<s>
Steganographic	B-Application
file	I-Application
systems	I-Application
are	O
a	O
kind	O
of	O
file	B-Application
system	I-Application
first	O
proposed	O
by	O
Ross	O
Anderson	O
,	O
Roger	O
Needham	O
,	O
and	O
Adi	O
Shamir	O
.	O
</s>
<s>
Their	O
paper	O
proposed	O
two	O
main	O
methods	O
of	O
hiding	O
data	O
:	O
in	O
a	O
series	O
of	O
fixed	O
size	O
files	O
originally	O
consisting	O
of	O
random	O
bits	O
on	O
top	O
of	O
which	O
'	O
vectors	O
 '	O
could	O
be	O
superimposed	O
in	O
such	O
a	O
way	O
as	O
to	O
allow	O
levels	O
of	O
security	O
to	O
decrypt	O
all	O
lower	O
levels	O
but	O
not	O
even	O
know	O
of	O
the	O
existence	O
of	O
any	O
higher	O
levels	O
,	O
or	O
an	O
entire	O
partition	B-Application
is	O
filled	O
with	O
random	O
bits	O
and	O
files	O
hidden	O
in	O
it	O
.	O
</s>
<s>
In	O
a	O
steganographic	B-Application
file	I-Application
system	I-Application
using	O
the	O
second	O
scheme	O
,	O
files	O
are	O
not	O
merely	O
stored	O
,	O
nor	O
stored	O
encrypted	O
,	O
but	O
the	O
entire	O
partition	B-Application
is	O
randomized	O
-	O
encrypted	O
files	O
strongly	O
resemble	O
randomized	O
sections	O
of	O
the	O
partition	B-Application
,	O
and	O
so	O
when	O
files	O
are	O
stored	O
on	O
the	O
partition	B-Application
,	O
there	O
is	O
no	O
easy	O
way	O
to	O
discern	O
between	O
meaningless	O
gibberish	O
and	O
the	O
actual	O
encrypted	O
files	O
.	O
</s>
<s>
While	O
there	O
may	O
seem	O
to	O
be	O
no	O
point	O
to	O
a	O
file	B-Application
system	I-Application
which	O
is	O
guaranteed	O
to	O
either	O
be	O
grossly	O
inefficient	O
storage	O
space-wise	O
or	O
to	O
cause	O
data	O
loss	O
and	O
corruption	O
either	O
from	O
data	O
collisions	O
or	O
loss	O
of	O
the	O
key	O
(	O
in	O
addition	O
to	O
being	O
a	O
complex	O
system	O
,	O
and	O
for	O
having	O
poor	O
read/write	O
performance	O
)	O
,	O
performance	O
was	O
not	O
the	O
goal	O
of	O
StegFS	B-Application
.	O
</s>
<s>
Rather	O
,	O
StegFS	B-Application
is	O
intended	O
to	O
thwart	O
"	O
rubberhose	O
attacks	O
"	O
,	O
which	O
usually	O
work	O
because	O
encrypted	O
files	O
are	O
distinguishable	O
from	O
regular	O
files	O
,	O
and	O
authorities	O
can	O
coerce	O
the	O
user	O
until	O
the	O
user	O
gives	O
up	O
the	O
keys	O
and	O
all	O
the	O
files	O
are	O
distinguishable	O
as	O
regular	O
files	O
.	O
</s>
<s>
However	O
,	O
since	O
in	O
a	O
steganographic	B-Application
file	I-Application
system	I-Application
,	O
the	O
number	O
of	O
files	O
are	O
unknown	O
and	O
every	O
byte	O
looks	O
like	O
an	O
encrypted	O
byte	O
,	O
the	O
authorities	O
cannot	O
know	O
how	O
many	O
files	O
(	O
and	O
hence	O
,	O
keys	O
)	O
are	O
stored	O
.	O
</s>
<s>
Poul-Henning	O
Kamp	O
has	O
criticized	O
the	O
threat	O
model	O
for	O
steganographic	B-Application
file	I-Application
systems	I-Application
in	O
his	O
paper	O
on	O
GBDE	B-Application
,	O
observing	O
that	O
in	O
certain	O
coercive	O
situations	O
,	O
especially	O
where	O
the	O
searched-for	O
information	O
is	O
in	O
fact	O
not	O
stored	O
in	O
the	O
steganographic	B-Application
file	I-Application
systems	I-Application
,	O
it	O
is	O
not	O
possible	O
for	O
a	O
subject	O
to	O
"	O
get	O
off	O
the	O
hook	O
"	O
by	O
proving	O
that	O
all	O
keys	O
have	O
been	O
surrendered	O
.	O
</s>
<s>
Other	O
methods	O
exist	O
;	O
the	O
method	O
laid	O
out	O
before	O
is	O
the	O
one	O
implemented	O
by	O
StegFS	B-Application
,	O
but	O
it	O
is	O
possible	O
to	O
steganographically	O
hide	O
data	O
within	O
image	O
(	O
e.g.	O
</s>
<s>
PNGDrive	O
)	O
or	O
audio	O
files	O
-	O
ScramDisk	B-Application
or	O
the	O
Linux	O
loop	B-Application
device	I-Application
can	O
do	O
this	O
.	O
</s>
<s>
Generally	O
,	O
a	O
steganographic	B-Application
file	I-Application
system	I-Application
is	O
implemented	O
over	O
a	O
steganographic	O
layer	O
,	O
which	O
supplies	O
just	O
the	O
storage	O
mechanism	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
steganographic	B-Application
file	I-Application
system	I-Application
layer	O
can	O
be	O
some	O
existing	O
MP3	O
files	O
,	O
each	O
file	B-Operating_System
contains	O
a	O
chunk	O
of	O
data	O
(	O
or	O
a	O
part	O
of	O
the	O
file	B-Application
system	I-Application
)	O
.	O
</s>
<s>
The	O
final	O
product	O
is	O
a	O
file	B-Application
system	I-Application
that	O
is	O
hardly	O
detected	O
(	O
depending	O
on	O
the	O
steganographic	O
layer	O
)	O
that	O
can	O
store	O
any	O
kind	O
of	O
file	B-Operating_System
in	O
a	O
regular	O
file	B-Application
system	I-Application
hierarchy	O
.	O
</s>
<s>
TrueCrypt	B-Application
allows	O
for	O
"	O
hidden	O
volumes	O
"	O
-	O
two	O
or	O
more	O
passwords	O
open	O
different	O
volumes	O
in	O
the	O
same	O
file	B-Operating_System
,	O
but	O
only	O
one	O
of	O
the	O
volumes	O
contains	O
secret	O
data	O
.	O
</s>
