<s>
In	O
computing	O
,	O
entropy	B-Algorithm
is	O
the	O
randomness	O
collected	O
by	O
an	O
operating	O
system	O
or	O
application	O
for	O
use	O
in	O
cryptography	O
or	O
other	O
uses	O
that	O
require	O
random	O
data	O
.	O
</s>
<s>
This	O
randomness	O
is	O
often	O
collected	O
from	O
hardware	O
sources	O
(	O
variance	O
in	O
fan	O
noise	O
or	O
HDD	O
)	O
,	O
either	O
pre-existing	O
ones	O
such	O
as	O
mouse	B-Device
movements	O
or	O
specially	O
provided	O
randomness	O
generators	O
.	O
</s>
<s>
A	O
lack	O
of	O
entropy	B-Algorithm
can	O
have	O
a	O
negative	O
impact	O
on	O
performance	O
and	O
security	O
.	O
</s>
<s>
The	O
Linux	B-Operating_System
kernel	I-Operating_System
generates	O
entropy	B-Algorithm
from	O
keyboard	B-Device
timings	O
,	O
mouse	B-Device
movements	O
,	O
and	O
IDE	O
timings	O
and	O
makes	O
the	O
random	O
character	O
data	O
available	O
to	O
other	O
operating	O
system	O
processes	O
through	O
the	O
special	B-Application
files	I-Application
/dev/random	B-Application
and	O
/dev/urandom	B-Application
.	O
</s>
<s>
This	O
capability	O
was	O
introduced	O
in	O
Linux	B-Application
version	O
1.3.30	O
.	O
</s>
<s>
There	O
are	O
some	O
Linux	B-Operating_System
kernel	I-Operating_System
patches	O
allowing	O
one	O
to	O
use	O
more	O
entropy	B-Algorithm
sources	O
.	O
</s>
<s>
The	O
project	O
,	O
which	O
is	O
included	O
in	O
some	O
operating	O
systems	O
such	O
as	O
Fedora	O
,	O
allows	O
audio	O
data	O
to	O
be	O
used	O
as	O
an	O
entropy	B-Algorithm
source	O
.	O
</s>
<s>
Also	O
available	O
are	O
which	O
calculates	O
random	O
data	O
from	O
a	O
video-source	O
and	O
which	O
includes	O
these	O
three	O
and	O
can	O
be	O
used	O
to	O
distribute	O
the	O
entropy	B-Algorithm
data	O
to	O
systems	O
not	O
capable	O
of	O
running	O
any	O
of	O
these	O
(	O
e.g.	O
</s>
<s>
virtual	B-Architecture
machines	I-Architecture
)	O
.	O
</s>
<s>
Furthermore	O
,	O
one	O
can	O
use	O
the	O
HAVEGE	O
algorithm	O
through	O
haveged	O
to	O
pool	O
entropy	B-Algorithm
.	O
</s>
<s>
In	O
some	O
systems	O
,	O
network	O
interrupts	B-Application
can	O
be	O
used	O
as	O
an	O
entropy	B-Algorithm
source	O
as	O
well	O
.	O
</s>
<s>
OpenBSD	B-Operating_System
has	O
integrated	O
cryptography	O
as	O
one	O
of	O
its	O
main	O
goals	O
and	O
has	O
always	O
worked	O
on	O
increasing	O
its	O
entropy	B-Algorithm
for	O
encryption	O
but	O
also	O
for	O
randomising	O
many	O
parts	O
of	O
the	O
OS	O
,	O
including	O
various	O
internal	O
operations	O
of	O
its	O
kernel	B-Operating_System
.	O
</s>
<s>
This	O
made	O
depletion	O
of	O
random	O
data	O
by	O
userland	O
programs	O
impossible	O
on	O
OpenBSD	B-Operating_System
once	O
enough	O
entropy	B-Algorithm
has	O
initially	O
been	O
gathered	O
.	O
</s>
<s>
A	O
driver	O
ported	O
from	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
has	O
been	O
made	O
available	O
for	O
the	B-Application
Hurd	I-Application
kernel	B-Operating_System
.	O
</s>
<s>
/dev/random	B-Application
and	O
/dev/urandom	B-Application
have	O
been	O
available	O
as	O
Sun	O
packages	O
or	O
patches	O
for	O
Solaris	B-Application
since	O
Solaris	B-Application
2.6	O
,	O
and	O
have	O
been	O
a	O
standard	O
