CS 682 - Network Security

1. CS 682 - Network Security Lecture 2 Prof. Katz

2. DES – Data Encryption Standard • Private key. Encrypts by series of substitution and transpositions. • Worldwide standard for more than 20 years. • Has a history of controversy. • Designed by IBM (Lucipher) with later help (interference?) from NSA. • No longer considered secure for highly sensitive applications. • Replacement standard (AES) currently in process of development. Lecture 2 - Data Encryption

3. DES - Overview Lecture 2 - Data Encryption

4. DES – Each iteration. Lecture 2 - Data Encryption

5. DES – Computation of F(Ri-1,Ki) Lecture 2 - Data Encryption

6. Computation of F: • Expansion function E: • maps bit string of length 32 to bit string of length 48. • Permutes bits in a fixed way and duplicates certain bits • Key schedule: each round uses a 48 bit key obtained by performing permutations, shifts, and discarding bits from the original 56 bit key. Fixed algorithm for each round • resulting 48 bit string broken into 8 6-bit strings Lecture 2 - Data Encryption

7. S-boxes: S1 Is the table entry from Lecture 2 - Data Encryption

8. Plain text Initial permutation (IP) Round-1 (key K1) Rounds 2-15 Round-16 (key K16) swap IP inverse Cipher text

9. IP Round-1 (K16) = Since encrypt IP inverse Cipher text decrypt

10. DES – Electronic Code Book Mode Lecture 2 - Data Encryption

11. DES – Cipher block chaining mode Lecture 2 - Data Encryption

12. DES Security • S-Box design not well understood (secret). • Has survived some recent sophisticated attacks (differential cryptanalysis). • Key is too short (thanks to NSA!). Hence is vulnerable to brute force attack. • 1998 distributed attack took 3 months. • $1,000,000 machine will crack DES in 35 minutes – 1997 estimate. 10,000 – 2.5 days. • In 1999 EFF achieved 245 billion keys per second rate to crack in 22 hours. Lecture 2 - Data Encryption

13. Double DES • Double DES is almost as easy to break as single DES! Lecture 2 - Data Encryption

14. Triple DES • Triple DES (2 keys) requires 2112 search. Is reasonably secure. 3 keys requires 2168 . Lecture 2 - Data Encryption

15. Other Private Key Cryptosystems • IDEA • Twofish • Blowfish • RC4, RC5, RC6 • Rijndael • Serpent • MARS • Feal Lecture 2 - Data Encryption

16. Message Authentication

17. Message Authentication • We must be able to certify that a message is from a particular person • We must be sure that the message has not been tampered with Lecture 2 - Data Encryption

18. Methods • Conventional Encryption • Message Authentication Code • One-way Hash • Using Conventional Encryption • Using Public-Key Encryption • Using Secret Value Lecture 2 - Data Encryption

19. Conventional Encryption • Modification of the cyphertext should produce unintelligible results in the plaintext. Lecture 2 - Data Encryption

20. One-Way Hash (using encryption) • The message is sent through a hashing function H(M) • The result is encrypted: C = E(K, H(M) • C is appended to the message: N = M||C • N is sent to the recipient • C is extracted from N: N -> M & C • C is decoded: H(M1) = D(K, C) • The recipient puts the message through the hashing function: H(M2) • If H(M1) = H(M2) the message is authentic Lecture 2 - Data Encryption

21. One-Way hash (Public Key) • Same as encryption but encryption Key is private key and decryption key is public key Lecture 2 - Data Encryption

22. One-Way Hash (secret value) • Secret Value (S) is concatenated onto M: N=S||M • N is put through the hash function: H(N) • The result is append to M: C = M||H(N) • C is sent to the recipient • H(N1) is extracted from C • Secret Value (S) is concatenated onto M: N=S||M • N is put through the hash function: H(N2) • If H(N1) = H(N2), the message is authentic. Lecture 2 - Data Encryption