Abstract: For improving the efficiency and the potential for actively protecting against attacks, a novel security-ensured public key cryptosystem is proposed. The idea is that a large set of published parameters, which are generated with almost no more overheads, is taken into account in addition to the original public and private keys. A new set of published parameters will be chosen when a group of data to be encrypted each time. The linear time quick algorithms for deciding the solution structure and computing the solution of the quartic congruential equations are proposed for implementing the operations on the published parameters. This cryptosystem can actively protect against the chosen plaintext and the various chosen ciphertext attacks including IND-CCA2 attacks. It achieves the same security like One-Time-Pad. For the repeated encryption/decryption for a set of data the requirements for computation and memory size are deeply decreased because only a series of XOR operations are needed after first data item has been encrypted/decrypted. Due to this reason, this cryptosystem may be used in very low-end devices, such as RFID tags, sensor networks, where the computation, memory and bandwidth are very limited.