1. A system for generating documents electronically comprising: a document generator generating electronically a plain document including a sequence of adjacent graphic elements; and a security image generator generating a security image including a plurality of discrete security elements arranged adjacent to each other, each security element being formed of at least two sets of parallel lines and corresponding to a respective graphic element of said sequence, wherein said sets of lines are not parallel to each other; and a modified document generator generating a modified document by merging said security image with said plain document by replacing said sequence of adjacent graphic elements with said security image.