An encryption and decryption method applied upon transmitting a plaintext in a communication network containing plural subscriber ends is provided. The method includes steps of: picking a synchronization variation secret key from a first subscriber end, the value of the synchronization variation secret key synchronously varying at the subscriber ends; executing a first operation on the synchronization variation secret key by the first subscriber end to obtain an automatically changed secret key; utilizing the automatically changed secret key to process a subsequent encryption to the plaintext by the first subscriber end so as to obtain a ciphertext to be transmitted to a second subscriber end; receiving the ciphertext and picking the synchronization variation secret key by the second subscriber end to execute the first operation and obtain the automatically changed secret key; and utilizing the automatically changed secret key to process a subsequent decryption to the ciphertext by the second subscriber end so as to obtain the plaintext.