<s>
An	O
anonymous	B-Application
remailer	I-Application
is	O
a	O
server	B-Application
that	O
receives	O
messages	O
with	O
embedded	O
instructions	O
on	O
where	O
to	O
send	O
them	O
next	O
,	O
and	O
that	O
forwards	O
them	O
without	O
revealing	O
where	O
they	O
originally	O
came	O
from	O
.	O
</s>
<s>
There	O
are	O
cypherpunk	B-Protocol
anonymous	I-Protocol
remailers	I-Protocol
,	O
mixmaster	B-Protocol
anonymous	I-Protocol
remailers	I-Protocol
,	O
and	O
nym	B-Protocol
servers	I-Protocol
,	O
among	O
others	O
,	O
which	O
differ	O
in	O
how	O
they	O
work	O
,	O
in	O
the	O
policies	O
they	O
adopt	O
,	O
and	O
in	O
the	O
type	O
of	O
attack	O
on	O
the	O
anonymity	O
of	O
e-mail	O
they	O
can	O
(	O
or	O
are	O
intended	O
to	O
)	O
resist	O
.	O
</s>
<s>
In	O
general	O
,	O
different	O
classes	O
of	O
anonymous	B-Application
remailers	I-Application
differ	O
with	O
regard	O
to	O
the	O
choices	O
their	O
designers/operators	O
have	O
made	O
.	O
</s>
<s>
These	O
choices	O
can	O
be	O
influenced	O
by	O
the	O
legal	O
ramifications	O
of	O
operating	O
specific	O
types	O
of	O
remailers	B-Application
.	O
</s>
<s>
It	O
must	O
be	O
understood	O
that	O
every	O
data	B-Protocol
packet	I-Protocol
traveling	O
on	O
the	O
Internet	O
contains	O
the	O
node	O
addresses	O
(	O
as	O
raw	O
IP	B-Protocol
bit	O
strings	O
)	O
of	O
both	O
the	O
sending	O
and	O
intended	O
recipient	O
nodes	O
,	O
and	O
so	O
no	O
data	B-Protocol
packet	I-Protocol
can	O
ever	O
actually	O
be	O
anonymous	O
at	O
this	O
level	O
.	O
</s>
<s>
Some	O
remailers	B-Application
change	O
both	O
types	O
of	O
address	O
in	O
messages	O
they	O
forward	O
,	O
and	O
the	O
list	O
of	O
forwarding	O
nodes	O
in	O
e-mail	O
messages	O
as	O
well	O
,	O
as	O
the	O
message	O
passes	O
through	O
;	O
in	O
effect	O
,	O
they	O
substitute	O
'	O
fake	O
source	O
addresses	O
 '	O
for	O
the	O
originals	O
.	O
</s>
<s>
The	O
'	O
IP	B-Protocol
source	O
address	O
 '	O
for	O
that	O
packet	B-Protocol
may	O
become	O
that	O
of	O
the	O
remailer	B-Application
server	B-Application
itself	O
,	O
and	O
within	O
an	O
e-mail	O
message	O
(	O
which	O
is	O
usually	O
several	O
packets	B-Protocol
)	O
,	O
a	O
nominal	O
'	O
user	O
 '	O
on	O
that	O
server	B-Application
.	O
</s>
<s>
Some	O
remailers	B-Application
forward	O
their	O
anonymized	O
e-mail	O
to	O
still	O
other	O
remailers	B-Application
,	O
and	O
only	O
after	O
several	O
such	O
hops	O
is	O
the	O
e-mail	O
actually	O
delivered	O
to	O
the	O
intended	O
address	O
.	O
</s>
<s>
There	O
are	O
,	O
more	O
or	O
less	O
,	O
four	O
types	O
of	O
remailers	B-Application
:	O
</s>
<s>
A	O
pseudonymous	B-Protocol
remailer	I-Protocol
simply	O
takes	O
away	O
the	O
e-mail	O
address	O
of	O
the	O
