<s>
The	O
xor	B-Algorithm
–	I-Algorithm
encrypt	I-Algorithm
–	I-Algorithm
xor	I-Algorithm
(	O
XEX	O
)	O
is	O
a	O
(	O
tweakable	O
)	O
mode	B-Algorithm
of	I-Algorithm
operation	I-Algorithm
of	I-Algorithm
a	I-Algorithm
block	I-Algorithm
cipher	I-Algorithm
.	O
</s>
<s>
In	O
tweaked-codebook	O
mode	O
with	O
ciphertext	B-Algorithm
stealing	I-Algorithm
(	O
XTS	O
mode	O
)	O
,	O
it	O
is	O
one	O
of	O
the	O
more	O
popular	O
modes	B-Algorithm
of	I-Algorithm
operation	I-Algorithm
for	O
whole-disk	O
encryption	O
.	O
</s>
<s>
XEX	O
is	O
also	O
a	O
common	O
form	O
of	O
key	B-Algorithm
whitening	I-Algorithm
,	O
and	O
part	O
of	O
some	O
smart	O
card	O
proposals	O
.	O
</s>
<s>
In	O
1984	O
,	O
to	O
protect	O
DES	O
against	O
exhaustive	O
search	O
attacks	O
,	O
Ron	O
Rivest	O
proposed	O
DESX	B-Algorithm
:	O
XOR	O
a	O
pre-whitening	O
key	O
to	O
the	O
plaintext	O
,	O
encrypt	O
the	O
result	O
with	O
DES	O
using	O
a	O
secret	O
key	O
,	O
and	O
then	O
XOR	O
a	O
postwhitening	O
key	O
to	O
the	O
encrypted	O
result	O
to	O
produce	O
the	O
final	O
ciphertext	O
.	O
</s>
<s>
In	O
1991	O
,	O
motivated	O
by	O
Rivest	O
's	O
DESX	B-Algorithm
construction	O
,	O
Even	O
and	O
Mansour	O
proposed	O
a	O
much	O
simpler	O
scheme	O
(	O
the	O
"	O
two-key	O
Even	O
–	O
Mansour	O
scheme	O
"	O
)	O
,	O
which	O
they	O
suggested	O
was	O
perhaps	O
the	O
simplest	O
possible	O
block	O
cipher	O
:	O
XOR	O
the	O
plaintext	O
with	O
a	O
prewhitening	O
key	O
,	O
apply	O
a	O
publicly	O
known	O
unkeyed	O
permutation	O
(	O
in	O
practice	O
,	O
a	O
pseudorandom	O
permutation	O
)	O
to	O
the	O
result	O
,	O
and	O
then	O
XOR	O
a	O
postwhitening	O
key	O
to	O
the	O
permuted	O
result	O
to	O
produce	O
the	O
final	O
ciphertext	O
.	O
</s>
<s>
Studying	O
simple	O
Even	O
–	O
Mansour	O
style	O
block	O
ciphers	O
gives	O
insight	O
into	O
the	O
security	O
of	O
Feistel	B-Algorithm
ciphers	I-Algorithm
(	O
DES-like	O
ciphers	O
)	O
and	O
helps	O
understand	O
block	O
cipher	O
design	O
in	O
general	O
.	O
</s>
<s>
Many	O
whole-disk	O
encryption	O
systems	O
–	O
BestCrypt	O
,	O
dm-crypt	O
,	O
FreeOTFE	O
,	O
TrueCrypt	O
,	O
DiskCryptor	O
,	O
FreeBSD	O
's	O
geli	O
,	O
OpenBSD	O
softraid	O
disk	O
encryption	O
software	O
,	O
and	O
Mac	O
OS	O
X	O
Lion	O
's	O
FileVault	O
2	O
–	O
support	O
XEX-based	O
tweaked-codebook	O
mode	O
with	O
ciphertext	B-Algorithm
stealing	I-Algorithm
(	O
XTS	O
mode	O
)	O
.	O
</s>
