Can one remove or revoke digital signature on PDF document?

Question

I have a theoretical question about PAdES. I want to know if it is possible to revoke a signature in PDF or remove it?

Answer 1

I don't know what exactly you technically mean by revoking a signature.

But it clearly is possible to remove a signature: An integrated PDF signature usually consists of a signature form field with a value that contains a CMS signature container.

You have the choice of either removing only that value or the whole field with the value.

The former option leaves an empty signature field, which can easily be used for a new signature with a visualization at the same location as your original signature (if it has any to start with).

The latter option removes your signature completely.

Two caveats, though:

• If you don't merely want the signature not to appear anymore, make sure that

  • you don't save this edit as an incremental update - if it was done as an incremental update, the document version with your signature could easily be restored;
  • you don't merely remove the reference to the the value from the signature field but that you actually clear the value object - the signature value object might be referenced from other locations in the PDF, too, so if you don't clear it, its information might remain accessible inside the PDF.

• If your PDF contains multiple signatures or document timestamps, and if the signature you want to remove is not the newest one, manipulating it will break at least all newer signatures / time stamps. This is due to the way multiple signatures are applied to PDFs:

  

  As you can recognize in this sketch, the bytes signed by newer signatures contain all older signatures.

  In such a situation, therefore, don't only implement "remove a single target signature" but instead "remove all signature starting at a single target signature".

For some more technical backgrounds on integrated PDF signatures cf. this answer and documents referenced from there.

Answer 2

Let's demonstrate with empirical evidence that it is indeed possible to remove a signature. Here's how it can be done.

Remove signature from a PDF

Disclaimer If you choose to edit or remove signatures from a PDF, be aware that you're doing so at your own risk. This action can have legal implications. It's advisable to seek professional guidance before proceeding.

1. Open the PDF in Google Chrome.
2. Click "Print" icon in the top right corner.
3. For "Destination", select "Save as PDF".

Verify that document has no signatures

You can use the command line tool pdfsig:

pdfsig some-file.pdf

The output should include the following line:

File 'some-file.pdf' does not contain any signatures