Abstract: The difference between teh fair exchange protocols and the non-repudiation protocols is proposed. Based on the publicly verifiable secret sharing scheme and group encryption scheme, a new multi-party fair exchange protocol with an off-line semi-trusted third party(off-STTP) is presented. The off-STTP who need not be completely trusted intervenes into the exchange in case of trouble. That the off-STTP can only decipher a sub-secret key makes sure the confidentiality of exchange data and the true fairness of the barter. The participants can barter with others at their will and the exchange topology is unknown to others including the off-STTP.

Key words: off-line trusted third party(off-STTP), non-repudiation, digital signature