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.
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. 15. The apparatus of claim 13 in which said spreading comprises modulating the message with a random carrier signal.