<s>
In	O
cryptography	O
,	O
a	O
classical	B-Algorithm
cipher	I-Algorithm
is	O
a	O
type	O
of	O
cipher	B-Application
that	O
was	O
used	O
historically	O
but	O
for	O
the	O
most	O
part	O
,	O
has	O
fallen	O
into	O
disuse	O
.	O
</s>
<s>
In	O
contrast	O
to	O
modern	O
cryptographic	O
algorithms	O
,	O
most	O
classical	B-Algorithm
ciphers	I-Algorithm
can	O
be	O
practically	O
computed	O
and	O
solved	O
by	O
hand	O
.	O
</s>
<s>
The	O
term	O
includes	O
the	O
simple	O
systems	O
used	O
since	O
Greek	O
and	O
Roman	O
times	O
,	O
the	O
elaborate	O
Renaissance	O
ciphers	B-Application
,	O
World	O
War	O
II	O
cryptography	O
such	O
as	O
the	O
Enigma	O
machine	O
and	O
beyond	O
.	O
</s>
<s>
Classical	B-Algorithm
ciphers	I-Algorithm
are	O
often	O
divided	O
into	O
transposition	B-Algorithm
ciphers	I-Algorithm
and	O
substitution	O
ciphers	B-Application
,	O
but	O
there	O
are	O
also	O
concealment	B-Algorithm
ciphers	I-Algorithm
.	O
</s>
<s>
In	O
a	O
substitution	O
cipher	B-Application
,	O
letters	O
(	O
or	O
groups	O
of	O
letters	O
)	O
are	O
systematically	O
replaced	O
throughout	O
the	O
message	O
for	O
other	O
letters	O
(	O
or	O
groups	O
of	O
letters	O
)	O
.	O
</s>
<s>
A	O
well-known	O
example	O
of	O
a	O
substitution	O
cipher	B-Application
is	O
the	O
Caesar	O
cipher	B-Application
.	O
</s>
<s>
To	O
encrypt	O
a	O
message	O
with	O
the	O
Caesar	O
cipher	B-Application
,	O
each	O
letter	O
of	O
message	O
is	O
replaced	O
by	O
the	O
letter	O
three	O
positions	O
later	O
in	O
the	O
alphabet	O
.	O
</s>
<s>
Another	O
method	O
of	O
substitution	O
cipher	B-Application
is	O
based	O
on	O
a	O
keyword	O
.	O
</s>
<s>
All	O
spaces	O
and	O
repeated	O
letters	O
are	O
removed	O
from	O
a	O
word	O
or	O
phrase	O
,	O
which	O
the	O
encoder	O
then	O
uses	O
as	O
the	O
start	O
of	O
the	O
cipher	B-Application
alphabet	O
.	O
</s>
<s>
The	O
end	O
of	O
the	O
cipher	B-Application
alphabet	O
is	O
the	O
rest	O
of	O
the	O
alphabet	O
in	O
order	O
without	O
repeating	O
the	O
letters	O
in	O
the	O
keyword	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
the	O
keyword	O
is	O
CIPHER	B-Application
,	O
the	O
cipher	B-Application
alphabet	O
would	O
look	O
like	O
this	O
:	O
</s>
<s>
The	O
previous	O
examples	O
were	O
all	O
examples	O
of	O
monoalphabetic	O
substitution	O
ciphers	B-Application
,	O
where	O
just	O
one	O
cipher	B-Application
alphabet	O
is	O
used	O
.	O
</s>
<s>
It	O
is	O
also	O
possible	O
to	O
have	O
a	O
polyalphabetic	B-Algorithm
substitution	I-Algorithm
cipher	I-Algorithm
,	O
where	O
multiple	O
cipher	B-Application
alphabets	O
are	O
used	O
.	O
</s>
<s>
The	O
encoder	O
would	O
make	O
up	O
two	O
or	O
more	O
cipher	B-Application
alphabets	O
using	O
whatever	O
techniques	O
they	O
choose	O
,	O
