Symmetric key cryptography plaintext encryption ciphertext decryption plaintext message,m algorithm algorithm Ks(m) m=Ks(Ks(m)) symmetric key crypto:Bob and Alice share same (symmetric)key:K e.g.,key is knowing substitution pattern in mono alphabetic substitution cipher Q:how do Bob and Alice agree on key value? Network Security 8-11
Network Security 8-11 Symmetric key cryptography symmetric key crypto: Bob and Alice share same (symmetric) key: K e.g., key is knowing substitution pattern in mono alphabetic substitution cipher Q: how do Bob and Alice agree on key value? ciphertext plaintext K S encryption algorithm decryption algorithm S K S plaintext message, m K (m) S m = KS (KS (m))
Simple encryption scheme substitution cipher:substituting one thing for another monoalphabetic cipher:substitute one letter for another plaintext:abcdefghijklmnopqrstuvwxyz ciphertext:mnbvcxzasdfghjklpoiuytrewq e.g.:plaintext:bob.i love you.alice ciphertext:nkn.s gktc wky.mgsbc Encryon key:mapping from set of 26 letters to set of 26 letters Network Security 8-12
Network Security 8-12 Simple encryption scheme substitution cipher: substituting one thing for another monoalphabetic cipher: substitute one letter for another plaintext: abcdefghijklmnopqrstuvwxyz ciphertext: mnbvcxzasdfghjklpoiuytrewq Plaintext: bob. i love you. alice ciphertext: nkn. s gktc wky. mgsbc e.g.: Encryption key: mapping from set of 26 letters to set of 26 letters
A more sophisticated encryption approach n substitution ciphers,M,M2,...,M cycling pattern: ■e.g,n=4:M1,M3,M4,M3,M2;M1,M3,M4,M3,M2; for each new plaintext symbol,use subsequent subsitution pattern in cyclic pattern dog:d from M1,o from M3,g from M4 ⑥学 Encryption key:n substitution ciphers,and cyclic pattern key need not be just n-bit pattern Network Security 8-13
Network Security 8-13 A more sophisticated encryption approach n substitution ciphers, M1 ,M2 ,…,M n cycling pattern: e.g., n=4: M1 ,M3 ,M4 ,M3 ,M2 ; M1 ,M3 ,M4 ,M3 ,M2 ; .. for each new plaintext symbol, use subsequent subsitution pattern in cyclic pattern dog: d from M1 , o from M3 , g from M 4 Encryption key: n substitution ciphers, and cyclic pattern key need not be just n-bit pattern
Symmetric key crypto:DES DES:Data Encryption Standard US encryption standard [NIST 1993] 56-bit symmetric key,64-bit plaintext input block cipher with cipher block chaining how secure is DES? DES Challenge:56-bit-key-encrypted phrase decrypted (brute force)in less than a day no known good analytic attack making DES more secure: 3DES:encrypt 3 times with 3 different keys Network Security 8-14
Network Security 8-14 Symmetric key crypto: DES DES: Data Encryption Standard US encryption standard [NIST 1993] 56-bit symmetric key, 64-bit plaintext input block cipher with cipher block chaining how secure is DES? DES Challenge: 56-bit-key-encrypted phrase decrypted (brute force) in less than a day no known good analytic attack making DES more secure: 3DES: encrypt 3 times with 3 different keys
Symmetric key 64-bit input 56-bit key permute crypto:DES 48-bi1K1 L1,R1K1) DES operation 12 R2 initial permutation 48-bit K2 2,R2,K2 16 identical“rounds”of function application, R3 each using different 48 bits of key final permutation 48-6itK16 117R17 permute 64-bit output Network Security 8-15
Network Security 8-15 Symmetric key crypto: DES initial permutation 16 identical “rounds” of function application, each using different 48 bits of key final permutation DES operation