RSA and digital signatures. Recall that in the RSA public-key cryptosystem, each user has...

RSA and digital signatures. Recall that in the RSA public-key cryptosystem, each user has a public key P = (N, e) and a secret key d. In a digital signature scheme, there are two algorithms, sign and verify. The sign procedure takes a message and a secret key, then outputs a signature σ. The verify procedure takes a public key (N, e), a signature σ, and a message M, then returns “true” if a could have been created by sign (when called with message M and the secret key corresponding to the public key (N, e)); “false” otherwise.

(a) Why would we want digital signatures?

(b) An RSA signature consists of sign (M, d) = Md (mod N), where d is a secret key and N is part of the public key. Show that anyone who knows the public key (N, e) can perform verify ((N, e), Md, M), i.e., they can check that a signature really was created by the private key. Give an implementation and prove its correctness.

(c) Generate your own RSA modulus N = pq, public key e, and private key d (you don’t need to use a computer). Pick p and q so you have a 4-digit modulus and work by hand. Now sign your name using the private exponent of this RSA modulus. To do this you will need to specify some one-to-one mapping from strings to integers in [0, N − 1]. Specify any mapping you like. Give the mapping from your name to numbers m1, m2, .. mk, then sign the first number by giving the value  (mod N), and finally show that  (mod N).

(d) Alice wants to write a message that looks like it was digitally signed by Bob. She notices that Bob’s public RSA key is (17, 391). To what exponent should she raise her message?