<s>
The	O
Data	B-Algorithm
Encryption	I-Algorithm
Standard	I-Algorithm
(	O
DES	B-Algorithm
)	O
is	O
a	O
symmetric-key	B-Algorithm
algorithm	I-Algorithm
for	O
the	O
encryption	O
of	O
digital	O
data	O
.	O
</s>
<s>
Controversies	O
arose	O
from	O
classified	O
design	O
elements	O
,	O
a	O
relatively	O
short	O
key	O
length	O
of	O
the	O
symmetric-key	B-Algorithm
block	O
cipher	B-Application
design	O
,	O
and	O
the	O
involvement	O
of	O
the	O
NSA	O
,	O
raising	O
suspicions	O
about	O
a	O
backdoor	O
.	O
</s>
<s>
The	O
S-boxes	B-Algorithm
that	O
had	O
prompted	O
those	O
suspicions	O
were	O
designed	O
by	O
the	O
NSA	O
to	O
remove	O
a	O
backdoor	O
they	O
secretly	O
knew	O
(	O
differential	O
cryptanalysis	O
)	O
.	O
</s>
<s>
However	O
,	O
the	O
NSA	O
also	O
ensured	O
that	O
the	O
key	O
size	O
was	O
drastically	O
reduced	O
so	O
that	O
they	O
could	O
break	O
the	O
cipher	B-Application
by	O
brute	O
force	O
attack	O
.	O
</s>
<s>
The	O
intense	O
academic	O
scrutiny	O
the	O
algorithm	O
received	O
over	O
time	O
led	O
to	O
the	O
modern	O
understanding	O
of	O
block	O
ciphers	B-Application
and	O
their	O
cryptanalysis	O
.	O
</s>
<s>
DES	B-Algorithm
is	O
insecure	O
due	O
to	O
the	O
relatively	O
short	O
56-bit	O
key	O
size	O
.	O
</s>
<s>
In	O
January	O
1999	O
,	O
distributed.net	O
and	O
the	O
Electronic	O
Frontier	O
Foundation	O
collaborated	O
to	O
publicly	O
break	O
a	O
DES	B-Algorithm
key	O
in	O
22	O
hours	O
and	O
15	O
minutes	O
(	O
see	O
chronology	O
)	O
.	O
</s>
<s>
There	O
are	O
also	O
some	O
analytical	O
results	O
which	O
demonstrate	O
theoretical	O
weaknesses	O
in	O
the	O
cipher	B-Application
,	O
although	O
they	O
are	O
infeasible	O
in	O
practice	O
.	O
</s>
<s>
The	O
algorithm	O
is	O
believed	O
to	O
be	O
practically	O
secure	O
in	O
the	O
form	O
of	O
Triple	B-Algorithm
DES	I-Algorithm
,	O
although	O
there	O
are	O
theoretical	O
attacks	O
.	O
</s>
<s>
This	O
cipher	B-Application
has	O
been	O
superseded	O
by	O
the	O
Advanced	B-Algorithm
Encryption	I-Algorithm
Standard	I-Algorithm
(	O
AES	B-Algorithm
)	O
.	O
</s>
<s>
DES	B-Algorithm
has	O
been	O
withdrawn	O
as	O
a	O
standard	O
by	O
the	O
National	O
Institute	O
of	O
Standards	O
and	O
Technology	O
.	O
</s>
<s>
Some	O
documents	O
distinguish	O
between	O
the	O
DES	B-Algorithm
standard	O
and	O
its	O
algorithm	O
,	O
referring	O
to	O
the	O
algorithm	O
as	O
the	O
DEA	O
(	O
Data	B-Algorithm
Encryption	I-Algorithm
Algorithm	I-Algorithm
)	O
.	O
</s>
<s>
The	O
origins	O
of	O
DES	B-Algorithm
date	O
to	O
1972	O
,	O
when	O
a	O
National	O
Bureau	O
of	O
Standards	O
study	O
of	O
US	O
government	O
computer	O
security	O
identified	O
a	O
need	O
for	O
a	O
government-wide	O
standard	O
for	O
encrypting	O
unclassified	O
,	O
sensitive	O
information	O
.	O
</s>
<s>
Atalla	O
was	O
an	O
early	O
competitor	O
to	O
IBM	O
in	O
the	O
banking	O
market	O
,	O
and	O
was	O
cited	O
as	O
an	O
influence	O
by	O
IBM	O
employees	O
who	O
worked	O
on	O
the	O
DES	B-Algorithm
standard	O
.	O
</s>
<s>
On	O
15	O
May	O
1973	O
,	O
after	O
consulting	O
with	O
the	O
NSA	O
,	O
NBS	O
solicited	O
proposals	O
for	O
a	O
cipher	B-Application
that	O
would	O
meet	O
rigorous	O
design	O
criteria	O
.	O
</s>
<s>
This	O
time	O
,	O
IBM	O
submitted	O
a	O
candidate	O
which	O
was	O
deemed	O
acceptable	O
—	O
a	O
cipher	B-Application
developed	O
during	O
the	O
period	O
1973	O
–	O
1974	O
based	O
on	O
an	O
earlier	O
algorithm	O
,	O
Horst	O
Feistel	O
's	O
Lucifer	B-Algorithm
cipher	I-Algorithm
.	O
</s>
<s>
The	O
team	O
at	O
IBM	O
involved	O
in	O
cipher	B-Application
design	O
and	O
analysis	O
included	O
Feistel	O
,	O
Walter	O
Tuchman	O
,	O
Don	O
Coppersmith	O
,	O
Alan	O
Konheim	O
,	O
Carl	O
Meyer	O
,	O
Mike	O
Matyas	O
,	O
Roy	O
Adler	O
,	O
Edna	O
Grossman	O
,	O
Bill	O
Notz	O
,	O
Lynn	O
Smith	O
