Hacklog Volume 1 Anonymity: it security & Ethical Hacking Handbook

top left green onion icon).
4.1.6.1 Bridges advanced use
If  you  wish  to  manually  set  your  bridges,  (e.g.  to  use  Tor  Expert  Bundle,
TOR-based  Linux  distros  like  Tails  or  TOR  Browser  through  advanced
configuration),  you  must  firstly  visit  the  Bridge  page  of  Torproject
(https://bridges.torproject.org/bridges), skip to step 2, complete the (impossible)
captcha on top and then obtain a value like the following (*** have been added):
92.***.0.174:9001 65B2F8E594190A3************59B0E32FC45720
194.***.208.26:27049 47063AFD4CB**********F16D6FE8DC68E6942DD6
107.191**.23:443 225A895211B179FDE2E8F8E3************ECC0B0
You  can  launch  TOR  Browser  and  pass  the  newly  obtained  bridges  (Figure
16 only).
Figure 16: entering bridges on TOR
4.1.7 Pluggable Transports
Keep  in  mind,  however,  that  bridges  may  be  blacklisted  as  well,  since
everyone can access them, censors included. In order to bypass that check, TOR
developers  introduced  a  new  feature,  known  as  pluggable  transports.  PTs  turn
the  TOR  traffic  flow  into  “clean”  traffic  between  client  and  bridge  that  may
otherwise  be  intercepted  by  the  ISP  with  the  Deep  Packet  Inspection  (DPI)
technique,  classifying  the  IP  traffic  flows  and,  once  the  pattern  is  compared,

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blocking them upstream.
At  the  moment,  PT  technology  is  under  active  development  and  requires
operators  and  developers,  in  order  to  be  efficiently  integrated  with  the  TOR
Project.  Learn  more  by  visiting  the  official  web  page
[33]
.  The  currently  most
common  PTs  are  defined  as  obfuscated  bridges,  since  they  obfuscate  traffic  in
order  to  make  it  hardly  interpretable  by  ISPs.  The  underlying  technology
leverages algorithms, identified by protocols, that mix the inbound and outbound
packets. There are three protocols of this kind: obfs2, obfs3 e obfs4.
Obfs2  (version  2,  also  known  as  “Twobfuscator”)  is  the  simplest  one:  the
underlying  technology  allows  to  fetch  inbound  and  outbound  traffic  data  and
resort them randomly. As shown by recent studies, this protocol can be cracked
by intercepting the initial handshake (just like what happens with WEP security
of  WiFi  networks),  thus  revealing  the  enclosed  information.  As  a  deprecated
version, it’s out of development and unsupported by TOR.
Obfs3 (“Threebfuscator”) is quite similar to the previous protocol; however,
it uses Diffie Hellman for keys swap during the handshake (we will explain this
topic in “Encryption”).
Obfs4  is  the  fourth  version  of  the  protocol,  although  “it  is  closer  to
ScrambleSuite  than  obfs2/obfs3”,  as  its  developer  said.  The  latest  version  is
seemingly  the  safest  one,  and  is  currently  available  in  Tor  Browser  by  default.
You can learn  more about the  protocol on the  official Github page
[34]
.  The  Tor
Project official page also includes an Obfs4 list
[35]
.

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4.1.7.1 MEEK & Scramblesuit Protocols
TOR can communicate with many other protocols, besides the Obfs family*
(Figure 17).
Figure 17: bridges selection on TOR
Meek-*
The  protocols  of  the  meek-*  family  have  been  created  in  2014  to  allow
tunneling  in  a  HTTPS  circuit.  Furthermore,  a  technique  known  as  “domain
fronting” hides TOR bridge communications to ISPs. As you can see, the meek-