A. n = pq, Phi (n) = (p-1) (q-1)
Pick block size b < min(p,q). Want b coprime to n.
Pick exponent e with gcd(e, Phi (n)) = 1.
Get e^(-1) (= s with 1 = se + t Phi (n)). Publish n and e.
If message block is x_i, then send y_i = x^s_i mod n.
B. receives y_i and computes y_i mod n = (x_i)^(se) mod n
= (x_i)^(1 + k Phi (n)) mod n
= x_i (x_i^(Phi (n)) mod n
= x_i 1^k mod n = x_i
E.g., Send X = "invest in bonds" using p = 61, q = 127.
So publish n = 7747, compute Phi (n) = 7560, pick and publish e = 3113
Compute e^(-1) = s = ... = 17
So X --> 0813 2104 1819 0813 0114 1303 1823 (added a X)
Raise each to 17th power and send mod 7747
2169 0628 5540 2169 6560 6401 4829