Скачать или смотреть Fixing the Invalid Signed PDF Error in iText7 with Deferred Signing

Learn how to solve the invalid signed PDF error when signing with iText7 using deferred signing techniques.
---
This video is based on the question https://stackoverflow.com/q/67889692/ asked by the user 'JoseSanches' ( https://stackoverflow.com/u/16155538/ ) and on the answer https://stackoverflow.com/a/68605398/ provided by the user 'JoseSanches' ( https://stackoverflow.com/u/16155538/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: java iText7 deferred with sign prefix producing invalid signed pdf

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Solving the Invalid Signed PDF Error in iText7: A Comprehensive Guide

When working on digital signatures in PDF documents, developers often encounter the dreaded error: "the document has been altered or corrupted after applying the signature." This issue can arise for various reasons, especially when integrating with external signing services. In this guide, we will explore a typical use case involving iText7 and explain how to implement a signing process that adheres to best practices, ensuring your signed PDFs are valid and trustworthy.

Understanding the Problem

In the scenario at hand, the objective is to use iText7 to sign a PDF with a signature from an external entity. The signing process involves several steps:

Obtaining certificates from a web service.

Getting the PDF's hash to sign, which requires adding a prefix before sending it to the external entity.

Creating a temporary PDF for signature processing.

Interacting with web services to obtain signed hashes.

Finalizing the PDF with the received signature.

The challenge arises when the implemented method fails, resulting in the error mentioning that the document has been altered or corrupted post-signature.

Solution Overview

To resolve the invalid signed PDF issue, a structured approach is needed:

Step 1: Set Up the Environment

Make sure to include the necessary libraries:

[[See Video to Reveal this Text or Code Snippet]]

Add the BouncyCastle security provider to allow cryptographic operations:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Obtain Certificates

Use a web service to acquire the necessary certificates:

[[See Video to Reveal this Text or Code Snippet]]

Step 3: Create a Temporary PDF and Generate the Hash

The next step is to create a temporary PDF file that will hold a placeholder for the signature. This includes calculating the hash that will be sent to the external signer:

[[See Video to Reveal this Text or Code Snippet]]

In the emptySignature method, a PdfSigner is instantiated. The empty signature container is utilized to create space for the signature and return the hash that is to be signed.

Step 4: Send the Hash to the External Signer

Once you have the hash, it needs to be sent to the external signing service. After sending, wait for a code confirmation via SMS and subsequently retrieve the signed hash:

[[See Video to Reveal this Text or Code Snippet]]

Step 5: Finalize the PDF with the Signed Hash

Once you've received the signed hash, you need to insert it into the temporary PDF to create a finalized, signed PDF:

[[See Video to Reveal this Text or Code Snippet]]

Step 6: Implement External Signature Containers

The signatures are managed with custom signature containers that handle the signing process. Here’s an example of the external signature container implementation:

[[See Video to Reveal this Text or Code Snippet]]

Make sure the signing process includes proper hashing and encoding to maintain PDF integrity and avoid the corruption error.

Final Note

Readers may note an important correction made during the implementation. Be on the lookout for updates in methods or classes that may not fully align with PDF signing standards.

By implementing this structured approach for obtaining signatures with iText7, you can overcome the obstacles commonly faced in signing PDFs and ensure a successful signing process that maintains document integrity.

Now that you have a comprehensive understanding of how to resolve the issue with signed PDFs in iText7, feel free to implement it into your projects or reach out for further assistance!