A recoverable semi fragile watermarking algorithm for image content authentication is proposed. The algorithm employs a compressed halftoned binary image as watermark and embeds it in the wavelet domain through quantization index modulation. It can not only detect the tampered area

but also recover it through decompressing and inverse halftoning the watermark image. Beside

a secret key is used to select the embedded watermark bits and their embedded positions to increase the security of watermarking. Experimental results show that this semi fragile watermarking algorithm is effective and practical for content authentication.