1. A method of passing encrypted files between data processing systems, comprising: at a source computer, providing at least one file which is encrypted with a key which is at least partially derived from at least one unique source computer system attribute; providing a transfer memory media; at said source computer, decrypting said at least one file; at said source computer, encrypting said at least one file with a key which is derived from at least one unique transfer memory media attribute; at said source computer, copying said encrypted at least one file to said transfer memory media; at a target computer, copying said encrypted at least one file to said target computer; at said target computer, decrypting said at least one file; at said target computer, encrypting said at least one file with a key which is at least partially derived from at least one target computer system attribute.