The market for Personal Digital Assistants (PDA) is growing rapidly and PDAs are becoming increasingly interesting for commercial transactions. One requirement for further growing of eCommerce with mobile devices is the provision of security. We implemented elliptic curves over binary fields on a Palm OS device. We chose the NIST recommended random and Koblitz curves over GF(2163 ) that are providing a sufficient level of security for most commercial applications. Using Koblitz curves a typical security protocol like Diffie-Hellman key exchange or ECDSA signature verification requires less than 2.4 seconds, while ECDSA signature generation can be done in less than 0.9 seconds. This should be tolerated by most users.