and	O
then	O
encode	O
their	O
message	O
,	O
alternating	O
what	O
cipher	B-Application
alphabet	O
is	O
used	O
with	O
every	O
letter	O
or	O
word	O
.	O
</s>
<s>
This	O
makes	O
the	O
message	O
much	O
harder	O
to	O
decode	O
because	O
the	O
codebreaker	O
would	O
have	O
to	O
figure	O
out	O
both	O
cipher	B-Application
alphabets	O
.	O
</s>
<s>
Another	O
example	O
of	O
a	O
polyalphabetic	B-Algorithm
substitution	I-Algorithm
cipher	I-Algorithm
that	O
is	O
much	O
more	O
difficult	O
to	O
decode	O
is	O
the	O
Vigenère	B-Algorithm
square	I-Algorithm
,	O
an	O
innovative	O
encoding	O
method	O
.	O
</s>
<s>
With	O
the	O
square	O
,	O
there	O
are	O
26	O
different	O
cipher	B-Application
alphabets	O
that	O
are	O
used	O
to	O
encrypt	O
text	O
.	O
</s>
<s>
Each	O
cipher	B-Application
alphabet	O
is	O
just	O
another	O
rightward	O
Caesar	O
shift	O
of	O
the	O
original	O
alphabet	O
.	O
</s>
<s>
This	O
is	O
what	O
a	O
Vigenère	B-Algorithm
square	I-Algorithm
looks	O
like	O
:	O
</s>
<s>
To	O
use	O
the	O
Vigenère	B-Algorithm
square	I-Algorithm
to	O
encrypt	O
a	O
message	O
,	O
a	O
coder	O
first	O
chooses	O
a	O
keyword	O
to	O
use	O
and	O
then	O
repeats	O
it	O
until	O
it	O
is	O
the	O
same	O
length	O
as	O
the	O
message	O
to	O
be	O
encoded	O
.	O
</s>
<s>
If	O
is	O
the	O
keyword	O
,	O
each	O
letter	O
of	O
the	O
repeated	O
keyword	O
will	O
tell	O
what	O
cipher	B-Application
(	O
what	O
row	O
)	O
to	O
use	O
for	O
each	O
letter	O
of	O
the	O
message	O
to	O
be	O
coded	O
.	O
</s>
<s>
The	O
cipher	B-Application
alphabet	O
on	O
the	O
second	O
row	O
uses	O
B	O
for	O
A	O
and	O
C	O
for	O
B	O
etc	O
.	O
</s>
<s>
That	O
is	O
cipher	B-Application
alphabet	O
'	O
B	O
 '	O
.	O
</s>
<s>
Each	O
cipher	B-Application
alphabet	O
is	O
named	O
by	O
the	O
first	O
letter	O
in	O
it	O
.	O
</s>
<s>
Some	O
substitution	O
ciphers	B-Application
involve	O
using	O
numbers	O
instead	O
of	O
letters	O
.	O
</s>
<s>
An	O
example	O
of	O
this	O
is	O
the	O
Great	O
Cipher	B-Application
,	O
where	O
numbers	O
were	O
used	O
to	O
represent	O
syllables	O
.	O
</s>
<s>
One	O
example	O
of	O
this	O
is	O
Zodiac	B-Algorithm
alphabet	O
,	O
where	O
signs	O
of	O
the	O
zodiac	B-Algorithm
were	O
used	O
to	O
represent	O
different	O
letters	O
,	O
for	O
example	O
,	O
the	O
symbols	O
for	O
the	O
sun	O
stood	O
for	O
A	O
,	O
Jupiter	O
stood	O
for	O
B	O
,	O
and	O
Saturn	O
stood	O
for	O
C	O
.	O
Dots	O
,	O
lines	O
,	O
or	O
dashes	O
could	O
also	O
be	O
used	O
,	O
one	O
example	O
of	O
this	O
being	O
Morse	O
Code	O
,	O
which	O
is	O
not	O
a	O
cipher	B-Application
,	O
but	O
uses	O
dots	O
and	O
dashes	O
as	O
letters	O
nonetheless	O
.	O
</s>
<s>
The	O
pigpen	B-Algorithm
cipher	I-Algorithm
uses	O
a	O
grid	O
system	O
or	O
lines	O
and	O
dots	O
to	O
