Embodiments of the present application provide an encryption method, a decryption method, and a related apparatus. The encryption method includes: generating a keystream, where the keystream is used to encrypt a part of data to be encrypted in an initial layer-3 message, and the part of data to be encrypted includes small data; generating, by performing an exclusive OR operation on the keystream and the initial layer-3 message, an initial layer-3 message in which the part of data is encrypted; and sending the initial layer-3 message in which the part of data is encrypted, where the initial layer-3 message includes an added encryption indication, and the encryption indication is used to indicate that the part of data to be encrypted in the initial layer-3 message is encrypted.