A fiduciary sends a signed service agreement to a client. Three months later, the client disputes the terms, claims the contract was never properly executed, and refuses to pay. The signature is there on the document. The date is there. The amount is there.
What happens next depends almost entirely on one variable: the tier of electronic signature that was used.
The legal test a challenged contract must pass
When a signed contract is disputed in Switzerland or the EU, a court does not simply look at whether a signature appears on the document. It examines three things:
- Signature tier: Was the signature simple, advanced, or qualified? Each tier carries a different legal status under ZertES (SR 943.03) and eIDAS (EU) 910/2014.
- Timestamp integrity: Can the system prove when the document was signed, and that it has not been altered since?
- Chain of custody: Is there a verifiable record linking the signer's identity to the signing event?
The answers determine something more important than whether the signature is valid. They determine who has to prove it.
The burden of proof: AES vs. QES
An Advanced Electronic Signature (AES) is legally valid. Under Swiss law and EU law, it can be used for most contracts. But it does not carry automatic legal presumption. If someone challenges a document signed with an AES, the party asserting its validity must prove it is authentic. That means audit logs, authentication records, and possibly expert testimony.
A Qualified Electronic Signature (QES) operates differently. Under ZertES Art. 11, a QES has the legal status of a handwritten signature. Under eIDAS Art. 25(2), a QES has the equivalent legal effect of a handwritten signature. Both statutes confer legal presumption. The burden now falls on the challenger: they must disprove the signature, not you.
That reversal, from proving to disproving, is the entire practical difference.
What a disputed AES costs in practice
When a contract signed with an AES is challenged, the enforcing party typically faces significant legal costs to gather authentication evidence, delays of months before reaching the substantive contractual question, and uncertainty that depends on the completeness of the signing platform's records. In regulated sectors, a disputed signature can also trigger supervisory inquiry. None of these consequences appear at signing. They surface only when the dispute arises.
How a QES changes the equation
A QES issued by a Qualified Trust Service Provider (QTSP) on the Swiss Federal Register (BAKOM) or the EU Trust List (EUTL) carries a cryptographic certificate tied to a verified identity, recorded with a qualified timestamp. When someone challenges a QES-signed document, the legal presumption under ZertES Art. 11 and eIDAS Art. 25(2) applies from the outset. The challenger must prove the signature is invalid. That is a considerably higher standard.
Documents a Swiss fiduciary should sign with a QES
Mandate agreements, investment confirmations, notarially-equivalent filings under OR Art. 14, and cross-border agreements benefiting from eIDAS recognition across all 27 EU member states should be signed with a QES. For a detailed breakdown, see the ZertES requirements guide.
The gap shows only in a dispute
AES and QES look identical on a document. The difference surfaces only when someone files a challenge. By then it is too late to change the signature tier. If you want to assess exposure on documents you have signed or are about to sign, the IP Exposure Calculator can help map risk by document type. For the specific requirements under ZertES, see the ZertES page.
