Copyright protection for digital data

If Gamma(p) is a p-ary code of length n, and a and b are two codewords, the n c is called a descendant of a and b if c(i) is an element of {a(i), b(i)} for i = 1, ... n. We are interested in codes Gamma(p) with the property th at 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 a nd b cannot frame a third user who is given the codeword c. Intersecting co des over primary field GF(p) with the above mentioned property are found.