<s>
In	O
cryptography	O
,	O
Crab	B-Algorithm
is	O
a	O
block	O
cipher	O
proposed	O
by	O
Burt	O
Kaliski	O
and	O
Matt	O
Robshaw	O
at	O
the	O
first	O
Fast	O
Software	O
Encryption	O
workshop	O
in	O
1993	O
.	O
</s>
<s>
Not	O
really	O
intended	O
for	O
use	O
,	O
Crab	B-Algorithm
was	O
developed	O
to	O
demonstrate	O
how	O
ideas	O
from	O
hash	B-Algorithm
functions	I-Algorithm
could	O
be	O
used	O
to	O
create	O
a	O
fast	O
cipher	O
.	O
</s>
<s>
Crab	B-Algorithm
has	O
an	O
unusually	O
large	O
block	O
size	O
of	O
8192	O
bits	O
.	O
</s>
<s>
The	O
authors	O
did	O
n't	O
specify	O
an	O
actual	O
key	B-Algorithm
schedule	I-Algorithm
,	O
only	O
that	O
the	O
key	O
is	O
used	O
to	O
generate	O
two	O
large	O
sets	O
of	O
subkeys	B-Algorithm
:	O
a	O
permutation	O
of	O
the	O
numbers	O
0	O
through	O
255	O
,	O
and	O
an	O
array	O
of	O
2048	O
32-bit	O
numbers	O
.	O
</s>
<s>
Then	O
the	O
algorithm	O
makes	O
four	O
passes	O
over	O
the	O
data	O
,	O
each	O
time	O
applying	O
one	O
of	O
four	O
transformations	O
adapted	O
from	O
MD5	B-Algorithm
.	O
</s>
<s>
A	O
brief	O
note	O
on	O
the	O
cryptanalysis	O
of	O
Crab	B-Algorithm
is	O
included	O
in	O
Markku-Juhani	O
Saarinen	O
's	O
paper	O
on	O
block	O
ciphers	O
based	O
on	O
SHA-1	B-Algorithm
and	O
MD5	B-Algorithm
,	O
published	O
at	O
FSE	O
2003	O
.	O
</s>
<s>
The	O
author	O
demonstrates	O
a	O
weakness	O
in	O
Crab	B-Algorithm
that	O
permits	O
a	O
distinguisher	O
using	O
no	O
more	O
than	O
a	O
dozen	O
chosen	O
plaintexts	O
,	O
and	O
speculates	O
that	O
this	O
can	O
be	O
converted	O
into	O
a	O
full	O
key-recovery	O
attack	O
using	O
no	O
more	O
than	O
216	O
chosen	O
plaintexts	O
.	O
</s>
<s>
Such	O
an	O
attack	O
would	O
depend	O
on	O
the	O
key	B-Algorithm
schedule	I-Algorithm
used	O
.	O
</s>
