One of essential functions of digital signatures is to guarantee the integrity of the signed data. That is achieved by encrypting the data (its checksum) with one of the asymmetric cryptography algorithms. When you make changes to the signed data, the checksum no longer corresponds to the value included in the signature, so the signature can be identified as invalid.
However, in real life, the data to be signed is often far more complicated than plain-text strings. When digitally signing document files, it is only guaranteed that the binary content of the document file is exactly as it was at the time of signing. However, it does not say anything about whether the visual representation or the document contents in the applications displaying these documents is the same.
We are demonstrating a number of ways in which, by using various functions available in .docx and .odt formats, it is possible to create documents whose file contents are unchanged and thus are not raising any doubts about the validity of the digital signature, but in which the actual content displayed to the user may vary.Continue reading