sender	O
,	O
gives	O
a	O
pseudonym	O
to	O
the	O
sender	O
,	O
and	O
sends	O
the	O
message	O
to	O
the	O
intended	O
recipient	O
(	O
that	O
can	O
be	O
answered	O
via	O
that	O
remailer	B-Application
)	O
.	O
</s>
<s>
A	O
Cypherpunk	B-Protocol
remailer	I-Protocol
sends	O
the	O
message	O
to	O
the	O
recipient	O
,	O
stripping	O
away	O
the	O
sender	O
address	O
on	O
it	O
.	O
</s>
<s>
One	O
can	O
not	O
answer	O
a	O
message	O
sent	O
via	O
a	O
Cypherpunk	B-Protocol
remailer	I-Protocol
.	O
</s>
<s>
The	O
message	O
sent	O
to	O
the	O
remailer	B-Application
can	O
usually	O
be	O
encrypted	O
,	O
and	O
the	O
remailer	B-Application
will	O
decrypt	O
it	O
and	O
send	O
it	O
to	O
the	O
recipient	O
address	O
hidden	O
inside	O
the	O
encrypted	O
message	O
.	O
</s>
<s>
In	O
addition	O
,	O
it	O
is	O
possible	O
to	O
chain	O
two	O
or	O
three	O
remailers	B-Application
,	O
so	O
that	O
each	O
remailer	B-Application
ca	O
n't	O
know	O
who	O
is	O
sending	O
a	O
message	O
to	O
whom	O
.	O
</s>
<s>
Cypherpunk	O
remailers	B-Application
do	O
not	O
keep	O
logs	O
of	O
transactions	O
.	O
</s>
<s>
In	O
Mixmaster	B-Protocol
,	O
the	O
user	O
composes	O
an	O
email	O
to	O
a	O
remailer	B-Application
,	O
which	O
is	O
relayed	O
through	O
each	O
node	O
in	O
the	O
network	O
using	O
SMTP	B-Protocol
,	O
until	O
it	O
finally	O
arrives	O
at	O
the	O
final	O
recipient	O
.	O
</s>
<s>
Mixmaster	B-Protocol
can	O
only	O
send	O
emails	O
one	O
way	O
.	O
</s>
<s>
Also	O
,	O
Mixmaster	B-Protocol
remailers	B-Application
require	O
the	O
use	O
of	O
a	O
computer	O
program	O
to	O
write	O
messages	O
.	O
</s>
<s>
A	O
Mixminion	B-Protocol
remailer	B-Application
attempts	O
to	O
address	O
the	O
following	O
challenges	O
in	O
Mixmaster	B-Protocol
remailers	B-Application
:	O
replies	O
,	O
forward	O
anonymity	O
,	O
replay	O
prevention	O
and	O
key	O
rotation	O
,	O
exit	O
policies	O
,	O
integrated	O
directory	O
servers	O
and	O
dummy	O
traffic	O
.	O
</s>
<s>
Some	O
remailers	B-Application
establish	O
an	O
internal	O
list	O
of	O
actual	O
senders	O
and	O
invented	O
names	O
such	O
that	O
a	O
recipient	O
can	O
send	O
mail	O
to	O
invented	O
name	O
AT	O
some-remailer.example	O
.	O
</s>
<s>
When	O
receiving	O
traffic	O
addressed	O
to	O
this	O
user	O
,	O
the	O
server	B-Application
software	I-Application
consults	O
that	O
list	O
,	O
and	O
forwards	O
the	O
mail	O
to	O
the	O
original	O
sender	O
,	O
thus	O
permitting	O
anonymous	O
—	O
though	O
traceable	O
with	O
access	O
to	O
the	O
list	O
—	O
two-way	O
communication	O
.	O
</s>
<s>
The	O
famous	O
"	O
penet.fi	B-Application
 "	O
remailer	B-Application
in	O
Finland	O
did	O
just	O
that	O
for	O
several	O
years	O
.	O
</s>
<s>
Because	O
of	O
the	O
