<s>
The	O
Common	B-Device
Scrambling	I-Device
Algorithm	I-Device
(	O
CSA	O
)	O
is	O
the	O
encryption	O
algorithm	O
used	O
in	O
the	O
DVB	B-Application
digital	B-Application
television	I-Application
broadcasting	I-Application
for	O
encrypting	O
video	O
streams	O
.	O
</s>
<s>
CSA	O
was	O
specified	O
by	O
ETSI	O
and	O
adopted	O
by	O
the	O
DVB	B-Application
consortium	O
in	O
May	O
1994	O
.	O
</s>
<s>
It	O
is	O
being	O
succeeded	O
by	O
CSA3	O
,	O
based	O
on	O
a	O
combination	O
of	O
128-bit	O
AES	B-Algorithm
and	O
a	O
confidential	O
block	O
cipher	O
,	O
XRC	O
.	O
</s>
<s>
However	O
,	O
CSA3	O
is	O
not	O
yet	O
in	O
any	O
significant	O
use	O
,	O
so	O
CSA	O
continues	O
to	O
be	O
the	O
dominant	O
cipher	O
for	O
protecting	O
DVB	B-Application
broadcasts	O
.	O
</s>
<s>
The	O
patent	O
papers	O
gave	O
some	O
hints	O
,	O
but	O
important	O
details	O
,	O
like	O
the	O
layout	O
of	O
the	O
so-called	O
S-boxes	B-Algorithm
,	O
remained	O
secret	O
.	O
</s>
<s>
Though	O
released	O
as	O
binary	B-Application
only	O
,	O
disassembly	O
revealed	O
the	O
missing	O
details	O
and	O
allowed	O
reimplementation	O
of	O
the	O
algorithm	O
in	O
higher-level	O
programming	O
languages	O
.	O
</s>
<s>
When	O
used	O
in	O
encryption	B-Algorithm
mode	I-Algorithm
the	O
data	O
are	O
first	O
encrypted	O
using	O
the	O
64-bit	O
block	O
cipher	O
in	O
CBC	B-Algorithm
mode	I-Algorithm
,	O
starting	O
from	O
packet	O
end	O
.	O
</s>
<s>
The	O
first	O
64	O
bits	O
of	O
data	O
are	O
used	O
as	O
initialization	B-Algorithm
vector	I-Algorithm
during	O
this	O
phase	O
and	O
are	O
left	O
unchanged	O
.	O
</s>
<s>
Were	O
CSA	O
to	O
be	O
broken	O
,	O
encrypted	O
DVB	B-Application
transmissions	O
would	O
be	O
decipherable	O
,	O
which	O
would	O
compromise	O
paid	O
digital	B-Application
television	I-Application
services	O
,	O
as	O
DVB	B-Application
has	O
been	O
standardised	O
for	O
digital	B-Application
terrestrial	I-Application
television	I-Application
in	O
Europe	O
and	O
elsewhere	O
,	O
and	O
is	O
used	O
by	O
many	O
satellite	O
television	O
providers	O
.	O
</s>
<s>
Most	O
attacks	O
on	O
the	O
pay-TV	O
system	O
have	O
not	O
targeted	O
CSA	O
itself	O
,	O
but	O
instead	O
the	O
various	O
key	O
exchange	O
systems	O
responsible	O
for	O
generating	O
the	O
CSA	O
keys	O
(	O
Conax	B-Device
,	O
Irdeto	O
,	O
Nagravision	B-Device
,	O
VideoGuard	B-Device
,	O
etc	O
.	O
</s>
<s>
)	O
,	O
either	O
by	O
reverse-engineering	O
and	O
breaking	O
the	O
algorithms	O
altogether	O
,	O
or	O
by	O
intercepting	O
the	O
keys	O
in	O
real-time	O
as	O
they	O
are	O
generated	O
on	O
a	O
legitimate	O
decoder	O
,	O
and	O
then	O
distributing	O
them	O
to	O
others	O
(	O
so-called	O
card	B-Device
sharing	I-Device
)	O
.	O
</s>
<s>
The	O
stream	O
cipher	O
part	O
of	O
CSA	O
is	O
prone	O
to	O
bit	B-General_Concept
slicing	I-General_Concept
,	O
a	O
software	O
implementation	O
technique	O
which	O
allows	O
decryption	O
of	O
many	O
blocks	O
,	O
or	O
the	O
same	O
block	O
with	O
many	O
different	O
keys	O
,	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
The	O
block	O
cipher	O
part	O
is	O
harder	O
to	O
bit	B-General_Concept
slice	I-General_Concept
,	O
as	O
the	O
S-boxes	B-Algorithm
involved	O
are	O
too	O
large	O
(	O
8x8	O
)	O
to	O
be	O
efficiently	O
implemented	O
using	O
logical	O
operations	O
,	O
a	O
prerequisite	O
for	O
bit	B-General_Concept
slicing	I-General_Concept
to	O
be	O
more	O
efficient	O
than	O
a	O
regular	O
implementation	O
.	O
</s>
<s>
However	O
,	O
as	O
all	O
operations	O
are	O
on	O
8-bit	O
subblocks	O
,	O
the	O
algorithm	O
can	O
be	O
implemented	O
using	O
regular	O
SIMD	B-Device
,	O
or	O
a	O
form	O
of	O
“	O
byteslicing	O
”	O
.	O
</s>
<s>
As	O
most	O
SIMD	B-Device
instruction	O
sets	O
,	O
(	O
with	O
the	O
exception	O
of	O
AVX2	O
)	O
do	O
not	O
support	O
parallel	O
look-up	O
tables	O
,	O
the	O
S-box	B-Algorithm
lookups	O
are	O
done	O
in	O
a	O
non-bytesliced	O
implementation	O
,	O
but	O
their	O
integration	O
into	O
the	O
rest	O
of	O
the	O
algorithm	O
is	O
not	O
hampered	O
markedly	O
by	O
the	O
byteslicing	O
.	O
</s>
<s>
In	O
2011	O
,	O
a	O
group	O
of	O
German	O
researchers	O
released	O
an	O
attack	O
on	O
CSA	O
as	O
used	O
in	O
the	O
DVB	B-Application
system	O
.	O
</s>
<s>
By	O
noting	O
that	O
MPEG-2	O
padding	O
frequently	O
requires	O
long	O
series	O
of	O
zeroes	O
,	O
leading	O
to	O
entire	O
184-byte	O
cells	O
being	O
encrypted	O
with	O
zeroes	O
only	O
,	O
it	O
is	O
possible	O
to	O
build	O
up	O
a	O
rainbow	B-Algorithm
table	I-Algorithm
recovering	O
the	O
key	O
from	O
such	O
a	O
known-zero	O
block	O
.	O
</s>
<s>
The	O
attack	O
described	O
would	O
require	O
about	O
7.9	O
TB	O
of	O
storage	O
,	O
and	O
enable	O
an	O
attacker	O
with	O
a	O
GPU	B-Architecture
to	O
recover	O
a	O
key	O
in	O
about	O
seven	O
seconds	O
with	O
96.8	O
%	O
certainty	O
.	O
</s>
<s>
This	O
differs	O
from	O
the	O
brute-force	O
attack	O
examples	O
already	O
mentioned	O
,	O
in	O
that	O
the	O
plain-text	O
contents	O
of	O
the	O
entire	O
cell	O
is	O
known	O
before	O
the	O
attack	O
,	O
not	O
just	O
parts	O
of	O
a	O
block	O
,	O
which	O
enables	O
the	O
ahead-of-time	O
computation	O
of	O
the	O
rainbow	B-Algorithm
table	I-Algorithm
.	O
</s>
