Symmetric Cipher Model Plaintext input 1- encryption algorithm 2- secret key Encryption Cipher text output Cipher text input 1- Decryption algorithm 2-

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Presentation transcript:

Symmetric Cipher Model Plaintext input 1- encryption algorithm 2- secret key Encryption Cipher text output Cipher text input 1- Decryption algorithm 2- 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 4 Ch.2 Classical Encryption Techniques Prepared 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 - Cryptology 1- 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 10 6 decryptions/µs = 4.3  µs= 35.8 minutes2.15 milliseconds = 7.2  µs= 1142 years10.01 hours = 3.4  µs= 5.4  years5.4  years = 3.7  µs= 5.9  years5.9  years 26 characters (permutation) 26! = 4   µs= 6.4  years6.4  10 6 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 abcdefghijklmnopqrstuvwxyz DEFGHIGKLMNOPQRSTUVWXYZABC Letter 3 rd letter

Caesar Cipher C = E(K, P) = (P + K) mod 26 P = D(K, C) = (C - K) mod 26 abcdefghijklm nopqrstuvwxyz

Monoalphabetic Ciphers

Playfair Cipher 5 × 5 matrix Allied forces “MONARCHY” MONAR CHYBD EFGI/JK LPQST UVWXZ 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 C i = (p i + k i mod m ) mod 26 p i = (C i - k i 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

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