,	O
and	O
Bryant	O
Tuckerman	O
.	O
</s>
<s>
On	O
17	O
March	O
1975	O
,	O
the	O
proposed	O
DES	B-Algorithm
was	O
published	O
in	O
the	O
Federal	O
Register	O
.	O
</s>
<s>
There	O
was	O
criticism	O
received	O
from	O
public-key	B-Application
cryptography	I-Application
pioneers	O
Martin	O
Hellman	O
and	O
Whitfield	O
Diffie	O
,	O
citing	O
a	O
shortened	O
key	O
length	O
and	O
the	O
mysterious	O
"	O
S-boxes	B-Algorithm
"	O
as	O
evidence	O
of	O
improper	O
interference	O
from	O
the	O
NSA	O
.	O
</s>
<s>
Alan	O
Konheim	O
(	O
one	O
of	O
the	O
designers	O
of	O
DES	B-Algorithm
)	O
commented	O
,	O
"	O
We	O
sent	O
the	O
S-boxes	B-Algorithm
off	O
to	O
Washington	O
.	O
</s>
<s>
Another	O
member	O
of	O
the	O
DES	B-Algorithm
team	O
,	O
Walter	O
Tuchman	O
,	O
stated	O
"	O
We	O
developed	O
the	O
DES	B-Algorithm
algorithm	O
entirely	O
within	O
IBM	O
using	O
IBMers	O
.	O
</s>
<s>
Some	O
of	O
the	O
suspicions	O
about	O
hidden	O
weaknesses	O
in	O
the	O
S-boxes	B-Algorithm
were	O
allayed	O
in	O
1990	O
,	O
with	O
the	O
independent	O
discovery	O
and	O
open	O
publication	O
by	O
Eli	O
Biham	O
and	O
Adi	O
Shamir	O
of	O
differential	O
cryptanalysis	O
,	O
a	O
general	O
method	O
for	O
breaking	O
block	O
ciphers	B-Application
.	O
</s>
<s>
The	O
S-boxes	B-Algorithm
of	O
DES	B-Algorithm
were	O
much	O
more	O
resistant	O
to	O
the	O
attack	O
than	O
if	O
they	O
had	O
been	O
chosen	O
at	O
random	O
,	O
strongly	O
suggesting	O
that	O
IBM	O
knew	O
about	O
the	O
technique	O
in	O
the	O
1970s	O
.	O
</s>
<s>
This	O
was	O
indeed	O
the	O
case	O
;	O
in	O
1994	O
,	O
Don	O
Coppersmith	O
published	O
some	O
of	O
the	O
original	O
design	O
criteria	O
for	O
the	O
S-boxes	B-Algorithm
.	O
</s>
<s>
Bruce	O
Schneier	O
observed	O
that	O
"	O
It	O
took	O
the	O
academic	O
community	O
two	O
decades	O
to	O
figure	O
out	O
that	O
the	O
NSA	O
'	O
tweaks	O
 '	O
actually	O
improved	O
the	O
security	O
of	O
DES.	O
"	O
</s>
<s>
Despite	O
the	O
criticisms	O
,	O
DES	B-Algorithm
was	O
approved	O
as	O
a	O
federal	O
standard	O
in	O
November	O
1976	O
,	O
and	O
published	O
on	O
15	O
January	O
1977	O
as	O
FIPS	O
PUB	O
46	O
,	O
authorized	O
for	O
use	O
on	O
all	O
unclassified	O
data	O
.	O
</s>
<s>
It	O
was	O
subsequently	O
reaffirmed	O
as	O
the	O
standard	O
in	O
1983	O
,	O
1988	O
(	O
revised	O
as	O
FIPS-46-1	O
)	O
,	O
1993	O
(	O
FIPS-46-2	O
)	O
,	O
and	O
again	O
in	O
1999	O
(	O
FIPS-46-3	O
)	O
,	O
the	O
latter	O
prescribing	O
"	O
Triple	B-Algorithm
DES	I-Algorithm
"	O
(	O
see	O
below	O
)	O
.	O
</s>
<s>
On	O
26	O
May	O
2002	O
,	O
DES	B-Algorithm
was	O
finally	O
superseded	O
by	O
the	O
Advanced	B-Algorithm
Encryption	I-Algorithm
Standard	I-Algorithm
(	O
AES	B-Algorithm
)	O
,	O
following	O
a	O
public	O
competition	O
.	O
</s>
<s>
On	O
19	O
May	O
2005	O
,	O
FIPS	O
46-3	O
was	O
officially	O
withdrawn	O
,	O
but	O
NIST	O
has	O
approved	O
Triple	B-Algorithm
DES	I-Algorithm
through	O
the	O
year	O
2030	O
for	O
sensitive	O
government	O
information	O
.	O
</s>
<s>
The	O
algorithm	O
is	O
also	O
specified	O
in	O
ANSI	O
X3.92	O
(	O
Today	O
X3	O
is	O
known	O
as	O
INCITS	O
and	O
ANSI	O
X3.92	O
as	O
ANSI	O
INCITS	O
92	O
)	O
,	O
NIST	O
SP	O
800-67	O
and	O
ISO/IEC	O
18033-3	O
(	O
as	O
a	O
component	O
of	O
TDEA	B-Algorithm
)	O
.	O
</s>
<s>
Another	O
theoretical	O
attack	O
,	O
linear	O
cryptanalysis	O
,	O
was	O
published	O
in	O
1994	O
,	O
but	O
it	O
was	O
the	O
Electronic	O
Frontier	O
Foundation	O
's	O
DES	B-Algorithm
cracker	I-Algorithm
in	O
1998	O
that	O
demonstrated	O
that	O
DES	B-Algorithm
could	O
be	O
attacked	O
very	O
practically	O
,	O
and	O
highlighted	O
the	O
need	O
for	O
a	O
replacement	O
algorithm	O
.	O
</s>
<s>
The	O
introduction	O
of	O
DES	B-Algorithm
is	O
considered	O
to	O
have	O
been	O
a	O
catalyst	O
for	O
the	O
academic	O
study	O