establish	O
symbols	O
for	O
letters	O
.	O
</s>
<s>
In	O
a	O
transposition	B-Algorithm
cipher	I-Algorithm
,	O
the	O
letters	O
themselves	O
are	O
kept	O
unchanged	O
,	O
but	O
their	O
order	O
within	O
the	O
message	O
is	O
scrambled	O
according	O
to	O
some	O
well-defined	O
scheme	O
.	O
</s>
<s>
Many	O
transposition	B-Algorithm
ciphers	I-Algorithm
are	O
done	O
according	O
to	O
a	O
geometric	O
design	O
.	O
</s>
<s>
A	O
scytale	O
is	O
a	O
machine	O
that	O
aids	O
in	O
the	O
transposition	B-Algorithm
of	O
methods	O
.	O
</s>
<s>
In	O
a	O
columnar	O
cipher	B-Application
,	O
the	O
original	O
message	O
is	O
arranged	O
in	O
a	O
rectangle	O
,	O
from	O
left	O
to	O
right	O
and	O
top	O
to	O
bottom	O
.	O
</s>
<s>
In	O
the	O
Chinese	O
cipher	B-Application
's	O
method	O
of	O
transposing	O
,	O
the	O
letters	O
of	O
the	O
message	O
are	O
written	O
from	O
right	O
to	O
left	O
,	O
down	O
and	O
up	O
columns	O
to	O
scramble	O
the	O
letters	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
the	O
message	O
needed	O
to	O
be	O
enciphered	O
was	O
THE	O
DOG	O
RAN	O
FAR	O
,	O
the	O
Chinese	O
cipher	B-Application
would	O
look	O
like	O
this	O
:	O
</s>
<s>
Many	O
transposition	B-Algorithm
ciphers	I-Algorithm
are	O
similar	O
to	O
these	O
two	O
examples	O
,	O
usually	O
involving	O
rearranging	O
the	O
letters	O
into	O
rows	O
or	O
columns	O
and	O
then	O
taking	O
them	O
in	O
a	O
systematic	O
way	O
to	O
transpose	O
the	O
letters	O
.	O
</s>
<s>
Other	O
examples	O
include	O
the	O
Vertical	O
Parallel	O
and	O
the	O
Double	O
Transposition	B-Algorithm
Cipher	I-Algorithm
.	O
</s>
<s>
More	O
complex	O
algorithms	O
can	O
be	O
formed	O
by	O
mixing	O
substitution	O
and	O
transposition	B-Algorithm
in	O
a	O
product	B-Algorithm
cipher	I-Algorithm
;	O
modern	O
block	O
ciphers	B-Application
such	O
as	O
DES	B-Algorithm
iterate	O
through	O
several	O
stages	O
of	O
substitution	O
and	O
transposition	B-Algorithm
.	O
</s>
<s>
Put	O
simply	O
,	O
a	O
concealment	O
,	O
or	O
null	O
,	O
cipher	B-Application
is	O
any	O
cipher	B-Application
which	O
involves	O
a	O
number	O
of	O
nulls	O
,	O
or	O
decoy	O
letters	O
.	O
</s>
<s>
A	O
null	B-Algorithm
cipher	I-Algorithm
could	O
be	O
plaintext	O
words	O
with	O
nulls	O
placed	O
in	O
designated	O
areas	O
or	O
even	O
a	O
plaintext	O
message	O
broken	O
up	O
in	O
different	O
positions	O
with	O
a	O
null	O
at	O
the	O
end	O
of	O
each	O
word	O
.	O
</s>
<s>
However	O
,	O
a	O
message	O
with	O
only	O
a	O
couple	O
nulls	O
(	O
for	O
example	O
,	O
one	O
at	O
the	O
beginning	O
and	O
one	O
at	O
the	O
end	O
)	O
is	O
not	O
a	O
null	B-Algorithm
cipher	I-Algorithm
.	O
</s>
<s>
A	O
dot	O
or	O
pinprick	O
null	B-Algorithm
cipher	I-Algorithm
is	O
a	O
common	O
classical	O
encryption	O
