1. An authentication apparatus comprising: a memory configured to store instructions, and a processing device configured to execute the stored instructions such that the processing device is configured to adjust a first similarity between an acquired fingerprint data and reference fingerprint data of a pre-registered user, and a second similarity between an acquired ECG waveform and a reference ECG waveform of the pre-registered user, based on an acquired humidity level, extract a combined similarity based on the adjusted first similarity and the adjusted second similarity; and authenticate whether the user is the pre-registered user based on the combined similarity.