feature	O
since	O
Solaris	B-Application
9	I-Application
.	O
</s>
<s>
As	O
of	O
Solaris	B-Application
10	I-Application
,	O
administrators	O
can	O
remove	O
existing	O
entropy	B-Algorithm
sources	O
or	O
define	O
new	O
ones	O
via	O
the	O
kernel-level	O
cryptographic	O
framework	O
.	O
</s>
<s>
A	O
3rd-party	O
kernel	B-Operating_System
module	O
implementing	O
/dev/random	B-Application
is	O
also	O
available	O
for	O
releases	O
dating	O
back	O
to	O
Solaris	B-Application
2.4	O
.	O
</s>
<s>
There	O
is	O
a	O
software	O
package	O
for	O
OS/2	B-Application
that	O
allows	O
software	O
processes	O
to	O
retrieve	O
random	O
data	O
.	O
</s>
<s>
Microsoft	B-Application
Windows	I-Application
releases	O
newer	O
than	O
Windows	O
95	O
use	O
CryptoAPI	O
to	O
gather	O
entropy	B-Algorithm
in	O
a	O
similar	O
fashion	O
to	O
Linux	B-Operating_System
kernel	I-Operating_System
's	O
/dev/random	B-Application
.	O
</s>
<s>
Windows	O
's	O
CryptoAPI	O
uses	O
the	O
binary	O
registry	B-Protocol
key	I-Protocol
HKEY_LOCAL_MACHINE	O
\SOFTWARE\Microsoft\Cryptography\RNG\Seed	O
to	O
store	O
a	O
seeded	O
value	O
from	O
all	O
of	O
its	O
entropy	B-Algorithm
sources	O
.	O
</s>
<s>
Because	O
CryptoAPI	O
is	O
closed-source	B-Application
,	O
some	O
free	B-Application
and	I-Application
open	I-Application
source	I-Application
software	I-Application
applications	O
running	O
on	O
the	O
Windows	O
platform	O
use	O
other	O
measures	O
to	O
get	O
randomness	O
.	O
</s>
<s>
Programmers	O
using	O
CAPI	O
can	O
get	O
entropy	B-Algorithm
by	O
calling	O
CAPI	O
's	O
CryptGenRandom( )	O
,	O
after	O
properly	O
initializing	O
it	O
.	O
</s>
<s>
CryptoAPI	O
was	O
deprecated	O
from	O
Windows	B-Application
Vista	I-Application
and	O
higher	O
.	O
</s>
<s>
Windows	O
's	O
CNG	O
uses	O
the	O
binary	O
registry	B-Protocol
key	I-Protocol
HKEY_LOCAL_MACHINE	O
\SYSTEM\RNG\Seed	O
to	O
store	O
a	O
seeded	O
value	O
.	O
</s>
<s>
Newer	O
version	O
of	O
Windows	O
are	O
able	O
to	O
use	O
a	O
variety	O
of	O
entropy	B-Algorithm
sources	O
:	O
</s>
<s>
Embedded	B-Architecture
Systems	I-Architecture
have	O
difficulty	O
gathering	O
enough	O
entropy	B-Algorithm
as	O
they	O
are	O
often	O
very	O
simple	O
devices	O
with	O
short	O
boot	O
times	O
,	O
and	O
key	O
generation	O
operations	O
that	O
require	O
sufficient	O
entropy	B-Algorithm
are	O
often	O
one	O
of	O
the	O
first	O
things	O
a	O
system	O
may	O
do	O
.	O
</s>
<s>
Common	O
entropy	B-Algorithm
sources	O
may	O
not	O
exist	O
on	O
these	O
devices	O
,	O
or	O
will	O
not	O
have	O
been	O
active	O
long	O
enough	O
during	O
boot	O
to	O
ensure	O
sufficient	O
entropy	B-Algorithm
exists	O
.	O
</s>
<s>
Embedded	B-Architecture
devices	I-Architecture
often	O
lack	O
rotating	O
disk	B-Device
drives	I-Device
,	O
human	O
interface	O
devices	O
,	O
and	O
even	O
fans	O
,	O
and	O
the	O
network	O
interface	O
,	O
if	O
any	O
,	O
will	O
not	O
have	O
been	O
active	O
for	O
long	O
enough	O
to	O
provide	O
much	O
entropy	B-Algorithm
.	O
</s>
<s>
Lacking	O
easy	O
access	O
to	O
entropy	B-Algorithm
,	O
some	O
devices	O
may	O
use	O
hard-coded	O
keys	O
to	O
seed	O
random	O
generators	O
,	O
or	O
seed	O
random	O
generators	O
