AES · Advanced Electronic Signature
SES + a second factor that uniquely identifies the signer.
What it is
An SES with four additional properties: (1) uniquely linked to the signatory, (2) capable of identifying the signatory, (3) created using signature-creation data the signatory can use under their sole control, (4) linked to the signed data so that any subsequent change is detectable. SMS one-time codes are the standard mechanism for satisfying (1)–(3).
Scope
Every EU member state. AES doesn't carry automatic equivalence to a handwritten signature (that's QES) — but it carries substantially more weight in court than SES.
What letssign.now does
Flip a toggle when sending. The signer provides their phone number, receives a 6-digit code by SMS, enters it before they can sign. The phone number is recorded in the audit trail (masked). €0.05/SMS in Europe, billed monthly. The PAdES envelope handles the 'subsequent change detectable' requirement.
Deeper detail
The four requirements, mechanicallyExpandClose
Unique link: each SMS code is tied to one signing session and one phone number. Identification: the phone holder is the signer. Sole control: only the phone holder can receive the SMS. Tamper detection: PAdES envelope cryptographically binds the signature to the PDF bytes.
Related standards
The frameworks above interlock. Each linked page covers one in full.
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