Copyright protection for digital data

If /spl Gamma//sub p/ is a p-ary code of length n, and a and b are two codewords, then c is called a descendant of a and b if c/sub i//spl isin/{a/sub i/, b/sub i/} for i=1,...,n. We are interested in codes /spl Gamma//sub p/ with the property that for any two codewords a and b their only descendant codewords are a and b themselves. This way a coalition of two users who are given codewords a and b cannot frame a third user who is given the codeword c. Intersecting codes over primary field GF(p) with the above mentioned property are found.