method	O
in	O
which	O
dot	O
or	O
pinprick	O
is	O
placed	O
above	O
or	O
below	O
certain	O
letters	O
in	O
a	O
piece	O
of	O
writing	O
.	O
</s>
<s>
Classical	B-Algorithm
ciphers	I-Algorithm
are	O
commonly	O
quite	O
easy	O
to	O
break	O
.	O
</s>
<s>
Many	O
of	O
the	O
classical	B-Algorithm
ciphers	I-Algorithm
can	O
be	O
broken	O
even	O
if	O
the	O
attacker	O
only	O
knows	O
sufficient	O
ciphertext	O
and	O
hence	O
they	O
are	O
susceptible	O
to	O
a	O
ciphertext-only	O
attack	O
.	O
</s>
<s>
Some	O
classical	B-Algorithm
ciphers	I-Algorithm
(	O
e.g.	O
,	O
the	O
Caesar	O
cipher	B-Application
)	O
have	O
a	O
small	O
key	O
space	O
.	O
</s>
<s>
These	O
ciphers	B-Application
can	O
be	O
broken	O
with	O
a	O
brute	O
force	O
attack	O
,	O
that	O
is	O
by	O
simply	O
trying	O
out	O
all	O
keys	O
.	O
</s>
<s>
Substitution	O
ciphers	B-Application
can	O
have	O
a	O
large	O
key	O
space	O
,	O
but	O
are	O
often	O
susceptible	O
to	O
a	O
frequency	O
analysis	O
,	O
because	O
for	O
example	O
frequent	O
letters	O
in	O
the	O
plaintext	O
language	O
correspond	O
to	O
frequent	O
letters	O
in	O
the	O
ciphertexts	O
.	O
</s>
<s>
Polyalphabetic	B-Algorithm
ciphers	I-Algorithm
such	O
as	O
the	O
Vigenère	B-Algorithm
cipher	I-Algorithm
prevent	O
a	O
simple	O
frequency	O
analysis	O
by	O
using	O
multiple	O
substitutions	O
.	O
</s>
<s>
However	O
,	O
more	O
advanced	O
techniques	O
such	O
as	O
the	O
Kasiski	O
examination	O
can	O
still	O
be	O
used	O
to	O
break	O
these	O
ciphers	B-Application
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
modern	O
ciphers	B-Application
are	O
designed	O
to	O
withstand	O
much	O
stronger	O
attacks	O
than	O
ciphertext-only	O
attacks	O
.	O
</s>
<s>
A	O
good	O
modern	O
cipher	B-Application
must	O
be	O
secure	O
against	O
a	O
wide	O
range	O
of	O
potential	O
attacks	O
including	O
known-plaintext	O
attacks	O
and	O
chosen-plaintext	O
attacks	O
as	O
well	O
as	O
chosen-ciphertext	O
attacks	O
.	O
</s>
<s>
For	O
these	O
ciphers	B-Application
an	O
attacker	O
should	O
not	O
be	O
able	O
to	O
find	O
the	O
key	O
even	O
if	O
they	O
know	O
any	O
amount	O
of	O
plaintext	O
and	O
corresponding	O
ciphertext	O
and	O
even	O
if	O
they	O
could	O
select	O
plaintext	O
or	O
ciphertext	O
themselves	O
.	O
</s>
<s>
Classical	B-Algorithm
ciphers	I-Algorithm
do	O
not	O
satisfy	O
these	O
much	O
stronger	O
criteria	O
and	O
hence	O
are	O
no	O
longer	O
of	O
interest	O
for	O
serious	O
applications	O
.	O
</s>
<s>
Some	O
techniques	O
from	O
classical	B-Algorithm
ciphers	I-Algorithm
can	O
be	O
used	O
to	O
strengthen	O
modern	O
ciphers	B-Application
.	O
</s>
<s>
For	O
example	O
,	O
the	O
MixColumns	B-Algorithm
step	O
in	O
AES	B-Algorithm
is	O
a	O
Hill	B-Algorithm
cipher	I-Algorithm
.	O
</s>
