By KhAn SaHaB


2013-05-23 09:37:44 8 Comments

I want to insert blank page at end of the digitally PDF using itext library.and my confusion is adding pages will invalidate the existing signatures? Thank you.

2 comments

@Joeri Hendrickx 2013-05-23 09:50:46

Yes it will. The signature is based on the content that is signed, so if you change the content, the signature is no longer valid.

If you're the signer, you can just sign the document again.

Whether the pages you add are blank is irrelevant.

@KhAn SaHaB 2013-05-23 09:54:10

but i am adding only blank pages in pdf..

@Joeri Hendrickx 2013-05-23 11:38:06

@NK123 that does not matter. See MKLs answer for a list of allowed changes. It's basically not allowing anything on the content.

@mkl 2013-05-23 10:37:53

To put Joeri's answer into context, have a look at the Adobe technical white paper Adobe Acrobat 9 Digital Signatures, Changes and Improvements, especially its section "Allowed and disallowed changes." Here Adobe clarifies the allowed changes (as seen by Acrobat 9 and up) that can be made to a certified or signed document without invalidating the signatures applied to the document.

Allowed actions for certified documents

Certified with no changes allowed

  • Allowed
    • No changes allowed
  • Disallowed
    • Digitally signing
    • Supplying form field values
    • Adding or editing annotations
    • Adding form fields
    • Changing page content

Certified with form fill-in and digital signatures allowed

  • Allowed
    • Supplying form field values
    • Digitally signing
  • Disallowed
    • Adding or editing annotations
    • Adding form fields
    • Changing page content

Certified with annotations, form fill-in, and digital signatures, allowed

  • Allowed
    • Adding or editing annotations
    • Supplying form field values
    • Digitally signing
  • Disallowed
    • Adding form fields
    • Changing page content

Allowed actions for signed but uncertified documents

  • Allowed
    • Adding signature fields (see Limitations on adding signature fields to signed but uncertified documents)
    • Adding or editing annotations
    • Supplying form field values
    • Digitally signing
  • Disallowed
    • Adding form fields other than signature fields
    • Changing page content

As you see "Changing page content" always is in the "Disallowed" category. As adding a page changes page content — afterwards there is at least a white page where there used to be nothing —, this is not allowed.

(Even though not explicitly mentioned here, instantiating page templates most likely also is allowed whenever form fill-ins are allowed as that would conform to the PDF standard, cf. ISO 32000-1 section 12.8.2.2.2. But making use of such page templates would at least require the document to be specially prepared before signing, and your question sounds like the documents are already signed without any such preparation...)

PS: Obviously even these allowed changes need to be added in append mode aka incremental updates. Anything else will break the signature for good. For some backgrounds see this answer.

@Egl 2017-01-11 16:29:01

Just an addition: if the blank page is added in append mode, the byte range of the signature is not modified, so the signature is correct from a cryptographic view. Another question is the disallowed changes that Adobe added from Acrobat version 9 that mkl has explained. So Adobe would consider this signature as invalid.

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