Method and apparatus for proving existence of a digital signature of a digital file

摘要

The method involves using a computer in which an electronic file whose signature to be verified is transformed from a first file format into a second file format. A first file signature is formed in dependence on the file in the first file format. A first transformation signature is formed in dependence on the file in the second file format. The verification value comprises the first file format, the first file signature and the first transformation signature. The first file format is a format which does not guarantee presenting all information included in the file. The second file format does not allow the user or a third person to add further information to the file which is not presented to the user. -