Patent Document ID: 8935681
Application ID: 11664431

Base Claim:
1. A method of protecting an original plain text file which comprises the steps of: a) encrypting the original plain text file and making the original plain text file available to a user as a protected file; b) issuing to said user a user program and a user license, operable on a computer, to enable said user to decrypt the protected file and view the original plain text file as 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 c) arranging that the user program comprises an editor program that allows the user to (i) edit the image of the original plain text file to create an edited image and (ii) to save changes made to the image of the original plain text file in an encrypted form, separate from the original plain text file, wherein the editor program enables the use to create 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 the user to view or edit the image of the original plain text file, the user program checks its own validity by checking a digital signature to ensure the user program 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.

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Claim 4:
4. The method as claimed in claim 1 in which said changes are encrypted by the editor program using said user license or a different license key.