we propose an encryption scheme which uses steganography . the encryption algorithm encrypts messages by embedding them in a data stream in such a way that an adversary cannot get information about the messages . since the embedding is the only computation requires this scheme is optimal in computational efficiency . however , since the size of the data stream is large , this scheme is most beneficial when the cost of bandwidth is less expensive than the cost of computation . the scheme embeds the message as specified by a pseudo random generator . therefore , the scheme &# 39 ; s security is based on a new weak assumption base on the pseudo random generator which we introduce .