Symmetric Cipher Model

Plaintext

input

1- encryption algorithm

2- secret key

Encryption

Cipher text

output

Cipher text

input

1- Decryption algorithm2- secret key

Decryption

Plaintext

output

Symmetric Cipher Model• Plaintext: The original intelligible message or data that is fed

into the algorithm as input.• Encryption algorithm: The encryption algorithm performs

various substitutions and transformations on the plaintext.• Secret key: The secret key is also an input to the encryption

algorithm.• Ciphertext: The scrambled unintelligible message produced as

output. • Decryption algorithm: It takes the ciphertext and the secret

key to produce the original plaintext.

Symmetric Cipher Model

Computer Security(CS4800)

Lecture 4Ch.2

Classical Encryption TechniquesPrepared by Dr. Lamiaa Elshenawy

Security Mathematics

• Encryption Y = E(K, X)

•Decryption X = D(K, Y)

Chipertext Plaintext

Secrete key

Encryption algorithm

Decryption algorithm

Security Requirements

• Strong encryption algorithm

• Secret key should be secret (sender/receiver)

Symmetric Cipher Model

- Cryptology1- Cryptography (enciphering)

2- Cryptanalysis (deciphering)

- Cryptanalyst

Cryptography

• Encryption techniques Substitution techniques Transposition techniques

• Secret keys Symmetric (single-key) Asymmetric (two-key)

• Plaintext processing Block cipher Stream cipher

Cryptanalysis and Brute-Force Attack

Cryptanalysis ( plaintext-ciphertext pairs)

Brute-force attack (try possible keys)

• Objective recover the key

Cryptanalysis and Brute-Force Attack

• Unconditionally secure

• Computationally secure

Cost of breaking cipher > value of encrypted information.

Time of breaking cipher > lifetime of information.

• 4-bit key • 56-bit key (DES)• 128-bit key (AES)

Key Size (bits) Number of Alternative Keys

Time required at 1 decryption/µs

Time required at 106 decryptions/µs

32 232 = 4.3 109 231 µs = 35.8 minutes 2.15 milliseconds

56 256 = 7.2 1016 255 µs = 1142 years 10.01 hours

128 2128 = 3.4 1038 2127 µs = 5.4 1024 years 5.4 1018 years

168 2168 = 3.7 1050 2167 µs = 5.9 1036 years 5.9 1030 years

26 characters (permutation)

26! = 4 1026 2 1026 µs = 6.4 1012 years 6.4 106 years

Cryptanalysis and Brute-Force Attack

Symmetric Cipher Techniques

• Caesar Cipher• Monoalphabetic Ciphers• Playfair Cipher• Hill Cipher• Polyalphabetic Ciphers (Vigenère cipher–Vernam cipher)• One-Time Pad

- Substitution techniques

- Transposition techniques• Rail fence

Caesar Cipher

• plain: meet me after the party• cipher: PHHW PH DIWHU WKH SDUWB

a b c d e f g h i j k l m n o p q r s t u v w x y z

D E F G H I G K L M N O P Q R S T U V W X Y Z A B C

• Letter 3rd letter

Caesar Cipher

• C = E(K, P) = (P + K) mod 26• P = D(K, C) = (C - K) mod 26

a b c d e f g h i j k l m0 1 2 3 4 5 6 7 8 9 10 11 12

n o p q r s t u v w x y z

13 14 15 16 17 18 19 20 21 22 23 24 25

Monoalphabetic Ciphers

Monoalphabetic Ciphers

Playfair Cipher

• 5 × 5 matrix• Allied forces “MONARCHY”

M O N A R

C H Y B D

E F G I/J K

L P Q S T

U V W X Z

World War II

Hill Cipher

• C = E(K, P) = PK mod 26• P = D(K, C) = CK-1 mod 26• For 3 × 3 matrix

Polyalphabetic Ciphers

• Vigenère Cipher• Ci = (pi + ki mod m) mod 26• pi = (Ci - ki mod m) mod 26

Vigenère Cipher

“deceptive”

Vernam cipher

One-Time Pad

• Random key• Unbreakable

Transposition techniques

• Rail fence

“meet me after the toga party”

Symmetric Cipher Techniques

• Rotor machines

Hebern rotor machine

• Steganography• Character marking• Invisible ink• Pin puncture• Type writer correction ribbon

Thank you for your attention