message is cascaded through several routers with the
following characteristics:
– a) The sender defines the whole path, choosing the routers that are
forming this path
– b) The message is encrypted in succession, first with the
symmetric key of the last router, the result of that operation with the
penultimate router, and so on:
● Suppose the path is: Router 1 (R1), Router 2 (R2) and Router 3
(R3), the message (MSG) is encrypted (by the E function) as
follow:
● E(E(E(MSG, symKeyR3), symKeyR2), symKeyR1)
– c) The encrypted message is sent through the predefined path and
each router only knows the previous router (or the sender) and the
next router

3. Tor – Glossary (3/3)
● Glossary:
– Tor:
● It is composed by:
– Protocols (Onion routing, Directory discovery, Rendezvous, etc.)
– Software
– Network (the volunteers → provide the “multitude” for effective anonymity)
● Main components:
– (1) Proxy schema:
● (1.1) Relays:
● (a) Tor relays
● It is an onion routing
● It is I charge of building circuits (routing and cryptography), sending traffic over
them
● (b) Tor Bridge:
● Tor relays that aren't listed in the main directory (so there is no complete public list
of them)
● How to get them: https://bridges.torproject.org/
● (1.2) Discovery:
● Learning what relays are available
● (1.3) Flash Proxy schema:
● Flash proxy
●
Facilitator
● Client Transport Plugin & Server Transport Plugin
– (2) Tor Client

4. Tor - Flash Proxies – Motivation (1/2)
● Against circumvention:
– Attacker goals:
● Restrict the flow of certain kinds of information
● Chill behavior by impression that on-line activities are monitored
– Attacker techniques:
● Blocking the directory authorities
● Blocking the Tor Relay IP addresses in the directory
● Intercept DNS requests and give bogus responses or redirects
● Filtering traffic based on Tor's network fingerprint
● Deep Packet Inspection
● Preventing users from finding Tor software
– Attacker restrictions:
● Economic, politic and social incentives not to block the whole Internet
● Not effective big repression against passive consumers or total control of the
clients
● It cannot noticeably slow legitimate traffic

5. Tor – Flash Proxies- Motivation (2/2)
● Where are needed Flash Proxies?: If I'm only interested in anonymity:
is it safer to use Flash proxies
anyway?
– Anonymity:
● I don't need Flash Proxies For sure is slower
– I'm able to connect directly to the Tor Network
● I've the Relay's public directory (and perhaps some of the Tor Bridges)
– Circumvention:
● I need Flash Proxies:
– I cannot connect to the Tor Network directly, cause probably the access points
are censored (blocked)
● I need to connect to anonymous proxies located at the reachable Internet,
and through them to the Tor Network
● One of the possible schemes for this kind of anonymous proxies are the
Flash Proxies
● Detailed explanation:
– https://crypto.stanford.edu/flashproxy/flashproxy.pdf
– https://gitweb.torproject.org/flashproxy.git/blob/HEAD:/doc/design.txt

6. Tor - Flash Proxies – Start connection
Tor bridge
Client computer
Tor Server
Tor Client Facilitator server (Onion router)
Facilitator 127.0.0.1 | ORPort
127.0.0.1 | 9001
HTTP POST
(Rendezvous) Server Transport
Client Transport Public IP | 9002 Plugin
Plugin Registration with
the Facilitator
HTTP GET Poll for the IPs of
Public IP | 9000 registered clients
Public IP |?
Websocket opened Websocket opened
Flash Proxy
by the Flash Proxy by the Flash Proxy
Web page The Server Transport Plugin is
Web Browser just a Websocket Transport Plugin
with the The Flash Proxy
BADGE (Running in an anonymous (i.e. there isn't the need to implement a
is executed while computer visiting the special Flash Proxy Plugin as in
visiting the web volunteer web server) the Client computer) ?
page with the
Web Server BADGE Where is it implemented?
(Volunteer server
serving a page
with the BADGE)

7. Tor - Flash Proxies – Sending data
Tor Relays Tor Bridges
Middle node
Target
web
site
Exit node
Entry node
Flash Proxies
Why isn't possible for the FP ask to the
client for the Entry node? Is for speed up Client
the connection stablishment?
Is the FP only choosing for Entry nodes
from the Tor Bridges?
* The client only chooses the Middle and Exit node
- The Flash proxy chooses the Entry node

8. Tor – Flash Proxies- Technical: Pluggable transport (1/3)
● Goal:
– Implement new transport mechanisms to connect the client
with the Tor Network decoupled from the core Tor Protocol:
● The idea is improve circumvention techniques (at the transport
level) that help people avoid censorship connecting to the Tor
Network
● More at:
– https://gitweb.torproject.org/torspec.git/blob/HEAD:/proposals/180-pluggable-transport.txt
– https://www.torproject.org/docs/pluggable-transports.html.en
● Implementation:
– To implement a new Transport Protocol is needed to
develop:
● A "Client Proxy" to run at the initiator side (Tor clients)
● A "Server Proxy" to run at the server side (Tor bridges)

