Cryptography Symmetric Encryption Week 9 Stallings: Ch 3 & 6 Stallings: Ch 4 CNT-4403: 19.March.2015 1 Today’s Class . Symmetric Ciphers . Multiple Encryption . Modes of Operation CNT-4403: 19.March.2015 2 Symmetric Cryptosystems Encryption Key Decryption Key Plaintext Plaintext Encryption Decryption Algorithm = Algorithm CNT-4403: 19.March.2015 3 Symmetric Ciphers . Stream Ciphers . Block Ciphers CNT-4403: 19.March.2015 4 Stream Ciphers . Encrypt one bit (byte) at a time . Example: Vigenere, Vernam . Length of key = length of (clear/cipher) text . Hard to share between sender and receiver Bit-stream Bit-stream Key (K) Key (K) generator generator Key ki Key ki Plaintext Ciphertext Plaintext Pi Ci Pi Plaintext Plaintext CNT-4403: 19.March.2015 5 Block Ciphers . Encrypt one block of text at a time . 64-128 bit long . Encryption key = Decryption key Focus: Block Ciphers! . Shared by sender and receiver 64 bits 64 bits Key (K) Encryption Algorithm Plaintext Ciphertext CNT-4403: 19.March.2015 6 Block Cipher Principles . n bit input to n bit output . 2n possible inputs . Each must produce a unique cipertext . Otherwise encryption is not reversible . No decryption possible CNT-4403: 19.March.2015 7 Ideal Block Cipher Need 2n table to encrypt! CNT-4403: 19.March.2015 8 Feistel Cipher . Introduced by Horst Feistel . 16 + 1 rounds Plaintext What is F ? LE0 RE0 F K1 Li = Ri–1 LE1=RE0 RE1 Ri = Li–1 F(Ri–1, Ki) CNT-4403: 19.March.2015 9 Feistel Cipher Structure CNT-4403: 19.March.2015 10 Data Encryption Standard (DES) . Most widely used block cipher in world . Adopted in 1977 by NBS (now NIST) . As FIPS PUB 46 . Encrypts 64-bit data using 56-bit key . Has been considerable controversy over its security CNT-4403: 19.March.2015 11 DES History . IBM developed Lucifer cipher . Team led by Feistel in late 60’s . Used 64-bit data blocks with 128-bit key . Redeveloped as a commercial cipher with input from NSA and others . 1973: National Bureau of Standards (NBS) issued request for proposals for a national cipher standard . IBM submitted their revised Lucifer which was eventually accepted as the DES CNT-4403: 19.March.2015 12 DES Controversy . DES standard is public . Considerable controversy over design . Choice of 56-bit key (vs Lucifer 128-bit) . Design criteria were classified . Subsequent events and public analysis show in fact design was appropriate . Use of DES has flourished . Especially in financial applications . Still standardised for legacy application use . To be replaced by AES CNT-4403: 19.March.2015 13 DES Encryption CNT-4403: 19.March.2015 14 Initial Permutation (IP) . First step of the data computation . IP reorders the input data bits . Even bits to LH half, odd bits to RH half . Quite regular in structure (easy in h/w) . Example: IP(675a6967 5e5a6b5a) = (ffb2194d 004df6fb) CNT-4403: 19.March.2015 15 DES “Round” Structure . Uses two 32-bit L & R halves . Feistel cipher: Li = Ri–1 Ri = Li–1 F(Ri–1, Ki) CNT-4403: 19.March.2015 16 DES Structure: Function F F takes 32-bit R half and 48-bit subkey: . Expands R to 48-bits using perm E . Adds to subkey using XOR . 8 S-boxes to get 32-bit result . Finally permutes using 32-bit perm P CNT-4403: 19.March.2015 17 .