Workflows & How-To6 min read·Updated Oct 2026

How SHA-256 Cryptographic Hashing Makes Signed PDFs Tamper-Evident

In the digital world, a simple visual image of a signature on a PDF can easily be cut, copied, or modified in Adobe Acrobat. Learn how dual SHA-256 cryptographic hashing creates an unbreakable mathematical seal that proves your document has never been tampered with.

The Vulnerability of Unsealed PDF Documents

Quick Answer / Definition

Anyone with a basic PDF editor can alter terms, numbers, or dates in an unsealed PDF, making visual signatures alone insufficient to prove authenticity during a legal dispute.

Imagine signing a quote for £5,000, but the counter-party alters the text in Adobe Acrobat to read £8,000. How do you prove what the original document said?

ChickenSign solves this using cryptographic SHA-256 hashing. The moment a document is created, ChickenSign computes its exact binary fingerprint (the content_hash). When all parties complete signing, a second completion fingerprint (completed_hash) is generated, locking the document forever.

If anyone alters the PDF, recalculating the SHA-256 hash immediately produces a completely different string, rendering the document invalid on the public verification portal at /verify/:code.

Frequently Asked Questions

What is a SHA-256 hash?

SHA-256 is an industry-standard cryptographic hash function designed by the US National Security Agency (NSA). It turns any digital file into an irreversible, unique 256-bit fingerprint.

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