23. The method of claim 18 , further comprising: representing the modified document with a modified distance field; detecting a modification associated with a subset of the modified distance values in the modified distance field; and determining the symbol of the message based on the modification.
23. The method of claim 18 , further comprising: representing the modified document with a modified distance field; detecting a modification associated with a subset of the modified distance values in the modified distance field; and determining the symbol of the message based on the modification. 25. The method of claim 23 , further comprising: determining the subset of values in the modified distance field according to a database of shape elements of glyphs.