Removing PDF Signature Certificates

There are a couple of ways that PDF’s can be locked down. The most common of these is the password, of which there are two variations: user and owner. The user password prevents the document from being opened without the password, while the owner password allows whoever created it to restrict access to certain functionality (like printing or image extraction). There’s a less common way of preventing a document from being altered: the digital signature. Applying a cryptographic digital signature effectively prevents anyone other than the document’s creator from altering it, or by extension, removing the signature. Or does it?

Back in 2005, Martin Backschat worked out a way by which the restrictions can be removed, which in turn allows the signature to be removed. Martin’s original Perl script (which was intended to work in Cygwin) was repackaged by cupcake with the necessary Cygwin DLL’s and a BAT file. All you have to do is extract the ZIP, copy the PDF’s you wish to clean into the Unsign directory, and run the BAT. The first time it runs, it creates a subdirectory called “unsigned” and copies the new PDF’s into it.

Once the script has worked its magic, it’s fairly trivial to remove the signature, though not exactly intuitive. In Acrobat 8...

Short version:

1 Right-click the signature and select Clear Signature from the context menu.

2 Go to Forms > Edit form in Acrobat.

3 Select the signature container and delete it.

Save your sparkly clean PDF.

Long version:

You don’t actually need to do this, but if you’re curious about what’s going on, walking through it once will help illustrate the process...

1 Down the left edge of the application window is a stack of icons which correspond to informational sidebar panels: page thumbnails, bookmarks, etc. Open the Signatures panel and you should see this warning just below the signature (the line beginning “Signed by ... ”):

This is because it’s been thoroughly mauled by the unsigning script.

2 You can either click the signature line and select Go to Signature field from the Options drop down just above, or right-click it and select the same command from the context menu. The page will shift to wherever the signature is, and highlight it with a box.

3 Right-click the signature and select Clear Signature from the context menu.

4 Go to Forms > Edit form in Acrobat.

5 Select the signature container and delete it.

Save your sparkly clean PDF.