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:Ks e.g.,key is knowing substitution pattern in mono alphabetic substitution cipher Q:how do Bob and Alice agree on key value? 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)) Security 8-11
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 Encryption key:mapping from set of 26 letters to set of 26 letters 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 Security 8-12
A more sophisticated encryption approach n substitution ciphers,M,M2,...,M cycling pattern: ·eg,n=4:M1,M3,M4,M3,M2;M1,M3,M4,M3,M2; for each new plaintext symbol,use subsequent substitution pattern in cyclic pattern dog:d from Mi,o from M3,g from M4 Encryption key:n substitution ciphers,and cyclic pattern key need not be just n-bit pattern Security 8-13
A more sophisticated encryption approach ▪ n substitution ciphers, M1 ,M2 ,…,Mn ▪ cycling pattern: • e.g., n=4: M1 ,M3 ,M4 ,M3 ,M2 ; M1 ,M3 ,M4 ,M3 ,M2 ; .. ▪ for each new plaintext symbol, use subsequent substitution pattern in cyclic pattern • dog: d from M1 , o from M3 , g from M4 Encryption key: n substitution ciphers, and cyclic pattern • key need not be just n-bit pattern Security 8-13
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 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 Security 8-14
64-bit inpul 56bit key Symmetric key permute crypto:DES P) 48-bi1K1 CL1,R1KID DES operation 12 R2 initial permutation 48-bit K2 I6 identical“rounds'”of f2.R2,K2 function application, R3 each using different 48 bits of key final permutation 48bi0K16 L17R17 permute 64-bit output 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 Security 8-15