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What is a digital signature?

A digital signature proves who signed and that nothing has changed since. See how the technology works, and what eIDAS means.

In short

A digital signature is a cryptographic signature that proves two things at once: that a document or file originates from a specific sender (authenticity), and that the content has not been altered since it was signed (integrity).

Technically, a hash of the content is computed and encrypted with the signer's private key, after which anyone can verify the signature using the public key. If even a single character in the document changes, the signature no longer matches.

Digital signatures are the foundation beneath signed contracts and MitID, Denmark's national digital ID, beneath signed software and beneath secure email, and in the EU the eIDAS regulation governs the levels, where a qualified electronic signature is legally equivalent to a handwritten one. For companies the point is twofold: signatures make paper processes digital and binding, and signed code and email protect against forgery.

MI Support IT helps with the certificates and the infrastructure behind them, from PKI management to signing in practice.

Back to the glossary

What is a digital signature?

A digital signature is a cryptographic signature on a document, an email or a file. It proves two things at once:

  • Authenticity: The content originates from the signer, and only from them.
  • Integrity: The content has not been altered by so much as a single character since it was signed.

On top of that comes non-repudiation: the signer cannot later credibly deny having signed. These are properties that a scanned handwritten signature or an image of a signature can never provide: they can be copied freely and prove nothing.

How does it work technically?

A digital signature combines two building blocks from cryptography: hashing and asymmetric encryption.

  1. A hash of the content is computed, a unique digital fingerprint.
  2. The hash is encrypted with the signer's private key. The result is the signature.
  3. The recipient computes their own hash of the content and decrypts the signature with the signer's public key. If the two match, both sender and content are verified.

The link between the public key and a real-world identity is provided by a digital certificate issued according to the X.509 standard, which means the whole system rests on a well-functioning PKI.

In the EU, the eIDAS regulation governs electronic signatures at three levels: simple, advanced and qualified electronic signature. The qualified version (based on a qualified certificate and secure signature creation hardware) is legally equivalent to a handwritten signature throughout the EU. That is the mechanism behind Danish businesses signing with MitID Erhverv, the business edition of Denmark's national digital ID, and it means contracts, annual reports and agreements can be signed fully digitally and bindingly.

Where are digital signatures used in practice?

In more places than most people realise: signed contracts and documents, code signing of software and drivers (so Windows can reject tampered programs), signed and encrypted email via S/MIME or PGP, the DKIM signatures on email in the DMARC ecosystem, and machine-to-machine trust throughout modern infrastructure. Common to them all: if the certificate behind the signature expires or is compromised, the trust collapses. That is why certificate management is an operational task, not a one-off project.

How MI Support IT helps

We deliver the foundation that makes digital signatures work reliably: PKI management with control of certificates, keys and renewals, protection of signing keys with an HSM and advice on signing documents, code and email. If you want to go deeper, download our free e-book PKI and HSM in practice, or contact us directly.

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