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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.

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