Are you sure you want to destroy (invalidate) the digital signature by changing stamps? This sounds like something to be completed before signing.
I do not want to invalidate the digital signature when applying a watermark.
The use case is when a user prints a hardcopy of a digitally signed SOP, a watermark needs to be placed on the page that states the hardcopy is an un-controlled document, the print date, and an expiration date.
I realize that any change to a digitally signed document is normally considered to be an operation that invalidates the signatures. However, it is possible to specify fields that may be changed AFTER the signatures are applied without invalidating the signature. I am looking for the same capablity with watermarks.
Watermarks are normal page contents; as far as I can see this MUST invalidate the signature, and correctly. A watermark could contain new content that changes the accredited meaning of a document.
However, perhaps someone has an idea. I don't know about the type of field you mention either.
It is possible allow any field to be modified after a signature has been applied without invalidating the signature. The author must specify the modifiable fields before the document is signed. In fact, it is even possible to specify fields that may be altered only AFTER the document has been signed. Take a look at section 2.4.2 in the Adobe's Digital Signature Workflow Guide for some details.
A page watermark is a type of field but unfortunately does not appear to have the same flexibility as other fields.