Symmetric key cryptography A-B A-B plaintext encryption ciphertext decryption plaintext message,m algorithm algorithm KA-m) m=KA-(K-e(m)) symmetric key crypto:Bob and Alice share know 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? 8:Network Security 8-11 8: Network Security 8-11 Symmetric key cryptography symmetric key crypto: Bob and Alice share know 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 KA-B encryption algorithm decryption algorithm A-B KA-B plaintext message, m K (m) A-B K (m) A-B m = K ( ) A-B