9. Tor – Flash Proxies- Technical: Pluggable transport (2/3)
● Implementation:
– "Client Proxy":
● At the “torrc” file:
– The “Bridge” option is modified in order to accept the <method> value:
● Bridge method address:port [[keyid=]id-fingerprint] [k=v] [k=v] [k=v]
– It is added the “ClientTransportPlugin” option:
● There are two kinds of “Client proxies”:
● The external ones: are running independent and the Tor Client must connect to them:
● ClientTransportPlugin <method> socks4 <address:port> [auth=X]
● ClientTransportPlugin <method> socks5 <address:port> [username=X] [password=Y]
● The managed ones: the Tor Client must execute them:
● ClientTransportPlugin <methods> exec <path> [options]
● Example:
– Configuration at the “torrc” file:
● Bridge trebuchet 200.178.1.34:3333 keyid=09F911 usr=xx pwd=yy
● ClientTransportPlugin trebuchet socks5 127.0.0.1:9999
– Meaning:
● The Tor Client knows that in order to connect to the Tor Network should use the “trebuchet” transport
(indicated by the “Brigde” option). In order to use this transport it must send its traffic to the local process
waiting at the port 9999 using SOCKS 5 (indicated by the “ClientTransportPlugin” option). This local
process must send the traffic to 200.178.1.34:3333, providing the strings "usr=xx;pwd=yy" as the user
and the password

10. Tor – Flash Proxies- Technical: Pluggable transport (3/3)
● Implementation:
– "Server Proxy":
● At the “torrc” file:
– It is added the “ServerTransportPlugin” option:
● There are two kinds of “Server proxies”:
● The external ones: are running independent and the Tor Server must connect to
them:
● ServerTransportPlugin <method> proxy <address:port> <param=val> ...
● The managed ones: the Tor Server must execute them:
● ServerTransportPlugin <methods> exec </path/to/binary> [options]
● Example:
– Configuration at the “torrc” file:
● ServerTransportPlugin trebuchet proxy 127.0.0.1:999 usr=aa
– Meaning:
How is working it related with the “Client proxy” configuration and
with the Tor Server (i.e. in which address and port the “Server
Proxy” is listening the outside connection and how is it
sending the information to the Tor Server)?
How knows the “Client Proxy” where to connect to the “Server Proxy”?

11. Tor – Flash Proxies- Technical: Use (1/8)
● Installation:
– (1) Tor:
● (1.1) Installing "libevent":
– Download the source tarball (from http://libevent.org/)
– Copy it to /usr/local
– Run: cd /usr/local
– Run: sudo tar xzf libevent-2.0.20-stable.tar.gz
– Run: cd ./libevent
– Run: sudo ./configure
– Run: sudo make
– Installing the “dev”:
● sudo apt-get install libevent-dev
● (1.2) Installing "openssl" and "zlib":
– sudo apt-get install libssl-dev
– sudo apt-get install zlib1g-dev

12. Tor – Flash Proxies- Technical: Use (2/8)
● Installation:
● (1.3) Installing "Tor" (alpha version):
– Download the source tarball (from https://www.torproject.org/download/download.html.en ->
"Source Tarball")
– Copy it to /usr/local
– Run: cd /usr/local
– Run: sudo tar xzf tor-0.2.3.24-rc.tar.gz
– Run: cd ./tor
– Run: sudo ./configure
– Run: sudo make install
– (2) Flashproxy
● Run: Cd /usr/local
● Run: git clone https://git.torproject.org/flashproxy.git

13. Tor – Flash Proxies- Technical: Use (3/8)
● Configuration:
● (1) If the “flashproxy-client” is running with a LAN IP (behind a
NAT), enable Port Forwarding in the router:
–

14. Tor – Flash Proxies- Technical: Use (4/8)
● Configuration:
● (2) If the “flashproxy-client” is running behind a Firewall open
the port where is listening (by default the 9000):
–

15. Tor – Flash Proxies- Technical: Use (5/8)
● Configuration:
● (3) Add the following lines to the “torrc” file of your Tor Client:
– Lines:
● ClientTransportPlugin websocket socks4 127.0.0.1:9001
● UseBridges 1
● Bridge websocket 0.0.1.0:1
– Meaning:
● That is saying to the Tor Client to use the websocket transport in
order to connect to the Tor Network (though the “Bridge” option) and
for that the Tor Client must forward its traffic through SOCKS 4 to the
process listening at the port 9001:
● It means also that the “flashproxy-client” must be started at the
port 9001 (the default one)

16. Tor – Flash Proxies- Technical: Use (6/8)
● Running:
● Fast test:
– Execute:
● tor ClientTransportPlugin "websocket socks4 tor-facilitator.bamsoftware.com:9999" UseBridges 1
Bridge "websocket 0.0.1.0:1" LearnCircuitBuildTimeout 0 CircuitBuildTimeout 60
● It is useful to test the Tor Client with the Flashproxy Transport Plugin and also the network
conditions, but isn't useful for a real scenario in a censored area:
● The problem is that the “flashproxy-client” is running at
“tor-facilitator.bamsoftware.com:9999”, instead of being local, so the censor only need
to filter this address to prevent you from connecting to the Tor Network
●

17. Tor – Flash Proxies- Technical: Use (7/8)
● Running:
● Normal running – Flash Proxy Client:
– Flashproxy-client --facilitator=xx.xx.xx.xx --register
● In a censored region you must use the “-- facilitator” option to define to which Facilitator to
connect in order to register your public IP, cause in the opposite the client is using the default
facilitator and it could be easily blocked
● Here the registration by email (“flashproxy-reg-email”) is failing cause the port 25 is filtered by
the ISP (also the ports 587 and 465 are blocked). So, finally the registration was made using
HTTP to the default Facilitator (https://tor-facilitator.bamsoftware.com )

18. Tor – Flash Proxies- Technical: Use (8/8)
● Running:
● Normal running – Tor Client:
– tor -f torrc