<s>
Mix	B-Protocol
networks	I-Protocol
are	O
routing	B-Protocol
protocols	O
that	O
create	O
hard-to-trace	O
communications	O
by	O
using	O
a	O
chain	O
of	O
proxy	B-Application
servers	I-Application
known	O
as	O
mixes	O
which	O
take	O
in	O
messages	O
from	O
multiple	O
senders	O
,	O
shuffle	O
them	O
,	O
and	O
send	O
them	O
back	O
out	O
in	O
random	O
order	O
to	O
the	O
next	O
destination	O
(	O
possibly	O
another	O
mix	O
node	O
)	O
.	O
</s>
<s>
Each	O
message	O
is	O
encrypted	O
to	O
each	O
proxy	O
using	O
public	B-Application
key	I-Application
cryptography	I-Application
;	O
the	O
resulting	O
encryption	O
is	O
layered	O
like	O
a	O
Russian	O
doll	O
(	O
except	O
that	O
each	O
"	O
doll	O
"	O
is	O
of	O
the	O
same	O
size	O
)	O
with	O
the	O
message	O
as	O
the	O
innermost	O
layer	O
.	O
</s>
<s>
Each	O
proxy	B-Application
server	I-Application
strips	O
off	O
its	O
own	O
layer	O
of	O
encryption	O
to	O
reveal	O
where	O
to	O
send	O
the	O
message	O
next	O
.	O
</s>
<s>
If	O
all	O
but	O
one	O
of	O
the	O
proxy	B-Application
servers	I-Application
are	O
compromised	O
by	O
the	O
tracer	O
,	O
untraceability	O
can	O
still	O
be	O
achieved	O
against	O
some	O
weaker	O
adversaries	O
.	O
</s>
<s>
The	O
concept	O
of	O
mix	B-Protocol
networks	I-Protocol
was	O
first	O
described	O
by	O
David	O
Chaum	O
in	O
1981	O
.	O
</s>
<s>
Applications	O
that	O
are	O
based	O
on	O
this	O
concept	O
include	O
anonymous	B-Application
remailers	I-Application
(	O
such	O
as	O
Mixmaster	B-Protocol
)	O
,	O
onion	B-Protocol
routing	I-Protocol
,	O
garlic	B-Protocol
routing	I-Protocol
,	O
and	O
key-based	B-Protocol
routing	I-Protocol
(	O
including	O
Tor	B-Application
,	O
I2P	B-Protocol
,	O
and	O
Freenet	B-Protocol
)	O
.	O
</s>
<s>
David	O
Chaum	O
published	O
the	O
concept	O
of	O
Mix	B-Protocol
Networks	I-Protocol
in	O
1979	O
in	O
his	O
paper	O
:	O
.	O
</s>
<s>
The	O
paper	O
was	O
for	O
his	O
master	O
's	O
degree	O
thesis	O
work	O
,	O
shortly	O
after	O
he	O
was	O
first	O
introduced	O
to	O
the	O
field	O
of	O
cryptography	O
through	O
the	O
work	O
of	O
public	B-Application
key	I-Application
cryptography	I-Application
,	O
Martin	O
Hellman	O
,	O
Whitfield	O
Diffie	O
and	O
Ralph	O
Merkle	O
.	O
</s>
<s>
While	O
public	B-Application
key	I-Application
cryptography	I-Application
encrypted	O
the	O
security	O
of	O
information	O
,	O
Chaum	O
believed	O
there	O
to	O
be	O
personal	O
privacy	O
vulnerabilities	O
in	O
the	O
meta	O
data	O
found	O
in	O
communications	O
.	O
</s>
<s>
Participant	O
A	O
prepares	O
a	O
message	O
for	O
delivery	O
to	O
participant	O
B	O
by	O
appending	O
a	O
random	O
value	O
R	O
to	O
the	O
message	O
,	O
sealing	O
it	O
with	O
the	O
addressee	O
's	O
public	B-Application
key	I-Application
,	O
appending	O
B	O
's	O
address	O
,	O
and	O
then	O
sealing	O
the	O
result	O
with	O
the	O
mix	O
's	O
public	B-Application
key	I-Application
.	O
</s>
<s>
M	O
opens	O
it	O
with	O
his	O
private	B-Application
key	I-Application
,	O
now	O
he	O
knows	O
B	O
's	O
address	O
,	O
and	O
he	O
sends	O
to	O
B	O
.	O
</s>
<s>
To	O
accomplish	O
this	O
,	O
the	O
sender	O
takes	O
the	O
mix	O
's	O
public	B-Application
key	I-Application
(	O
)	O
,	O
and	O
uses	O
it	O
to	O
encrypt	O
an	O
envelope	O
containing	O
a	O
random	O
string	O
(	O
)	O
,	O
a	O
nested	O
envelope	O
addressed	O
to	O
the	O
recipient	O
,	O
and	O
the	O
email	O
address	O
of	O
the	O
recipient	O
(	O
B	O
)	O
.	O
</s>
<s>
This	O
nested	O
envelope	O
is	O
encrypted	O
with	O
the	O
recipient	O
's	O
public	B-Application
key	I-Application
(	O
)	O
,	O
and	O
contains	O
another	O
random	O
string	O
(	O
R0	O
)	O
,	O
along	O
with	O
the	O
body	O
of	O
the	O
message	O
being	O
sent	O
.	O
</s>
<s>
Where	O
:	O
=	O
B	O
’s	O
public	B-Application
key	I-Application
,	O
=	O
the	O
mix	O
's	O
public	B-Application
key	I-Application
.	O
</s>
<s>
Although	O
mix	B-Protocol
networks	I-Protocol
provide	O
security	O
even	O
if	O
an	O
adversary	O
is	O
able	O
to	O
view	O
the	O
entire	O
path	O
,	O
mixing	O
is	O
not	O
absolutely	O
perfect	O
.	O
</s>
<s>
An	O
adversary	O
can	O
perform	O
a	O
passive	O
attack	O
by	O
monitoring	O
the	O
traffic	O
to	O
and	O
from	O
the	O
mix	B-Protocol
network	I-Protocol
.	O
</s>
<s>
In	O
a	O
worst	O
case	O
of	O
an	O
attack	O
,	O
we	O
assume	O
that	O
all	O
the	O
links	O
of	O
the	O
network	O
are	O
observable	O
by	O
the	O
adversary	O
and	O
the	O
strategies	O
and	O
infrastructure	O
of	O
the	O
mix	B-Protocol
network	I-Protocol
are	O
known	O
.	O
</s>
<s>
Mix	B-Protocol
networks	I-Protocol
derive	O
security	O
by	O
changing	O
order	O
of	O
messages	O
they	O
receive	O
to	O
avoid	O
creating	O
significant	O
relation	O
between	O
the	O
incoming	O
and	O
outgoing	O
messages	O
.	O
</s>
