1. INTRODUCTION
The success of digital technologies such as JPEG, MPEG, MP3, and the Internet has brought the world into a multimedia era, in which an enormous amount of multimedia data is being produced and manipulated every second. Consequently, two challenging issues have arisen: 1) it is difficult to identify multimedia content in a large database; 2) it is difficult to authenticate multimedia content due to the advance of editing software. These two issues both suffer from the fact, that the same multimedia content may have different digital representations. On one hand, the same content can be represented by various file formats, qualities, etc.; on the other hand, it might undergo signal processing such as smoothing, denoising, etc. – they all result in almost the same content but the underlying binary files can be quite different. In order to cope with this problem, content-based hashing, also known as perceptual or robust hashing, was proposed as a promising solution.