existence	O
of	O
such	O
lists	O
in	O
this	O
type	O
of	O
remailing	O
server	B-Application
,	O
it	O
is	O
possible	O
to	O
break	O
the	O
anonymity	O
by	O
gaining	O
access	O
to	O
the	O
list(s )	O
,	O
by	O
breaking	O
into	O
the	O
computer	O
,	O
asking	O
a	O
court	O
(	O
or	O
merely	O
the	O
police	O
in	O
some	O
places	O
)	O
to	O
order	O
that	O
the	O
anonymity	O
be	O
broken	O
,	O
and/or	O
bribing	O
an	O
attendant	O
.	O
</s>
<s>
This	O
happened	O
to	O
penet.fi	B-Application
as	O
a	O
result	O
of	O
some	O
traffic	O
passed	O
through	O
it	O
about	O
Scientology	O
.	O
</s>
<s>
The	O
Church	O
claimed	O
copyright	O
infringement	O
and	O
sued	O
fi	O
's	O
operator	O
.	O
</s>
<s>
Penet	O
's	O
operator	O
shut	O
it	O
down	O
after	O
destroying	O
its	O
records	O
(	O
including	O
the	O
list	O
)	O
to	O
retain	O
identity	B-Application
confidentiality	O
for	O
its	O
users	O
;	O
though	O
not	O
before	O
being	O
forced	O
to	O
supply	O
the	O
court	O
with	O
the	O
real	O
e-mail	O
addresses	O
of	O
two	O
of	O
its	O
users	O
.	O
</s>
<s>
More	O
recent	O
remailer	B-Application
designs	O
use	O
cryptography	O
in	O
an	O
attempt	O
to	O
provide	O
more	O
or	O
less	O
the	O
same	O
service	O
,	O
but	O
without	O
so	O
much	O
risk	O
of	O
loss	O
of	O
user	O
confidentiality	O
.	O
</s>
<s>
These	O
are	O
generally	O
termed	O
nym	B-Protocol
servers	I-Protocol
or	O
pseudonymous	B-Protocol
remailers	I-Protocol
.	O
</s>
<s>
Multiple	O
anonymous	O
forwarding	O
among	O
cooperating	O
remailers	B-Application
in	O
different	O
jurisdictions	O
may	O
retain	O
,	O
but	O
cannot	O
guarantee	O
,	O
anonymity	O
against	O
a	O
determined	O
attempt	O
by	O
one	O
or	O
more	O
governments	O
,	O
or	O
civil	O
litigators	O
.	O
</s>
<s>
If	O
users	O
accept	O
the	O
loss	O
of	O
two-way	O
interaction	O
,	O
identity	B-Application
anonymity	O
can	O
be	O
made	O
more	O
secure	O
.	O
</s>
<s>
By	O
not	O
keeping	O
any	O
list	O
of	O
users	O
and	O
corresponding	O
anonymizing	O
labels	O
for	O
them	O
,	O
a	O
remailer	B-Application
can	O
ensure	O
that	O
any	O
message	O
that	O
has	O
been	O
forwarded	O
leaves	O
no	O
internal	O
information	O
behind	O
that	O
can	O
later	O
be	O
used	O
to	O
break	O
identity	B-Application
confidentiality	O
.	O
</s>
<s>
However	O
,	O
while	O
being	O
handled	O
,	O
messages	O
remain	O
vulnerable	O
within	O
the	O
server	B-Application
(	O
e.g.	O
,	O
to	O
Trojan	O
software	O
in	O
a	O
compromised	O
server	B-Application
,	O
to	O
a	O
compromised	O
server	B-Application
operator	O
,	O
or	O
to	O
mis-administration	O
of	O
the	O
server	B-Application
)	O
,	O
and	O
traffic	O
analysis	O
comparison	O
of	O
traffic	O
into	O
and	O
out	O
of	O
such	O
a	O
server	B-Application
can	O
suggest	O
quite	O
