What Is a Digital Signature? Definition and How It Works
Summary:
Learn what a digital signature is, how it verifies identity and document integrity, how it differs from an e-signature, and what it means for PDFs.
Table of Contents
A digital signature is often mentioned in contracts, PDFs, government forms, and secure online documents, but it is not the same as simply adding a signature image to a file.
This guide explains what a digital signature is, how it works, how it differs from an electronic signature, what it means in a PDF, and when it may be legally valid.
What Is Digital Signature?
A digital signature is a secure way to prove two things: who signed a digital file and whether the file has been changed after signing.
In simple terms:
A digital signature works like a tamper-evident seal with an ID check. The ID helps verify the signer, and the seal helps show whether the document has been changed.
Because of this, digital signatures provide three important benefits:
- Authentication: helps verify the identity of the signer.
- Integrity: helps confirm that the document has not been modified after it was signed.
- Non-repudiation: makes it harder for the signer to deny signing the document.
A digital signature is often confused with other types of signatures, but they are not the same. A typed name, scanned handwritten signature, or signature image can show signing intent, but they cannot verify identity or detect document changes by themselves.
Likewise, a digital signature is not the same as document encryption. Encryption protects document privacy, while a digital signature verifies authenticity and integrity.
How Does Digital Signature Work?
When someone digitally signs a document, the goal is simple: to prove who signed it and to make it easy to tell if the document has been changed afterward.
Here’s what happens behind the scenes:
1. Create a unique fingerprint
Every document has its own unique fingerprint, called a hash. If even one character changes, the fingerprint changes, too.
2. Sign it with a private key
This key, known as a private key, belongs only to the signer. Instead of signing the entire document, it signs the document’s fingerprint.
3. Attach a digital certificate
A digital certificate is attached as well. It tells others who the signer is and provides the information needed to verify the signature.
4. Verify the signature
The PDF reader creates a new fingerprint from the received document and uses the signer’s public key to verify the signature.
5. Compare the fingerprints
If they match, the document hasn’t changed since it was signed. If they don’t, the signature may be marked as invalid because the document was modified, or something doesn’t match.
Why Should You Trust a Digital Signature?
You may also come across terms like Certificate Authority (CA) and Public Key Infrastructure (PKI).
A CA verifies the signer’s identity and issues the digital certificate, while PKI is the framework that manages certificates and encryption keys. Together, they help ensure that a digital signature can be trusted.
Digital Signature vs Electronic Signature
The terms digital signature and electronic signature are often used interchangeably, but they are different.
An electronic signature is any electronic way of showing your intent to sign a document, such as typing your name, drawing a signature, or inserting a signature image.
A digital signature is a specific type of electronic signature that uses cryptography and a digital certificate to help verify the signer’s identity and detect whether the document has been changed after signing.
In other words, every digital signature is an electronic signature, but not every electronic signature is a digital signature.
For a more detailed comparison, see our guide to Digital Signature vs. Electronic Signature.
What Digital Signature Means in PDF
In a PDF, a digital signature is not just a visible signature mark. It can also include verification details, such as the signer’s identity, signing time, digital certificate, and signature status.
Examples of digital signatures in PDF
The main value is tamper detection. If the PDF is edited after signing, the signature may become invalid or show a warning because the file no longer matches the version that was originally signed.
Invalid Digital Signature Warning in PDF
Are Digital Signatures Legally Binding?
Yes, digital signatures can be legally binding because they help prove the signer’s identity, signing intent, and document integrity.
However, whether a digital signature is legally recognized ultimately depends on factors such as your country’s laws, the type of document, the digital certificate used, and how the signer’s identity is verified.
For example, many regions recognize digital signatures under laws such as the ESIGN Act and UETA in the United States or eIDAS in the European Union. However, specific requirements vary by jurisdiction, so the same signing method may not be suitable for every document or country.
In general, digital signatures are commonly used for:
- Important contracts and business agreements.
- Government and regulatory documents.
- Financial and tax forms.
- Sensitive PDF files that require proof of authenticity.
Note: This article provides general information only and should not be considered legal advice.
FAQs About Digital Signatures
Is a digital signature the same as an electronic signature?
No. A digital signature is a specific type of electronic signature that uses cryptography and digital certificates to verify the signer’s identity and help protect document integrity.
An electronic signature is a broader term that includes typed names, signature images, and electronically added handwritten signatures.
Does a digital signature encrypt the document?
Not exactly. Digital signatures are used to verify a file’s authenticity and integrity, not to keep its contents private. If you want to prevent others from viewing the file, you need to protect it separately, such as by adding a password.
What is a digital signature certificate?
A digital signature certificate links a signer’s identity to a public key. It allows others to verify that a digital signature belongs to the claimed signer and has not been forged.
What happens if a digitally signed PDF is edited?
If a digitally signed PDF is modified after signing, the signature may become invalid or show a warning like “Signature is invalid,” “Document has been altered,” or “At least one signature has problems.”
Can I create a digital signature for a PDF for free?
Yes. With PDFgear, you can create or import a Digital ID and use it to add a certificate-based digital signature to a PDF for free.
Download PDFgear, open your PDF, then go to Protect > Certificate Manager > Digital IDs to create or import your certificate. After that, choose Protect > Encrypt with Certificate, place the signature field, enter your certificate password, and save the signed PDF.
Conclusion
A digital signature is more than a way to sign a document. It helps verify who signed a file, whether it has been modified, and, in many cases, supports legal and business requirements for secure digital documents.