of	O
cryptography	O
,	O
particularly	O
of	O
methods	O
to	O
crack	O
block	O
ciphers	B-Application
.	O
</s>
<s>
According	O
to	O
a	O
NIST	O
retrospective	O
about	O
DES	B-Algorithm
,	O
</s>
<s>
The	O
DES	B-Algorithm
can	O
be	O
said	O
to	O
have	O
"	O
jump-started	O
"	O
the	O
nonmilitary	O
study	O
and	O
development	O
of	O
encryption	O
algorithms	O
.	O
</s>
<s>
A	O
generation	O
of	O
cryptanalysts	O
has	O
cut	O
its	O
teeth	O
analyzing	O
(	O
that	O
is	O
,	O
trying	O
to	O
"	O
crack	O
"	O
)	O
the	O
DES	B-Algorithm
algorithm	O
.	O
</s>
<s>
In	O
the	O
words	O
of	O
cryptographer	O
Bruce	O
Schneier	O
,	O
"	O
DES	B-Algorithm
did	O
more	O
to	O
galvanize	O
the	O
field	O
of	O
cryptanalysis	O
than	O
anything	O
else	O
.	O
</s>
<s>
An	O
astonishing	O
share	O
of	O
the	O
open	O
literature	O
in	O
cryptography	O
in	O
the	O
1970s	O
and	O
1980s	O
dealt	O
with	O
the	O
DES	B-Algorithm
,	O
and	O
the	O
DES	B-Algorithm
is	O
the	O
standard	O
against	O
which	O
every	O
symmetric	B-Algorithm
key	I-Algorithm
algorithm	I-Algorithm
since	O
has	O
been	O
compared	O
.	O
</s>
<s>
Date	O
Year	O
Event	O
15	O
May	O
1973	O
NBS	O
publishes	O
a	O
first	O
request	O
for	O
a	O
standard	O
encryption	O
algorithm	O
27	O
August	O
1974	O
NBS	O
publishes	O
a	O
second	O
request	O
for	O
encryption	O
algorithms	O
17	O
March	O
1975	O
DES	B-Algorithm
is	O
published	O
in	O
the	O
Federal	O
Register	O
for	O
comment	O
August	O
1976	O
First	O
workshop	O
on	O
DES	B-Algorithm
September	O
1976	O
Second	O
workshop	O
,	O
discussing	O
mathematical	O
foundation	O
of	O
DES	B-Algorithm
November	O
1976	O
DES	B-Algorithm
is	O
approved	O
as	O
a	O
standard	O
15	O
January	O
1977	O
DES	B-Algorithm
is	O
published	O
as	O
a	O
FIPS	O
standard	O
FIPS	O
PUB	O
46	O
June	O
1977	O
Diffie	O
and	O
Hellman	O
argue	O
that	O
the	O
DES	B-Algorithm
cipher	B-Application
can	O
be	O
broken	O
by	O
brute	O
force	O
.	O
</s>
<s>
1983	O
DES	B-Algorithm
is	O
reaffirmed	O
for	O
the	O
first	O
time	O
1986	O
Videocipher	O
II	O
,	O
a	O
TV	O
satellite	O
scrambling	O
system	O
based	O
upon	O
DES	B-Algorithm
,	O
begins	O
use	O
by	O
HBO	O
22	O
January	O
1988	O
DES	B-Algorithm
is	O
reaffirmed	O
for	O
the	O
second	O
time	O
as	O
FIPS	O
46-1	O
,	O
superseding	O
FIPS	O
PUB	O
46	O
July	O
1991	O
Biham	O
and	O
Shamir	O
rediscover	O
differential	O
cryptanalysis	O
,	O
and	O
apply	O
it	O
to	O
a	O
15-round	O
DES-like	O
cryptosystem	O
.	O
</s>
<s>
30	O
December	O
1993	O
DES	B-Algorithm
is	O
reaffirmed	O
for	O
the	O
third	O
time	O
as	O
FIPS	O
46-2	O
1994	O
The	O
first	O
experimental	O
cryptanalysis	O
of	O
DES	B-Algorithm
is	O
performed	O
using	O
linear	O
cryptanalysis	O
(	O
Matsui	O
,	O
1994	O
)	O
.	O
</s>
<s>
June	O
1997	O
The	O
DESCHALL	B-Algorithm
Project	I-Algorithm
breaks	O
a	O
message	O
encrypted	O
with	O
DES	B-Algorithm
for	O
the	O
first	O
time	O
in	O
public	O
.	O
</s>
<s>
July	O
1998	O
The	O
EFF	O
's	O
DES	B-Algorithm
cracker	I-Algorithm
(	O
Deep	B-Algorithm
Crack	I-Algorithm
)	O
breaks	O
a	O
DES	B-Algorithm
key	O
in	O
56	O
hours	O
.	O
</s>
<s>
January	O
1999	O
Together	O
,	O
Deep	B-Algorithm
Crack	I-Algorithm
and	O
distributed.net	O
break	O
a	O
DES	B-Algorithm
key	O
in	O
22	O
hours	O
and	O
15	O
minutes	O
.	O
</s>
<s>
25	O
October	O
1999	O
DES	B-Algorithm
is	O
reaffirmed	O
for	O
the	O
fourth	O
time	O
as	O
FIPS	O
46-3	O
,	O
which	O
specifies	O
the	O
preferred	O
use	O
of	O
Triple	B-Algorithm
DES	I-Algorithm
,	O
with	O
single	B-Algorithm
DES	I-Algorithm
permitted	O
only	O
in	O
legacy	O
systems	O
.	O
</s>
<s>
26	O
November	O
2001	O
The	O
Advanced	B-Algorithm
Encryption	I-Algorithm
Standard	I-Algorithm
is	O
published	O
in	O
FIPS	B-Algorithm
197	I-Algorithm
26	O
May	O
2002	O
The	O
AES	B-Algorithm
becomes	O
effective	O
26	O
July	O
2004	O
The	O
withdrawal	O
of	O
