How Can I Add Blank Pages to a Digitally Signed PDF without Invalidating the Signature?

Adding Blank Pages to Digitally Signed PDFs with iText and Preserving Signatures

Inserting an empty page at the end of a digitally signed PDF using iText remains a common requirement. However, a significant concern arises: does this operation invalidate the existing signatures?

Allowed and Disallowed Changes

To understand the impact of this operation, it's essential to refer to Adobe's technical white paper on digital signatures. Here, Adobe clarifies the allowed changes that do not void signatures in certified or signed documents.

Certified vs. Signed Documents

Within certified documents, changing page content is always prohibited, regardless of the allowed actions. For uncertified documents, page content changes are also disallowed. Thus, adding a page, which alters page content, presents a challenge.

Possible Solution

Unfortunately, iText does not provide a direct solution to append a page without invalidating signatures. However, the following workarounds may be considered:

• Using Page Templates: Prepare the document beforehand by creating a page template that anticipates the need for additional pages. Then, reference this page template when signing. Making use of page templates falls under the allowed category of form fill-ins.
• Splitting, Signing, and Merging: Split the signed PDF into two segments, sign the first segment, and then insert a blank page to the second segment. Finally, merge the segments back together, ensuring to preserve signatures using incremental updates.

Note: Remember that even allowed changes (including page templates) must be applied incrementally to retain signatures. Failure to do so can irreparably break the signature.

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