<s>
In	O
cryptography	O
,	O
Ladder-DES	B-Algorithm
is	O
a	O
block	O
cipher	O
designed	O
in	O
1994	O
by	O
Terry	O
Ritter	O
.	O
</s>
<s>
It	O
is	O
a	O
4-round	O
Feistel	B-Algorithm
cipher	I-Algorithm
with	O
a	O
block	O
size	O
of	O
128	O
bits	O
,	O
using	O
DES	B-Algorithm
as	O
the	O
round	O
function	O
.	O
</s>
<s>
It	O
has	O
no	O
actual	O
key	B-Algorithm
schedule	I-Algorithm
,	O
so	O
the	O
total	O
key	O
size	O
is	O
4×56	O
=	O
224	O
bits	O
.	O
</s>
<s>
In	O
1997	O
,	O
Eli	O
Biham	O
found	O
two	O
forms	O
of	O
cryptanalysis	O
for	O
Ladder-DES	B-Algorithm
that	O
depend	O
on	O
the	O
birthday	O
paradox	O
;	O
the	O
key	O
is	O
deduced	O
from	O
the	O
presence	O
or	O
absence	O
of	O
collisions	O
,	O
plaintexts	O
that	O
give	O
equal	O
intermediate	O
values	O
in	O
the	O
encryption	O
process	O
.	O
</s>
