Cryptography Reference
In-Depth Information
We begin with some very basic examples of historical cryptosystems.
2.1.1
Caesar Cipher
The
Caesar Cipher
tends to be the first example in any introduction to
cryptography. Although it is a very straightforward cryptosystem, there are
basic lessons to be learned from studying it. The cryptosystem that we describe
is sometimes referred to as the
Shift Cipher
, with the name 'Caesar Cipher'
occasionally used in a more restricted way to describe the Shift Cipher when
the particular shift of three is used.
DESCRIPTION OF THE CAESAR CIPHER
The idea of the Caesar Cipher is to encrypt each letter of the alphabet by the
letter obtained by 'shifting' the alphabet a secret number of positions. One way of
visualising this is to:
1. Write out the plaintext letters of the alphabet A to Z, in order, twice (one after
the other).
2. Imagine a 'sliding ruler' consisting of one copy of the ciphertext letters of the
alphabet A to Z, in order. This 'sliding ruler' can be shifted to any position
underneath the plaintext letters.
3. Sender Alice and receiver Bob both agree on a secret
shift
, which is represented
by a number between 0 and 25.
In order to encrypt a plaintext letter, Alice positions the sliding ruler underneath
the first set of plaintext letters and slides it to the right by the number of positions
of the secret shift. The plaintext letter is then encrypted to the ciphertext letter
underneath it on the sliding ruler.
The result of this process is depicted in Figure 2.1 for an agreed shift of three
positions. In this case, plaintext HOBBY is encrypted to ciphertext KREEB.
On receiving the ciphertext, Bob, who also knows the secret shift, positions his
sliding ruler in the same way as Alice. He then replaces the ciphertext letter on
the sliding ruler with the plaintext letter above it. Hence ciphertext NHVWUHO
is decrypted to KESTREL.
A B C D
E
F G H
I
J
... O ... W X
Y
Z
D
E
F
G
H
I
J
K
L
M
...
R
...
Z
A
B
C
Figure 2.1.
Encryption using the Caesar Cipher with a shift of three