FIPS	O
46-3	O
(	O
and	O
a	O
couple	O
of	O
related	O
standards	O
)	O
is	O
proposed	O
in	O
the	O
Federal	O
Register	O
19	O
May	O
2005	O
NIST	O
withdraws	O
FIPS	O
46-3	O
(	O
see	O
Federal	O
Register	O
vol	O
70	O
,	O
number	O
96	O
)	O
April	O
2006	O
The	O
FPGA-based	O
parallel	O
machine	O
COPACOBANA	O
of	O
the	O
Universities	O
of	O
Bochum	O
and	O
Kiel	O
,	O
Germany	O
,	O
breaks	O
DES	B-Algorithm
in	O
9	O
days	O
at	O
a	O
$	O
10,000	O
hardware	O
cost.S.	O
</s>
<s>
Kumar	O
,	O
C	O
.	O
Paar	O
,	O
J	O
.	O
Pelzl	O
,	O
G	O
.	O
Pfeiffer	O
,	O
A	O
.	O
Rupp	O
,	O
M	O
.	O
Schimmler	O
,	O
"	O
How	O
to	O
Break	O
DES	B-Algorithm
for	O
Euro	O
8,980	O
"	O
.	O
</s>
<s>
August	O
2016	O
The	O
Open	O
Source	O
password	O
cracking	O
software	O
hashcat	O
added	O
in	O
DES	B-Algorithm
brute	O
force	O
searching	O
on	O
general	O
purpose	O
GPUs	O
.	O
</s>
<s>
Benchmarking	O
shows	O
a	O
single	O
off	O
the	O
shelf	O
Nvidia	B-Device
GeForce	I-Device
GTX	I-Device
1080	I-Device
Ti	I-Device
GPU	O
costing	O
$1000	O
USD	O
recovers	O
a	O
key	O
in	O
an	O
average	O
of	O
15	O
days	O
(	O
full	O
exhaustive	O
search	O
taking	O
30	O
days	O
)	O
.	O
</s>
<s>
Systems	O
have	O
been	O
built	O
with	O
eight	O
GTX	B-Device
1080	I-Device
Ti	I-Device
GPUs	O
which	O
can	O
recover	O
a	O
key	O
in	O
an	O
average	O
of	O
under	O
2	O
days	O
.	O
</s>
<s>
July	O
2017	O
A	O
chosen-plaintext	O
attack	O
utilizing	O
a	O
rainbow	B-Algorithm
table	I-Algorithm
can	O
recover	O
the	O
DES	B-Algorithm
key	O
for	O
a	O
single	O
specific	O
chosen	O
plaintext	O
1122334455667788	O
in	O
25	O
seconds	O
.	O
</s>
<s>
A	O
new	O
rainbow	B-Algorithm
table	I-Algorithm
has	O
to	O
be	O
calculated	O
per	O
plaintext	O
.	O
</s>
<s>
A	O
limited	O
set	O
of	O
rainbow	B-Algorithm
tables	I-Algorithm
have	O
been	O
made	O
available	O
for	O
download	O
.	O
</s>
<s>
DES	B-Algorithm
is	O
the	O
archetypal	O
block	O
cipher	B-Application
—	O
an	O
algorithm	O
that	O
takes	O
a	O
fixed-length	O
string	O
of	O
plaintext	O
bits	O
and	O
transforms	O
it	O
through	O
a	O
series	O
of	O
complicated	O
operations	O
into	O
another	O
ciphertext	O
bitstring	O
of	O
the	O
same	O
length	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
DES	B-Algorithm
,	O
the	O
block	O
size	O
is	O
64	O
bits	O
.	O
</s>
<s>
DES	B-Algorithm
also	O
uses	O
a	O
key	O
to	O
customize	O
the	O
transformation	O
,	O
so	O
that	O
decryption	O
can	O
supposedly	O
only	O
be	O
performed	O
by	O
those	O
who	O
know	O
the	O
particular	O
key	O
used	O
to	O
encrypt	O
.	O
</s>
<s>
Eight	O
bits	O
are	O
used	O
solely	O
for	O
checking	O
parity	B-Error_Name
,	O
and	O
are	O
thereafter	O
discarded	O
.	O
</s>
<s>
The	O
key	O
is	O
nominally	O
stored	O
or	O
transmitted	O
as	O
8	O
bytes	B-Application
,	O
each	O
with	O
odd	B-Error_Name
parity	I-Error_Name
.	O
</s>
<s>
Like	O
other	O
block	O
ciphers	B-Application
,	O
DES	B-Algorithm
by	O
itself	O
is	O
not	O
a	O
secure	O
means	O
of	O
encryption	O
,	O
but	O
must	O
instead	O
be	O
used	O
in	O
a	O
mode	B-Algorithm
of	I-Algorithm
operation	I-Algorithm
.	O
</s>
<s>
FIPS-81	O
specifies	O
several	O
modes	O
for	O
use	O
with	O
DES	B-Algorithm
.	O
</s>
<s>
Further	O
comments	O
on	O
the	O
usage	O
of	O
DES	B-Algorithm
are	O
contained	O
in	O
FIPS-74	O
.	O
</s>
<s>
There	O
is	O
also	O
an	O
initial	O
and	O
final	O
permutation	B-Algorithm
,	O
termed	O
IP	O
and	O
FP	O
,	O
which	O
are	O
inverses	O
(	O
IP	O
"	O
undoes	O
"	O
the	O
action	O
of	O
FP	O
,	O
and	O
vice	O
versa	O
)	O
.	O
</s>
<s>
Before	O
the	O
main	O
rounds	O
,	O
the	O
block	O
is	O
divided	O
into	O
two	O
32-bit	O
halves	O
and	O
processed	O
alternately	O
;	O
this	O
criss-crossing	O
is	O
known	O
as	O
the	O
Feistel	B-Algorithm
scheme	I-Algorithm
.	O
</s>
<s>
The	O
Feistel	B-Algorithm
structure	I-Algorithm
ensures	O
that	O
