Key independent watermark detection

Many types of pseudo-random signals have been used to embed signatures as watermarks, with spread spectrum signal techniques used to recover the signature from the encrypted data. Legendre sequences are a suitable candidate for signature encryption as they exhibit 'perfect' two level auto-correlation. Additionally Legendre sequences have the unusual and interesting property of invariance under Fourier transformation; the spatial and frequency representation of each sequence is identical up to a phase factor. The presence of a Legendre-based watermark, embedded in the pixel or transform domain, can be detected by cross-correlating a sequence-encrypted image with its Fourier transform. This property enables verification of the presence of a watermark (of specified length), without requiring prior knowledge of the sequence type or key used for the encryption.