<s>
The	O
Texas	O
Instruments	O
digital	B-Device
signature	I-Device
transponder	I-Device
(	O
DST	O
)	O
is	O
a	O
cryptographically	O
enabled	O
radio-frequency	B-Application
identification	I-Application
(	O
RFID	B-Application
)	O
device	O
used	O
in	O
a	O
variety	O
of	O
wireless	O
authentication	O
applications	O
.	O
</s>
<s>
Each	O
DST	O
tag	O
contains	O
a	O
quantity	O
of	O
non-volatile	B-General_Concept
RAM	I-General_Concept
,	O
which	O
stores	O
a	O
40-bit	O
encryption	O
key	O
.	O
</s>
<s>
Once	O
the	O
cipher	O
design	O
was	O
known	O
,	O
the	O
team	O
programmed	O
several	O
FPGA	B-Architecture
devices	O
to	O
perform	O
brute-force	O
key	O
searches	O
based	O
on	O
known	O
challenge/response	O
pairs	O
.	O
</s>
<s>
Using	O
a	O
single	O
FPGA	B-Architecture
device	O
,	O
the	O
team	O
was	O
able	O
to	O
recover	O
a	O
key	O
from	O
two	O
known	O
challenge/response	O
pairs	O
in	O
approximately	O
11	O
hours	O
(	O
average	O
case	O
)	O
.	O
</s>
<s>
With	O
an	O
array	O
of	O
16	O
FPGA	B-Architecture
devices	O
,	O
they	O
reduced	O
this	O
time	O
to	O
less	O
than	O
one	O
hour	O
.	O
</s>
<s>
Additionally	O
,	O
researchers	O
from	O
the	O
COSIC	B-General_Concept
research	O
group	O
of	O
KU	O
Leuven	O
implemented	O
a	O
time/memory	O
trade-off	O
attack	O
for	O
which	O
a	O
5.6	O
TB	O
lookup	B-Data_Structure
table	I-Data_Structure
has	O
to	O
be	O
precomputed	O
,	O
after	O
which	O
a	O
key	O
can	O
be	O
recovered	O
in	O
two	O
seconds	O
using	O
a	O
Raspberry	B-Operating_System
Pi	I-Operating_System
3B	I-Operating_System
by	O
sniffing	B-Protocol
two	O
challenge-response	O
pairs	O
.	O
</s>
<s>
DST40	O
is	O
a	O
200-round	O
unbalanced	B-Algorithm
Feistel	I-Algorithm
cipher	I-Algorithm
,	O
in	O
which	O
L0	O
is	O
38	O
bits	O
,	O
and	O
R0	O
is	O
2	O
bits	O
.	O
</s>
<s>
The	O
key	B-Algorithm
schedule	I-Algorithm
is	O
a	O
simple	O
linear	B-Architecture
feedback	I-Architecture
shift	I-Architecture
register	I-Architecture
,	O
which	O
updates	O
every	O
three	O
rounds	O
,	O
resulting	O
in	O
some	O
weak	O
keys	O
(	O
e.g.	O
,	O
the	O
zero	O
key	O
)	O
.	O
</s>
<s>
When	O
used	O
in	O
the	O
protocol	O
with	O
the	O
40	O
–	O
24-bit	O
output	O
truncation	O
,	O
the	O
resulting	O
primitive	O
is	O
more	O
aptly	O
described	O
as	O
a	O
Message	B-Algorithm
Authentication	I-Algorithm
Code	I-Algorithm
rather	O
than	O
an	O
encryption	O
function	O
.	O
</s>
<s>
The	O
DST40	O
cipher	O
is	O
one	O
of	O
the	O
most	O
widely	O
used	O
unbalanced	B-Algorithm
Feistel	I-Algorithm
ciphers	I-Algorithm
in	O
existence	O
.	O
</s>
<s>
As	O
,	O
like	O
its	O
predecessor	O
,	O
DST80	O
documentation	O
was	O
never	O
released	O
to	O
the	O
public	O
either	O
,	O
researchers	O
from	O
the	O
COSIC	B-General_Concept
research	O
group	O
of	O
KU	O
Leuven	O
managed	O
to	O
extract	O
the	O
firmware	O
of	O
several	O
vehicle	O
immobilizer	O
implementations	O
,	O
and	O
reverse-engineer	O
the	O
cipher	O
from	O
these	O
implementations	O
.	O
</s>
<s>
DST80	O
is	O
still	O
an	O
unbalanced	B-Algorithm
Feistel	I-Algorithm
network	I-Algorithm
of	O
200	O
rounds	O
with	O
the	O
same	O
values	O
for	O
L0	O
and	O
R0	O
.	O
</s>
<s>
The	O
key	B-Algorithm
schedule	I-Algorithm
,	O
now	O
updating	O
every	O
round	O
and	O
using	O
80-bit	O
keys	O
,	O
is	O
still	O
based	O
on	O
an	O
LSFR	B-Architecture
.	O
</s>
