Biometric cryptosystems are widely studied recently, which bind a cryptographic key with the biometric characteristic of a user in such a way that the key cannot be recovered without a successful biometric authentication. By now, the key is generated from extra random number generators, which makes the system more complex. A novel method is proposed, which generates the key from biometric characteristics directly, and makes the biometric cryptosystems much cheaper, simpler and more convenient. To make the keys secure enough, the TD-ERCS is adopted and revised in the proposed algorithm. Experiments show that the algorithm is sensitive to biometric characteristics, free of collision, and the generated keys seem also random. It is believed that such a algorithm can be practically used for various biometric cryptosystems.