Nowadays, with the proliferation of data exchange across the Internet and the storage of sensitive data on open networks, intensive works are done to secure data. Recently, researchers made more considerable effort to utilize biometric features instead of a memorable password to authenticate the user identity. Based on the theories of chaos map, which are known to possess desirable properties of pseudorandomness, high sensitivity to initial conditions, and very large key space, we propose a biometric cryptosystem for online users’ authentications. The biggest advantage of our proposal is the secure nature of the key. In fact, our system is designed first so that the key never needs to be transmitted or revealed but it will be derived from biometric template and second even if we introduce the same biometric template twice, our system generates different key values. In order to assess the effectiveness of our approach, a biometric-based online automatic teller machine system is proposed and evaluated. Experimental results and statistical analysis, using palmprint as well as palm-vein databases of 200 users, are presented. The obtained results show that the proposed scheme is more secure, fast, and points at increased authentication accuracy. Therefore, it can be implemented in real-time applications, which will undoubtedly help to promote more widespread use of biometric cryptosystems based on chaos theory.