<s>
A	O
polyalphabetic	B-Algorithm
cipher	I-Algorithm
substitution	O
,	O
using	O
multiple	O
substitution	O
alphabets	O
.	O
</s>
<s>
The	O
Vigenère	B-Algorithm
cipher	I-Algorithm
is	O
probably	O
the	O
best-known	O
example	O
of	O
a	O
polyalphabetic	B-Algorithm
cipher	I-Algorithm
,	O
though	O
it	O
is	O
a	O
simplified	O
special	O
case	O
.	O
</s>
<s>
The	O
Enigma	O
machine	O
is	O
more	O
complex	O
but	O
is	O
still	O
fundamentally	O
a	O
polyalphabetic	B-Algorithm
substitution	I-Algorithm
cipher	I-Algorithm
.	O
</s>
<s>
The	O
work	O
of	O
Al-Qalqashandi	O
(	O
1355	O
–	O
1418	O
)	O
,	O
based	O
on	O
the	O
earlier	O
work	O
of	O
Ibn	O
al-Durayhim	O
(	O
1312	O
–	O
1359	O
)	O
,	O
contained	O
the	O
first	O
published	O
discussion	O
of	O
the	O
substitution	O
and	O
transposition	O
of	O
ciphers	O
,	O
as	O
well	O
as	O
the	O
first	O
description	O
of	O
a	O
polyalphabetic	B-Algorithm
cipher	I-Algorithm
,	O
in	O
which	O
each	O
plaintext	O
letter	O
is	O
assigned	O
more	O
than	O
one	O
substitute	O
.	O
</s>
<s>
However	O
,	O
it	O
has	O
been	O
claimed	O
that	O
polyalphabetic	B-Algorithm
ciphers	I-Algorithm
may	O
have	O
been	O
developed	O
by	O
the	O
Arab	O
cryptologist	O
Al	O
Kindi	O
(	O
801	O
–	O
873	O
)	O
centuries	O
earlier	O
.	O
</s>
<s>
The	O
Alberti	B-Algorithm
cipher	I-Algorithm
by	O
Leon	O
Battista	O
Alberti	O
around	O
1467	O
was	O
an	O
early	O
polyalphabetic	B-Algorithm
cipher	I-Algorithm
.	O
</s>
<s>
For	O
this	O
encipherment	O
Alberti	O
used	O
a	O
decoder	O
device	O
,	O
his	O
cipher	O
disk	O
,	O
which	O
implemented	O
a	O
polyalphabetic	B-Algorithm
substitution	I-Algorithm
with	O
mixed	O
alphabets	O
.	O
</s>
<s>
Johannes	O
Trithemius	O
—	O
in	O
his	O
book	O
Polygraphiae	O
libri	O
sex	O
(	O
Six	O
books	O
of	O
polygraphia	O
)	O
,	O
which	O
was	O
published	O
in	O
1518	O
after	O
his	O
death	O
—	O
invented	O
a	O
progressive	O
key	O
polyalphabetic	B-Algorithm
cipher	I-Algorithm
called	O
the	O
Trithemius	O
cipher	O
.	O
</s>
<s>
He	O
started	O
with	O
a	O
tabula	B-Algorithm
recta	I-Algorithm
,	O
a	O
square	O
with	O
26	O
letters	O
in	O
it	O
(	O
although	O
Trithemius	O
,	O
writing	O
in	O
Latin	O
,	O
used	O
24	O
letters	O
)	O
.	O
</s>
<s>
Even	O
Alberti	O
's	O
implementation	O
of	O
his	O
polyalphabetic	B-Algorithm
cipher	I-Algorithm
was	O
rather	O
easy	O
to	O
break	O
(	O
the	O
capitalized	O
letter	O
is	O
a	O
major	O
clue	O
to	O
the	O
cryptanalyst	O
)	O
.	O
</s>
<s>
Polyalphabetic	B-Algorithm
substitution	I-Algorithm
cipher	I-Algorithm
designers	O
seem	O
to	O
have	O
concentrated	O
on	O
obscuring	O
the	O
choice	O
of	O
a	O
few	O
such	O
alphabets	O
(	O
repeating	O
as	O
needed	O
)	O
,	O
not	O
on	O
the	O
increased	O
security	O
possible	O
by	O
using	O
many	O
and	O
never	O
repeating	O
any	O
.	O
</s>
<s>
It	O
was	O
not	O
until	O
the	O
mid-19th	O
century	O
(	O
in	O
Babbage	O
's	O
secret	O
work	O
during	O
the	O
Crimean	O
War	O
and	O
Friedrich	O
Kasiski	O
's	O
generally	O
equivalent	O
public	O
disclosure	O
some	O
years	O
later	O
)	O
,	O
that	O
cryptanalysis	O
of	O
well-implemented	O
polyalphabetic	B-Algorithm
ciphers	I-Algorithm
got	O
anywhere	O
at	O
all	O
.	O
</s>
