So if you want to talk to a particular person, you use their particular public key. Even with full knowledge of the public key, the “size of the wheel” is really hard to pin down. Once you have k and φ(m) you can find ℓ by solving a Diophantine equation, kx+φ(m)y=1, for x and y, and then ℓ = [x] φ(m) . But you keep ℓ, φ(m), p, and q private. Without p and q, there’s no (easy) way to find φ(m), and without φ(m) there’s no (easy) way to find ℓ.