11. Encryption (shared key) - Sender and receiver agree on a key K - No one else knows K - K is used to derive encryption key EK & decryption key DK - Sender computes and sends EK (Message) - Receiver computes DK ( EK (Message)) - Example: DES: Data Encryption Standard m : message k : shared key
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13. Digital signature Sign: sign(sk,m) = Dsk (m) Verify: Epk (sign(sk,m)) = m Sign on small hash function to reduce cost
14. Signed and secret messages sign(sk1, m) Encrypt(pk2) m Decrypt(sk2) Verify-sign Encrypt(pk1) Epk2 ( Dsk1 (m)) pk1 pk2 First sign, then encrypt: order is important.