decryption	O
and	O
encryption	O
are	O
very	O
similar	O
processes	O
—	O
the	O
only	O
difference	O
is	O
that	O
the	O
subkeys	O
are	O
applied	O
in	O
the	O
reverse	O
order	O
when	O
decrypting	O
.	O
</s>
<s>
The	O
⊕	O
symbol	O
denotes	O
the	O
exclusive-OR	O
(	O
XOR	B-Application
)	O
operation	O
.	O
</s>
<s>
The	O
F-function	B-Algorithm
scrambles	O
half	O
a	O
block	O
together	O
with	O
some	O
of	O
the	O
key	O
.	O
</s>
<s>
The	O
output	O
from	O
the	O
F-function	B-Algorithm
is	O
then	O
combined	O
with	O
the	O
other	O
half	O
of	O
the	O
block	O
,	O
and	O
the	O
halves	O
are	O
swapped	O
before	O
the	O
next	O
round	O
.	O
</s>
<s>
After	O
the	O
final	O
round	O
,	O
the	O
halves	O
are	O
swapped	O
;	O
this	O
is	O
a	O
feature	O
of	O
the	O
Feistel	B-Algorithm
structure	I-Algorithm
which	O
makes	O
encryption	O
and	O
decryption	O
similar	O
processes	O
.	O
</s>
<s>
The	O
F-function	B-Algorithm
,	O
depicted	O
in	O
Figure	O
2	O
,	O
operates	O
on	O
half	O
a	O
block	O
(	O
32	O
bits	O
)	O
at	O
a	O
time	O
and	O
consists	O
of	O
four	O
stages	O
:	O
</s>
<s>
Expansion	O
:	O
the	O
32-bit	O
half-block	O
is	O
expanded	O
to	O
48	O
bits	O
using	O
the	O
expansion	O
permutation	B-Algorithm
,	O
denoted	O
E	O
in	O
the	O
diagram	O
,	O
by	O
duplicating	O
half	O
of	O
the	O
bits	O
.	O
</s>
<s>
Key	O
mixing	O
:	O
the	O
result	O
is	O
combined	O
with	O
a	O
subkey	O
using	O
an	O
XOR	B-Application
operation	O
.	O
</s>
<s>
Sixteen	O
48-bit	O
subkeys	O
—	O
one	O
for	O
each	O
round	O
—	O
are	O
derived	O
from	O
the	O
main	O
key	O
using	O
the	O
key	B-Algorithm
schedule	I-Algorithm
(	O
described	O
below	O
)	O
.	O
</s>
<s>
Substitution	O
:	O
after	O
mixing	O
in	O
the	O
subkey	O
,	O
the	O
block	O
is	O
divided	O
into	O
eight	O
6-bit	O
pieces	O
before	O
processing	O
by	O
the	O
S-boxes	B-Algorithm
,	O
or	O
substitution	B-Algorithm
boxes	I-Algorithm
.	O
</s>
<s>
Each	O
of	O
the	O
eight	O
S-boxes	B-Algorithm
replaces	O
its	O
six	O
input	O
bits	O
with	O
four	O
output	O
bits	O
according	O
to	O
a	O
non-linear	O
transformation	O
,	O
provided	O
in	O
the	O
form	O
of	O
a	O
lookup	B-Data_Structure
table	I-Data_Structure
.	O
</s>
<s>
The	O
S-boxes	B-Algorithm
provide	O
the	O
core	O
of	O
the	O
security	O
of	O
DES	B-Algorithm
—	O
without	O
them	O
,	O
the	O
cipher	B-Application
would	O
be	O
linear	O
,	O
and	O
trivially	O
breakable	O
.	O
</s>
<s>
Permutation	B-Algorithm
:	O
finally	O
,	O
the	O
32	O
outputs	O
from	O
the	O
S-boxes	B-Algorithm
are	O
rearranged	O
according	O
to	O
a	O
fixed	O
permutation	B-Algorithm
,	O
the	O
P-box	O
.	O
</s>
<s>
This	O
is	O
designed	O
so	O
that	O
,	O
after	O
permutation	B-Algorithm
,	O
the	O
bits	O
from	O
the	O
output	O
of	O
each	O
S-box	B-Algorithm
in	O
this	O
round	O
are	O
spread	O
across	O
four	O
different	O
S-boxes	B-Algorithm
in	O
the	O
next	O
round	O
.	O
</s>
<s>
The	O
alternation	O
of	O
substitution	O
from	O
the	O
S-boxes	B-Algorithm
,	O
and	O
permutation	B-Algorithm
of	O
bits	O
from	O
the	O
P-box	O
and	O
E-expansion	O
provides	O
so-called	O
"	O
confusion	O
and	O
diffusion	O
"	O
respectively	O
,	O
a	O
concept	O
identified	O
by	O
Claude	O
Shannon	O
in	O
the	O
1940s	O
as	O
a	O
necessary	O
condition	O
for	O
a	O
secure	O
yet	O
practical	O
cipher	B-Application
.	O
</s>
<s>
Figure	O
3	O
illustrates	O
the	O
key	B-Algorithm
schedule	I-Algorithm
for	O
encryption	O
—	O
the	O
algorithm	O
which	O
generates	O
the	O
subkeys	O
.	O
</s>
<s>
Initially	O
,	O
56	O
bits	O
of	O
the	O
key	O
are	O
selected	O
from	O
the	O
initial	O
64	O
by	O
Permuted	O
Choice	O
1	O
(	O
PC-1	O
)	O
—	O
the	O
remaining	O
eight	O
bits	O
are	O
either	O
discarded	O
or	O
used	O
as	O
parity	B-Error_Name
check	I-Error_Name
bits	O
.	O
</s>
<s>
The	O
key	B-Algorithm
schedule	I-Algorithm
for	O
decryption	O
is	O
similar	O
—	O
