1. A user authentication method comprising: extracting, by a processor, a facial landmark from each of a first input image and a second input image; generating, by the processor, a first sparse code of the facial landmark extracted from the first input image and a second sparse code of the facial landmark extracted from the second input image; and determining, by the processor, whether a user is to be authenticated based on the first sparse code of the first input image and the second sparse code of the second input image, the determining including, calculating a similarity between the first sparse code and the second sparse code, determining a final similarity based on the calculated similarity, and determining whether a user represented in the first input image is similar to a user represented in the second input image based on whether the final similarity satisfies a desired condition.