10. A computer tool, stored in a non-transitory medium, for a user to access an original plain text file which has been protected by being encrypted in a protected file, the computer tool being adapted to decrypt the protected file once authorized by a user license issued by an authority responsible for the protected file to produce an image of the original plain text file whilst protecting the image of the original plain text file from being copied to any file, other than a further protected file, wherein the image of the original plain text file cannot be found by other programs on the computer, and an editor program that (i) edits the image of the original plain text file to create an edited image of the original plaint text file and then (ii) saves changes made to the image of the original plain text file in an encrypted form, separate from the original plain text file, wherein the computer tool creates the edited image of the original plain text file from the protected file and a difference file using the editor program and user license, wherein, before allowing to produce the image of the original plain text file or to edit the image of the original plain text file, the computer tool checks its own validity by checking a digital signature to ensure the computer tool has not been modified, and wherein parts of the original plain text file are marked as non-editable, and the editor program prevents such parts being edited so that they will always be present in any image created from the original plain text file and any difference file or files.
10. A computer tool, stored in a non-transitory medium, for a user to access an original plain text file which has been protected by being encrypted in a protected file, the computer tool being adapted to decrypt the protected file once authorized by a user license issued by an authority responsible for the protected file to produce an image of the original plain text file whilst protecting the image of the original plain text file from being copied to any file, other than a further protected file, wherein the image of the original plain text file cannot be found by other programs on the computer, and an editor program that (i) edits the image of the original plain text file to create an edited image of the original plaint text file and then (ii) saves changes made to the image of the original plain text file in an encrypted form, separate from the original plain text file, wherein the computer tool creates the edited image of the original plain text file from the protected file and a difference file using the editor program and user license, wherein, before allowing to produce the image of the original plain text file or to edit the image of the original plain text file, the computer tool checks its own validity by checking a digital signature to ensure the computer tool has not been modified, and wherein parts of the original plain text file are marked as non-editable, and the editor program prevents such parts being edited so that they will always be present in any image created from the original plain text file and any difference file or files. 15. The computer tool as claimed in claim 10 in which parts of the original file are marked as invisible, and the editor program prevents such parts from being displayed in any image created from the original plain text file and any difference file or files.