A method of providing signals for detecting forged part in an image is proposed. The solution involves extracting image key points. Next, pairing every two different image key points is performed so that every key point is compared. The inverse cosine angle of dot products between every paired image is determined. If the result is less than threshold, the paired image key points are prematched. If the prematched key points have Euclidean distance more than 10, the paired image key points are matched. Hence, the forged part of image can be determined.