Watermarking electronic text documents
A text watermarking method embeds an auxiliary message in an original electronic text document to form a watermarked text document. The method applies a spreading function to message symbols to spread the symbols over a carrier, which forms a modulated carrier. It maps elements of the modulated carrier to corresponding inter-word spaces in the electronic text document, and applies an embedding function to modify the corresponding inter-word spaces according to elements of the modulated carrier signal such that the modified inter-word spaces hide the modulated carrier signal in the watermarked text document. The message symbols are automatically decodable from the watermarked document without the original electronic text document. A compatible decoder extracts the auxiliary message from a printed or electronic watermarked text document. The decoder automatically measures inter-word spaces in the watermarked text document. It estimates elements of a modulated carrier signal embedded in the inter-word spaces to form an estimated modulated carrier signal, and applies a de-spreading function to the estimated modulated carrier signal to extract message symbols.
1. A method of encoding hidden auxiliary data in a text document comprising:
using an electronic processor for:
forming a message;
spreading the message over a carrier signal to yield a spread spectrum message;
modifying distances of existing spaces between words to encode a machine-detectable interword modification in the document based on elements of the spread spectrum message, wherein interword modifications are controlled to form a machine-detectable message in the text document.
2. The method of claim 1 wherein forming the message includes error correction encoding a message.
3. The method of claim 1 wherein the spreading comprises modulating the message with a random carrier signal.
4. The method of claim 3 wherein the random carrier signal comprises a pseudorandom carrier signal.
5. The method of claim 1 comprising modifying spaces between lines to encode hidden auxiliary data.
6. The method of claim 1 comprising modifying spaces between text characters to encode hidden auxiliary data.
7. The method of claim 1 wherein the modifying comprises increasing a space distance to represent a first symbol, and decreasing a space distance to represent a second symbol.
8. An electronic device programmed to perform the method of claim 7 .
9. A programmed computing device including electronic memory with instructions stored therein, the instructions are for causing said programmed computing device to perform the method of claim 7 .
10. A non-transitory computer readable medium on which is stored instructions to cause an electronic processor to perform the method of claim 1 .
11. An electronic device programmed to perform the method of claim 1 .
12. A programmed computing device including electronic memory with instructions stored therein, the instructions are for causing said programmed computing device to perform the method of claim 1 .
13. An apparatus comprising:
electronic memory for storing data representing at least a portion of a text document;
an electronic processor programmed for:
forming a message;
spreading the message over a carrier signal to yield a spread spectrum message;
modifying distances of existing spaces between words to encode a machine-detectable interword modification in the document based on elements of the spread spectrum message, in which interword modifications are controlled to form a machine-detectable message in the text document.
14. The apparatus of claim 13 in which said forming the message includes error correction encoding a message.
15. The apparatus of claim 13 in which said spreading comprises modulating the message with a random carrier signal.
16. The apparatus of claim 15 in which the random carrier signal comprises a pseudorandom carrier signal.
17. The apparatus of claim 13 in which said electronic processor is further programmed for modifying spaces between lines to encode hidden auxiliary data.
18. The apparatus of claim 13 in which said one or more electronic processor is further programmed for modifying spaces between text characters to encode hidden auxiliary data.
19. The apparatus of claim 13 in which said modifying comprises increasing a space distance to represent a first symbol, and decreasing a space distance to represent a second symbol.
20. The apparatus of claim 13 in which said electronic processor is operating to perform one of the functions recited therein.