The present invention relates to a system for copy protection of an information carrier, said system comprising a diffractive layer for delivering a speckle pattern when illuminated by a light source, a spatial filter, which is aligned with respect to the diffractive layer, for delivering a filtered optical signal from the speckle pattern and a detector array for delivering, when illuminated by said filtered optical signal, an electrical signal. Said system further comprises means for computing a cryptographic key from the electrical signal, and means for decrypting encrypted data contained in the information carrier from the cryptographic key. It finds its application in copy protection of content carriers such as optical discs or in smart cards.