What four properties should a (hand-written or digital) signature provide?

Question

Accepted Answer

Integrity, Authenticity, Non-repudiation, and "Willenserklärung" (consent / intentionality).

Property
What it means
How digital signatures provide it

Integrity
The document cannot be (undetectably) changed after signing
The signature is computed over the document's hash; any change → hash mismatch → signature invalid

Authenticity
The signature can be unambiguously attributed to a specific person
Only the holder of the private key can produce a valid signature verifiable with their public key

Non-repudiation
The signer cannot later deny having signed
Only the private key holder could have made the signature → courts can use it as proof

Willenserklärung (consent / declaration of intent)
The signature can only have been placed deliberately
Provided by UX — the signing software must require explicit user action (PIN, biometric, "Sign" button)

The first three are purely cryptographic. The fourth is a UX/legal property — a signature has to mean "I, the human, intended to sign this", not "my software automatically signed it without my knowledge."
Legal weight (qualified electronic signatures, EU eIDAS, Swiss ZertES):

Need to be made with a secure signature creation device (SmartCard, HSM, or qualified mobile app).
Use a certificate from a qualified trust service provider (eID services like Swiss SuisseID, Mobile-ID, etc.).
Then they have the same legal force as a handwritten signature under Swiss / EU law.

Tip: A "self-signed PDF with a checkbox" provides integrity and (maybe) authenticity but not non-repudiation or proper Willenserklärung — it's not a qualified signature. For contracts where legal force matters, use a recognised QES provider.

Property	What it means	How digital signatures provide it
Integrity	The document cannot be (undetectably) changed after signing	The signature is computed over the document's hash; any change → hash mismatch → signature invalid
Authenticity	The signature can be unambiguously attributed to a specific person	Only the holder of the private key can produce a valid signature verifiable with their public key
Non-repudiation	The signer cannot later deny having signed	Only the private key holder could have made the signature → courts can use it as proof
Willenserklärung (consent / declaration of intent)	The signature can only have been placed deliberately	Provided by UX — the signing software must require explicit user action (PIN, biometric, "Sign" button)