Abstract. Chosen-message attack on RSA is usually considered as an inherent property of its homomorphic structure. In this paper, we show that nonhomomorphic RSA-type cryptosystems are also susceptible to a chosen-message attack. In particular, we prove that only one message is needed to mount a successful chosen-message attack against the Lucas-based systems and Demytko’s elliptic curve system. Keywords. Chosen-message attack, signature forgery, RSA, Lucas-based systems, Demytko’s elliptic curve system.