20. A watermark detecting method for detecting from a document a secret message sequence embedded therein, 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 public key driven finite state machine; extracting each secret message of said secret message sequence based on layout changes in each of the calculated embedded positions, respectively.
20. A watermark detecting method for detecting from a document a secret message sequence embedded therein, 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 public key driven finite state machine; extracting each secret message of said secret message sequence based on layout changes in each of the calculated embedded positions, respectively. 21. The watermark detecting method according to claim 20 , further comprising repeating said calculating the embedded positions and said extracting the secret message sequence until it is determined that all the embedded secret messages have been extracted based on current state, said digest of the document and each of said extracted secret messages.