the	O
subkeys	O
are	O
in	O
reverse	O
order	O
compared	O
to	O
encryption	O
.	O
</s>
<s>
Pseudocode	B-Language
for	O
the	O
DES	B-Algorithm
algorithm	O
follows	O
.	O
</s>
<s>
Although	O
more	O
information	O
has	O
been	O
published	O
on	O
the	O
cryptanalysis	O
of	O
DES	B-Algorithm
than	O
any	O
other	O
block	O
cipher	B-Application
,	O
the	O
most	O
practical	O
attack	O
to	O
date	O
is	O
still	O
a	O
brute-force	O
approach	O
.	O
</s>
<s>
For	O
any	O
cipher	B-Application
,	O
the	O
most	O
basic	O
method	O
of	O
attack	O
is	O
brute	O
force	O
—	O
trying	O
every	O
possible	O
key	O
in	O
turn	O
.	O
</s>
<s>
For	O
DES	B-Algorithm
,	O
questions	O
were	O
raised	O
about	O
the	O
adequacy	O
of	O
its	O
key	O
size	O
early	O
on	O
,	O
even	O
before	O
it	O
was	O
adopted	O
as	O
a	O
standard	O
,	O
and	O
it	O
was	O
the	O
small	O
key	O
size	O
,	O
rather	O
than	O
theoretical	O
cryptanalysis	O
,	O
which	O
dictated	O
a	O
need	O
for	O
a	O
replacement	O
algorithm	O
.	O
</s>
<s>
In	O
academia	O
,	O
various	O
proposals	O
for	O
a	O
DES-cracking	O
machine	O
were	O
advanced	O
.	O
</s>
<s>
In	O
1977	O
,	O
Diffie	O
and	O
Hellman	O
proposed	O
a	O
machine	O
costing	O
an	O
estimated	O
US$20	O
million	O
which	O
could	O
find	O
a	O
DES	B-Algorithm
key	O
in	O
a	O
single	O
day	O
.	O
</s>
<s>
The	O
vulnerability	O
of	O
DES	B-Algorithm
was	O
practically	O
demonstrated	O
in	O
the	O
late	O
1990s	O
.	O
</s>
<s>
In	O
1997	O
,	O
RSA	O
Security	O
sponsored	O
a	O
series	O
of	O
contests	O
,	O
offering	O
a	O
$	O
10,000	O
prize	O
to	O
the	O
first	O
team	O
that	O
broke	O
a	O
message	O
encrypted	O
with	O
DES	B-Algorithm
for	O
the	O
contest	O
.	O
</s>
<s>
That	O
contest	O
was	O
won	O
by	O
the	O
DESCHALL	B-Algorithm
Project	I-Algorithm
,	O
led	O
by	O
Rocke	O
Verser	O
,	O
Matt	O
Curtin	O
,	O
and	O
Justin	O
Dolske	O
,	O
using	O
idle	O
cycles	O
of	O
thousands	O
of	O
computers	O
across	O
the	O
Internet	O
.	O
</s>
<s>
The	O
feasibility	O
of	O
cracking	O
DES	B-Algorithm
quickly	O
was	O
demonstrated	O
in	O
1998	O
when	O
a	O
custom	O
DES-cracker	O
was	O
built	O
by	O
the	O
Electronic	O
Frontier	O
Foundation	O
(	O
EFF	O
)	O
,	O
a	O
cyberspace	O
civil	O
rights	O
group	O
,	O
at	O
the	O
cost	O
of	O
approximately	O
US$	O
250,000	O
(	O
see	O
EFF	B-Algorithm
DES	I-Algorithm
cracker	I-Algorithm
)	O
.	O
</s>
<s>
Their	O
motivation	O
was	O
to	O
show	O
that	O
DES	B-Algorithm
was	O
breakable	O
in	O
practice	O
as	O
well	O
as	O
in	O
theory	O
:	O
"	O
There	O
are	O
many	O
people	O
who	O
will	O
not	O
believe	O
a	O
truth	O
until	O
they	O
can	O
see	O
it	O
with	O
their	O
own	O
eyes	O
.	O
</s>
<s>
Showing	O
them	O
a	O
physical	O
machine	O
that	O
can	O
crack	O
DES	B-Algorithm
in	O
a	O
few	O
days	O
is	O
the	O
only	O
way	O
to	O
convince	O
some	O
people	O
that	O
they	O
really	O
cannot	O
trust	O
their	O
security	O
to	O
DES.	O
"	O
</s>
<s>
The	O
next	O
confirmed	O
DES	B-Algorithm
cracker	I-Algorithm
was	O
the	O
COPACOBANA	O
machine	O
built	O
in	O
2006	O
by	O
teams	O
of	O
the	O
Universities	O
of	O
Bochum	O
and	O
Kiel	O
,	O
both	O
in	O
Germany	O
.	O
</s>
<s>
120	O
of	O
these	O
field-programmable	B-Architecture
gate	I-Architecture
arrays	I-Architecture
(	O
FPGAs	B-Architecture
)	O
of	O
type	O
XILINX	O
Spartan-3	O
1000	O
run	O
in	O
parallel	O
.	O
</s>
<s>
They	O
are	O
grouped	O
in	O
20	O
DIMM	O
modules	O
,	O
each	O
containing	O
6	O
FPGAs	B-Architecture
.	O
</s>
<s>
Since	O
2007	O
,	O
SciEngines	B-Device
GmbH	I-Device
,	O
a	O
spin-off	O
company	O
of	O
the	O
two	O
project	O
partners	O
of	O
COPACOBANA	O
has	O
enhanced	O
and	O
developed	O
successors	O
of	O
COPACOBANA	O
.	O
</s>
<s>
In	O
2008	O
their	O
COPACOBANA	O
RIVYERA	O
reduced	O
the	O
time	O
to	O
break	O
DES	B-Algorithm
to	O
less	O
than	O
one	O
day	O
,	O
using	O
128	O
Spartan-3	O
5000	O
's	O
