<s>
Executable	B-Application
compression	I-Application
is	O
any	O
means	O
of	O
compressing	B-General_Concept
an	O
executable	B-Application
file	I-Application
and	O
combining	O
the	O
compressed	B-General_Concept
data	I-General_Concept
with	O
decompression	O
code	O
into	O
a	O
single	O
executable	B-Application
.	O
</s>
<s>
When	O
this	O
compressed	B-Application
executable	I-Application
is	O
executed	O
,	O
the	O
decompression	O
code	O
recreates	O
the	O
original	O
code	O
from	O
the	O
compressed	O
code	O
before	O
executing	O
it	O
.	O
</s>
<s>
In	O
most	O
cases	O
this	O
happens	O
transparently	O
so	O
the	O
compressed	B-Application
executable	I-Application
can	O
be	O
used	O
in	O
exactly	O
the	O
same	O
way	O
as	O
the	O
original	O
.	O
</s>
<s>
Executable	B-Application
compressors	I-Application
are	O
often	O
referred	O
to	O
as	O
"	O
runtime	B-Application
packers	I-Application
"	O
,	O
"	O
software	B-Application
packers	I-Application
"	O
,	O
"	O
software	B-Application
protectors	I-Application
"	O
(	O
or	O
even	O
"	O
polymorphic	O
packers	O
"	O
and	O
"	O
obfuscating	O
tools	O
"	O
)	O
.	O
</s>
<s>
A	O
compressed	B-Application
executable	I-Application
can	O
be	O
considered	O
a	O
self-extracting	B-General_Concept
archive	I-General_Concept
,	O
where	O
a	O
compressed	B-Application
executable	I-Application
is	O
packaged	O
along	O
with	O
the	O
relevant	O
decompression	O
code	O
in	O
an	O
executable	B-Application
file	I-Application
.	O
</s>
<s>
Some	O
compressed	B-Application
executables	I-Application
can	O
be	O
decompressed	O
to	O
reconstruct	O
the	O
original	O
program	O
file	O
without	O
being	O
directly	O
executed	O
.	O
</s>
<s>
Most	O
compressed	B-Application
executables	I-Application
decompress	O
the	O
original	O
code	O
in	O
memory	O
and	O
most	O
require	O
slightly	O
more	O
memory	O
to	O
run	O
(	O
because	O
they	O
need	O
to	O
store	O
the	O
decompressor	O
code	O
,	O
the	O
compressed	B-General_Concept
data	I-General_Concept
and	O
the	O
decompressed	O
code	O
)	O
.	O
</s>
<s>
Moreover	O
,	O
some	O
compressed	B-Application
executables	I-Application
have	O
additional	O
requirements	O
,	O
such	O
as	O
those	O
that	O
write	O
the	O
decompressed	O
executable	B-Application
to	O
the	O
file	O
system	O
before	O
executing	O
it	O
.	O
</s>
<s>
Executable	B-Application
compression	I-Application
is	O
not	O
limited	O
to	O
binary	B-Application
executables	I-Application
,	O
but	O
can	O
also	O
be	O
applied	O
to	O
scripts	O
,	O
such	O
as	O
JavaScript	B-Language
.	O
</s>
<s>
Because	O
most	O
scripting	O
languages	O
are	O
designed	O
to	O
work	O
on	O
human-readable	O
code	O
,	O
which	O
has	O
a	O
high	O
redundancy	B-General_Concept
,	O
compression	O
can	O
be	O
very	O
effective	O
and	O
as	O
simple	O
as	O
replacing	O
long	O
names	O
used	O
to	O
identify	O
variables	O
and	O
functions	O
with	O
shorter	O
versions	O
and/or	O
removing	O
white-space	O
.	O
</s>
<s>
Software	O
distributors	O
use	O
executable	B-Application
compression	I-Application
for	O
a	O
variety	O
of	O
reasons	O
,	O
primarily	O
to	O
reduce	O
the	O
secondary	O
storage	O
requirements	O
of	O
their	O
software	O
;	O
as	O
executable	B-Application
compressors	I-Application
are	O
specifically	O
designed	O
to	O
compress	O
executable	B-Application
code	I-Application
,	O
they	O
often	O
achieve	O
better	O
compression	O
ratio	O
than	O
standard	O
data	B-General_Concept
compression	I-General_Concept
facilities	O
such	O
as	O
gzip	B-Application
,	O
zip	B-General_Concept
or	O
bzip2	B-Application
.	O
</s>
<s>
This	O
allows	O
software	O
distributors	O
to	O
stay	O
within	O
the	O
constraints	O
of	O
their	O
chosen	O
distribution	O
media	O
(	O
such	O
as	O
CD-ROM	B-Device
,	O
DVD-ROM	O
,	O
or	O
floppy	B-Device
disk	I-Device
)	O
,	O
or	O
to	O
reduce	O
the	O
time	O
and	O
bandwidth	O
customers	O
require	O
