A method for determining authenticity of a video in a surveillance system, the method comprising: capturing a sequence of image frames of a scene, tracking an object in the sequence of image frames, determining a first current image quality measure and a second current image quality measure, the second current image quality measure being different from the first current image quality measure, in an image area corresponding to the tracked object in at least a first image frame and a second image frame of the image frames in which the object is tracked, wherein the first and second image frames are chosen such that the object has moved at least a predetermined distance in a predetermined direction between the first and second image frames, identifying a first current image quality measure variation for the object from the first determined image quality measure in the first image frame and the second image frame, and identifying a second current image quality measure variation from the second determined image quality measure in the first image frame and the second image frame, the image quality measure variations describing the image quality measures as functions of position of the object in the first and second image frames, comparing the first current image quality measure variation to a first known image quality measure variation, and comparing the second current image quality measure variation to a second known image quality measure variation, and in response to the first current image quality measure variation deviating from the first known image quality measure variation and the second current image quality measure variation deviating from the second known image quality variation by less than a first predetermined amount, determining that the video is authentic, and in response to the first current image quality measure variation deviating from the first known image quality measure variation and the second current image quality measure variation deviating from the second known image quality variation by more than a second predetermined amount, issuing an alert indicative of a possible replayed video attack, wherein the image quality measure is at least one from the group consisting of pixel contrast, focus, motion blur, pixel range fill, and noise level. capturing a sequence of image frames of a scene, tracking an object in the sequence of image frames, determining a first current image quality measure and a second current image quality measure, the second current image quality measure being different from the first current image quality measure, in an image area corresponding to the tracked object in at least a first image frame and a second image frame of the image frames in which the object is tracked, wherein the first and second image frames are chosen such that the object has moved at least a predetermined distance in a predetermined direction between the first and second image frames, identifying a first current image quality measure variation for the object from the first determined image quality measure in the first image frame and the second image frame, and identifying a second current image quality measure variation from the second determined image quality measure in the first image frame and the second image frame, the image quality measure variations describing the image quality measures as functions of position of the object in the first and second image frames, comparing the first current image quality measure variation to a first known image quality measure variation, and comparing the second current image quality measure variation to a second known image quality measure variation, and in response to the first current image quality measure variation deviating from the first known image quality measure variation and the second current image quality measure variation deviating from the second known image quality variation by less than a first predetermined amount, determining that the video is authentic, and in response to the first current image quality measure variation deviating from the first known image quality measure variation and the second current image quality measure variation deviating from the second known image quality variation by more than a second predetermined amount, issuing an alert indicative of a possible replayed video attack, wherein the image quality measure is at least one from the group consisting of pixel contrast, focus, motion blur, pixel range fill, and noise level.