<s>
A	O
null	B-Algorithm
cipher	I-Algorithm
,	O
also	O
known	O
as	O
concealment	O
cipher	B-Application
,	O
is	O
an	O
ancient	O
form	O
of	O
encryption	O
where	O
the	O
plaintext	O
is	O
mixed	O
with	O
a	O
large	O
amount	O
of	O
non-cipher	O
material	O
.	O
</s>
<s>
This	O
is	O
one	O
of	O
3	O
categories	O
of	O
cipher	B-Application
used	O
in	O
classical	B-Algorithm
cryptography	I-Algorithm
along	O
with	O
Substitution	O
ciphers	B-Application
and	O
Transposition	B-Algorithm
ciphers	I-Algorithm
.	O
</s>
<s>
In	O
classical	B-Algorithm
cryptography	I-Algorithm
,	O
a	O
null	B-Language
is	O
intended	O
to	O
confuse	O
the	O
cryptanalyst	O
.	O
</s>
<s>
In	O
the	O
most	O
common	O
form	O
of	O
a	O
null	B-Algorithm
cipher	I-Algorithm
,	O
the	O
plaintext	O
is	O
included	O
within	O
the	O
ciphertext	O
and	O
one	O
needs	O
to	O
discard	O
certain	O
characters	O
in	O
order	O
to	O
decrypt	O
the	O
message	O
(	O
such	O
as	O
first	O
letter	O
,	O
last	O
letter	O
,	O
third	O
letter	O
of	O
every	O
second	O
word	O
,	O
etc	O
.	O
)	O
</s>
<s>
Most	O
characters	O
in	O
such	O
a	O
cryptogram	O
are	O
nulls	B-Language
,	O
only	O
some	O
are	O
significant	O
,	O
and	O
some	O
others	O
can	O
be	O
used	O
as	O
pointers	O
to	O
the	O
significant	O
ones	O
.	O
</s>
<s>
Here	O
is	O
an	O
example	O
null	B-Algorithm
cipher	I-Algorithm
message	O
,	O
sent	O
by	O
a	O
German	O
during	O
World	O
War	O
I	O
:	O
</s>
<s>
Historically	O
,	O
users	O
of	O
concealment	O
ciphers	B-Application
often	O
used	O
substitution	O
and	O
transposition	B-Algorithm
ciphers	I-Algorithm
on	O
the	O
data	O
prior	O
to	O
concealment	O
.	O
</s>
<s>
For	O
example	O
,	O
Cardinal	O
Richelieu	O
is	O
said	O
to	O
have	O
used	O
a	O
grille	B-Algorithm
to	O
write	O
secret	O
messages	O
,	O
after	O
which	O
the	O
blank	O
spaces	O
were	O
filled	O
out	O
with	O
extraneous	O
matter	O
to	O
create	O
the	O
impression	O
of	O
a	O
continuous	O
text	O
.	O
</s>
<s>
A	O
dot	O
or	O
pinprick	O
concealment	O
cipher	B-Application
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>
The	O
acrostic	O
puzzle	O
is	O
an	O
extended	O
form	O
of	O
null	B-Algorithm
cipher	I-Algorithm
,	O
but	O
not	O
an	O
Anacrostic	O
(	O
in	O
which	O
you	O
use	O
a	O
set	O
of	O
lettered	O
clues	O
with	O
numbered	O
blanks	O
representing	O
the	O
letters	O
of	O
the	O
answer	O
to	O
figure	O
out	O
the	O
second	O
part	O
,	O
a	O
long	O
series	O
of	O
numbered	O
blanks	O
and	O
spaces	O
representing	O
a	O
message	O
into	O
which	O
the	O
answers	O
for	O
the	O
clues	O
fit	O
)	O
.	O
</s>
<s>
Acrostics	O
in	O
poems	O
and	O
other	O
written	O
works	O
that	O
are	O
not	O
referenced	O
to	O
are	O
a	O
type	O
of	O
Null	B-Algorithm
cipher	I-Algorithm
.	O
</s>
<s>
Put	O
simply	O
,	O
a	O
null	B-Algorithm
cipher	I-Algorithm
is	O
any	O
cipher	B-Application
which	O
involves	O
a	O
number	O
of	O
nulls	B-Language
,	O
or	O
decoy	O
letters	O
.	O
</s>
<s>
As	O
well	O
as	O
the	O
methods	O
shown	O
above	O
,	O
a	O
null	B-Algorithm
cipher	I-Algorithm
could	O
be	O
plaintext	O
words	O
with	O
nulls	B-Language
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	B-Language
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	B-Language
(	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
null	B-Algorithm
cipher	I-Algorithm
is	O
technically	O
only	O
a	O
term	O
for	O
hidden	O
letters	O
or	O
words	O
within	O
an	O
otherwise	O
unimportant	O
message	O
,	O
however	O
,	O
it	O
is	O
occasionally	O
used	O
for	O
the	O
entire	O
category	O
of	O
concealment	O
ciphers	B-Application
.	O
</s>
<s>
Null	B-Algorithm
ciphers	I-Algorithm
in	O
modern	O
times	O
are	O
used	O
by	O
prison	O
inmates	O
in	O
an	O
attempt	O
to	O
have	O
their	O
messages	O
pass	O
inspection	O
.	O
</s>
<s>
Null	B-Algorithm
ciphers	I-Algorithm
are	O
one	O
of	O
three	O
major	O
cipher	B-Application
types	O
in	O
classical	B-Algorithm
cryptography	I-Algorithm
,	O
but	O
it	O
is	O
less	O
well	O
known	O
as	O
the	O
others	O
.	O
</s>
<s>
It	O
is	O
often	O
marked	O
as	O
a	O
subcategory	O
of	O
transposition	B-Algorithm
ciphers	I-Algorithm
,	O
but	O
that	O
is	O
not	O
true	O
,	O
as	O
transposition	B-Algorithm
ciphers	I-Algorithm
are	O
scrambled	O
messages	O
.	O
</s>
