Ch 3: Block Ciphers and the Data Encryption Standard

1. Information and Network Security Ch 3: Block Ciphers and the Data Encryption Standard

2. Modern Block Ciphers • will now look at modern block ciphers • one of the most widely used types of cryptographic algorithms • provide secrecy and/or authentication services • in particular will introduce DES (Data Encryption Standard)

3. Block vs Stream Ciphers • block ciphers process messages in into blocks, each of which is then en/decrypted • like a substitution on very big characters • 64-bits or more • stream ciphers process messages a bit or byte at a time when en/decrypting • many current ciphers are block ciphers • hence are focus of course

4. Exclusive Or • Encryption: • Plain text 10011110000 xor • Key 10011001100 • ---------------------------------- • Cipher text 00000111100 • Decryption: • Cipher text 00000111100 xor • Key 10011001100 • ---------------------------------- • Plain text 10011110000

22. K1 XOR XOR Fk Steps for S-DES

23. Another Complete Example for S-DES

33. Data Encryption Standard

34. Data Encryption Standard (DES( Key Generation

36. More Symmetric Ciphers • IDEA (International Data Encryption Algorithm) • - block size 64 bits • - key size 128 bits • - Easy to implement

37. More Symmetric Ciphers • BLOWFISH • Block size 64 bits • Key size variable up to 448 bits. • Fast, simple, and compact (can run in less than 5K of memory)

38. More Symmetric Ciphers • RC5 • Block size variable 16, 32, and 64 bits • Key size (in number of bytes) 0-255 • Number of rounds 0-255 • Fast, Simple, High security (with suitable parameters). • Suitable for hardware or software.

39. More Symmetric Ciphers • CAST-128 • Block size 64 bits • Key size variable 40 - 128 bits • Similar to DES but with two keys at each round and function F depends on the round. • Used with PGP (Pretty Good Privacy)