<s>
Crowds	B-Protocol
is	O
a	O
proposed	O
anonymity	O
network	O
for	O
anonymous	B-Protocol
web	I-Protocol
browsing	I-Protocol
.	O
</s>
<s>
The	O
main	O
idea	O
behind	O
Crowds	B-Protocol
anonymity	O
protocol	O
is	O
to	O
hide	O
each	O
user	O
's	O
communications	O
by	O
routing	O
them	O
randomly	O
within	O
a	O
group	O
of	O
similar	O
users	O
.	O
</s>
<s>
Crowds	B-Protocol
was	O
designed	O
by	O
Michael	O
K	O
.	O
Reiter	O
and	O
Aviel	O
D	O
.	O
Rubin	O
.	O
</s>
<s>
It	O
defends	O
against	O
internal	O
attackers	O
and	O
a	O
corrupt	O
receiver	O
,	O
but	O
provides	O
no	O
anonymity	O
against	O
a	O
global	O
attacker	O
or	O
a	O
local	O
eavesdropper	O
(	O
see	O
"	O
Crowds	B-Protocol
:	O
Anonymity	O
For	O
Web	O
Transactions	O
"	O
)	O
.	O
</s>
<s>
Crowds	B-Protocol
is	O
vulnerable	O
to	O
the	O
predecessor	O
attack	O
;	O
this	O
was	O
discussed	O
in	O
Reiter	O
and	O
Rubin	O
's	O
paper	O
and	O
further	O
expanded	O
in	O
"	O
The	O
Predecessor	O
Attack	O
:	O
An	O
Analysis	O
of	O
a	O
Threat	O
to	O
Anonymous	O
Communications	O
Systems	O
"	O
by	O
Matthew	O
K	O
.	O
Wright	O
,	O
Micah	O
Adler	O
,	O
And	O
Brian	O
Neil	O
Levine	O
.	O
</s>
<s>
Crowds	B-Protocol
introduced	O
the	O
concept	O
of	O
users	O
blending	O
into	O
a	O
crowd	O
of	O
computers	O
.	O
</s>
<s>
Crowds	B-Protocol
uses	O
and	O
defines	O
the	O
following	O
terms	O
:	O
</s>
<s>
Crowds	B-Protocol
works	O
by	O
making	O
each	O
node	O
seem	O
equally	O
likely	O
to	O
be	O
the	O
initiator	O
of	O
the	O
message	O
.	O
</s>
<s>
We	O
consider	O
the	O
question	O
of	O
what	O
information	O
an	O
attacker	O
can	O
learn	O
about	O
the	O
senders	O
and	O
receivers	O
of	O
web	O
transactions	O
,	O
given	O
the	O
mechanisms	O
of	O
Crowds	B-Protocol
we	O
described	O
.	O
</s>
<s>
This	O
suggests	O
that	O
crowds	B-Protocol
should	O
be	O
able	O
to	O
grow	O
quite	O
large	O
.	O
</s>
<s>
Crowds	B-Protocol
provides	O
perfect	O
anonymity	O
against	O
a	O
corrupt	O
receiver	O
(	O
i.e.	O
</s>
<s>
see	O
Degree	B-Application
of	I-Application
anonymity	I-Application
)	O
as	O
all	O
members	O
appear	O
equally	O
likely	O
to	O
have	O
been	O
the	O
initiator	O
.	O
</s>
<s>
As	O
we	O
showed	O
against	O
collaborating	O
corrupt	O
nodes	O
Crowds	B-Protocol
provides	O
probable	O
innocence	O
as	O
long	O
as	O
(	O
see	O
the	O
paper	O
for	O
the	O
derivation	O
of	O
this	O
)	O
,	O
and	O
provides	O
a	O
degree	B-Application
of	I-Application
anonymity	I-Application
.	O
</s>
<s>
Against	O
the	O
predecessor	O
attack	O
Crowds	B-Protocol
succumbs	O
in	O
;	O
this	O
attack	O
works	O
by	O
a	O
corrupt	O
node	O
retaining	O
the	O
previous	O
hop	O
in	O
the	O
path	O
,	O
as	O
this	O
will	O
be	O
the	O
sender	O
more	O
than	O
any	O
other	O
node	O
over	O
the	O
rounds	O
of	O
rebuilding	O
the	O
network	O
it	O
will	O
become	O
apparent	O
who	O
the	O
initiator	O
is	O
.	O
</s>
<s>
Crowds	B-Protocol
is	O
unable	O
to	O
protect	O
against	O
a	O
global	O
eavesdropper	O
as	O
it	O
cannot	O
use	O
encryption	O
on	O
the	O
links	O
,	O
this	O
is	O
because	O
each	O
node	O
in	O
Crowds	B-Protocol
is	O
able	O
to	O
communicate	O
with	O
every	O
other	O
node	O
(	O
a	O
fully	O
connected	O
graph	O
)	O
,	O
because	O
of	O
this	O
setting	O
up	O
symmetric	O
keys	O
requires	O
pairwise	O
keys	O
;	O
this	O
is	O
too	O
large	O
of	O
a	O
number	O
to	O
be	O
feasible	O
.	O
</s>
<s>
Against	O
a	O
local	O
eavesdropper	O
again	O
Crowds	B-Protocol
provides	O
no	O
protection	O
as	O
the	O
eavesdropper	O
will	O
see	O
a	O
message	O
coming	O
out	O
of	O
a	O
node	O
that	O
did	O
not	O
enter	O
,	O
and	O
this	O
positively	O
identifies	O
the	O
node	O
as	O
the	O
sender	O
.	O
</s>
