说明：  　It has been proved that network coding can be used to improve the throughput for multicast t ransmission , but such systems are very vulnerable to contamination at tack , and the signatures for t raditional network are no longer suitable for network coding. Network coding allows the forwarding and receiving nodes to verify the coded messages against the contamina2 tion at tacks. In this paper , we proposed a network coding signature scheme based on homomorphism public cryptography which uses the one time secrete key and it changes along with the change of the messages , so that it is infeasible for adversary to get the secrete key and forge the signature to contaminate the message