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Glossary / Digital Signature

What Is a Digital Signature in PDFs?

A digital signature in a PDF is a cryptographic mechanism that verifies the signer's identity and ensures the document has not been modified after signing. Unlike a simple image of a handwritten signature, a digital signature uses public-key infrastructure (PKI) and is legally binding in many jurisdictions under regulations like eIDAS and ESIGN.

How digital signatures work

When a PDF is digitally signed, the signing software computes a cryptographic hash of the document contents and encrypts it with the signer's private key. The resulting signature, along with the signer's certificate, is embedded in the PDF. A reader can verify the signature by decrypting the hash with the signer's public key and comparing it to a freshly computed hash of the document.

Per ISO 32000-2, PDF signatures use a dedicated signature dictionary that records the byte range of the signed content, the signature algorithm (typically RSA or ECDSA), and the certificate chain. If even a single byte of the signed content changes, verification fails.

Digital vs electronic signatures

An electronic signature is any electronic mark indicating intent to sign (including a typed name or scanned image). A digital signature is a specific type of electronic signature that uses cryptography for verification. Digital signatures offer stronger proof of identity and document integrity.

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