1. A method of authenticating an audio-visual signal comprising: formation of a progressive signature by generating a variable number of signature bits, splitting said audio-visual signal into blocks, calculating an DC-value for each of said blocks, assigning blocks with similar DC-values to regions, calculating differences between DC-values of said regions, and generating said number of signature bits based on said differences between said DC-values of said regions; wherein the assigning said blocks to regions with similar DC values further comprises repeating: picking a first block not yet assigned to a region according to a pseudo-random sequence wherein said first block becomes the first block of a new region and the DC-value of said first block becomes the DC-value of said new region, and examining each neighbouring block of said first block whereby a further block of said neighbouring blocks is assigned to said new region and the DC-value of the new region is updated with the DC-value of the further block if the DC-value of said further block is less than a threshold, until all blocks are assigned to a region.