The present invention provides methods, systems and apparatus for encrypting and for decrypting a data stream, for securely sending a data stream and for securely receiving a data stream, and for secure transmission of a data stream. The data stream, after at least a part of it being encrypted, is transmitted from a sender to a receiver via a channel. An exemplary method for encrypting comprises: adjusting encryption attributes during transmission; encrypting the data stream according to the adjusted encryption attributes; and transmitting the encrypted data stream and information of the encryption attributes to the receiver.