1. 1 Symmetric key cryptography: DES DES: Data Encryption Standard US encryption standard [NIST 1993] 56-bit symmetric key, 64 bit plaintext input How secure is DES? –DES Challenge: 56-bit-key-encrypted phrase (“Strong cryptography makes the world a safer place”) decrypted (brute force) in 4 months –no known “backdoor” decryption approach making DES more secure –use three keys sequentially (3-DES) on each datum –use cipher-block chaining

2. 2 Secret Key (DES) Plaintext Encrypt with secret key Ciphertext Plaintext Decrypt with secret key

3. 3 Symmetric key crypto: DES initial permutation 16 identical “rounds” of function application, each using different 48 bits of key final permutation DES operation Note: Refer to text for exclusive details

4. 4 Cipher Block Chaining Block 1 IV DES Cipher 1 Block 2 DES Block 3 DES Block 4 DES + Cipher 2 3 4 +++ IV : Initialization vector is used instead of for nonexistent ciphertext for block 0