.	O
</s>
<s>
SciEngines	O
RIVYERA	O
held	O
the	O
record	O
in	O
brute-force	O
breaking	O
DES	B-Algorithm
,	O
having	O
utilized	O
128	O
Spartan-3	O
5000	O
FPGAs	B-Architecture
.	O
</s>
<s>
In	O
2012	O
,	O
David	O
Hulton	O
and	O
Moxie	O
Marlinspike	O
announced	O
a	O
system	O
with	O
48	O
Xilinx	O
Virtex-6	O
LX240T	O
FPGAs	B-Architecture
,	O
each	O
FPGA	B-Architecture
containing	O
40	O
fully	O
pipelined	O
DES	B-Algorithm
cores	O
running	O
at	O
400MHz	O
,	O
for	O
a	O
total	O
capacity	O
of	O
768	O
gigakeys/sec	O
.	O
</s>
<s>
The	O
system	O
can	O
exhaustively	O
search	O
the	O
entire	O
56-bit	O
DES	B-Algorithm
key	O
space	O
in	O
about	O
26	O
hours	O
and	O
this	O
service	O
is	O
offered	O
for	O
a	O
fee	O
online	O
.	O
</s>
<s>
There	O
are	O
three	O
attacks	O
known	O
that	O
can	O
break	O
the	O
full	O
16	O
rounds	O
of	O
DES	B-Algorithm
with	O
less	O
complexity	O
than	O
a	O
brute-force	O
search	O
:	O
differential	O
cryptanalysis	O
(	O
DC	O
)	O
,	O
linear	O
cryptanalysis	O
(	O
LC	O
)	O
,	O
and	O
Davies	B-Algorithm
 '	I-Algorithm
attack	I-Algorithm
.	O
</s>
<s>
DES	B-Algorithm
was	O
designed	O
to	O
be	O
resistant	O
to	O
DC	O
.	O
</s>
<s>
Linear	O
cryptanalysis	O
was	O
discovered	O
by	O
Mitsuru	O
Matsui	O
,	O
and	O
needs	O
243	O
known	O
plaintexts	O
(	O
Matsui	O
,	O
1993	O
)	O
;	O
the	O
method	O
was	O
implemented	O
(	O
Matsui	O
,	O
1994	O
)	O
,	O
and	O
was	O
the	O
first	O
experimental	O
cryptanalysis	O
of	O
DES	B-Algorithm
to	O
be	O
reported	O
.	O
</s>
<s>
There	O
is	O
no	O
evidence	O
that	O
DES	B-Algorithm
was	O
tailored	O
to	O
be	O
resistant	O
to	O
this	O
type	O
of	O
attack	O
.	O
</s>
<s>
Junod	O
(	O
2001	O
)	O
performed	O
several	O
experiments	O
to	O
determine	O
the	O
actual	O
time	O
complexity	O
of	O
linear	O
cryptanalysis	O
,	O
and	O
reported	O
that	O
it	O
was	O
somewhat	O
faster	O
than	O
predicted	O
,	O
requiring	O
time	O
equivalent	O
to	O
239	O
–	O
241	O
DES	B-Algorithm
evaluations	O
.	O
</s>
<s>
Improved	O
Davies	B-Algorithm
 '	I-Algorithm
attack	I-Algorithm
:	O
while	O
linear	O
and	O
differential	O
cryptanalysis	O
are	O
general	O
techniques	O
and	O
can	O
be	O
applied	O
to	O
a	O
number	O
of	O
schemes	O
,	O
Davies	B-Algorithm
 '	I-Algorithm
attack	I-Algorithm
is	O
a	O
specialized	O
technique	O
for	O
DES	B-Algorithm
,	O
first	O
suggested	O
by	O
Donald	O
Davies	O
in	O
the	O
eighties	O
,	O
and	O
improved	O
by	O
Biham	O
and	O
Biryukov	O
(	O
1997	O
)	O
.	O
</s>
<s>
There	O
have	O
also	O
been	O
attacks	O
proposed	O
against	O
reduced-round	O
versions	O
of	O
the	O
cipher	B-Application
,	O
that	O
is	O
,	O
versions	O
of	O
DES	B-Algorithm
with	O
fewer	O
than	O
16	O
rounds	O
.	O
</s>
<s>
An	O
enhanced	O
version	O
of	O
the	O
attack	O
can	O
break	O
9-round	O
DES	B-Algorithm
with	O
215.8	O
chosen	O
plaintexts	O
and	O
has	O
a	O
229.2	O
time	O
complexity	O
(	O
Biham	O
and	O
others	O
,	O
2002	O
)	O
.	O
</s>
<s>
By	O
definition	O
,	O
this	O
property	O
also	O
applies	O
to	O
TDES	B-Algorithm
cipher	B-Application
.	O
</s>
<s>
DES	B-Algorithm
also	O
has	O
four	O
so-called	O
weak	O
keys	O
.	O
</s>
<s>
Encryption	O
(	O
E	O
)	O
and	O
decryption	O
(	O
D	O
)	O
under	O
a	O
weak	O
key	O
have	O
the	O
same	O
effect	O
(	O
see	O
involution	B-Algorithm
)	O
:	O
</s>
<s>
DES	B-Algorithm
has	O
also	O
been	O
proved	O
not	O
to	O
be	O
a	O
group	O
,	O
or	O
more	O
precisely	O
,	O
the	O
set	O
(	O
for	O
all	O
possible	O
keys	O
)	O
under	O
functional	B-Application
composition	I-Application
is	O
not	O
a	O
group	O
,	O
nor	O
"	O
close	O
"	O
to	O
being	O
a	O
group	O
.	O
</s>
<s>
This	O
was	O
an	O
open	O
question	O
for	O
some	O
time	O
,	O
and	O
if	O
it	O
had	O
been	O
the	O
case	O
,	O
it	O
would	O
have	O
been	O
possible	O
to	O
break	O
DES	B-Algorithm
,	O
and	O
multiple	O
encryption	B-Algorithm
