<s>
GBDE	B-Application
,	O
standing	O
for	O
GEOM	B-Application
Based	I-Application
Disk	I-Application
Encryption	I-Application
,	O
is	O
a	O
block	O
device-layer	O
disk	O
encryption	O
system	O
written	O
for	O
FreeBSD	B-Operating_System
,	O
initially	O
introduced	O
in	O
version	O
5.0	O
.	O
</s>
<s>
It	O
is	O
based	O
on	O
the	O
GEOM	B-Application
disk	O
framework	O
.	O
</s>
<s>
GBDE	B-Application
was	O
designed	O
and	O
implemented	O
by	O
Poul-Henning	O
Kamp	O
and	O
Network	O
Associates	O
Inc	O
.	O
(	O
now	O
known	O
as	O
McAfee	O
)	O
.	O
</s>
<s>
Unlike	O
most	O
disk	O
encryption	O
software	O
,	O
GBDE	B-Application
does	O
not	O
attempt	O
to	O
defeat	O
watermarking	O
attacks	O
through	O
the	O
use	O
of	O
disk	O
encryption-specific	O
modes	O
of	O
operation	O
(	O
see	O
disk	B-Algorithm
encryption	I-Algorithm
theory	I-Algorithm
)	O
,	O
but	O
instead	O
generates	O
a	O
random	O
key	O
each	O
time	O
a	O
sector	B-Application
is	O
written	O
.	O
</s>
<s>
Unlike	O
some	O
alternatives	O
,	O
such	O
as	O
CBC	O
with	O
sector-specific	O
initialization	B-Algorithm
vectors	I-Algorithm
,	O
this	O
approach	O
does	O
not	O
reveal	O
any	O
information	O
to	O
the	O
attacker	O
even	O
if	O
they	O
have	O
access	O
to	O
snapshots	O
of	O
the	O
disk	O
image	O
from	O
different	O
points	O
in	O
time	O
,	O
since	O
encryption	O
keys	O
are	O
never	O
re-used	O
.	O
</s>
<s>
The	O
one	O
time	O
sector	B-Application
key	O
is	O
encrypted	O
using	O
a	O
pseudorandom	O
key	O
.	O
</s>
<s>
This	O
pseudorandom	O
key	O
is	O
derived	O
from	O
the	O
sector	B-Application
number	O
and	O
a	O
static	O
2048-bit	O
master	O
key	O
with	O
128	O
bits	O
of	O
salt	O
.	O
</s>
<s>
The	O
pseudorandom	B-Algorithm
number	I-Algorithm
generator	I-Algorithm
used	O
for	O
this	O
purpose	O
is	O
called	O
the	O
Cherry	O
Picker	O
.	O
</s>
<s>
This	O
is	O
not	O
a	O
well	O
established	O
PRNG	O
,	O
but	O
rather	O
one	O
invented	O
for	O
GBDE	B-Application
.	O
</s>
<s>
Due	O
to	O
this	O
unique	O
approach	O
,	O
GBDE	B-Application
only	O
supports	O
128-bit	O
AES	B-Algorithm
.	O
</s>
<s>
Using	O
a	O
different	O
key	O
for	O
each	O
write	O
also	O
introduces	O
a	O
significant	O
CPU	O
overhead	O
,	O
as	O
most	O
block	O
ciphers	O
use	O
key-specific	O
precomputations	O
,	O
and	O
makes	O
disk	O
updates	O
non-atomic	O
since	O
the	O
keys	O
are	O
written	O
separately	O
from	O
the	O
data	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
data	O
loss	O
can	O
occur	O
on	O
unexpected	O
power	O
drops	O
,	O
even	O
when	O
used	O
with	O
journaling	B-Application
file	I-Application
systems	I-Application
.	O
</s>
<s>
GBDE	B-Application
also	O
has	O
a	O
disk	O
space	O
overhead	O
of	O
about	O
3%	O
to	O
store	O
the	O
per-sector	O
keys	O
.	O
</s>
<s>
To	O
address	O
these	O
shortcomings	O
,	O
a	O
more	O
typical	O
disk	O
encryption	O
solution	O
for	O
FreeBSD	B-Operating_System
,	O
GELI	B-Application
,	O
was	O
written	O
later	O
by	O
Pawel	O
Jakub	O
Dawidek	O
.	O
</s>
