Tracing and proving ownership of digital objects

If C is a p-ary code of length n and a/sup (1)/ and a/sup (2)/ are two codewords, then a/sup (3)/ is called a descendant of a/sup (1)/ and a/sup (2)/ if a/sub i//sup (3)//spl isin/{a/sub i//sup (1)/, a/sub i//sup (2)/} for i=1,..., n. We are interested in codes C with the property that if a/sup (1)/, a/sup (2)//spl isin/C then a/sup (3)/ is not a codeword in C. This way a coalition of 2 users a/sup (1)/ and a/sup (2)/ cannot frame a 3rd user a/sub i//sup (3)/. Intersecting codes over GF(p) with the above mentioned property are found.