1
Classical Encryption
Techniques
Chapter 2
Internet
Security
2
EE 282
°
classical cipher techniques and terminology
°
monoalphabetic substitution ciphers
°
cryptanalysis using letter frequencies
°
Playfair ciphers
°
polyalphabetic ciphers
°
transposition ciphers
°
product ciphers and rotor machines
°
stenography
Outline
Please also read Chapter2,
– you can skip section on
“Hill Cipher”

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2
Internet
Security
3
EE 282
Introduction: Symmetric Encryption
p
All traditional schemes are
symmetric
/
single key
/
private-key
encryption
algorithms,
p
with a
single key
, used for both
encryption and decryption,
p
both sender and receiver are equivalent,
either can encrypt or decrypt messages
using that common key.
p
was only type of encryption scheme prior
to invention of public-key in 1970’s
Internet
Security
4
EE 282
Basic Terminology
The following
is a brief
review of some of terminology used
throughout the course.
p
plaintext
- the original message
p
ciphertext
- the coded message
p
cipher
- algorithm for transforming plaintext to ciphertext
p
key
- info used in cipher known only to sender/receiver
p
encipher (encrypt)
- converting plaintext to ciphertext
p
decipher (decrypt)
- recovering plaintext from ciphertext
p
cryptography
- study of encryption principles/methods
p
cryptanalysis (codebreaking)
- the study of principles/
methods of deciphering ciphertext
without
knowing key
p
cryptology
- the field of both cryptography and
cryptanalysis

3
Internet
Security
5
EE 282
Symmetric Cipher Model
Symmetric cipher model consists of 5 components (figure below):
•plaintext
•encryption algorithm – performs substitutions/transformations
on plaintext
•secret key – for performing substitutions/transformations
needed in encryption algorithm
•ciphertext,
? decryption algorithm
Internet
Security
6
EE 282
Requirements
p
two requirements for secure use of symmetric
encryption:
°
a strong encryption algorithm
°
a secret key known only to the sender &
receiver
p
For a plaintext
X
,
its cipher text
Y
,
and a key
K
,
the
encryption
transformation
E
K
is denoted by:
Y
= E(K,
X
),
or
E
K
(
X
)
“Y is an encryption of
message
X
under key
K
”
Similarly,
the
decryption algorithm
D
K
is denoted by:
X
= D(K, Y),
or
D
K
(
Y
)
“X is the decryption of Y under
key K”

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