Cryptography Lecture 22

Q and A; bring the written answers to TA before the class 1. Why plain RSA should not be used? 2. What is RSA-OAEP? 3. What’s a digital signature? 4. What are the differences between digital signatures and message authentication codes?

RSA-based PKE

Recall… (informal) Choose random, equal-length primes p, q Compute modulus N=pq Choose e, d such that e · d = 1 mod (N) The eth root of x modulo N is [xd mod N] (xd)e = xde = x[ed mod (N)] = x mod N RSA assumption: given N, e only, it is hard to compute the eth root of a uniform cℤN*

“Plain” RSA encryption N, e c (N, e, d)  RSAGen(1n) pk = (N, e) sk = d c = [me mod N] m = [cd mod N]

Is this scheme secure?

Plain RSA should never be used! Security? This scheme is deterministic Cannot be CPA-secure! RSA assumption only refers to hardness of computing the eth roots of uniform c c is not uniform unless m is Easy to recover “small” m from c RSA assumption only refers to hardness of computing the eth root of c in its entirety Partial information about the eth root may be leaked (In fact, this is the case) Plain RSA should never be used!

PKCS #1 v1.5 Standard issued by RSA labs in 1993 Idea: add random padding To encrypt m, choose random r c = [ (r|m)e mod N] Issues: No proof of CPA-security (unless m is very short) Chosen-plaintext attacks known if r is too short Chosen-ciphertext attacks possible

PKCS #1 v2.0 Optimal asymmetric encryption padding (OAEP) applied to message first This padding introduces redundancy, so that not every cℤ*N is a valid ciphertext Need to check for proper format upon decryption Return error if not properly formatted

OAEP m || 0…0 r G H e c = s t mod N

Security? RSA-OAEP can be proven CCA-secure under the RSA assumption, if G and H are modeled as random oracles Widely used in practice…

Digital signatures

Digital signatures Provide integrity in the public-key setting Analogous to message authentication codes, but some key differences…

Digital signatures pk pk pk m,  pk pk, sk 1 = Vrfypk(m, ) ?  = Signsk(m)

Public-key encryption pk pk pk c pk pk, sk c  Encpk(m) m = Decsk(c)

Security (informal) Even after observing signatures on multiple messages, an attacker should be unable to forge a valid signature on a new message

Prototypical application pk patch’, ’ patch,  pk, sk pk  = Signsk(patch) pk

Comparison to MACs? t’ = Mack(patch’) k patch’, t’ patch, t k k

Comparison to MACs? patch, t1 k1 patch, t2 k1, k2, k3 patch, t3 k2 t1 = Mack1(patch) t2 = Mack2(patch) t3 = Mack3(patch) k3

Comparison to MACs? Public verifiability Transferability “Anyone” can verify a signature (Only a holder of the key can verify a MAC tag) Transferability Can forward a signature to someone else… Non-repudiation

Non-repudiation Signer cannot (easily) deny issuing a signature Crucial for legal applications Judge can verify signature using public copy of pk MACs cannot provide this functionality! Without access to the key, no way to verify a tag Even if receiver leaks key to judge, how can the judge verify that the key is correct? Even if key is correct, receiver could have generated the tag also!