Abstract: Multi-decrypter encryption is a typical application in multi-usercryptographic branches. In multi-decrypter encryption, a message isencrypted under multiple decrypters' public keys in the way thatonly when all the decrypters cooperate, can the message be read.However, trivial implementation of multi-decrypter encryption usingstandard approaches leads to heavy computation costs and longciphertext which grows as the receiver group expands. This consumesmuch precious bandwidth in wireless environment, such as mobile ad hocnetwork. In this paper, we propose an efficient identity basedmulti-decrypter encryption scheme, which needs only one or zero (ifprecomputed) pairing computation and the ciphertext contains only threegroup elements no matter how many the receivers are. Moreover, we give aformal security definition for the scheme, and prove the scheme to bechosen ciphertext secure in the random oracle model, and discuss how tomodify the scheme to resist chosen ciphertext attack.