Cryptography Reference
In-Depth Information
For instance, the row labelled
f
intersects with the column labelled
x
at the
ciphertext letter
C
, and so on. To decipher, the receiver knows the priming
key
x
, so this letter is placed above the ciphertext letter
C
and looks in the row
labelled
x
to find the letter
C
, then the label of the column in which
C
sits is the
plaintext, namely
f
, and so on, as follows.
x
f
o
r
m
s
e
c
r
e
t
d
i
c
t
i
o
C
T
F
D
E
W G T
V
X
W L
K V
B
W B
f
o
r
m
s
e
c
r
e
t
d
i
c
t
i
o
n
Unfortunately, as is the case too often, Vigenere's idea was forgotten and
reinvented at the end of the nineteenth century. However, what was rediscovered
was a weakened version of his idea. Essentially it amounted to exactly what
Belaso has discovered, which we discussed on page 53, applied to the Vigenere
square rather than that of Trithemius, so we need not replay it here.
One obvious improvement to the above is to have not a single priming key
letter, but rather a priming
keyphrase
. Moreover, in the interests of security,
the keyphrase should be as long as possible and feasible. Later we will see a
very secure cipher where the key is as long as the plaintext itself, called the
one-time pad
(page 83). For instance, consider the following depiction of the
more general idea of extending Vigenere's idea to a keyphrase.
Example 1.4
Suppose that we want to encipher, again:
form secret diction
,
but this time using the priming
keyphrase
:
“xanadu”
. Then we proceed as in
steps
(a)-(c)
on page 56, this time with our more general keyphrase sitting below
plaintext letters before introducing the plaintext into the key, as follows.
f
o
r
m
s
e
c
r
e
t
d
i
c
t
i
o
n
x
a
n
a
d
u
f
o
r
m
s
e
c
r
e
t
d
C
O E M V
Y
H
F V
F
V
M E K M H
Q
Then to decipher, we proceed as in Example 1.3, but with the full keyphrase
this time, rather than the key letter, as follows.
x
a
n
a
d
u
f
o
r
m
s
e
c
r
e
t
d
C
O E M V
Y
H
F V
F
V
M E K M H
Q
f
o
r
m
s
e
c
r
e
t
d
i
c
t
i
o
n
The Vatican and Cipher Secretariats
Before we meet our next character, who will help us close the door on the
Renaissance and this section, we must backtrack a bit in time to set the stage
in another scene populated by the Italian City States, the Vatican, and Cipher
Secretariats.
In Pavia, Italy on July 4, 1474, Cicco Simonetta, secretary to the Dukes of
Sfoza, oligarchs of Milan, wrote the first known manuscript devoted
solely
to
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