modes	I-Algorithm
such	O
as	O
Triple	B-Algorithm
DES	I-Algorithm
would	O
not	O
increase	O
the	O
security	O
,	O
because	O
repeated	O
encryption	O
(	O
and	O
decryptions	O
)	O
under	O
different	O
keys	O
would	O
be	O
equivalent	O
to	O
encryption	O
under	O
another	O
,	O
single	O
key	O
.	O
</s>
<s>
Simplified	O
DES	B-Algorithm
(	O
SDES	O
)	O
was	O
designed	O
for	O
educational	O
purposes	O
only	O
,	O
to	O
help	O
students	O
learn	O
about	O
modern	O
cryptanalytic	O
techniques	O
.	O
</s>
<s>
SDES	O
has	O
similar	O
structure	O
and	O
properties	O
to	O
DES	B-Algorithm
,	O
but	O
has	O
been	O
simplified	O
to	O
make	O
it	O
much	O
easier	O
to	O
perform	O
encryption	O
and	O
decryption	O
by	O
hand	O
with	O
pencil	O
and	O
paper	O
.	O
</s>
<s>
Some	O
people	O
feel	O
that	O
learning	O
SDES	O
gives	O
insight	O
into	O
DES	B-Algorithm
and	O
other	O
block	O
ciphers	B-Application
,	O
and	O
insight	O
into	O
various	O
cryptanalytic	O
attacks	O
against	O
them	O
.	O
</s>
<s>
Concerns	O
about	O
security	O
and	O
the	O
relatively	O
slow	O
operation	O
of	O
DES	B-Algorithm
in	O
software	O
motivated	O
researchers	O
to	O
propose	O
a	O
variety	O
of	O
alternative	O
block	O
cipher	B-Application
designs	O
,	O
which	O
started	O
to	O
appear	O
in	O
the	O
late	O
1980s	O
and	O
early	O
1990s	O
:	O
examples	O
include	O
RC5	B-Algorithm
,	O
Blowfish	B-Algorithm
,	O
IDEA	B-Algorithm
,	O
NewDES	B-Algorithm
,	O
SAFER	B-Algorithm
,	O
CAST5	B-Algorithm
and	O
FEAL	B-Algorithm
.	O
</s>
<s>
Most	O
of	O
these	O
designs	O
kept	O
the	O
64-bit	O
block	O
size	O
of	O
DES	B-Algorithm
,	O
and	O
could	O
act	O
as	O
a	O
"	O
drop-in	O
"	O
replacement	O
,	O
although	O
they	O
typically	O
used	O
a	O
64-bit	O
or	O
128-bit	O
key	O
.	O
</s>
<s>
In	O
the	O
Soviet	O
Union	O
the	O
GOST	B-Algorithm
28147-89	I-Algorithm
algorithm	O
was	O
introduced	O
,	O
with	O
a	O
64-bit	O
block	O
size	O
and	O
a	O
256-bit	O
key	O
,	O
which	O
was	O
also	O
used	O
in	O
Russia	O
later	O
.	O
</s>
<s>
DES	B-Algorithm
itself	O
can	O
be	O
adapted	O
and	O
reused	O
in	O
a	O
more	O
secure	O
scheme	O
.	O
</s>
<s>
Many	O
former	O
DES	B-Algorithm
users	O
now	O
use	O
Triple	B-Algorithm
DES	I-Algorithm
(	O
TDES	B-Algorithm
)	O
which	O
was	O
described	O
and	O
analysed	O
by	O
one	O
of	O
DES	B-Algorithm
's	O
patentees	O
(	O
see	O
FIPS	O
Pub	O
46-3	O
)	O
;	O
it	O
involves	O
applying	O
DES	B-Algorithm
three	O
times	O
with	O
two	O
(	O
2TDES	O
)	O
or	O
three	O
(	O
3TDES	O
)	O
different	O
keys	O
.	O
</s>
<s>
TDES	B-Algorithm
is	O
regarded	O
as	O
adequately	O
secure	O
,	O
although	O
it	O
is	O
quite	O
slow	O
.	O
</s>
<s>
A	O
less	O
computationally	O
expensive	O
alternative	O
is	O
DES-X	B-Algorithm
,	O
which	O
increases	O
the	O
key	O
size	O
by	O
XORing	O
extra	O
key	O
material	O
before	O
and	O
after	O
DES	B-Algorithm
.	O
</s>
<s>
GDES	B-Algorithm
was	O
a	O
DES	B-Algorithm
variant	O
proposed	O
as	O
a	O
way	O
to	O
speed	O
up	O
encryption	O
,	O
but	O
it	O
was	O
shown	O
to	O
be	O
susceptible	O
to	O
differential	O
cryptanalysis	O
.	O
</s>
<s>
On	O
January	O
2	O
,	O
1997	O
,	O
NIST	O
announced	O
that	O
they	O
wished	O
to	O
choose	O
a	O
successor	O
to	O
DES	B-Algorithm
.	O
</s>
<s>
In	O
2001	O
,	O
after	O
an	O
international	O
competition	O
,	O
NIST	O
selected	O
a	O
new	O
cipher	B-Application
,	O
the	O
Advanced	B-Algorithm
Encryption	I-Algorithm
Standard	I-Algorithm
(	O
AES	B-Algorithm
)	O
,	O
as	O
a	O
replacement	O
.	O
</s>
<s>
The	O
algorithm	O
which	O
was	O
selected	O
as	O
the	O
AES	B-Algorithm
was	O
submitted	O
by	O
its	O
designers	O
under	O
the	O
name	O
Rijndael	B-Algorithm
.	O
</s>
<s>
Other	O
finalists	O
in	O
the	O
NIST	O
AES	B-Algorithm
competition	O
included	O
RC6	B-Algorithm
,	O
Serpent	B-Algorithm
,	O
MARS	B-Algorithm
,	O
and	O
Twofish	B-Algorithm
.	O
</s>
