A network function performs a method to identify an invalid subscription concealed identifier, SUCI. When the network function receives a message containing a SUCI, it determines a size of the SUCI contained in the received message, and also determines an expected size of the SUCI in the received message. The network function then determines whether the size of the SUCI contained in the received message satisfies a criterion associated with the expected size. If the size of the SUCI contained in the received message does not satisfy the criterion associated with the expected size, the network function determines that the SUCI in the received message is invalid, and it rejects the SUCI in the received message if it is determined to be invalid.