<s>
The	O
Cryptomeria	B-Algorithm
cipher	I-Algorithm
,	O
also	O
called	O
C2	O
,	O
is	O
a	O
proprietary	B-Application
block	O
cipher	O
defined	O
and	O
licensed	O
by	O
the	O
4C	O
Entity	O
.	O
</s>
<s>
It	O
is	O
the	O
successor	O
to	O
CSS	B-Algorithm
algorithm	O
(	O
used	O
for	O
DVD-Video	O
)	O
and	O
was	O
designed	O
for	O
the	O
CPRM/CPPM	O
digital	O
rights	O
management	O
scheme	O
which	O
are	O
used	O
by	O
DRM-restricted	O
Secure	B-Device
Digital	I-Device
cards	I-Device
and	O
DVD-Audio	O
discs	O
.	O
</s>
<s>
The	O
C2	O
symmetric	B-Algorithm
key	I-Algorithm
algorithm	I-Algorithm
is	O
a	O
10-round	O
Feistel	B-Algorithm
cipher	I-Algorithm
.	O
</s>
<s>
Like	O
DES	B-Algorithm
,	O
it	O
has	O
a	O
key	O
size	O
of	O
56	O
bits	O
and	O
a	O
block	O
size	O
of	O
64	O
bits	O
.	O
</s>
<s>
The	O
encryption	O
and	O
decryption	O
algorithms	O
are	O
available	O
for	O
peer	O
review	O
,	O
but	O
implementations	O
require	O
the	O
so-called	O
"	O
secret	O
constant	O
"	O
,	O
the	O
values	O
of	O
the	O
substitution	B-Algorithm
box	I-Algorithm
(	O
S-box	B-Algorithm
)	O
,	O
which	O
are	O
only	O
available	O
under	O
a	O
license	O
from	O
the	O
4C	O
Entity	O
.	O
</s>
<s>
The	O
4C	O
Entity	O
licenses	O
a	O
different	O
set	O
of	O
S-boxes	B-Algorithm
for	O
each	O
application	O
(	O
such	O
as	O
DVD-Audio	O
,	O
DVD-Video	O
and	O
CPRM	O
)	O
.	O
</s>
<s>
In	O
2008	O
,	O
an	O
attack	O
was	O
published	O
against	O
a	O
reduced	O
8-round	O
version	O
of	O
Cryptomeria	O
to	O
discover	O
the	O
S-box	B-Algorithm
in	O
a	O
chosen-key	O
scenario	O
.	O
</s>
<s>
In	O
a	O
practical	O
experiment	O
,	O
the	O
attack	O
succeeded	O
in	O
recovering	O
parts	O
of	O
the	O
S-box	B-Algorithm
in	O
15	O
hours	O
of	O
CPU	O
time	O
,	O
using	O
2	O
plaintext-ciphertext	O
pairs	O
.	O
</s>
<s>
A	O
paper	O
by	O
Julia	O
Borghoff	O
,	O
Lars	O
Knudsen	O
,	O
Gregor	O
Leander	O
and	O
Krystian	O
Matusiewicz	O
in	O
2009	O
breaks	O
the	O
full-round	O
cipher	O
in	O
three	O
different	O
scenarios	O
;	O
it	O
presents	O
a	O
224	O
time	O
complexity	O
attack	O
to	O
recover	O
the	O
S-box	B-Algorithm
in	O
a	O
chosen-key	O
scenario	O
,	O
a	O
248	O
boomerang	O
attack	O
to	O
recover	O
the	O
key	O
with	O
a	O
known	O
S-box	B-Algorithm
using	O
244	O
adaptively	O
chosen	O
plaintexts/ciphertexts	O
,	O
and	O
a	O
253.5	O
attack	O
when	O
both	O
the	O
key	O
and	O
S-box	B-Algorithm
are	O
unknown	O
.	O
</s>
<s>
Following	O
an	O
announcement	O
by	O
Japanese	O
HDTV	B-Device
broadcasters	O
that	O
they	O
would	O
start	O
broadcasting	O
programs	O
with	O
the	O
copy-once	O
broadcast	B-Device
flag	I-Device
starting	O
with	O
2004-04-05	O
,	O
a	O
distributed	O
Cryptomeria	B-Algorithm
cipher	I-Algorithm
brute	O
force	O
cracking	O
effort	O
was	O
launched	O
on	O
2003-12-21	O
.	O
</s>
<s>
To	O
enforce	O
the	O
broadcast	B-Device
flag	I-Device
,	O
digital	B-Application
video	I-Application
recorders	I-Application
employ	O
CPRM-compatible	O
storage	O
devices	O
,	O
which	O
the	O
project	O
aimed	O
to	O
circumvent	O
.	O
</s>
<s>
Because	O
the	O
attack	O
was	O
based	O
on	O
S-box	B-Algorithm
values	O
from	O
DVD-Audio	O
,	O
it	O
was	O
suggested	O
that	O
CPRM	O
may	O
use	O
different	O
S-boxes	B-Algorithm
.	O
</s>
