<s>
In	O
cryptography	O
,	O
key	B-Algorithm
whitening	I-Algorithm
is	O
a	O
technique	O
intended	O
to	O
increase	O
the	O
security	O
of	O
an	O
iterated	O
block	O
cipher	O
.	O
</s>
<s>
The	O
most	O
common	O
form	O
of	O
key	B-Algorithm
whitening	I-Algorithm
is	O
xor-encrypt-xor	B-Algorithm
--	O
using	O
a	O
simple	O
XOR	B-Application
before	O
the	O
first	O
round	O
and	O
after	O
the	O
last	O
round	O
of	O
encryption	O
.	O
</s>
<s>
The	O
first	O
block	O
cipher	O
to	O
use	O
a	O
form	O
of	O
key	B-Algorithm
whitening	I-Algorithm
is	O
DES-X	B-Algorithm
,	O
which	O
simply	O
uses	O
two	O
extra	O
64-bit	O
keys	O
for	O
whitening	O
,	O
beyond	O
the	O
normal	O
56-bit	O
key	O
of	O
DES	B-Algorithm
.	O
</s>
<s>
DES-X	B-Algorithm
'	O
s	O
inventor	O
,	O
Ron	O
Rivest	O
,	O
named	O
the	O
technique	O
whitening	O
.	O
</s>
<s>
The	O
cipher	O
FEAL	B-Algorithm
(	O
followed	O
by	O
Khufu	B-Algorithm
and	I-Algorithm
Khafre	I-Algorithm
)	O
introduced	O
the	O
practice	O
of	O
key	B-Algorithm
whitening	I-Algorithm
using	O
portions	O
of	O
the	O
same	O
key	O
used	O
in	O
the	O
rest	O
of	O
the	O
cipher	O
.	O
</s>
<s>
In	O
a	O
Feistel	B-Algorithm
cipher	I-Algorithm
or	O
similar	O
algorithm	O
,	O
key	B-Algorithm
whitening	I-Algorithm
can	O
increase	O
security	O
by	O
concealing	O
the	O
specific	O
inputs	O
to	O
the	O
first	O
and	O
last	O
round	O
functions	O
.	O
</s>
<s>
This	O
form	O
of	O
key	B-Algorithm
whitening	I-Algorithm
has	O
been	O
adopted	O
as	O
a	O
feature	O
of	O
many	O
later	O
block	O
ciphers	O
,	O
including	O
AES	O
,	O
MARS	B-Algorithm
,	O
RC6	B-Algorithm
,	O
and	O
Twofish	B-Algorithm
.	O
</s>
