1. A watermark embedding method for embedding a secret message sequence in a document, said method comprising: obtaining layout information of said document; extracting a digest of said document by using a Hash function; calculating by a computer processor embedded positions where said secret message sequence is embedded in said document, wherein said calculating the embedded positions is performed by means of a Hash digest of said document and a finite state machine driven by a public key encrypted algorithm, wherein said calculating the embedded positions further comprises calculating an initial state based on the extracted document digest and said secret message sequence, calculating a state sequence based on the initial state and a public key, and calculating the embedded position sequence based on the state sequence; and dispersedly hiding said secret message sequence in each of said calculated embedded positions by altering the layout of said document.