Removing Digital Signatures From a PDF Document

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Vincent Ycasas

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May 1, 2013, 2:41:09 PM5/1/13
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Question:
How can I remove a digital signature from a PDF document? If I have a PDF document that has two signatures in it, can I remove one signature without invalidating the other signature?

Vincent Ycasas

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May 1, 2013, 2:41:46 PM5/1/13
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Answer:

Depending on a given use case scenario, removing a signature field would involve the following:

1.) Assuming there is only one valid signature in a document. In order to remove this digital signature, you will need to determine whether the digital signature was applied on the first time the PDF was created, or the digital signature was added after the PDF was created. If the former case (digital signature was added during PDF creation) is correct, then you will need to find the signature field and delete the value (which is the signature dictionary). You may also need to delete the AP field if you wish to remove the signature appearance.

If it is the latter case (where the signature is added after the PDF was created), usually these documents are saved in incremental mode. To remove an existing signature, you simply go back to the previous version of the document. Adding a new signature on an exisitng document requires preserving the current version which is why signed PDF documents are saved in incremental mode. One way to go back to a previous version of the PDF is to find the document's 2nd to last "%%EOF\r\n" (assuming the most recent incremental saving involved adding signatures only) and delete anything after this string. This will give you the version of the document before it was signed.

2.) If there are more than one signatures in the document, you will only be able to remove the most recent signature so that other signatures do not get invalidated. As mentioned above, signed PDF's are usually saved in incremental mode -especially if counter signatures are involved. To remove the most recent signature, you will have to revert back to the version before this most recent signature is applied (see previous use case scenario). If you want to remove an older signature, you will need to remove all counter signatures that were added after.

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