Digital Signature Security Question

Hi Phil,

Under normal circumstances, Acrobat and Reader allow some change to occur that will not invalidate the signature, but they do show up as document modification. A couple of examples would be adding a sticky note, or filling out an unlocked text form field after it was signed. Other operations, such as changing the PDF content (form data and comments are not PDF content, but are part of what is known as the annotation layer) will invalidate the signature.

It looks like you are trying to prevent anyone from changing the document at all after it's signed, and if they did, then you want the signature to become invalid.

There are a couple of ways to go here. When you sign the document if you elect to apply a Certifying signature you can set the Permitted Actions to "No changes allowed". That will give you the effect you want. Another way is if you are adding the last digital signature (that is, signing the last unsigned signature field) you have the option of selecting the "Lock Document After Signing" checkbox. That will give you he same effect as certifying the document. You want to be careful though because once the document is locked the only way to unlock it is to clear the signature and the only person that can clear the signature is the signer.

Steve

Thank you, Steve.
That's what I was needing to know.
The document doesn't have any fields (aside from the signature field I am applying), I am certifying it when signing, and setting the "Lock Document After Signing" checkbox.

From what I read of what you and others have written in these forums, I think the intent of the state law allowing digital signatures on engineering documents that the signature be invalidated if the document is modified is met (primarily because the document cannot be modified by the typical user - it would take someone who is really determined to modify the document and has a high degree of knowledge about PDF formats, content, etc. to defeat the document security and modify the document without my signature being marked invalid).