Digital signature

In short: A digital signature proves that a document comes from a particular person and has not been altered since signing.

In more detail: The sender computes a hash of the document and encrypts it with their private key. Anyone can use the public key to check whether the signature matches. If even one character changes, the check fails.

In Depth

• Uses: software updates, TLS certificates, emails (S/MIME, PGP), PDFs, JWT, eIDAS signatures (eIDAS).
• Methods: RSA, ECDSA, Ed25519.
• Difference to encryption: The signature does not make the content secret; it secures origin and integrity.
• Legal levels: simple, advanced and qualified electronic signature.
• Trust: The public key must be securely tied to the person (certificate).

See also: Hashing, Asymmetric encryption