<s>
The	O
D'Agapeyeff	B-Algorithm
cipher	I-Algorithm
is	O
a	O
cipher	B-Application
that	O
appears	O
in	O
the	O
first	O
edition	O
of	O
Codes	O
and	O
Ciphers	B-Application
,	O
an	O
elementary	O
book	O
on	O
cryptography	O
published	O
by	O
the	O
Russian-born	O
English	O
cryptographer	O
and	O
cartographer	O
Alexander	B-Algorithm
D'Agapeyeff	I-Algorithm
in	O
1939	O
.	O
</s>
<s>
Offered	O
as	O
a	O
"	O
challenge	O
cipher	B-Application
"	O
at	O
the	O
end	O
of	O
the	O
book	O
,	O
the	O
ciphertext	O
is	O
:	O
</s>
<s>
It	O
was	O
not	O
included	O
in	O
later	O
editions	O
,	O
and	O
D'Agapeyeff	B-Algorithm
is	O
said	O
to	O
have	O
admitted	O
later	O
to	O
having	O
forgotten	O
how	O
he	O
had	O
encrypted	O
it	O
.	O
</s>
<s>
It	O
is	O
possible	O
that	O
not	O
all	O
the	O
ciphertext	O
characters	O
are	O
used	O
in	O
decryption	O
and	O
that	O
some	O
characters	O
are	O
nulls	B-Algorithm
.	O
</s>
<s>
Evidence	O
for	O
this	O
is	O
given	O
by	O
the	O
author	O
on	O
p.111	O
of	O
the	O
text	O
under	O
the	O
sub-section	O
heading	O
Military	O
Codes	O
and	O
Ciphers	B-Application
:	O
</s>
<s>
"	O
The	O
cipher	B-Application
is	O
of	O
course	O
easily	O
made	O
out	O
,	O
but	O
if	O
every	O
third	O
,	O
fourth	O
,	O
or	O
fifth	O
letter	O
,	O
as	O
may	O
be	O
previously	O
arranged	O
,	O
is	O
a	O
dummy	O
inserted	O
after	O
a	O
message	O
has	O
been	O
put	O
into	O
cipher	B-Application
,	O
it	O
is	O
then	O
extremely	O
difficult	O
to	O
decipher	O
unless	O
you	O
are	O
in	O
the	O
secret.	O
"	O
</s>
<s>
While	O
the	O
index	B-General_Concept
of	I-General_Concept
coincidence	I-General_Concept
for	O
the	O
D'Agapeyeff	B-Algorithm
cipher	I-Algorithm
is	O
1.812	O
when	O
taken	O
in	O
pairs	O
horizontally	O
(	O
e.g.	O
,	O
'	O
75	O
 '	O
'	O
62	O
 '	O
'	O
82	O
 '	O
)	O
,	O
the	O
letter	O
frequency	O
distribution	O
is	O
too	O
flat	O
for	O
a	O
196	O
character	O
message	O
written	O
in	O
English	O
.	O
</s>
<s>
Additionally	O
,	O
D'Agapeyeff	B-Algorithm
left	O
two	O
ciphers	B-Application
for	O
the	O
reader	O
to	O
solve	O
.	O
</s>
<s>
Each	O
are	O
approximately	O
100	O
characters	O
in	O
length	O
and	O
have	O
an	O
index	B-General_Concept
of	I-General_Concept
coincidence	I-General_Concept
much	O
higher	O
than	O
what	O
is	O
expected	O
for	O
English	O
plaintext	O
.	O
</s>
<s>
The	O
structure	O
of	O
the	O
D'Agapeyeff	B-Algorithm
Cipher	I-Algorithm
has	O
similarities	O
to	O
the	O
Polybius	O
square	O
,	O
which	O
the	O
author	O
used	O
as	O
examples	O
in	O
his	O
book	O
.	O
</s>
<s>
He	O
explicitly	O
solves	O
an	O
example	O
of	O
a	O
Polybius	O
square	O
based	O
cipher	B-Application
from	O
a	O
friend	O
in	O
his	O
cryptanalysis	O
section	O
of	O
the	O
book	O
.	O
</s>
