1. A method for decoding a watermark embedded in a target video comprising blocks of samples representative of said target video, wherein watermark is embedded in an original video by modifying motion vectors of a compressed representation of the original video, the method comprising: determining, in said target video, a location of a watermarked block from a first frame identifier and a first block identifier in a pixel-domain representation of said target video; determining a location for each reference block among a plurality of reference blocks wherein each reference block is a potential prediction block also located in said target video and wherein said location of a potential prediction block is determined from a second frame identifier in a pixel-domain representation of said target video, the determined location of said watermarked block, and a potential motion vector, the potential motion vector being associated to a watermark value among a plurality of watermark values; comparing said watermarked block with said each reference block; identifying watermark value of said watermarked block, wherein the identified watermark value is associated to the potential motion vector used to determine the reference block that corresponds to the best prediction for the watermarked block from the results of the plurality of comparisons.