<s>
Churning	B-Algorithm
is	O
an	O
encryption	O
function	O
used	O
to	O
scramble	O
downstream	O
user	O
data	O
of	O
the	O
ATM	O
passive	B-Architecture
optical	I-Architecture
network	I-Architecture
system	O
defined	O
by	O
the	O
ITU	O
G.983.1	O
standard	O
.	O
</s>
<s>
The	O
standard	O
states	O
that	O
churning	B-Algorithm
"	O
offers	O
a	O
low	O
level	O
of	O
protection	O
for	O
data	O
confidentiality	O
"	O
.	O
</s>
<s>
Cryptanalysis	O
had	O
shown	O
that	O
"	O
the	O
churning	B-Algorithm
cipher	O
is	O
robustly	O
weak	O
"	O
.	O
</s>
<s>
Churning	B-Algorithm
uses	O
24	O
bits	O
of	O
the	O
key	O
,	O
designated	O
X1	O
..	O
X8	O
and	O
P1	O
..	O
P16	O
.	O
</s>
<s>
The	O
churning	B-Algorithm
transforms	O
eight	O
bits	O
into	O
eight	O
bits	O
:	O
</s>
<s>
the	O
churning	B-Algorithm
function	O
is	O
entirely	O
linear	O
,	O
so	O
it	O
can	O
be	O
broken	O
using	O
linear	O
algebra	O
.	O
</s>
<s>
Due	O
to	O
extreme	O
weakness	O
of	O
the	O
churning	B-Algorithm
cipher	O
,	O
PON	O
systems	O
frequently	O
use	O
the	O
"	O
triple	O
churning	B-Algorithm
"	O
technique	O
,	O
where	O
the	O
three	O
churning	B-Algorithm
operations	O
are	O
combined	O
with	O
two	O
XORs	O
with	O
adjacent	O
data	O
in	O
the	O
stream	O
.	O
</s>
<s>
PMC	O
Sierra	O
holds	O
patents	O
on	O
triple	O
churning	B-Algorithm
(	O
)	O
.	O
</s>
