Using Cryptography

1. Using Cryptography More on Symmetric Ciphers, Signatures, Hash functions, and Certificates

2. More on Symmetric Ciphers Block Cipher Modes and Stream Ciphers

3. Using a Block Cipher • ECB Mode • Problem? P1 P2 … AES AES Key Key C1 C2

4. Using a Block Cipher • CBC Mode • What is the benefit? P1 P2 IV   C1 … AES AES Key Key C1 C2

5. How bad is ECB? • An image representation

6. Stream Ciphers • OTP • Good, but impractical • Can encrypt one letter at a time • A computerized approximation 10100101010010101001101001… PT  CT

7. Randomness • Algorithm • Not random (by definition!) • [example] • Something better • RC4 • Key used to “seed” • Better randomness properties

8. Digital Signatures

9. Signatures • Suppose … • Alice could encrypt with her private key • And Bob could decrypt it with her public key • Why? • Sign (given private key) S = EKRa(M) • Verify (given public key) M = EKUa(S)

10. More for your Metaphor • A special lockbox KRa KUa

11. Test Protocols • What security properties do you get? 1. A to B: E[KRB,M] 2. B to A: E[KRB,Ks], E[Ks,M] 3. A to B: E[KUB,E[KRA,M]] 4. B to A: E[KRB,Ks||E[Ks,M]] 5. A to B: E[KRA,M]

12. Cryptographic Hash Functions

13. Hash Functions • Purpose • pack data into a fixed size container • Random-looking • Reproducible • Does NOT provide integrity • Benefits • Fixed size • typically smaller than the data • Can keep data in clear text

14. 0 0 1 Example • A simple hash • Bitwise XOR of all blocks

15. A More Complex Hash • Using a block cipher • Use known IV for the key (e.g. all 0’s) • XOR all ciphertexts together • CBC Mode • Take the last ciphertext P1 P2 0   C1 … AES AES Key Key C1 C2

16. What is a Hash Function? • Basic Requirements: • take input of any size • produce a fixed-length output • easy to compute • Collision Resistance • given x, hard to find y s.t. H(y) = H(x)

17. Signatures • PKC is expensive • Sign a large message M • Sign the hash • Verify by doing what? • Nobody can modify M • Unless …

18. Collision (non-)Resistance Alice sends this message M: To LocalBank: I have enclosed $10,000 to be deposited in my beloved son Anthony’s money market account (#12345) so that I may help him pay for graduate school. • -Alice She signs a 16-bit hash h = H(M) and sends this signature

19. An Evilsmith Production To LocalBank: I have enclosed a check for $10,000 to be deposited in my beloved friend Mallory Evilsmith’s money market account (#98765) so that I may help him pay for graduate school. He’s such a hard-working, honest student, and he always follows the code of conduct for his classes in information security. • -Alice

20. Many Equivalent Letters { - || To} LocalBank { - || , Inc.} : I {have enclosed || am including} { - || a check for} {$10,000 || ten thousand dollars} to {be deposited || deposit} in my {beloved || favorite} {friend || pal} {- || Mr.} {Mallory || M.} Evilsmith’s money market account {(#98765) || (no. 98765)} so that I {may || can} help him {pay for || afford} {attending || - } graduate school { - || at UTA}. {He’s || he is} such a {hard-working || diligent}, {honest || responsible} {student || person}, and he always follows the code of conduct for his {classes || courses} in {computer || information} security. • - Alice • 221 possibilities

21. Collision Non-Resistance • Alternately • “space-space-backspace” • Replace some with “space-backspace-space” • Images • Small low-order bit changes in a bitmap • Re-compress into JPEG • Cost to the attacker • How much?

22. MAC • “Keyed hash” • Provides authentication and integrity • MAC algorithms • CBC-MAC (Uses key in the hashing) • HMAC (Hash with a secret)

23. Fix These Protocols • Authentication For 2. and 3. Assume both know Ks • Conf. and Auth. 1. A to B: M|E[KUB,H(M)] 2. B to A: H(Ks),M 3. A to B: E[Ks,H(M)] 4. B to A: E[KUA,Ks|E[KRB,H(M)]], E[Ks,M]

24. Certificates

25. Public Keys for All! • Why not always use PKC? • Slow • About 10K times slower than symmetric • Large • 1024-bit+ keys • Block size = key size

26. Using Public Keys • New York Times • expensive • out-of-band broadcast is difficult • hard to validate • Email • Easy to spoof • No standard way to check

27. Certificates • CA • Signs a hash of the certificate • Collision resistance is critical! • Users • Can see certificate (in plaintext) • Verify signature on the hash • Verify once, then store

28. Certificates • Simple Certificates • Set of info • ID (“Alice”, etc.) • Public Key (KUa) • Timestamp • Linked together • Authenticated • Ca = EKRt[IDa, KUa, Time1]

29. Requirements • Certificate Authority • Known and trusted • Public key known by all • Individuals • In person or • Other secure communication

30. Using Certificates • User may distribute • Anyone can check the authorities’ signature • Anyone can read the information • Can still use a DB • DB can verify the signature before insertion

31. Certificates and the Web • SSL/TLS • X.509 Certificates • Major CAs • How many do you trust? • The little lock • Just some certificate • Signed by a CA in the list • That CA matched the cert. name to the owner (hopefully)

32. X.509 Format

33. CAs that I trust • A sampling • Autoridad de Certificacion Firmaprofesional • ABA.ECOM, Inc. • AC Camerfirma SA CIF A82743287 • AOL Time Warner • AddTrust AB • America Online, Inc. • 42 more …