to	O
access	O
software	O
distributed	O
via	O
the	O
Internet	O
.	O
</s>
<s>
Executable	B-Application
compression	I-Application
is	O
also	O
frequently	O
used	O
to	O
deter	O
reverse	O
engineering	O
or	O
to	O
obfuscate	O
the	O
contents	O
of	O
the	O
executable	B-Application
(	O
for	O
example	O
,	O
to	O
hide	O
the	O
presence	O
of	O
malware	O
from	O
antivirus	B-Application
scanners	I-Application
)	O
by	O
proprietary	O
methods	O
of	O
compression	O
and/or	O
added	O
encryption	O
.	O
</s>
<s>
Executable	B-Application
compression	I-Application
can	O
be	O
used	O
to	O
prevent	O
direct	O
disassembly	O
,	O
mask	O
string	O
literals	O
and	O
modify	O
signatures	O
.	O
</s>
<s>
A	O
compressed	B-Application
executable	I-Application
requires	O
less	O
storage	O
space	O
in	O
the	O
file	O
system	O
,	O
thus	O
less	O
time	O
to	O
transfer	O
data	O
from	O
the	O
file	O
system	O
into	O
memory	O
.	O
</s>
<s>
Thus	O
the	O
compressed	B-Application
executable	I-Application
will	O
load	O
faster	O
on	O
most	O
common	O
systems	O
.	O
</s>
<s>
On	O
modern	O
desktop	O
computers	O
,	O
this	O
is	O
rarely	O
noticeable	O
unless	O
the	O
executable	B-Application
is	O
unusually	O
big	O
,	O
so	O
loading	O
speed	O
is	O
not	O
a	O
primary	O
reason	O
for	O
or	O
against	O
compressing	B-General_Concept
an	O
executable	B-Application
.	O
</s>
<s>
On	O
operating	O
systems	O
which	O
page	B-Architecture
executable	B-Application
images	O
on	O
demand	O
from	O
the	O
disk	O
,	O
compressed	B-Application
executables	I-Application
make	O
this	O
process	O
less	O
efficient	O
.	O
</s>
<s>
The	O
decompressor	O
stub	O
allocates	O
a	O
block	O
of	O
memory	O
to	O
hold	O
the	O
decompressed	O
data	O
,	O
which	O
stays	O
allocated	O
as	O
long	O
as	O
the	O
executable	B-Application
stays	O
loaded	O
,	O
whether	O
it	O
is	O
used	O
or	O
not	O
,	O
competing	O
for	O
memory	O
resources	O
with	O
other	O
applications	O
all	O
along	O
.	O
</s>
<s>
If	O
the	O
operating	O
system	O
uses	O
a	O
swap	O
file	O
,	O
the	O
decompressed	O
data	O
has	O
to	O
be	O
written	O
to	O
it	O
to	O
free	O
up	O
the	O
memory	O
instead	O
of	O
simply	O
discarding	O
unused	O
data	O
blocks	O
and	O
reloading	O
them	O
from	O
the	O
executable	B-Application
image	O
if	O
needed	O
again	O
.	O
</s>
<s>
This	O
is	O
usually	O
not	O
noticeable	O
,	O
but	O
it	O
becomes	O
a	O
problem	O
when	O
an	O
executable	B-Application
is	O
loaded	O
more	O
than	O
once	O
at	O
the	O
same	O
time	O
—	O
the	O
operating	O
system	O
cannot	O
reuse	O
data	O
blocks	O
it	O
has	O
already	O
loaded	O
,	O
the	O
data	O
has	O
to	O
be	O
decompressed	O
into	O
a	O
new	O
memory	O
block	O
,	O
and	O
will	O
be	O
swapped	O
out	O
independently	O
if	O
not	O
used	O
.	O
</s>
<s>
The	O
additional	O
storage	O
and	O
time	O
requirements	O
mean	O
that	O
it	O
has	O
to	O
be	O
weighed	O
carefully	O
whether	O
to	O
compress	O
executables	B-Application
which	O
are	O
typically	O
run	O
more	O
than	O
once	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
Another	O
disadvantage	O
is	O
that	O
some	O
utilities	O
can	O
no	O
longer	O
identify	O
run-time	B-Library
library	I-Library
dependencies	O
,	O
as	O
only	O
the	O
statically	B-Library
linked	I-Library
extractor	O
stub	O
is	O
visible	O
.	O
</s>
<s>
Also	O
,	O
some	O
older	O
virus	B-Application
scanners	I-Application
simply	O
report	O
all	O
compressed	B-Application
executables	I-Application
as	O
viruses	O
because	O
the	O
decompressor	O
stubs	O
share	O
some	O
characteristics	O
with	O
those	O
.	O
</s>
<s>
Most	O
modern	O
virus	B-Application
scanners	I-Application
can	O
unpack	O
several	O
different	O
executable	B-Application
compression	I-Application
layers	O
to	O
check	O
the	O
actual	O
executable	B-Application
inside	O
,	O
but	O
some	O
popular	O
anti-virus	B-Application
and	O
anti-malware	B-Application
scanners	O
have	O
