Digital Signature Method for Images without Additional Files, Robust to JPEG Compression and Metadata Removal

Abstract

The article discusses a method of digitally signing images that does not use metadata or additional files. The object for signature is an array of 8x8 pixel blocks with a discrete cosine transformation applied, which make up a JPEG file. Any known algorithm can be used as a signature algorithm, for example, RSA.

The resulting digital signature is converted from a binary form to an image. The conversion method is based on encoding every few bits of the signature of one of the 64 basic functions of the discrete cosine transform. Next, the basic function is transformed into an image by an inverse discrete cosine transformation. The resulting signature, encoded as an image, is attached to the original image on the right to form a signed image.

The digital signature is resistant to JPEG compression within adjustable limits. The following method is used to achieve compression resistance. Since the values in the blocks can change during compression, quantization is used – reducing the accuracy of the values. It is designed in such a way that when quantizing compressed and uncompressed images, the values in the blocks after quantization are becoming the same, which allows you to verify the validity of the digital signature. Quantization also includes a step of checking the parity of the received value to avoid misinterpretation of values when compression is strong.

The quantization step does not apply to the part of the image containing the digital signature. This is due to the fact that this part consists of the basic functions of the discrete cosine transform, and even with strong compression, the corresponding basic function will still retain a large coefficient and will be unambiguously interpreted.