<s>
In	O
cryptography	O
,	O
the	O
tabula	B-Algorithm
recta	I-Algorithm
(	O
from	O
Latin	O
tabula	O
rēcta	O
)	O
is	O
a	O
square	O
table	O
of	O
alphabets	O
,	O
each	O
row	O
of	O
which	O
is	O
made	O
by	O
shifting	O
the	O
previous	O
one	O
to	O
the	O
left	O
.	O
</s>
<s>
Trithemius	O
used	O
the	O
tabula	B-Algorithm
recta	I-Algorithm
to	O
define	O
a	O
polyalphabetic	B-Algorithm
cipher	I-Algorithm
,	O
which	O
was	O
equivalent	O
to	O
Leon	O
Battista	O
Alberti	O
's	O
cipher	B-Algorithm
disk	I-Algorithm
except	O
that	O
the	O
order	O
of	O
the	O
letters	O
in	O
the	O
target	O
alphabet	O
is	O
not	O
mixed	O
.	O
</s>
<s>
The	O
tabula	B-Algorithm
recta	I-Algorithm
is	O
often	O
referred	O
to	O
in	O
discussing	O
pre-computer	O
ciphers	O
,	O
including	O
the	O
Vigenère	B-Algorithm
cipher	I-Algorithm
and	O
Blaise	O
de	O
Vigenère	O
's	O
less	O
well-known	O
autokey	B-Algorithm
cipher	I-Algorithm
.	O
</s>
<s>
All	O
polyalphabetic	B-Algorithm
ciphers	I-Algorithm
based	O
on	O
the	O
Caesar	O
cipher	O
can	O
be	O
described	O
in	O
terms	O
of	O
the	O
tabula	B-Algorithm
recta	I-Algorithm
.	O
</s>
<s>
The	O
tabula	B-Algorithm
recta	I-Algorithm
uses	O
a	O
letter	O
square	O
with	O
the	O
26	O
letters	O
of	O
the	O
alphabet	O
followed	O
by	O
26	O
rows	O
of	O
additional	O
letters	O
,	O
each	O
shifted	O
once	O
to	O
the	O
left	O
from	O
the	O
one	O
above	O
it	O
.	O
</s>
<s>
In	O
1553	O
,	O
an	O
important	O
extension	O
to	O
Trithemius	O
's	O
method	O
was	O
developed	O
by	O
Giovan	O
Battista	O
Bellaso	O
,	O
now	O
called	O
the	O
Vigenère	B-Algorithm
cipher	I-Algorithm
.	O
</s>
<s>
This	O
method	O
was	O
misattributed	O
to	O
Blaise	O
de	O
Vigenère	O
,	O
who	O
published	O
a	O
similar	O
autokey	B-Algorithm
cipher	I-Algorithm
in	O
1586	O
.	O
</s>
<s>
The	O
classic	O
Trithemius	O
cipher	O
(	O
using	O
a	O
shift	O
of	O
one	O
)	O
is	O
equivalent	O
to	O
a	O
Vigenère	B-Algorithm
cipher	I-Algorithm
with	O
ABCDEFGHIJKLMNOPQRSTUVWXYZ	O
as	O
the	O
key	O
.	O
</s>
<s>
Within	O
the	O
body	O
of	O
the	O
tabula	B-Algorithm
recta	I-Algorithm
,	O
each	O
alphabet	O
is	O
shifted	O
one	O
letter	O
to	O
the	O
left	O
from	O
the	O
one	O
above	O
it	O
.	O
</s>
<s>
The	O
tabula	B-Algorithm
recta	I-Algorithm
can	O
be	O
used	O
in	O
several	O
equivalent	O
ways	O
to	O
encrypt	O
and	O
decrypt	O
text	O
.	O
</s>
<s>
In	O
order	O
to	O
decrypt	O
a	O
Trithemius	O
cipher	O
,	O
one	O
first	O
locates	O
in	O
the	O
tabula	B-Algorithm
recta	I-Algorithm
the	O
letters	O
to	O
decrypt	O
:	O
first	O
letter	O
in	O
the	O
first	O
interior	O
column	O
,	O
second	O
letter	O
in	O
the	O
second	O
column	O
,	O
etc	O
.	O
</s>
<s>
Data	O
is	O
encrypted	O
in	O
the	O
opposite	O
fashion	O
,	O
by	O
first	O
locating	O
each	O
plaintext	O
letter	O
of	O
the	O
message	O
in	O
the	O
leftmost	O
header	O
column	O
of	O
the	O
tabula	B-Algorithm
recta	I-Algorithm
,	O
and	O
mapping	O
it	O
to	O
the	O
appropriate	O
corresponding	O
letter	O
in	O
the	O
interior	O
columns	O
.	O
</s>
<s>
may	O
be	O
defined	O
as	O
the	O
value	O
of	O
a	O
letter	O
from	O
a	O
companion	O
ciphertext	O
in	O
a	O
running	B-Algorithm
key	I-Algorithm
cipher	I-Algorithm
,	O
a	O
constant	O
for	O
a	O
Caesar	O
cipher	O
,	O
or	O
a	O
zero-based	O
counter	O
with	O
some	O
period	O
in	O
Trithemius	O
's	O
usage	O
.	O
</s>