had	O
troubles	O
with	O
false	O
positive	O
alarms	O
on	O
compressed	B-Application
executables	I-Application
.	O
</s>
<s>
In	O
an	O
attempt	O
to	O
solve	O
the	O
problem	O
of	O
malware	O
obfuscated	O
with	O
the	O
help	O
of	O
runtime	B-Application
packers	I-Application
the	O
IEEE	O
Industry	O
Connections	O
Security	O
Group	O
has	O
introduced	O
a	O
software	B-Algorithm
taggant	I-Algorithm
system	O
.	O
</s>
<s>
Executable	B-Application
compression	I-Application
used	O
to	O
be	O
more	O
popular	O
when	O
computers	O
were	O
limited	O
to	O
the	O
storage	O
capacity	O
of	O
floppy	B-Device
disks	I-Device
,	O
which	O
were	O
both	O
slow	O
and	O
low	O
capacity	O
media	O
,	O
and	O
small	O
hard	B-Device
drives	I-Device
;	O
it	O
allowed	O
the	O
computer	O
to	O
store	O
more	O
software	O
in	O
the	O
same	O
amount	O
of	O
space	O
,	O
without	O
the	O
inconvenience	O
of	O
having	O
to	O
manually	O
unpack	O
an	O
archive	O
file	O
every	O
time	O
the	O
user	O
wanted	O
to	O
use	O
the	O
software	O
.	O
</s>
<s>
However	O
,	O
executable	B-Application
compression	I-Application
has	O
become	O
less	O
popular	O
because	O
of	O
increased	O
storage	O
capacity	O
on	O
computers	O
.	O
</s>
<s>
Only	O
very	O
sophisticated	O
compression	O
formats	O
,	O
which	O
add	O
to	O
load	O
time	O
,	O
keep	O
an	O
executable	B-Application
small	O
enough	O
to	O
enter	O
these	O
competitions	O
.	O
</s>
<s>
Known	O
executable	B-Application
compressors	I-Application
for	O
CP/M	B-Application
-80	I-Application
/	O
MSX-DOS	B-Operating_System
.COM	O
files	O
:	O
</s>
<s>
Known	O
executable	B-Application
compressors	I-Application
for	O
DOS	B-Device
executable	B-Application
files	I-Application
(	O
.COM	O
or	O
.EXE	O
)	O
:	O
</s>
<s>
Known	O
executable	B-Application
compressors	I-Application
under	O
OS/2	B-Application
:	O
</s>
<s>
Known	O
executable	B-Application
compressors	I-Application
for	O
New	O
Executables	B-Application
:	O
</s>
<s>
Known	O
executable	B-Application
compressors	I-Application
for	O
Portable	O
Executables	B-Application
:	O
</s>
<s>
Known	O
executable	B-Application
compressors	I-Application
for	O
ELF	O
files	O
:	O
</s>
<s>
Known	O
executable	B-Application
compressors	I-Application
for	O
CLI	O
assembly	O
files	O
:	O
</s>
<s>
Executable	B-Application
compressors	I-Application
for	O
Mac	B-Application
OS	I-Application
Classic	I-Application
applications	O
:	O
</s>
<s>
Known	O
executable	B-Application
compressors	I-Application
for	O
Mach-O	B-Operating_System
(	O
Apple	O
Mac	B-Application
OS	I-Application
X	O
)	O
files	O
:	O
</s>
<s>
Known	O
executable	B-Application
compressors	I-Application
for	O
executables	B-Application
on	O
the	O
Commodore	O
64	O
and	O
VIC-20	B-Device
:	O
</s>
<s>
Known	O
executable	B-Application
compressors	I-Application
for	O
executables	B-Application
on	O
the	O
Amiga	B-Device
series	O
:	O
</s>
<s>
Known	O
executable	B-Application
compressors	I-Application
for	O
Java	B-Device
:	O
</s>
<s>
JAR	B-Language
files	I-Language
:	O
</s>
<s>
WAR	B-Language
files	O
:	O
</s>
<s>
There	O
are	O
two	O
types	O
of	O
compression	O
that	O
can	O
be	O
applied	O
to	O
JavaScript	B-Language
scripts	O
:	O
</s>
<s>
Reduce	O
the	O
redundancy	B-General_Concept
in	O
the	O
script	O
(	O
by	O
removing	O
comments	O
,	O
white	O
space	O
and	O
shorten	O
variable	O
and	O
functions	O
names	O
)	O
.	O
</s>
<s>
Compress	O
the	O
original	O
script	O
and	O
create	O
a	O
new	O
script	O
that	O
contains	O
decompression	O
code	O
and	O
compressed	B-General_Concept
data	I-General_Concept
.	O
</s>
<s>
This	O
is	O
similar	O
to	O
binary	B-Application
executable	I-Application
compression	O
.	O
</s>
<s>
These	O
compress	O
the	O
original	O
script	O
and	O
output	O
a	O
new	O
script	O
that	O
has	O
a	O
decompressor	O
and	O
compressed	B-General_Concept
data	I-General_Concept
.	O
</s>
