<s>
In	O
cryptography	O
,	O
DES-X	B-Algorithm
(	O
or	O
DESX	B-Algorithm
)	O
is	O
a	O
variant	O
on	O
the	O
DES	B-Algorithm
(	O
Data	B-Algorithm
Encryption	I-Algorithm
Standard	I-Algorithm
)	O
symmetric-key	B-Algorithm
block	O
cipher	O
intended	O
to	O
increase	O
the	O
complexity	O
of	O
a	O
brute-force	O
attack	O
.	O
</s>
<s>
The	O
technique	O
used	O
to	O
increase	O
the	O
complexity	O
is	O
called	O
key	B-Algorithm
whitening	I-Algorithm
.	O
</s>
<s>
The	O
original	O
DES	B-Algorithm
algorithm	O
was	O
specified	O
in	O
1976	O
with	O
a	O
56-bit	O
key	O
size	O
:	O
256	O
possibilities	O
for	O
the	O
key	O
.	O
</s>
<s>
One	O
scheme	O
to	O
increase	O
the	O
key	O
size	O
of	O
DES	B-Algorithm
without	O
substantially	O
altering	O
the	O
algorithm	O
was	O
DES-X	B-Algorithm
,	O
proposed	O
by	O
Ron	O
Rivest	O
in	O
May	O
1984	O
.	O
</s>
<s>
The	O
algorithm	O
has	O
been	O
included	O
in	O
RSA	O
Security	O
's	O
BSAFE	B-Protocol
cryptographic	O
library	O
since	O
the	O
late	O
1980s	O
.	O
</s>
<s>
DES-X	B-Algorithm
augments	O
DES	B-Algorithm
by	O
XORing	O
an	O
extra	O
64	O
bits	O
of	O
key	O
(	O
K1	O
)	O
to	O
the	O
plaintext	O
before	O
applying	O
DES	B-Algorithm
,	O
and	O
then	O
XORing	O
another	O
64	O
bits	O
of	O
key	O
(	O
K2	O
)	O
after	O
the	O
encryption	O
:	O
</s>
<s>
DES-X	B-Algorithm
also	O
increases	O
the	O
strength	O
of	O
DES	B-Algorithm
against	O
differential	O
cryptanalysis	O
and	O
linear	O
cryptanalysis	O
,	O
although	O
the	O
improvement	O
is	O
much	O
smaller	O
than	O
in	O
the	O
case	O
of	O
brute	O
force	O
attacks	O
.	O
</s>
<s>
It	O
is	O
estimated	O
that	O
differential	O
cryptanalysis	O
would	O
require	O
261	O
chosen	O
plaintexts	O
(	O
vs	O
.	O
247	O
for	O
DES	B-Algorithm
)	O
,	O
while	O
linear	O
cryptanalysis	O
would	O
require	O
260	O
known	O
plaintexts	O
(	O
vs	O
.	O
243	O
for	O
DES	B-Algorithm
or	O
261	O
for	O
DES	B-Algorithm
with	O
independent	O
subkeys	O
.	O
)	O
</s>
<s>
Note	O
that	O
with	O
264	O
plaintexts	O
(	O
known	O
or	O
chosen	O
being	O
the	O
same	O
in	O
this	O
case	O
)	O
,	O
DES	B-Algorithm
(	O
or	O
indeed	O
any	O
other	O
block	O
cipher	O
with	O
a	O
64	O
bit	O
block	O
size	O
)	O
is	O
totally	O
broken	O
as	O
the	O
whole	O
cipher	O
's	O
codebook	O
becomes	O
available	O
.	O
</s>