from	O
easily-guessed	O
unique	O
identifiers	O
such	O
as	O
the	O
device	O
's	O
MAC	O
address	O
.	O
</s>
<s>
A	O
simple	O
study	O
demonstrated	O
the	O
widespread	O
use	O
of	O
weak	O
keys	O
by	O
finding	O
many	O
embedded	B-Architecture
systems	I-Architecture
such	O
as	O
routers	O
using	O
the	O
same	O
keys	O
.	O
</s>
<s>
It	O
was	O
thought	O
that	O
the	O
number	O
of	O
weak	O
keys	O
found	O
would	O
have	O
been	O
far	O
higher	O
if	O
simple	O
and	O
often	O
attacker	O
determinable	O
one-time	O
unique	O
identifiers	O
had	O
not	O
been	O
incorporated	O
into	O
the	O
entropy	B-Algorithm
of	O
some	O
of	O
these	O
systems	O
.	O
</s>
<s>
The	O
Cardano	O
platform	O
uses	O
the	O
participants	O
of	O
their	O
decentralized	O
proof-of-stake	B-Protocol
protocol	O
to	O
generate	O
random	O
numbers	O
.	O
</s>
<s>
There	O
are	O
some	O
software	O
packages	O
that	O
allow	O
one	O
to	O
use	O
a	O
userspace	O
process	O
to	O
gather	O
random	O
characters	O
,	O
exactly	O
what	O
/dev/random	B-Application
does	O
,	O
such	O
as	O
EGD	O
,	O
the	O
Entropy	B-Algorithm
Gathering	O
Daemon	O
.	O
</s>
<s>
Modern	O
CPUs	B-Device
and	O
hardware	O
often	O
feature	O
integrated	O
generators	O
that	O
can	O
provide	O
high-quality	O
and	O
high-speed	O
entropy	B-Algorithm
to	O
operating	O
systems	O
.	O
</s>
<s>
On	O
systems	O
based	O
on	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
,	O
one	O
can	O
read	O
the	O
entropy	B-Algorithm
generated	O
from	O
such	O
a	O
device	O
through	O
/dev/hw_random	O
.	O
</s>
<s>
However	O
,	O
sometimes	O
/dev/hw_random	O
may	O
be	O
slow	O
;	O
</s>
<s>
There	O
are	O
some	O
companies	O
manufacturing	O
entropy	B-Algorithm
generation	O
devices	O
,	O
and	O
some	O
of	O
them	O
are	O
shipped	O
with	O
drivers	O
for	O
Linux	B-Application
.	O
</s>
<s>
On	O
Linux	B-Application
system	O
,	O
one	O
can	O
install	O
the	O
package	O
that	O
supports	O
the	O
true	O
random	O
number	O
generators	O
(	O
TRNGs	O
)	O
found	O
in	O
CPUs	B-Device
supporting	O
the	O
RDRAND	B-Language
instruction	O
,	O
Trusted	O
Platform	O
Modules	O
and	O
in	O
some	O
Intel	O
,	O
AMD	O
,	O
or	O
VIA	O
chipsets	B-Device
,	O
effectively	O
increasing	O
the	O
entropy	B-Algorithm
collected	O
into	O
/dev/random	B-Application
and	O
potentially	O
improving	O
the	O
cryptographic	O
potential	O
.	O
</s>
<s>
This	O
is	O
especially	O
useful	O
on	O
headless	B-Application
systems	I-Application
that	O
have	O
no	O
other	O
sources	O
of	O
entropy	B-Algorithm
.	O
</s>
<s>
System	B-Application
administrators	I-Application
,	O
especially	O
those	O
supervising	O
Internet	O
servers	O
,	O
have	O
to	O
ensure	O
that	O
the	O
server	B-Application
processes	O
will	O
not	O
halt	O
because	O
of	O
entropy	B-Algorithm
depletion	O
.	O
</s>
<s>
Entropy	B-Algorithm
on	O
servers	O
utilising	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
,	O
or	O
any	O
other	O
kernel	B-Operating_System
or	O
userspace	O
process	O
that	O
generates	O
entropy	B-Algorithm
from	O
the	O
console	O
and	O
the	O
storage	O
subsystem	O
,	O
is	O
often	O
less	O
than	O
ideal	O
because	O
of	O
the	O
lack	O
of	O
a	O
mouse	B-Device
and	O
keyboard	B-Device
,	O
thus	O
