<s>
The	O
Two-square	B-Algorithm
cipher	I-Algorithm
,	O
also	O
called	O
double	B-Algorithm
Playfair	I-Algorithm
,	O
is	O
a	O
manual	O
symmetric	B-Algorithm
encryption	I-Algorithm
technique	O
.	O
</s>
<s>
It	O
was	O
developed	O
to	O
ease	O
the	O
cumbersome	O
nature	O
of	O
the	O
large	O
encryption/decryption	O
matrix	O
used	O
in	O
the	O
four-square	B-Algorithm
cipher	I-Algorithm
while	O
still	O
being	O
slightly	O
stronger	O
than	O
the	O
single-square	O
Playfair	B-Algorithm
cipher	I-Algorithm
.	O
</s>
<s>
In	O
1987	O
,	O
Noel	O
Currer‐Briggs	O
described	O
the	O
double	B-Algorithm
Playfair	I-Algorithm
cipher	O
used	O
by	O
Germans	O
in	O
World	O
War	O
II	O
.	O
</s>
<s>
In	O
this	O
case	O
,	O
double	B-Algorithm
Playfair	I-Algorithm
refers	O
to	O
a	O
method	O
using	O
two	O
Polybius	O
squares	O
plus	O
seriation	O
.	O
</s>
<s>
Even	O
variants	O
of	O
Double	B-Algorithm
Playfair	I-Algorithm
that	O
encipher	O
each	O
pair	O
of	O
letters	O
twice	O
are	O
considered	O
weaker	O
than	O
the	O
double	O
transposition	O
cipher	O
.	O
</s>
<s>
The	O
two-square	B-Algorithm
cipher	I-Algorithm
is	O
not	O
described	O
in	O
some	O
other	O
20th	O
century	O
popular	O
cryptography	O
books	O
e.g.	O
</s>
<s>
by	O
Helen	O
Fouché	O
Gaines	O
(	O
1939	O
)	O
or	O
William	O
Maxwell	O
Bowers	O
(	O
1959	O
)	O
,	O
although	O
both	O
describe	O
the	O
Playfair	B-Algorithm
cipher	I-Algorithm
and	O
four-square	B-Algorithm
cipher	I-Algorithm
.	O
</s>
<s>
The	O
two-square	B-Algorithm
cipher	I-Algorithm
uses	O
two	O
5x5	O
matrices	O
and	O
comes	O
in	O
two	O
varieties	O
,	O
horizontal	O
and	O
vertical	O
.	O
</s>
<s>
Encryption	O
using	O
two-square	O
is	O
basically	O
the	O
same	O
as	O
the	O
system	O
used	O
in	O
four-square	B-Algorithm
,	O
except	O
that	O
the	O
plaintext	O
and	O
ciphertext	O
digraphs	O
use	O
the	O
same	O
matrixes	O
.	O
</s>
<s>
Just	O
like	O
Playfair	O
(	O
and	O
unlike	O
four-square	B-Algorithm
)	O
,	O
there	O
are	O
special	O
circumstances	O
when	O
the	O
two	O
letters	O
in	O
a	O
digraph	O
are	O
in	O
the	O
same	O
column	O
for	O
vertical	O
two-square	O
or	O
in	O
the	O
same	O
row	O
for	O
horizontal	O
two-square	O
.	O
</s>
<s>
Like	O
most	O
pre-modern	O
era	O
ciphers	O
,	O
the	O
two-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>
A	O
good	O
tutorial	O
on	O
reconstructing	O
the	O
key	O
for	O
a	O
two-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>
