<s>
Onion	B-Protocol
routing	I-Protocol
is	O
a	O
technique	O
for	O
anonymous	O
communication	O
over	O
a	O
computer	B-Architecture
network	I-Architecture
.	O
</s>
<s>
In	O
an	O
onion	B-Protocol
network	I-Protocol
,	O
messages	O
are	O
encapsulated	O
in	O
layers	O
of	O
encryption	O
,	O
analogous	O
to	O
the	O
layers	O
of	O
an	O
onion	O
.	O
</s>
<s>
The	O
encrypted	O
data	O
is	O
transmitted	O
through	O
a	O
series	O
of	O
network	B-Protocol
nodes	I-Protocol
called	O
"	O
onion	B-Protocol
routers	I-Protocol
,	O
"	O
each	O
of	O
which	O
"	O
peels	O
"	O
away	O
a	O
single	O
layer	O
,	O
revealing	O
the	O
data	O
's	O
next	O
destination	O
.	O
</s>
<s>
While	O
onion	B-Protocol
routing	I-Protocol
provides	O
a	O
high	O
level	O
of	O
security	O
and	O
anonymity	O
,	O
there	O
are	O
methods	O
to	O
break	O
the	O
anonymity	O
of	O
this	O
technique	O
,	O
such	O
as	O
timing	O
analysis	O
.	O
</s>
<s>
Onion	B-Protocol
routing	I-Protocol
was	O
developed	O
in	O
the	O
mid-1990s	O
at	O
the	O
U.S.	O
</s>
<s>
The	O
most	O
important	O
part	O
of	O
this	O
research	O
is	O
the	O
configurations	O
and	O
applications	O
of	O
onion	B-Protocol
routing	I-Protocol
on	O
the	O
existing	O
e-services	O
,	O
such	O
as	O
Virtual	B-Application
private	I-Application
network	I-Application
,	O
Web-browsing	O
,	O
Email	O
,	O
Remote	B-Protocol
login	I-Protocol
,	O
and	O
Electronic	O
cash	O
.	O
</s>
<s>
Based	O
on	O
the	O
existing	O
onion	B-Protocol
routing	I-Protocol
technology	O
,	O
computer	O
scientists	O
Roger	O
Dingledine	O
and	O
Nick	O
Mathewson	O
joined	O
Paul	O
Syverson	O
in	O
2002	O
to	O
develop	O
what	O
has	O
become	O
the	O
largest	O
and	O
best-known	O
implementation	O
of	O
onion	B-Protocol
routing	I-Protocol
,	O
then	O
called	O
The	B-Application
Onion	I-Application
Routing	I-Application
project	I-Application
(	O
Tor	B-Application
project	I-Application
)	O
.	O
</s>
<s>
After	O
the	O
Naval	O
Research	O
Laboratory	O
released	O
the	O
code	O
for	O
Tor	B-Application
under	O
a	O
free	B-License
license	I-License
,	O
Dingledine	O
,	O
Mathewson	O
and	O
five	O
others	O
founded	O
The	O
Tor	B-Application
Project	I-Application
as	O
a	O
non-profit	O
organization	O
in	O
2006	O
,	O
with	O
the	O
financial	O
support	O
of	O
the	O
Electronic	O
Frontier	O
Foundation	O
and	O
several	O
other	O
organizations	O
.	O
</s>
<s>
Likewise	O
,	O
no	O
node	O
in	O
the	O
circuit	O
is	O
able	O
to	O
tell	O
how	O
many	O
other	O
nodes	O
are	O
in	O
the	O
circuit	O
and	O
only	O
the	O
final	O
node	O
,	O
the	O
"	O
exit	B-Application
node	I-Application
"	O
,	O
is	O
able	O
to	O
determine	O
its	O
own	O
location	O
in	O
the	O
chain	O
.	O
</s>
<s>
Using	O
asymmetric	B-Application
key	I-Application
cryptography	I-Application
,	O
the	O
originator	O
obtains	O
a	O
public	B-Application
key	I-Application
from	O
the	O
directory	O
node	O
to	O
send	O
an	O
