Digital Mobile Key: how to sign documents with legal validity
The Digital Mobile Key (Chave Móvel Digital, CMD) is the Portuguese State's digital identification and signature system. A signature made with the CMD is a qualified electronic signature — the highest level provided for in the eIDAS Regulation — with a legal effect equivalent to a handwritten signature; the signed document counts as a private document with recognised authorship.
What the Digital Mobile Key is
The Digital Mobile Key is a means of authentication and digital signature created by the Portuguese State and managed by AMA — Agência para a Modernização Administrativa (the Agency for Administrative Modernisation). It links a citizen's mobile phone number to their civil identification number, so they can authenticate with public and private services and sign documents without a card reader or installed software.
To sign, the citizen enters the signature PIN they have set and confirms with a code sent by SMS or generated in an authenticator app. The certificate behind the signature is issued by a qualified trust service provider and remains under the holder's sole control.
Registration is free and can be done online with a Citizen Card (Cartão de Cidadão) and card reader, through the Autenticação.gov app (the Portuguese government's authentication app), or in person at a Loja do Cidadão (citizen service centre), an IRN (Registries and Notary Institute) desk or participating tax offices.
What legal value a Digital Mobile Key signature has
A Digital Mobile Key signature is a qualified electronic signature. Under Article 25 of Regulation (EU) No 910/2014 (eIDAS), a qualified electronic signature has a legal effect equivalent to that of a handwritten signature and is recognised in all Member States of the European Union.
Under Portuguese law, an electronic document bearing a qualified electronic signature is treated as a private document with recognised authorship, under Article 376 of the Código Civil (Portuguese Civil Code). In practice, this reverses the burden of proof: it is the person disputing the signature who must challenge it, not the other way round.
The national framework is set out in Decreto-Lei n.º 290-D/99, as amended by Decreto-Lei n.º 88/2009, which governs electronic documents and digital signatures in Portugal.
How to sign a document with the Digital Mobile Key in AssinaJá
Signing with the CMD in AssinaJá takes five steps and less than two minutes, with nothing to install on your computer.
- Upload the document (PDF, Word or image) to AssinaJá.
- Mark the areas of the document where the signatures should be placed.
- Choose "Digital Mobile Key" as the signature method.
- Enter the mobile phone number linked to your CMD and your signature PIN.
- Confirm with the security code received by SMS or in the authenticator app.
Digital Mobile Key with Professional Attributes
Professional Attributes let you sign not just as a citizen, but in your certified professional capacity — lawyer, doctor, engineer, certified accountant, solicitor, among others. The attribute is validated with the relevant professional order or association and is recorded in the signature itself.
This matters whenever a document only has effect if it is signed by someone with a specific capacity: a court filing signed by a lawyer, a declaration signed by a certified accountant, a project signed by an engineer.
AssinaJá integrates the Digital Mobile Key and Professional Attributes natively. The Digital Mobile Key is included in every plan, including the free plan; Professional Attributes are available from the Growth plan upwards.
Digital Mobile Key or Digital Certificate: which to choose
Both produce legally valid signatures, but they solve different problems. The CMD is simpler to use and needs no hardware; a digital certificate on a card or token is often preferred in business settings where the certificate is tied to a role rather than to an individual.
| Digital Mobile Key | Digital Certificate (card/token) | |
|---|---|---|
| Signature level | Qualified | Qualified |
| Equipment needed | Mobile phone only | Card reader or USB token |
| Software to install | None | Issuer's middleware |
| Registration cost | Free | Paid, depending on the issuer |
| Professional Attributes | Yes | Depends on the issuer |
| Signing on the move | Yes | Difficult |
Why use AssinaJá instead of the State's app
The Autenticação.gov app, provided by the State, is free and lets you sign documents with the CMD. It is a good option for occasional signatures you make yourself.
AssinaJá handles the next step: sending documents to other people for signature, tracking who has already signed, keeping audit trails, generating contracts from templates and sending dozens of personalised documents at once. It accepts PDF, Word and images, runs entirely in the browser on any device, and keeps data stored in Portugal in compliance with the GDPR.
In other words: the State's app signs; AssinaJá manages the whole signing workflow, with the same level of legal validity.