A Digital Watermarking Algorithm for Authentication and Tamper Detection

An important element of a communications system is the trustworthiness of the data being received, especially where the data must travel through insecure channels or pass through several hands. Digital watermarking provides a partial solution to the problem of trustworthiness, especially with regard to image and video data. This paper describes a hybrid watermarking algorithm that combines robust and fragile watermarking to allow source authentication of an image and detection of malicious tampering. The embedded watermark is relatively immune to moderate JPEG and SPIHT compression. Encryption is used to ensure that the watermark cannot be forged without knowledge of a secret key. The watermark is generated from the host image and is a content hash containing a single bit of data for every sixty-four pixels of the host image. This watermark is embedded in a region of the discrete wavelet transform of the image. An encrypted version of the watermark is embedded in another region for use as a source authenticator. The embedding is performed by the quantisation of groups of discrete wavelet transform coefficients. The watermark generation requires a symmetric session key which is randomly determined during watermark generation and stored (in an encrypted form) in a robust watermark in a third region of the discrete wavelet transform.