A decryption engine includes an update circuit, a key generator, a decryption circuit and a detection circuit. The update circuit generates a first updating information based on a premise of that a currently received frame is encrypted, and generates a second updating information based on a premise of that the currently received frame is non-encrypted. The key generator produces a first key according to the first updating information, and produces a second key according to the second updating information. The decryption circuit generates a first decrypted frame according to the first key and the currently received frame, and generates a second decrypted frame according to the second key and the currently received frame. The detection circuit detects whether the currently received frame is decrypted according to the first decrypted frame and the second decrypted frame, to generate an encryption detection result.