<s>
The	O
four-square	B-Algorithm
cipher	I-Algorithm
is	O
a	O
manual	O
symmetric	B-Algorithm
encryption	I-Algorithm
technique	O
.	O
</s>
<s>
The	O
four-square	B-Algorithm
cipher	I-Algorithm
uses	O
four	O
5	O
by	O
5	O
(	O
5x5	O
)	O
matrices	O
arranged	O
in	O
a	O
square	O
.	O
</s>
<s>
Like	O
most	O
pre-modern	O
era	O
ciphers	O
,	O
the	O
four-square	B-Algorithm
cipher	I-Algorithm
can	O
be	O
easily	O
cracked	O
if	O
there	O
is	O
enough	O
text	O
.	O
</s>
<s>
Unlike	O
the	O
Playfair	B-Algorithm
cipher	I-Algorithm
,	O
a	O
four-square	B-Algorithm
cipher	I-Algorithm
will	O
not	O
show	O
reversed	O
ciphertext	O
digraphs	O
for	O
reversed	O
plaintext	O
digraphs	O
(	O
e.g.	O
</s>
<s>
A	O
good	O
tutorial	O
on	O
reconstructing	O
the	O
key	O
for	O
a	O
four-square	B-Algorithm
cipher	I-Algorithm
can	O
be	O
found	O
in	O
chapter	O
7	O
,	O
"	O
Solution	O
to	O
Polygraphic	O
Substitution	O
Systems	O
,	O
"	O
of	O
,	O
produced	O
by	O
the	O
United	O
States	O
Army	O
.	O
</s>
