A data decryption circuit for decrypting a current encrypted data packet is provided. The current encrypted data packet includes a header and a payload. The data decryption circuit includes an operation unit and a decryption calculation unit. The operation unit generates first data according to the header and a pseudo-random number, second data according to a session key and a constant, and length information and start position information of the payload according to the header. The operation unit generates the first data, the second data, the length information and the start position information by executing a program code. The decryption calculation circuit, coupled to the operation unit, generates a decryption key according to the first and second data, retrieves the payload from the current encrypted data packet according to the start position information and the length information, and decrypts the payload by the decryption key.