Verifying Digital Signatures: Hashing & Public Keys

Unlike a simple hand-drawn graphic or image stamp, a Digital Signature provides mathematical proof of document authenticity and integrity. It binds a cryptographic key to the document binary data.

When you sign a PDF digitally, the signing software calculates a cryptographic hash (such as SHA-256) of the document. It then encrypts this hash using the signer's private key, embedding the signature block into the PDF.

When a recipient opens the file, the PDF reader decrypts the signature using the signer's public key, recalculates the document hash, and verifies that they match. If any character, image, or page is altered after signing, the hash validation will fail, marking the document invalid.