key usage control file 19 if file x is file 00100 then the previous accesses to this file resulted in its being re stored encrypted with key code k 2 at entry 37 the search of the key usage control file 19 thus indicates that file 00100 was previously accessed twice and now requires decryption with key code k 2 if authorization of the requesting user is still valid 39 then the key code generator 23 is set to supply the key code k 2 to decryption module 27 in order to furnish the data in this file in clear text 29 re storing the data from this file in modified or unmodified form is accomplished by resetting 38 the key code generator 23 to supply key code k3 entry 41 in fig4 to the encryption module 17 for encryption therein of the returned data with the new key code k3 all retrievals of data in storage 17 may be by destructive read of information in the addressed file so that data for restoring therein may be written in the newlyencrypted form after numerous accesses to files in storage 17 the key usage control file 19 will typically include entries as illustrated in fig4 such file optionally may also include codes to identify the particular users who gained access to each.