<s>
An	O
autokey	B-Algorithm
cipher	I-Algorithm
(	O
also	O
known	O
as	O
the	O
autoclave	B-Algorithm
cipher	I-Algorithm
)	O
is	O
a	O
cipher	B-Application
that	O
incorporates	O
the	O
message	O
(	O
the	O
plaintext	O
)	O
into	O
the	O
key	O
.	O
</s>
<s>
There	O
are	O
two	O
forms	O
of	O
autokey	B-Algorithm
cipher	I-Algorithm
:	O
key-autokey	O
and	O
text-autokey	O
ciphers	O
.	O
</s>
<s>
A	O
key-autokey	O
cipher	O
uses	O
previous	O
members	O
of	O
the	O
keystream	B-Algorithm
to	O
determine	O
the	O
next	O
element	O
in	O
the	O
keystream	B-Algorithm
.	O
</s>
<s>
A	O
text-autokey	O
uses	O
the	O
previous	O
message	O
text	O
to	O
determine	O
the	O
next	O
element	O
in	O
the	O
keystream	B-Algorithm
.	O
</s>
<s>
This	O
cipher	B-Application
was	O
invented	O
in	O
1586	O
by	O
Blaise	O
de	O
Vigenère	O
with	O
a	O
reciprocal	O
table	O
of	O
ten	O
alphabets	O
.	O
</s>
<s>
More	O
popular	O
autokeys	O
use	O
a	O
tabula	B-Algorithm
recta	I-Algorithm
,	O
a	O
square	O
with	O
26	O
copies	O
of	O
the	O
alphabet	O
,	O
the	O
first	O
line	O
starting	O
with	O
'	O
A	O
 '	O
,	O
the	O
next	O
line	O
starting	O
with	O
'	O
B	O
 '	O
etc	O
.	O
</s>
<s>
The	O
autokey	B-Algorithm
cipher	I-Algorithm
,	O
as	O
used	O
by	O
members	O
of	O
the	O
American	O
Cryptogram	O
Association	O
,	O
starts	O
with	O
a	O
relatively-short	O
keyword	O
,	O
the	O
primer	O
,	O
and	O
appends	O
the	O
message	O
to	O
it	O
.	O
</s>
<s>
The	O
first	O
letter	O
of	O
the	O
key	O
,	O
Q	O
,	O
would	O
then	O
be	O
taken	O
,	O
and	O
that	O
row	O
would	O
be	O
found	O
in	O
a	O
tabula	B-Algorithm
recta	I-Algorithm
.	O
</s>
<s>
With	O
Vigenère	O
's	O
autokey	B-Algorithm
cipher	I-Algorithm
,	O
a	O
single	O
mistake	O
in	O
encryption	O
renders	O
the	O
rest	O
of	O
the	O
message	O
unintelligible	O
.	O
</s>
<s>
Autokey	B-Algorithm
ciphers	I-Algorithm
are	O
somewhat	O
more	O
secure	O
than	O
polyalphabetic	O
ciphers	B-Application
that	O
use	O
fixed	O
keys	O
since	O
the	O
key	O
does	O
not	O
repeat	O
within	O
a	O
single	O
message	O
.	O
</s>
<s>
Therefore	O
,	O
methods	O
like	O
the	O
Kasiski	O
examination	O
or	O
index	B-General_Concept
of	I-General_Concept
coincidence	I-General_Concept
analysis	O
will	O
not	O
work	O
on	O
the	O
ciphertext	O
,	O
unlike	O
for	O
similar	O
ciphers	B-Application
that	O
use	O
a	O
single	O
repeated	O
key	O
.	O
</s>
<s>
The	O
key	O
can	O
be	O
attacked	O
by	O
using	O
a	O
dictionary	O
of	O
common	O
words	O
,	O
bigrams	O
,	O
trigrams	B-General_Concept
etc	O
.	O
</s>
<s>
The	O
message	O
is	O
then	O
encrypted	O
by	O
using	O
the	O
key	O
and	O
the	O
substitution	O
alphabets	O
,	O
here	O
a	O
tabula	B-Algorithm
recta	I-Algorithm
:	O
</s>
