This paper proposes different low-cost coprocessors for public key authentication on 8-bit smart cards. Elliptic curve cryptography is used for its efficiency per bit of key and the Elliptic Curve Digital Signature Algorithm is chosen. For this functionality, an area constrained coprocessor is probably the best approach to perform the most computer-intensive operations at an acceptable speed considering the limited memory and power of the selected platform. For that purpose, the scalar point multiplication in GF(2m) in both affine and projective coordinates was implemented in order to compare their performances with the same level of optimization and the same technology. A hardware/ software co-design strategy was also used to avoid the need of a dedicated register file.