servers	O
have	O
to	O
generate	O
their	O
entropy	B-Algorithm
from	O
a	O
limited	O
set	O
of	O
resources	O
such	O
as	O
IDE	O
timings	O
.	O
</s>
<s>
The	O
entropy	B-Algorithm
pool	O
size	O
in	O
Linux	B-Application
is	O
viewable	O
through	O
the	O
file	O
/proc/sys/kernel/random/entropy_avail	O
and	O
should	O
generally	O
be	O
at	O
least	O
2000	O
bits	O
(	O
out	O
of	O
a	O
maximum	O
of	O
4096	O
)	O
.	O
</s>
<s>
Entropy	B-Algorithm
changes	O
frequently	O
.	O
</s>
<s>
Administrators	O
responsible	O
for	O
systems	O
that	O
have	O
low	O
or	O
zero	O
entropy	B-Algorithm
should	O
not	O
attempt	O
to	O
use	O
/dev/urandom	B-Application
as	O
a	O
substitute	O
for	O
/dev/random	B-Application
as	O
this	O
may	O
cause	O
SSL/TLS	O
connections	O
to	O
have	O
lower-grade	O
encryption	O
.	O
</s>
<s>
Some	O
software	O
systems	O
change	O
their	O
Diffie-Hellman	B-Protocol
keys	O
often	O
,	O
and	O
this	O
may	O
in	O
some	O
cases	O
help	O
a	O
server	B-Application
to	O
continue	O
functioning	O
normally	O
even	O
with	O
an	O
entropy	B-Algorithm
bottleneck	O
.	O
</s>
<s>
On	O
servers	O
with	O
low	O
entropy	B-Algorithm
,	O
a	O
process	O
can	O
appear	O
hung	O
when	O
it	O
is	O
waiting	O
for	O
random	O
characters	O
to	O
appear	O
in	O
/dev/random	B-Application
(	O
on	O
Linux-based	O
systems	O
)	O
.	O
</s>
<s>
For	O
example	O
,	O
there	O
was	O
a	O
known	O
problem	O
in	O
Debian	O
that	O
caused	O
exim4	B-Protocol
to	O
hang	O
in	O
some	O
cases	O
because	O
of	O
this	O
.	O
</s>
<s>
Entropy	B-Algorithm
sources	O
can	O
be	O
used	O
for	O
keyboard	B-Device
timing	O
attacks	O
.	O
</s>
<s>
Entropy	B-Algorithm
can	O
affect	O
the	O
cryptography	O
(	O
TLS/SSL	O
)	O
of	O
a	O
server	B-Application
:	O
If	O
a	O
server	B-Application
fails	O
to	O
use	O
a	O
proper	O
source	O
of	O
randomness	O
,	O
the	O
keys	O
generated	O
by	O
the	O
server	B-Application
will	O
be	O
insecure	O
.	O
</s>
<s>
In	O
some	O
cases	O
a	O
cracker	O
(	O
malicious	O
attacker	O
)	O
can	O
guess	O
some	O
bits	O
of	O
entropy	B-Algorithm
from	O
the	O
output	O
of	O
a	O
pseudorandom	B-Algorithm
number	I-Algorithm
generator	I-Algorithm
(	O
PRNG	O
)	O
,	O
and	O
this	O
happens	O
when	O
not	O
enough	O
entropy	B-Algorithm
is	O
introduced	O
into	O
the	O
PRNG	O
.	O
</s>
<s>
Commonly	O
used	O
entropy	B-Algorithm
sources	O
include	O
the	O
mouse	B-Device
,	O
keyboard	B-Device
,	O
and	O
IDE	O
timings	O
,	O
but	O
there	O
are	O
other	O
potential	O
sources	O
.	O
</s>
<s>
For	O
example	O
,	O
one	O
could	O
collect	O
entropy	B-Algorithm
from	O
the	O
computer	O
's	O
microphone	B-Application
,	O
or	O
by	O
building	O
a	O
sensor	O
to	O
measure	O
the	O
air	O
turbulence	O
inside	O
a	O
disk	B-Device
drive	I-Device
.	O
</s>
<s>
For	O
Unix/BSD	O
derivatives	O
there	O
exists	O
a	O
USB	O
based	O
solution	O
that	O
utilizes	O
an	O
ARM	O
Cortex	O
CPU	B-Device
for	O
filtering	O
/	O
securing	O
the	O
bit	O
stream	O
generated	O
by	O
two	O
entropy	B-Algorithm
generator	O
sources	O
in	O
the	O
system	O
.	O
</s>
<s>
CloudFlare	B-Application
use	O
an	O
image	O
feed	O
from	O
a	O
rack	O
of	O
80	O
lava	O
lamps	O
as	O
an	O
additional	O
source	O
of	O
entropy	B-Algorithm
.	O
</s>
