An identity-based threshold unsigncryption scheme is proposed, which is the integration of the signcryption scheme, the (
) threshold scheme and zero knowledge proof for the equality of two discrete logarithms based on the bilinear map. In this scheme, a signcrypted message is decrypted only when more than
members join an unsigncryption protocol and the signature can be verified by any third party. A formal proof of security of this scheme is provided in the random oracle model, assuming the Decisional Bilinear Diffie-Hellman problem is computationally hard.