Glossary
What is a digital signature?
Short answer
A digital signature is a cryptographic seal: software hashes the document, then encrypts that hash with a private key, and anyone with the matching public key and certificate can check it. If even one byte of the file changes, the check fails. It is a technique, while an electronic signature is a legal category, and many e-signature services use a digital signature to lock the finished document.
Updated 25 October 2026
- Digital signature
- A digital signature is a cryptographic value created with a private key over a hash of a document, which anyone can check with the matching public key. It shows which key signed the file and that the file has not changed since.
How a digital signature works
Signing software first runs the document through a hash function such as SHA-256, which produces a short fixed-length fingerprint. It then encrypts that fingerprint with the signer's private key, using an algorithm such as RSA or ECDSA. The result, plus the signer's certificate, is stored with the document.
To verify, a reader recomputes the hash of the file, decrypts the stored value with the public key from the certificate, and compares the two. A match means the file is unchanged and was signed by whoever held that private key. The certificate, issued under a public key infrastructure, ties the key to a named person or organisation.
Digital signature versus electronic signature
People use the two terms interchangeably, but they answer different questions. 'Electronic signature' is the legal term in ESIGN, UETA and eIDAS for any electronic mark made with intent to sign. 'Digital signature' describes a specific piece of cryptography.
A signer drawing their name in a browser is making an electronic signature. The service then applying a certificate-based seal to the finished PDF is making a digital signature. One document often carries both.
Where digital signatures sit under eIDAS
eIDAS does not use the phrase 'digital signature', but its advanced and qualified electronic signatures are in practice built on certificate-based digital signatures. A qualified electronic signature needs a qualified certificate from a trust service provider on an EU trusted list and a qualified signature creation device.
A digital signature applied by a platform with its own certificate is not, by itself, an advanced or qualified signature of the individual signer, because the key belongs to the platform, not to the person.
How Sign PDF uses digital signatures
When the last person signs, Sign PDF writes the fields into the PDF, adds the certificate of completion, and seals the whole file with a PAdES-style digital signature carrying an independent timestamp from DigiCert's timestamp authority. If anyone edits the file afterwards, the seal breaks and PDF readers such as Adobe Acrobat show a warning.
That seal protects the document's integrity. The signers' own signatures remain simple electronic signatures backed by email code confirmation and the audit trail. We do not issue personal signing certificates or qualified signatures.
Questions people ask
Do I need a certificate to make a digital signature?
To make one that others can verify, yes. The certificate links your public key to your identity. For everyday contracts most people do not need their own certificate, because the e-signature service seals the finished file for them.
Can a digitally signed PDF be edited?
It can be edited, but the edit breaks the signature and PDF readers will flag it as changed or invalid. That is the point of the seal: changes become visible rather than silent.
Is a digital signature more legally binding than an electronic one?
Not automatically. Both are valid for most contracts. A certificate-based signature gives stronger evidence of integrity, and under eIDAS only a qualified signature has the automatic equivalence to a handwritten one. This is not legal advice.
How do I check a digital signature on a PDF?
Open the file in Adobe Acrobat Reader and look at the signature panel, which shows whether the signature is valid and whether the file changed after signing. For Sign PDF documents you can also use signpdf.ai/verify.
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