encrypted	O
message	O
to	O
the	O
first	O
(	O
"	O
entry	O
"	O
)	O
node	O
,	O
establishing	O
a	O
connection	O
and	O
a	O
shared	O
secret	O
(	O
"	O
session	O
key	O
"	O
)	O
.	O
</s>
<s>
For	O
example	O
,	O
when	O
a	O
person	O
accesses	O
a	O
particular	O
website	O
,	O
the	O
data	O
itself	O
may	O
be	O
secured	O
through	O
a	O
connection	O
like	O
HTTPS	B-Protocol
such	O
that	O
the	O
user	O
's	O
password	O
,	O
emails	O
,	O
or	O
other	O
content	O
is	O
not	O
visible	O
to	O
an	O
outside	O
party	O
,	O
but	O
there	O
is	O
a	O
record	O
of	O
the	O
connection	O
itself	O
,	O
what	O
time	O
it	O
occurred	O
,	O
and	O
the	O
amount	O
of	O
data	O
transferred	O
.	O
</s>
<s>
Onion	B-Protocol
routing	I-Protocol
creates	O
and	O
obscures	O
a	O
path	O
between	O
two	O
computers	O
such	O
that	O
there	O
is	O
no	O
discernible	O
connection	O
directly	O
from	O
a	O
person	O
to	O
a	O
website	O
,	O
but	O
there	O
still	O
exist	O
records	O
of	O
connections	O
between	O
computers	O
.	O
</s>
<s>
Garlic	B-Protocol
routing	I-Protocol
is	O
a	O
variant	O
of	O
onion	B-Protocol
routing	I-Protocol
associated	O
with	O
the	O
I2P	B-Protocol
network	O
that	O
encrypts	O
multiple	O
messages	O
together	O
,	O
which	O
both	O
increases	O
the	O
speed	O
of	O
data	O
transfer	O
and	O
makes	O
it	O
more	O
difficult	O
for	O
attackers	O
to	O
perform	O
traffic	O
analysis	O
.	O
</s>
<s>
Although	O
the	O
message	O
being	O
sent	O
is	O
transmitted	O
inside	O
several	O
layers	O
of	O
encryption	O
,	O
the	O
job	O
of	O
the	O
exit	B-Application
node	I-Application
,	O
as	O
the	O
final	O
node	O
in	O
the	O
chain	O
,	O
is	O
to	O
decrypt	O
the	O
final	O
layer	O
and	O
deliver	O
the	O
message	O
to	O
the	O
recipient	O
.	O
</s>
<s>
A	O
compromised	O
exit	B-Application
node	I-Application
is	O
thus	O
able	O
to	O
acquire	O
the	O
raw	O
data	O
being	O
transmitted	O
,	O
potentially	O
including	O
passwords	O
,	O
private	O
messages	O
,	O
bank	O
account	O
numbers	O
,	O
and	O
other	O
forms	O
of	O
personal	O
information	O
.	O
</s>
<s>
Exit	B-Application
node	I-Application
vulnerabilities	O
are	O
similar	O
to	O
those	O
on	O
unsecured	O
wireless	O
networks	O
,	O
where	O
the	O
data	O
being	O
transmitted	O
by	O
a	O
user	O
on	O
the	O
network	O
may	O
be	O
intercepted	O
by	O
another	O
user	O
or	O
by	O
the	O
router	O
operator	O
.	O
</s>
<s>
Both	O
issues	O
are	O
solved	O
by	O
using	O
a	O
secure	O
end-to-end	O
connection	O
like	O
SSL/TLS	B-Protocol
or	O
secure	B-Protocol
HTTP	I-Protocol
(	O
S-HTTP	B-Protocol
)	O
.	O
</s>
<s>
If	O
there	O
is	O
end-to-end	O
encryption	O
between	O
the	O
sender	O
and	O
the	O
recipient	O
,	O
and	O
the	O
sender	O
is	O
n't	O
lured	O
into	O
trusting	O
a	O
false	O
SSL	O
certificate	O
offered	O
by	O
the	O
exit	B-Application
node	I-Application
,	O
then	O
not	O
even	O
the	O
last	O
intermediary	O
can	O
view	O
the	O
original	O
message	O
.	O
</s>
