<s>
In	O
cryptography	O
,	O
RC5	B-Algorithm
is	O
a	O
symmetric-key	B-Algorithm
block	O
cipher	O
notable	O
for	O
its	O
simplicity	O
.	O
</s>
<s>
Designed	O
by	O
Ronald	O
Rivest	O
in	O
1994	O
,	O
RC	O
stands	O
for	O
"	O
Rivest	O
Cipher	O
"	O
,	O
or	O
alternatively	O
,	O
"	O
Ron	O
's	O
Code	O
"	O
(	O
compare	O
RC2	B-Algorithm
and	O
RC4	B-Algorithm
)	O
.	O
</s>
<s>
The	O
Advanced	B-Algorithm
Encryption	I-Algorithm
Standard	I-Algorithm
(	O
AES	B-Algorithm
)	O
candidate	O
RC6	B-Algorithm
was	O
based	O
on	O
RC5	B-Algorithm
.	O
</s>
<s>
Unlike	O
many	O
schemes	O
,	O
RC5	B-Algorithm
has	O
a	O
variable	O
block	O
size	O
(	O
32	O
,	O
64	O
or	O
128	O
bits	O
)	O
,	O
key	O
size	O
(	O
0	O
to	O
2040	O
bits	O
)	O
and	O
number	O
of	O
rounds	O
(	O
0	O
to	O
255	O
)	O
.	O
</s>
<s>
A	O
key	O
feature	O
of	O
RC5	B-Algorithm
is	O
the	O
use	O
of	O
data-dependent	O
rotations	O
;	O
one	O
of	O
the	O
goals	O
of	O
RC5	B-Algorithm
was	O
to	O
prompt	O
the	O
study	O
and	O
evaluation	O
of	O
such	O
operations	O
as	O
a	O
cryptographic	O
primitive	O
.	O
</s>
<s>
RC5	B-Algorithm
also	O
consists	O
of	O
a	O
number	O
of	O
modular	O
additions	O
and	O
eXclusive	O
OR	O
(	O
XOR	B-Application
)	O
s	O
.	O
The	O
general	O
structure	O
of	O
the	O
algorithm	O
is	O
a	O
Feistel-like	O
network	O
.	O
</s>
<s>
The	O
tantalising	O
simplicity	O
of	O
the	O
algorithm	O
together	O
with	O
the	O
novelty	O
of	O
the	O
data-dependent	O
rotations	O
has	O
made	O
RC5	B-Algorithm
an	O
attractive	O
object	O
of	O
study	O
for	O
cryptanalysts	O
.	O
</s>
<s>
The	O
RC5	B-Algorithm
is	O
basically	O
denoted	O
as	O
RC5-w/r/b	O
where	O
w	O
=	O
word	O
size	O
in	O
bits	O
,	O
r	O
=	O
number	O
of	O
rounds	O
,	O
b	O
=	O
number	O
of	O
8-bit	O
bytes	O
in	O
the	O
key	O
.	O
</s>
<s>
RC5	B-Algorithm
encryption	O
and	O
decryption	O
both	O
expand	O
the	O
random	O
key	O
into	O
2( r+1	O
)	O
words	O
that	O
will	O
be	O
used	O
sequentially	O
(	O
and	O
only	O
once	O
each	O
)	O
during	O
the	O
encryption	O
and	O
decryption	O
processes	O
.	O
</s>
<s>
All	O
of	O
the	O
below	O
comes	O
from	O
Rivest	O
's	O
revised	O
paper	O
on	O
RC5	B-Algorithm
.	O
</s>
<s>
The	O
example	O
source	O
code	O
is	O
provided	O
from	O
the	O
appendix	O
of	O
Rivest	O
's	O
paper	O
on	O
RC5	B-Algorithm
.	O
</s>
<s>
12-round	O
RC5	B-Algorithm
(	O
with	O
64-bit	O
blocks	O
)	O
is	O
susceptible	O
to	O
a	O
differential	O
attack	O
using	O
244	O
chosen	O
plaintexts	O
.	O
</s>
<s>
A	O
number	O
of	O
these	O
challenge	O
problems	O
have	O
been	O
tackled	O
using	O
distributed	B-Architecture
computing	I-Architecture
,	O
organised	O
by	O
Distributed.net	O
.	O
</s>
<s>
Distributed.net	O
has	O
brute-forced	O
RC5	B-Algorithm
messages	O
encrypted	O
with	O
56-bit	O
and	O
64-bit	O
keys	O
and	O
has	O
been	O
working	O
on	O
cracking	O
a	O
72-bit	O
key	O
since	O
November	O
3	O
,	O
2002	O
.	O
</s>
<s>
RSA	O
Security	O
,	O
which	O
had	O
a	O
patent	O
on	O
the	O
algorithm	O
,	O
offered	O
a	O
series	O
of	O
US$	O
10,000	O
prizes	O
for	O
breaking	O
ciphertexts	O
encrypted	O
with	O
RC5	B-Algorithm
,	O
but	O
these	O
contests	O
have	O
been	O
discontinued	O
as	O
of	O
May	O
2007	O
.	O
</s>
<s>
The	O
individual	O
who	O
discovers	O
the	O
winning	O
key	O
will	O
receive	O
US$1	O
,	O
000	O
,	O
their	O
team	O
(	O
if	O
applicable	O
)	O
will	O
receive	O
US$	O
1,000	O
and	O
the	B-Operating_System
Free	I-Operating_System
Software	I-Operating_System
Foundation	I-Operating_System
will	O
receive	O
US$	O
2,000	O
.	O
</s>
