1. A data transfer method for transferring digital data from a data transmitter device to a data receiver device, said method comprising a data transmitting step performed by said data transmitter device and a data reproducing step performed by said data receiver device, each of said data transmitter device and said data receiver device using a cryptographic algorithm in which bidirectional data displacement of object data from a head to an end thereof and from the end to the head thereof is performed, said data transmitting step comprising the steps of: producing variable-length data having a data length corresponding to said digital data; encrypting said variable-length data using at least one natural random number to produce a first variable-length random number sequence based on said cryptographic algorithm; performing an exclusive OR operation per bit between said first variable-length random number sequence and said digital data to derive transfer data; transmitting said transfer data to said data receiver device; and transmitting said variable-length data and said at least one natural random number in an encrypted manner to said data receiver device, and said data reproducing step comprising the steps of: reproducing said variable-length data and said at least one natural random number transmitted from said data transmitter device in the encrypted manner; encrypting said variable-length data using said at least one natural random number to produce a second variable-length random number sequence based on said cryptographic algorithm, said second variable-length random number sequence identical with said first variable-length random number sequence; and performing an exclusive OR operation per bit between said second variable-length random number sequence and said transfer data transmitted from said data transmitter device to reproduce said digital data.