<s>
In	O
cryptography	O
,	O
the	O
so-called	O
product	B-Algorithm
ciphers	I-Algorithm
are	O
a	O
certain	O
kind	O
of	O
cipher	O
,	O
where	O
the	O
(	O
de	O
-	O
)	O
ciphering	O
of	O
data	O
is	O
typically	O
done	O
as	O
an	O
iteration	O
of	O
rounds	O
.	O
</s>
<s>
The	O
setup	O
for	O
each	O
round	O
is	O
generally	O
the	O
same	O
,	O
except	O
for	O
round-specific	O
fixed	O
values	O
called	O
a	O
round	O
constant	O
,	O
and	O
round-specific	O
data	O
derived	O
from	O
the	O
cipher	O
key	O
called	O
a	O
round	B-Algorithm
key	I-Algorithm
.	O
</s>
<s>
A	O
key	B-Algorithm
schedule	I-Algorithm
is	O
an	O
algorithm	O
that	O
calculates	O
all	O
the	O
round	B-Algorithm
keys	I-Algorithm
from	O
the	O
key	O
.	O
</s>
<s>
Some	O
ciphers	O
have	O
simple	O
key	B-Algorithm
schedules	I-Algorithm
.	O
</s>
<s>
For	O
example	O
,	O
the	O
block	O
cipher	O
TEA	B-Algorithm
splits	O
the	O
128-bit	O
key	O
into	O
four	O
32-bit	O
pieces	O
and	O
uses	O
them	O
repeatedly	O
in	O
successive	O
rounds	O
.	O
</s>
<s>
DES	B-Algorithm
has	O
a	O
key	B-Algorithm
schedule	I-Algorithm
in	O
which	O
the	O
56-bit	O
key	O
is	O
divided	O
into	O
two	O
28-bit	O
halves	O
;	O
each	O
half	O
is	O
thereafter	O
treated	O
separately	O
.	O
</s>
<s>
In	O
successive	O
rounds	O
,	O
both	O
halves	O
are	O
rotated	O
left	O
by	O
one	O
or	O
two	O
bits	O
(	O
specified	O
for	O
each	O
round	O
)	O
,	O
and	O
then	O
48	O
round	B-Algorithm
key	I-Algorithm
bits	O
are	O
selected	O
by	O
Permuted	O
Choice	O
2	O
(	O
PC-2	O
)	O
–	O
24	O
bits	O
from	O
the	O
left	O
half	O
and	O
24	O
from	O
the	O
right	O
.	O
</s>
<s>
The	O
rotations	O
have	O
the	O
effect	O
that	O
a	O
different	O
set	O
of	O
bits	O
is	O
used	O
in	O
each	O
round	B-Algorithm
key	I-Algorithm
;	O
each	O
bit	O
is	O
used	O
in	O
approximately	O
14	O
out	O
of	O
the	O
16	O
round	B-Algorithm
keys	I-Algorithm
.	O
</s>
<s>
To	O
avoid	O
simple	O
relationships	O
between	O
the	O
cipher	O
key	O
and	O
the	O
round	B-Algorithm
keys	I-Algorithm
,	O
in	O
order	O
to	O
resist	O
such	O
forms	O
of	O
cryptanalysis	O
as	O
related-key	O
attacks	O
and	O
slide	O
attacks	O
,	O
many	O
modern	O
ciphers	O
use	O
more	O
elaborate	O
key	B-Algorithm
schedules	I-Algorithm
to	O
generate	O
an	O
"	O
expanded	O
key	O
"	O
from	O
which	O
round	B-Algorithm
keys	I-Algorithm
are	O
drawn	O
.	O
</s>
<s>
Some	O
ciphers	O
,	O
such	O
as	O
Rijndael	O
(	O
AES	O
)	O
and	O
Blowfish	B-Algorithm
,	O
use	O
the	O
same	O
operations	O
as	O
those	O
used	O
in	O
the	O
data	O
path	O
of	O
the	O
cipher	O
algorithm	O
for	O
their	O
key	O
expansion	O
,	O
sometimes	O
initialized	O
with	O
some	O
"	O
nothing-up-my-sleeve	O
numbers	O
"	O
.	O
</s>
<s>
Other	O
ciphers	O
,	O
such	O
as	O
RC5	B-Algorithm
,	O
expand	O
keys	O
with	O
functions	O
that	O
are	O
somewhat	O
or	O
completely	O
different	O
from	O
the	O
encryption	O
functions	O
.	O
</s>