a	O
lot	O
—	O
far	O
more	O
than	O
almost	O
any	O
would	O
credit	O
.	O
</s>
<s>
The	O
Mixmaster	B-Protocol
strategy	O
is	O
designed	O
to	O
defeat	O
such	O
attacks	O
,	O
or	O
at	O
least	O
to	O
increase	O
their	O
cost	O
(	O
i.e.	O
,	O
to	O
'	O
attackers	O
 '	O
)	O
beyond	O
feasibility	O
.	O
</s>
<s>
And	O
,	O
since	O
many	O
different	O
servers	O
and	O
server	B-Application
operators	O
are	O
involved	O
,	O
subversion	O
of	O
any	O
(	O
i.e.	O
,	O
of	O
either	O
system	O
or	O
operator	O
)	O
becomes	O
less	O
effective	O
also	O
since	O
no	O
one	O
(	O
most	O
likely	O
)	O
will	O
be	O
able	O
to	O
subvert	O
the	O
entire	O
chain	O
of	O
remailers	B-Application
.	O
</s>
<s>
Random	O
padding	B-Algorithm
of	O
messages	O
,	O
random	O
delays	O
before	O
forwarding	O
,	O
and	O
encryption	O
of	O
forwarding	O
information	O
between	O
forwarding	O
remailers	B-Application
,	O
increases	O
the	O
degree	O
of	O
difficulty	O
for	O
attackers	O
still	O
further	O
as	O
message	O
size	O
and	O
timing	O
can	O
be	O
largely	O
eliminated	O
as	O
traffic	O
analysis	O
clues	O
,	O
and	O
lack	O
of	O
easily	O
readable	O
forwarding	O
information	O
renders	O
ineffective	O
simple	O
automated	O
traffic	O
analysis	O
algorithms	O
.	O
</s>
<s>
These	O
services	O
do	O
not	O
provide	O
the	O
anonymity	O
of	O
real	O
remailers	B-Application
,	O
but	O
they	O
are	O
easier	O
to	O
use	O
.	O
</s>
<s>
When	O
using	O
a	O
web-based	O
anonymous	O
e-mail	O
or	O
anonymous	B-Application
remailer	I-Application
service	O
,	O
its	O
reputation	O
should	O
first	O
be	O
analyzed	O
,	O
since	O
the	O
service	O
stands	O
between	O
senders	O
and	O
recipients	O
.	O
</s>
<s>
In	O
most	O
cases	O
,	O
remailers	B-Application
are	O
owned	O
and	O
operated	O
by	O
individuals	O
,	O
and	O
are	O
not	O
as	O
stable	O
as	O
they	O
might	O
ideally	O
be	O
.	O
</s>
<s>
In	O
fact	O
,	O
remailers	B-Application
can	O
,	O
and	O
have	O
,	O
gone	O
down	O
without	O
warning	O
.	O
</s>
<s>
It	O
is	O
important	O
to	O
use	O
up-to-date	O
statistics	O
when	O
choosing	O
remailers	B-Application
.	O
</s>
<s>
Until	O
technical	O
changes	O
were	O
made	O
in	O
the	O
remailers	B-Application
concerned	O
in	O
the	O
mid-2000s	O
,	O
some	O
re-mailers	O
(	O
notably	O
nym.alias.net	O
based	O
systems	O
)	O
were	O
seemingly	O
willing	O
to	O
use	O
any	O
genuine	O
(	O
and	O
thus	O
valid	O
)	O
but	O
otherwise	O
forged	O
address	O
.	O
</s>
<s>
In	O
2014	O
,	O
dizum.com	O
(	O
a	O
Netherlands-based	O
remailer	B-Application
)	O
was	O
seemingly	O
blocked	O
by	O
authorities	O
in	O
Pakistan	O
,	O
because	O
comments	O
an	O
(	O
anonymous	O
)	O
user	O
of	O
that	O
service	O
had	O
made	O
concerning	O
key	O
figures	O
in	O
Islam	O
.	O
</s>
