<s>
A	O
Hardware	B-Device
Trojan	I-Device
(	O
HT	O
)	O
is	O
a	O
malicious	O
modification	O
of	O
the	O
circuitry	O
of	O
an	O
integrated	O
circuit	O
.	O
</s>
<s>
A	O
hardware	B-Device
Trojan	I-Device
is	O
completely	O
characterized	O
by	O
its	O
physical	O
representation	O
and	O
its	O
behavior	O
.	O
</s>
<s>
Hardware	B-Device
Trojans	I-Device
may	O
be	O
introduced	O
as	O
hidden	O
"	O
Front-doors	O
"	O
that	O
are	O
inserted	O
while	O
designing	O
a	O
computer	O
chip	O
,	O
by	O
using	O
a	O
pre-made	O
application-specific	O
integrated	O
circuit	O
(	O
ASIC	O
)	O
semiconductor	B-Architecture
intellectual	I-Architecture
property	I-Architecture
core	I-Architecture
(	O
IP	B-Architecture
Core	I-Architecture
)	O
that	O
have	O
been	O
purchased	O
from	O
a	O
non-reputable	O
source	O
,	O
or	O
inserted	O
internally	O
by	O
a	O
rogue	O
employee	O
,	O
either	O
acting	O
on	O
their	O
own	O
,	O
or	O
on	O
behalf	O
of	O
rogue	O
special	O
interest	O
groups	O
,	O
or	O
state	O
sponsored	O
spying	O
and	O
espionage	O
.	O
</s>
<s>
In	O
high	O
security	O
governmental	O
IT	O
departments	O
,	O
hardware	B-Device
Trojans	I-Device
are	O
a	O
well	O
known	O
problem	O
when	O
buying	O
hardware	O
such	O
as	O
:	O
a	O
KVM	B-Device
switch	I-Device
,	O
keyboards	O
,	O
mice	O
,	O
network	O
cards	O
,	O
or	O
other	O
network	O
equipment	O
.	O
</s>
<s>
This	O
is	O
especially	O
the	O
case	O
when	O
purchasing	O
such	O
equipment	O
from	O
non-reputable	O
sources	O
that	O
could	O
have	O
placed	O
hardware	B-Device
Trojans	I-Device
to	O
leak	O
keyboard	O
passwords	O
,	O
or	O
provide	O
remote	O
unauthorized	O
entry	O
.	O
</s>
<s>
In	O
a	O
diverse	O
global	O
economy	O
,	O
outsourcing	B-Architecture
of	O
production	O
tasks	O
is	O
a	O
common	O
way	O
to	O
lower	O
a	O
product	O
's	O
cost	O
.	O
</s>
<s>
A	O
Trojan	O
is	O
functional	O
if	O
the	O
adversary	O
adds	O
or	O
deletes	O
any	O
transistors	B-Application
or	O
gates	O
to	O
the	O
original	O
chip	O
design	O
.	O
</s>
<s>
thinning	O
of	O
wires	O
,	O
weakening	O
of	O
flip-flops	O
or	O
transistors	B-Application
,	O
subjecting	O
the	O
chip	O
to	O
radiation	O
,	O
or	O
using	O
Focused	O
Ion-Beams	O
(	O
FIB	O
)	O
to	O
reduce	O
the	O
reliability	O
of	O
a	O
chip	O
.	O
</s>
<s>
The	O
typical	O
Trojan	O
is	O
condition-based	O
:	O
It	O
is	O
triggered	O
by	O
sensors	B-Application
,	O
internal	O
logic	O
states	O
,	O
a	O
particular	O
input	O
pattern	O
or	O
an	O
internal	O
counter	O
value	O
.	O
</s>
<s>
Hardware	B-Device
Trojans	I-Device
can	O
be	O
triggered	O
in	O
different	O
ways	O
.	O
</s>
<s>
There	O
can	O
be	O
malicious	O
logic	O
inside	O
a	O
chip	O
,	O
that	O
uses	O
an	O
antenna	O
or	O
other	O
sensors	B-Application
the	O
adversary	O
can	O
reach	O
from	O
outside	O
the	O
chip	O
.	O
</s>
<s>
Extraction	O
of	O
secret	O
keys	O
by	O
means	O
of	O
a	O
hardware	B-Device
Trojan	I-Device
without	O
detecting	O
the	O
Trojan	O
requires	O
that	O
the	O
Trojan	O
uses	O
a	O
random	O
signal	O
or	O
some	O
cryptographic	O
implementation	O
itself	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
USB	B-Protocol
keyboard	O
that	O
hides	O
all	O
malicious	O
processing	O
cycles	O
from	O
the	O
target	O
network	O
endpoint	O
to	O
which	O
it	O
is	O
attached	O
by	O
communicating	O
with	O
the	O
target	O
network	O
endpoint	O
using	O
unintended	O
USB	B-Protocol
channels	O
.	O
</s>
<s>
The	O
only	O
known	O
hardware	B-Device
Trojan	I-Device
using	O
unique	O
crypto	O
keys	O
but	O
having	O
the	O
same	O
structure	O
is	O
.	O
</s>
<s>
The	O
extra	O
logic	O
monitors	O
input	O
stimulus	O
and	O
internal	O
signals	O
or	O
memory	O
states	O
,	O
generally	O
by	O
computing	O
checksums	B-Algorithm
or	O
by	O
exposing	O
internal	O
registers	O
via	O
a	O
customized	O
scanning	O
technique	O
.	O
</s>
