<s>
The	O
Alberti	B-Algorithm
Cipher	I-Algorithm
,	O
created	O
in	O
1467	O
by	O
Italian	O
architect	O
Leon	O
Battista	O
Alberti	O
,	O
was	O
one	O
of	O
the	O
first	O
polyalphabetic	B-Algorithm
ciphers	I-Algorithm
.	O
</s>
<s>
In	O
the	O
opening	O
pages	O
of	O
his	O
treatise	O
he	O
explained	O
how	O
his	O
conversation	O
with	O
the	O
papal	O
secretary	O
Leonardo	O
Dati	O
about	O
a	O
recently	O
developed	O
movable	B-Protocol
type	I-Protocol
printing	O
press	O
led	O
to	O
the	O
development	O
of	O
his	O
cipher	B-Application
wheel	O
.	O
</s>
<s>
Alberti	O
's	O
cipher	B-Application
disk	O
embodies	O
the	O
first	O
example	O
of	O
polyalphabetic	B-Algorithm
substitution	I-Algorithm
with	O
mixed	O
alphabets	O
and	O
variable	O
period	O
.	O
</s>
<s>
For	O
an	O
unequivocal	O
study	O
of	O
this	O
cipher	B-Application
two	O
chapters	O
of	O
De	O
Cifris	O
are	O
herewith	O
reproduced	O
in	O
English	O
.	O
</s>
<s>
The	O
Alberti	O
disk	O
encipherment	O
has	O
nothing	O
to	O
do	O
with	O
Affine	O
Shifts	O
,	O
Keyword	O
shifts	O
,	O
Caesar	O
shift	O
or	O
Vigenère	B-Algorithm
ciphers	I-Algorithm
.	O
</s>
<s>
Caesar	O
’s	O
cipher	B-Application
is	O
a	O
simple	O
substitution	O
based	O
on	O
the	O
sliding	O
of	O
a	O
single	O
ordinary	O
alphabet	O
with	O
fixed	O
key	O
.	O
</s>
<s>
Once	O
the	O
equivalent	O
of	O
a	O
letter	O
is	O
discovered	O
all	O
the	O
equivalent	O
cipher	B-Application
letters	O
are	O
known	O
.	O
</s>
<s>
With	O
the	O
Alberti	B-Algorithm
cipher	I-Algorithm
there	O
are	O
two	O
mixed	O
alphabets	O
and	O
the	O
key	O
varies	O
continuously	O
during	O
encryption	O
,	O
therefore	O
the	O
discovery	O
of	O
a	O
single	O
letter	O
does	O
not	O
permit	O
further	O
progress	O
.	O
</s>
<s>
The	O
Vigenère	B-Algorithm
cipher	I-Algorithm
is	O
based	O
on	O
a	O
single	O
ordinary	O
alphabet	O
like	O
that	O
of	O
Caesar	O
and	O
is	O
easily	O
solved	O
after	O
discovering	O
its	O
fixed	O
period	O
by	O
means	O
of	O
the	O
Kasiski	O
exam	O
.	O
</s>
<s>
Compared	O
to	O
previous	O
ciphers	B-Application
of	O
the	O
time	O
the	O
Alberti	B-Algorithm
Cipher	I-Algorithm
was	O
impossible	O
to	O
break	O
without	O
knowledge	O
of	O
the	O
method	O
.	O
</s>
<s>
This	O
was	O
because	O
the	O
frequency	O
distribution	O
of	O
the	O
letters	O
was	O
masked	O
and	O
frequency	O
analysis	O
-	O
the	O
only	O
known	O
technique	O
for	O
attacking	O
ciphers	B-Application
at	O
that	O
time	O
-	O
was	O
no	O
help	O
.	O
</s>
<s>
It	O
was	O
also	O
more	O
convenient	O
than	O
the	O
Vigenère	B-Algorithm
cipher	I-Algorithm
.	O
</s>
