<s>
A	O
straddling	B-Algorithm
checkerboard	I-Algorithm
is	O
a	O
device	O
for	O
converting	O
an	O
alphanumeric	O
plaintext	O
into	O
digits	O
whilst	O
simultaneously	O
achieving	O
fractionation	O
(	O
a	O
simple	O
form	O
of	O
information	O
diffusion	O
)	O
and	O
data	B-General_Concept
compression	I-General_Concept
relative	O
to	O
other	O
schemes	O
using	O
digits	O
.	O
</s>
<s>
Matteo	O
is	O
credited	O
for	O
designing	O
what	O
is	O
now	O
called	O
the	O
straddling	B-Algorithm
checkerboard	I-Algorithm
cipher	O
.	O
</s>
<s>
It	O
was	O
later	O
used	O
as	O
the	O
basis	O
for	O
the	O
message-to-digits	O
step	O
in	O
the	B-Algorithm
VIC	I-Algorithm
cipher	I-Algorithm
.	O
</s>
<s>
A	O
straddling	B-Algorithm
checkerboard	I-Algorithm
is	O
set	O
up	O
something	O
like	O
this	O
:	O
</s>
<s>
The	O
slash	O
'	O
/	O
'	O
is	O
used	O
as	O
a	O
numeric	B-Algorithm
escape	I-Algorithm
character	I-Algorithm
(	O
indicating	O
that	O
a	O
numeral	O
follows	O
)	O
.	O
</s>
<s>
Single	O
Digit	O
Escape	O
:	O
Encode	O
the	O
numerical	O
escape	B-Algorithm
character	I-Algorithm
(	O
i.e.	O
</s>
<s>
This	O
means	O
a	O
digit	O
is	O
encrypted	O
by	O
3	O
ciphertext	O
characters	O
;	O
2	O
for	O
the	O
escape	B-Algorithm
character	I-Algorithm
,	O
1	O
for	O
the	O
digit	O
itself	O
.	O
</s>
<s>
In	O
this	O
scheme	O
,	O
each	O
digit	O
requires	O
an	O
escape	B-Algorithm
character	I-Algorithm
encoded	O
before	O
it	O
.	O
</s>
<s>
Double-Digit	O
Scheme	O
:	O
If	O
the	O
escape	B-Algorithm
character	I-Algorithm
is	O
encoded	O
by	O
two	O
different	O
digits	O
(	O
e.g.	O
</s>
<s>
To	O
'	O
escape	O
 '	O
back	O
to	O
text	O
the	O
escape	B-Algorithm
character	I-Algorithm
is	O
used	O
.	O
</s>
<s>
In	O
this	O
way	O
a	O
stream	O
of	O
digits	O
can	O
be	O
encoded	O
with	O
only	O
one	O
escape	B-Algorithm
character	I-Algorithm
.	O
</s>
<s>
This	O
method	O
cannot	O
be	O
used	O
if	O
the	O
escape	B-Algorithm
character	I-Algorithm
is	O
itself	O
encoded	O
by	O
a	O
double	O
digit	O
combination	O
.	O
</s>
<s>
The	O
table	O
below	O
shows	O
the	O
conversion	O
of	O
string	O
"	O
ATTACK	O
AT	O
DAWN	O
"	O
into	O
numbers	O
using	O
the	O
straddling	B-Algorithm
checkerboard	I-Algorithm
setup	O
above	O
.	O
</s>
<s>
The	O
resulting	O
message	O
,	O
3113212731223655	O
has	O
to	O
be	O
secured	O
by	O
other	O
means	O
if	O
the	O
straddling	B-Algorithm
checkerboard	I-Algorithm
table	O
is	O
not	O
scrambled	O
.	O
</s>
<s>
By	O
passing	O
digits	O
through	O
an	O
additional	O
transposition	B-Algorithm
or	O
substitution	O
cipher	O
stage	O
can	O
be	O
used	O
to	O
secure	O
message	O
--	O
to	O
whatever	O
extent	O
transposition	B-Algorithm
or	O
substitution	O
can	O
be	O
considered	O
secure	O
.	O
</s>
<s>
For	O
example	O
,	O
securing	O
the	O
message	O
above	O
using	O
as	O
a	O
secret	O
key	O
digits	O
"	O
0452	O
"	O
--	O
digits	O
of	O
the	O
secret	O
key	O
are	O
repeated	O
to	O
get	O
at	O
least	O
as	O
many	O
digits	O
as	O
there	O
are	O
digits	O
in	O
the	O
message	O
--	O
and	O
adding	O
modulo	O
10	O
for	O
transposition	B-Algorithm
produces	O
:	O
</s>
<s>
The	O
message	O
can	O
also	O
be	O
ciphered	O
with	O
a	O
substitution	O
cipher	O
using	O
the	O
same	O
straddling	B-Algorithm
checkerboard	I-Algorithm
to	O
convert	O
the	O
digits	O
back	O
to	O
letters	O
again	O
:	O
</s>
<s>
If	O
the	O
letter	O
decoded	O
is	O
the	O
numerical	O
escape	B-Algorithm
character	I-Algorithm
'	O
/	O
'	O
then	O
the	O
next	O
1+	O
digits	O
(	O
depending	O
on	O
scheme	O
)	O
are	O
to	O
be	O
interpreted	O
as	O
a	O
digit	O
.	O
</s>
<s>
Fractionation	O
:	O
Unlike	O
in	O
the	O
Polybius	O
Square	O
(	O
where	O
every	O
character	O
is	O
represented	O
by	O
a	O
pair	O
of	O
digits	O
)	O
,	O
a	O
straddling	B-Algorithm
checkerboard	I-Algorithm
will	O
not	O
encrypt	O
each	O
character	O
with	O
the	O
same	O
number	O
of	O
ciphertext	O
digits	O
.	O
</s>
<s>
This	O
may	O
be	O
combined	O
with	O
a	O
transposition	B-Algorithm
(	O
as	O
it	O
is	O
in	O
the	B-Algorithm
VIC	I-Algorithm
cipher	I-Algorithm
)	O
in	O
order	O
to	O
locate	O
the	O
ciphertext	O
letters	O
of	O
the	O
same	O
plaintext	O
character	O
at	O
unknown	O
locations	O
in	O
the	O
ciphertext	O
.	O
</s>
