Cryptography Reference
In-Depth Information
letter in the message is shifted by a specific number of letters that has been agreed on by
the sender and the receiver. This cipher is categorized under monoalphabetic ciphers and
can be easily broken by listing the alphabet repeatedly, aligning each letter sequentially
with succeeding letters, until the message is decrypted.
Most early cryptographic schemes can be categorized as symmetric key cryptographic
systems. The Middle Ages did not see any major advancement in the art and science of
cryptography. In 1467, Leon Battista Alberti invented the polyalphabetic cipher. This is
simply a block cipher in which the plain text is mapped to one of several possibilities in
the cipher text and vice versa. However, polyalphabetic ciphers went through a variety
of changes, and the modern version is attributed to Vigenere. For example, let the key-
word be “MEC” and the plain text be “We need more supplies fast.” Using the Vigenere
square table (Table 4.1), the Vigenere cipher is
M
E
C
M
E
C
M
E
C
M
E
C
M
E
C
M
E
C
M
E
C
M
w Ğ Ŷ
e
e
d
m
o
r
e
s
u
p
p
l
i
e
s Ĩ
a
s
t
I
I
P
Q
I
F
Y
S
T
Q
W
W
B
T
N
U
I
U
R
E
U
F
Table 4.1. Vigenere Cipher Square Table
a b c d e f g h i j k l mn op q r s t u v w x y z
1 B C D E F G H I J K L MNOP QRS T U V W X Y Z A
2 C D E F G H I J K L MNOP QRS T UV W X Y Z AB
3 D E F G H I J K L MNOP QRS T UVW X Y Z ABC
4 E F G H I J K L MNOP QRS T UVWX Y Z A BCD
5 F G H I J K L M NOP QRS T UVWX Y Z A B CDE
6 G H I J K L M N OP QRS T UVWX Y Z A B C DE F
7 H I J K L M N O P QRS T UVWX Y Z A B C D E F G
8 I J K L MN O P QRS T UVWX Y Z AB C D E F GH
9 J K L M N O P Q RS T UVWX Y Z ABC D E F GHI
10 K L M N O P Q R S T UVWX Y Z ABCD E F G HI J
11 L M N O P Q R S T UVWX Y Z ABCDE F G H I J K
12 M N O P Q R S T UVWX Y Z ABCDE F G H I J K L
13 N O P Q R S T U VWX Y Z ABCDE F G H I J K L M
14 O P Q R S T U V WX Y Z ABCDE F GH I J K L MN
15 P Q R S T U V W X Y Z ABCDE F GHI J K L MNO
16 Q R S T U V W X Y Z ABCDE F GHI J K L MNOP
17 R S T U V W X Y Z ABCDE F GHI J K L M N OP Q
18 S T U V W X Y Z ABCDE F GHI J K L M N O P QR
19 T U V W X Y Z A BCDE F GHI J K L M N O P QRS
20 U V W X Y Z A B CDE F GHI J K L MN O P Q RS T
21 V W X Y Z A B C DE F GHI J K L MNO P Q R S T U
22 W X Y Z A B C D E F GHI J K L MNOP Q R S T UV
23 X Y Z A B C D E F GHI J K L MNOP Q R S T UVW
24 Y Z A B C D E F GHI J K L MNOP QR S T U VWX
25 Z A B C D E F G HI J K L MNOP QRS T U V WX Y
26 A B C D E F G H I J K L MNOP QRS T U V WX Y Z
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