Authentication watermarking approaches can be classified into two kinds: fragile and semi-fragile. In contrast to the latter one, fragile watermarking does not tolerate modifications of any single bit of the watermarked data. Since the transmission of digital data often requires lossy compression, an authentication system should accept non-malicious modifications such as JPEG compression. Semi-fragile techniques aim to discriminate malicious manipulations from admissible manipulations. In our approach, we extract image content dependent information, which is hashed afterwards and encrypted using secure methods known from the classical cryptography. The image data is partitioned into nonoverlapping 4x4 pixel blocks in the spatial domain. The mean values of these blocks form <i>n</i>-dimensional vectors, which are quantized to the nearest lattice point neighbours. Based on the changed vector values, a hash is calculated and asymmetrically encrypted, resulting in a digital signature. Traditional dual subspace approaches divide the signal space into a region for signature generation and a region for signature embedding. To ensure the security of the whole image, we join the two subspaces. The vectors, where to embed the bits using quantization-based data hiding techniques, are predistorted and also used for the signature generation. Our scheme applies error correction coding to gain the robustness of the embedded signature to non-malicious distortions. A second quantization run finally embeds the signature.
This paper presents a new watermarking framework, suitable for authentication of H.264 compressed videos. The
authentication data is embedded as fragile, blind and erasable watermark with low video quality degradations. Because
of using a fragile watermark, hard authentication is possible. In contrast to other approaches, the watermarking is done
after the H.264 compression process. Hence, the authentication information can be embedded in already encoded videos.
To reconstruct the original H.264 compressed video the watermark can be removed. The framework is based on a new
transcoder, which analyses the original H.264 bit stream, computes a watermark, embeds the watermark and generates a
new H.264 bit stream. To authenticate the video a hash value is used. This value is encrypted with a private key of an
asymmetric cryptosystem. The payload of the watermark consists of the encrypted hash value and a certificate with the
public key. Some skipped macroblock of the H.264 video are used to embed the watermark. A special process selects
these macroblocks. This process sets the distribution and the number of skipped blocks as well as the number of
embedded bits per block to achieve low video quality degradations and low data rate. To embed the watermark the
performance of several approaches is discussed and analyzed. The result of the framework is a new watermarked H.264
bit stream. All data necessary for authentication are embedded and cannot get lost.