We propose variations of the class of hidden monomial cryptosystems in order to make it resistant to all known attacks. We use identities built upon a single bivariate polynomial equation with coefficients in a finite field. Indeed, it can be replaced by a ``small'' ideal, as well. Throughout, we set up probabilistic encryption protocols, too. The same ideas extend to digital signature algorithms, as well. Our schemes work as well on differential fields of positive characteristic, and elsewhere.