CMSC 414 Computer and Network Security Lecture 7

1. CMSC 414Computer and Network SecurityLecture 7 Jonathan Katz

2. Review: El Gamal encryption • (Some aspects of the actual scheme are simplified) • Key generation • Choose a large prime p, and an element g Zp* • Choose random x  {0, …, p-2}, set h=gx • The public key is (p, g, h), and the private key is x • Encryption • View the message m as an element of Zp* • Choose random r  {0, …, p-2} • The ciphertext is (gr, hr m) • To decrypt ciphertext (c1, c2) output c2/c1x • Correctness?

3. Security? • Security of El Gamal encryption is based on the decisional Diffie-Hellman assumption • Best current algorithm for the decisional Diffie-Hellman problem in Zp* runs in time ≈ exp(|p|1/3) • So if p is a 1024-bit prime, best current attack on El Gamal encryption requires time ≈ 260 • In other groups, the Diffie-Hellman problem is currently ‘harder’ • E.g., for elliptic curve groups, best current algorithms require time exp(|p|/2) • Can use 120-bit primes to get 260 security

4. RSA background • N=pq, p and q distinct, odd primes • (N) = (p-1)(q-1) = |ZN*| • Easy to compute (N) given the factorization of N • Hard to compute (N) without the factorization of N • For all x  ZN*, it holds that x(N) = 1 mod N • If ed=1 mod (N), then for all m: (me)d = m mod NI.e., given d, we can compute eth roots

5. We have an asymmetry! • Let e be relatively prime to (N) • Needed so that ed=1 mod (N) has a solution • Given e and the factors of N, can compute d and hence compute eth roots • Without the factorization of N, no apparent way to compute eth roots

6. Hardness of computing eth roots? • The RSA problem: • Given N, e, and c, compute c1/e mod N • If factoring is easy, then the RSA problem is easy • We know of no other way to solve the RSA problem besides factoring N • But we do not know how to prove that the RSA problem is as hard as factoring • The upshot: we believe factoring is hard, and we believe the RSA problem is hard

7. How hard is factoring? • Best current algorithms for factoring N=pq a product of two equal-length primes, run in time ≈ exp(|N|1/3) • So need |N| ≈ 1024 for reasonable security • Currently |N| ≈ 2048 recommended for good security margins

8. We have an asymmetry! • Given d (which can be computed from e and the factorization of N), possible to compute eth roots • Without the factorization of N, no apparent way to compute eth roots • Let’s use this to encrypt…

9. RSA key generation • Generate random p, q of sufficient length • Compute N=pq and (N) = (p-1)(q-1) • Compute e and d such that ed = 1 mod (N) • e must be relatively prime to (N) • Typical choice: e = 3; other choices possible • Public key = (N, e); private key = (N, d)

10. “Textbook RSA” encryption • Public key (N, e); private key (N, d) • To encrypt a message m  ZN*, compute c = me mod N • To decrypt a ciphertext c, compute m = cd mod N • Correctness… • …what about security?

11. Textbook RSA is insecure! • It is deterministic! • Furthermore, it can be shown that the ciphertext leaks specific information about the plaintext

12. Padded RSA • Introduce randomization… • Public key (N, e); private key (N, d) • Say |N| = 1024 bits • To encrypt m  {0,1}895, • Choose random r  {0,1}128 • Compute c = (r | m)e mod N • Decryption done in the natural way… • Essentially this is standardized as PKCS #1 v1.5

13. Hybrid encryption • Public-key encryption is “slow” • Encrypting “block-by-block” would be inefficient for long messages • Hybrid encryption gives the functionality of public-key encryption at the (asymptotic) efficiency of private-key encryption!

14. Enc’ Enc Hybrid encryption message pk “encrypted message” ciphertext “encapsulated key” k random! Enc = public-key encryption scheme Enc’ = private-key encryption scheme

15. Security • If public-key component and private-key component are secure against chosen-plaintext attacks, then hybrid encryption is secure against chosen-plaintext attacks

16. Extension • How should hybrid encryption be done when sending the same message to multiple recipients (e.g., email encryption)?

17. Malleability • All the public-key encryption schemes we have seen so far are malleable • Given ciphertext c that encrypts (unknown) message m, possible to generate a ciphertext c’ that encrypts a related message m’ • In the public-key setting, security against chosen-ciphertext attacks implies non-malleability • In many scenarios, malleability/chosen-ciphertext attacks are problematic • E.g., auction example; password example; Bleichenbacher attack…

18. Bleichenbacher’s attack • RSA PKCS #1 v1.5 is actually defined as: c = (00 || 02 || r || 0 || m)e mod N • When decrypting, return an error if formatting is not obeyed • This enables a chosen-ciphertext attack that relies only on the ability to detect errors upon decryption

19. c1 If the {ci} are carefully constructed, this information is enough to determine m! error/no error c999 error/no error Bleichenbacher’s attack c = Encpk(m) …

20. Malleability • All the public-key encryption schemes we have seen so far are malleable • Given a ciphertext c that encrypts an (unknown) message m, possible to generate a ciphertext c’ that encrypts a related message m’ • Note: the problem is not integrity (there is no integrity in public-key encryption, anyway), but malleability and/or the ability to conduct a chosen-ciphertext attack

21. Malleability in private-key setting • Malleability is an issue in the private-key setting as well • Recall that CBC, OFB, CTR mode are all vulnerable to chosen-ciphertext attacks, and are all malleable • Authenticated encryption schemes (e.g., “encrypt-then-authenticate”) are secure against chosen-ciphertext attacks (and non-malleable)