<s>
KeeLoq	B-Algorithm
is	O
a	O
proprietary	O
hardware-dedicated	O
block	O
cipher	B-Application
that	O
uses	O
a	O
non-linear	B-Algorithm
feedback	I-Algorithm
shift	I-Algorithm
register	I-Algorithm
(	O
NLFSR	B-Algorithm
)	O
.	O
</s>
<s>
KeeLoq	B-Algorithm
was	O
sold	O
to	O
Microchip	O
Technology	O
Inc	O
in	O
1995	O
for	O
$10	O
million	O
.	O
</s>
<s>
KeeLoq	B-Algorithm
is	O
or	O
was	O
used	O
in	O
many	O
remote	O
keyless	O
entry	O
systems	O
by	O
such	O
companies	O
as	O
Chrysler	O
,	O
Daewoo	O
,	O
Fiat	O
,	O
GM	O
,	O
Honda	O
,	O
Toyota	O
,	O
Volvo	O
,	O
Volkswagen	O
Group	O
,	O
Clifford	O
,	O
Shurlok	O
,	O
and	O
Jaguar	O
.	O
</s>
<s>
KeeLoq	B-Algorithm
"	O
code	O
hopping	O
"	O
encoders	O
encrypt	O
a	O
0-filled	O
32-bit	O
block	O
with	O
KeeLoq	B-Algorithm
cipher	B-Application
to	O
produce	O
a	O
32-bit	O
"	O
hopping	O
code	O
"	O
.	O
</s>
<s>
A	O
32-bit	O
initialization	B-Algorithm
vector	I-Algorithm
is	O
linearly	O
added	O
(	O
XORed	O
)	O
to	O
the	O
32	O
least	O
significant	O
bits	O
of	O
the	O
key	O
prior	O
to	O
encryption	O
and	O
after	O
decryption	O
.	O
</s>
<s>
KeeLoq	B-Algorithm
cipher	B-Application
accepts	O
64-bit	O
keys	O
and	O
encrypts	O
32-bit	O
blocks	O
by	O
executing	O
its	O
single-bit	O
NLFSR	B-Algorithm
for	O
528	O
rounds	O
.	O
</s>
<s>
KeeLoq	B-Algorithm
uses	O
bits	O
1	O
,	O
9	O
,	O
20	O
,	O
26	O
and	O
31	O
of	O
the	O
NLFSR	B-Algorithm
state	O
as	O
its	O
inputs	O
during	O
encryption	O
and	O
bits	O
0	O
,	O
8	O
,	O
19	O
,	O
25	O
and	O
30	O
during	O
decryption	O
.	O
</s>
<s>
Its	O
output	O
is	O
linearly	O
combined	O
(	O
XORed	O
)	O
with	O
two	O
of	O
the	O
bits	O
of	O
the	O
NLFSR	B-Algorithm
state	O
(	O
bits	O
0	O
and	O
16	O
on	O
encryption	O
and	O
bits	O
31	O
and	O
15	O
on	O
decryption	O
)	O
and	O
with	O
a	O
key	O
bit	O
(	O
bit	O
0	O
of	O
the	O
key	O
state	O
on	O
encryption	O
and	O
bit	O
15	O
of	O
the	O
key	O
state	O
on	O
decryption	O
)	O
and	O
is	O
fed	O
back	O
into	O
the	O
NLFSR	B-Algorithm
state	O
on	O
every	O
round	O
.	O
</s>
<s>
The	O
Dual	O
KeeLoq	B-Algorithm
system	O
is	O
a	O
timer-based	O
algorithm	O
enhancing	O
the	O
Classic	O
KeeLoq	B-Algorithm
system	O
.	O
</s>
<s>
The	O
goal	O
of	O
this	O
newer	O
version	O
is	O
to	O
have	O
a	O
timer-driven	O
counter	O
which	O
continuously	O
increments	O
,	O
which	O
is	O
the	O
opposite	O
of	O
the	O
Classic	O
KeeLoq	B-Algorithm
where	O
the	O
counter	O
increments	O
based	O
on	O
the	O
events	O
it	O
received	O
.	O
</s>
<s>
KeeLoq	B-Algorithm
was	O
first	O
cryptanalyzed	O
by	O
Andrey	O
Bogdanov	O
using	O
sliding	O
techniques	O
and	O
efficient	O
linear	B-Algorithm
approximations	I-Algorithm
.	O
</s>
<s>
Nicolas	O
Courtois	O
attacked	O
KeeLoq	B-Algorithm
using	O
sliding	O
and	O
algebraic	O
methods	O
.	O
</s>
<s>
The	O
attacks	O
by	O
Bogdanov	O
and	O
Courtois	O
do	O
not	O
pose	O
any	O
threat	O
to	O
the	O
actual	O
implementations	O
that	O
seem	O
to	O
be	O
much	O
more	O
vulnerable	O
to	O
simple	O
brute-force	O
of	O
the	O
key	O
space	O
that	O
is	O
reduced	O
in	O
all	O
the	O
code-hopping	O
implementations	O
of	O
the	O
cipher	B-Application
known	O
to	O
date	O
.	O
</s>
<s>
Some	O
KeeLoq	B-Algorithm
"	O
code	O
grabbers	O
"	O
use	O
FPGA-based	O
devices	O
to	O
break	O
KeeLoq-based	O
keys	O
by	O
brute	O
force	O
within	O
about	O
two	O
weeks	O
due	O
to	O
the	O
reduced	O
key	O
length	O
in	O
the	O
real	O
world	O
implementations	O
.	O
</s>
<s>
In	O
2007	O
,	O
researchers	O
in	O
the	O
COSIC	B-General_Concept
group	O
at	O
the	O
university	O
at	O
Leuven	O
,	O
Belgium	O
,	O
(	O
K.U.Leuven	O
)	O
in	O
cooperation	O
with	O
colleagues	O
from	O
Israel	O
found	O
a	O
new	O
attack	O
against	O
the	O
system	O
.	O
</s>
<s>
Microchip	O
introduced	O
in	O
1996	O
a	O
version	O
of	O
KeeLoq	B-Algorithm
ICs	O
which	O
use	O
a	O
60-bit	O
seed	O
.	O
</s>
<s>
In	O
March	O
2008	O
,	O
researchers	O
from	O
the	O
Chair	O
for	O
Embedded	O
Security	O
of	O
Ruhr	O
University	O
Bochum	O
,	O
Germany	O
,	O
presented	O
a	O
complete	O
break	O
of	O
remote	O
keyless	O
entry	O
systems	O
based	O
on	O
the	O
KeeLoq	B-Algorithm
RFID	O
technology	O
.	O
</s>
<s>
Their	O
attack	O
works	O
on	O
all	O
known	O
car	O
and	O
building	O
access	O
control	O
systems	O
that	O
rely	O
on	O
the	O
KeeLoq	B-Algorithm
cipher	B-Application
.	O
</s>
<s>
Unlike	O
the	O
cryptanalytic	O
attack	O
described	O
above	O
which	O
requires	O
about	O
65536	O
chosen	O
plaintext-ciphertext	O
pairs	O
and	O
days	O
of	O
calculation	O
on	O
a	O
PC	O
to	O
recover	O
the	O
key	O
,	O
the	O
side-channel	O
attack	O
can	O
also	O
be	O
applied	O
to	O
the	O
so-called	O
KeeLoq	B-Algorithm
Code	O
Hopping	O
mode	O
of	O
operation	O
(	O
a.k.a.	O
</s>
