What is an Advanced Electronic Signature (AES)?

An advanced electronic signature (AES) is an electronic signature that meets the four requirements set out in Article 26 of the eIDAS Regulation (EU) No 910/2014. In terms of evidential strength an advanced electronic signature (AES) sits between a plain electronic signature and a qualified electronic signature (QES).

What are the four requirements that make a signature “advanced”?

Article 26 sets four tests. A signature that fails any one of them is not an AES, this is regardless of what the vendor calls it in the product name.

Article 26 is technology neutral. It tells you what the signature must achieve but not which cryptography or identity method to use. In practice, almost every compliant implementation uses public key infrastructure with a hash of the document, because nothing else satisfies the fourth requirement cleanly.

Signer Identification

Identification is the requirement most implementations get wrong, because an email address is not an identity. In order to satisfy Article 26(b), there needs to be evidence that the person holding the credential is who they claim to be, captured before or at the point of signing. That normally means a document check, a biometric face check against the document photo, or a reusable electronic identification (eID) credential issued after an equivalent check.

The failure mode, however, is subtle. A flow that verifies identity once at account creation, then lets anyone with the password sign months later, identifies an account rather than a signatory. If the signature is challenged, you end up defending your password policy instead of the identity.

Sole control of the signature creation data

Sole control means the signatory, and only the signatory, can trigger the signature. Two models are common for this:

• Local keys: The private key lives on a smart card, a USB token, or in the secure element of the signer’s phone. The control is physical.
• Remote signing: The key is held by a trust service provider inside a hardware security module and released only when the signer authenticates. Control is procedural and depends entirely on the strength of that authentication.

Remote signing is where sole control usually breaks. If the one-time passcode goes to a shared inbox, if an administrator can trigger signing on a user’s behalf, or if the authentication step is a single reused password, you no longer have “a high level of confidence” in sole control, and you no longer have an AES.

Tamper Detection

The signature must be linked to the signed data so that any subsequent change is detectable. In practice, the signing service hashes the document, signs the hash with the private key, and embeds the result in the file. Change one character and the recomputed hash stops matching, so validation fails.

This is what separates an AES from a drawn signature image, which proves nothing about the content it sits on. Keep the signed artefact rather than a re-exported copy, because flattening or re-processing a PDF can strip the signature structure and leave you unable to validate later.

How AES compares with SES and QES

eIDAS defines three tiers. “Simple electronic signature” (SES) is industry shorthand rather than a legal term, and it covers any electronic signature under Article 3(10) that does not meet the Article 26 bar.

The line that matters for a compliance decision is Article 25. No electronic signature can be denied legal effect or admissibility just for being electronic, but only a QES carries automatic equivalence to a handwritten signature. An AES is not automatically equivalent. What it gives you is a much stronger evidential record to put in front of a court or a regulator if the signature is disputed.

How does an Advanced Electronic Signature Work?

Public key infrastructure is used for almost every compliant infrastructure and runs the same five steps, this is regardless of whether the key sits on a smart card or with a remote provider. Following are the steps that define the working of an AES.

1. Verification of the Signer, identity is established by a document and biometric check or an existing eID credential, which is then bound to the signing credential.
2. The signature creation data must be issued, a key pair is generated for that person, it can be held on their own device or a provider’s hardware security module. 
3. Authentication at signing is also a crucial step. The signer passes a step which can only be cleared by them.
4. Hash and sign, the service hashes the document and that hash is signed with the private key, it binds the signature to that exact content. 
5. Packaging and logging is the final step for the functioning of an AES. The signature, certificate and time stamp go into the file, with an audit trail of the identity and authentication evidence.

Where is AES used?

AES is the default choice where you need defensible proof of who signed, but the law does not demand a qualified signature:

• Account opening and customer agreements in banking, lending, and insurance
• Employment contracts, NDAs, and supplier agreements
• Loan documentation, leases, and mandates where the counterparty may later dispute the terms
• Cross-border business contracts inside the EU, where a single AES standard beats reconciling national rules
• Public sector online services, where Commission Implementing Regulation (EU) 2026/248 sets the formats of advanced electronic signatures that public sector bodies must recognise, replacing Commission Implementing Decision (EU) 2015/1506

The residual risk is repudiation. An AES shifts the argument from “did you sign this” to “how do you know it was me”, so the identity evidence captured at signing time is what you will actually be defending.