Polygraphic Substitution
• ADFGVX
– Uses a random alphabet table
– Keyword in secondary grid to rearrange
message
• Nihilist cipher
– Uses a Polybius square to change letters into
numbers
– Adds numbers derived from a keyword to
create ciphertext
Polygraphic Substitution
• Straddling checkerboard
– Converts plaintext into digits
– A monome-binome cipher
• VIC
– Uses a straddling checkerboard to obtain
numerical values for letters
– Adds a random number key (using modular
arithmetic)
– Optionally, converts these numbers back into
letters using the straddling checkerboard to
create the final ciphertext
Straddling Checkerboard
0
1
E
T
2
B
C
6
P
Q
2
3
4
5
A
O
N
D
F
G
H
/
U
V
W
6
7
8
9
R
I
S
J
K
L
M
X
Y
Z
.
Polygraphic Substitution
• Bifid cipher
– Combines Polybius square with transposition
– Writing numbers on two lines
Polygraphic Substitution
• Trifid cipher
– Similar to bifid cipher, except each letter is
assigned a 3-digit number using digits 1-3
– Fractionation is accomplished by splitting the
numbers onto three lines and then writing the
resulting ciphertext letters
Polyalphabetic Substitution Ciphers
• Della Porta table
Polyalphabetic Substitution Ciphers
• Vigenère table
– use columns to look up letters of the text
– the key indicates which row to use to encipher
or decipher the message
– autokey improves security
Pictured below:
Open Vigenère table in Excel
Open Rogue’s Gallery next
key numbers used in
Gronsfeld variation
Polyalphabetic Substitution Ciphers
• Rogue’s gallery
– Vigenère table with double columnar
transposition
– advantage is it creates a cipher that is tough
to break
– drawback is it is a complex operation, and
prone to errors
– recipient needs to know the keyword that was
used to create the table and the keyword that
was used to encipher the text
Modern Cryptography
• Hill cipher
– Uses matrices and modular arithmetic
• One-way functions
– Alice & Bob example
The Unbreakable Cipher
• One-time pad is unbreakable if it meets
the following conditions:
– The key must be random.
– The key is as long as the plaintext.
– It must be used only once.
– It must be kept secret from everyone except
the sender and the intended receiver.
Unsolved Mysteries
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Linear A
Etruscan
Iberian
Phaistos Disk
Bronze Age Indus Script
Beale Ciphers
Rongorongo
Runic
Ogham