<s>
OneHalf	B-Application
is	O
a	O
DOS-based	O
polymorphic	O
computer	O
virus	O
(	O
hybrid	O
boot	O
and	O
file	O
infector	O
)	O
discovered	O
in	O
October	O
1994	O
.	O
</s>
<s>
It	O
is	O
also	O
known	O
as	O
Slovak	O
Bomber	B-Application
,	O
Freelove	O
or	O
Explosion-II	O
.	O
</s>
<s>
It	O
infects	O
the	O
master	O
boot	O
record	O
(	O
MBR	O
)	O
of	O
the	O
hard	B-Device
disk	I-Device
,	O
and	O
any	O
files	O
with	O
extensions	O
.COM	O
,	O
.SCR	O
and	O
.EXE	O
.	O
</s>
<s>
It	O
is	O
also	O
known	O
as	O
one	O
of	O
the	O
first	O
viruses	O
to	O
implement	O
a	O
technique	O
of	O
"	O
patchy	O
infection	O
"	O
,	O
introduced	O
in	O
Bomber	B-Application
.	O
</s>
<s>
OneHalf	B-Application
has	O
about	O
20	O
different	O
variants	O
,	O
all	O
with	O
functionally	O
similar	O
behaviour	O
.	O
</s>
<s>
OneHalf	B-Application
is	O
known	O
for	O
its	O
peculiar	O
payload	O
:	O
at	O
every	O
boot	O
,	O
it	O
encrypts	O
two	O
unencrypted	O
cylinders	O
of	O
the	O
user	O
's	O
hard	B-Device
disk	I-Device
,	O
but	O
then	O
temporarily	O
decrypts	O
them	O
when	O
they	O
are	O
accessed	O
.	O
</s>
<s>
This	O
makes	O
sure	O
the	O
user	O
does	O
not	O
notice	O
that	O
their	O
hard	B-Device
disk	I-Device
is	O
being	O
encrypted	O
like	O
this	O
,	O
and	O
lets	O
the	O
encryption	O
continue	O
further	O
.	O
</s>
<s>
OneHalf	B-Application
's	O
unique	O
payload	O
makes	O
removal	O
harder	O
:	O
simply	O
removing	O
the	O
virus	O
and	O
cleaning	O
the	O
MBR	O
will	O
leave	O
the	O
data	O
encrypted	O
,	O
requiring	O
backups	O
to	O
restore	O
it	O
.	O
</s>
<s>
As	O
such	O
,	O
special	O
tools	O
are	O
needed	O
to	O
decrypt	O
the	O
hard	B-Device
disk	I-Device
before	O
removing	O
the	O
virus	O
.	O
</s>
