For a long time, signing a document meant printing, initialing, signing, and, in certain cases, certifying the signature in person. Paper was the natural support for contracting, and the handwritten signature was the main mechanism for identifying the signatory and demonstrating their intent. 

Today, a contract can be concluded between people who never meet physically, through a digital platform, and signed in a few minutes, allowing paper to disappear completely. This transformation did not eliminate the need for a signature. Instead, it changed how the Law ensures its authenticity, the identification of the signatory, and the integrity of the document. 

The European framework is based on Regulation (EU) No. 910/2014, known as the eIDAS Regulation (amended in the meantime by Regulation (EU) 2024/1183), which establishes the European framework applicable to electronic identification and trust services. In Portugal, the regime is complemented by Decree-Law No. 12/2021, which ensures the implementation of that Regulation within the national legal system.

One of the main particularities of this regime is precisely the distinction between different levels of signature. An electronic signature corresponds to the broadest concept and can take relatively simple forms. An advanced electronic signature requires additional guarantees, namely the ability to identify the signatory, the signatory's control over the means used to create the signature, and the ability to detect subsequent changes to the document. At the highest level is the qualified electronic signature: an advanced electronic signature created by a qualified signature creation device and based on a qualified certificate.

This distinction has legal consequences. Article 25 of eIDAS establishes that an electronic signature shall not be denied legal effect or admissibility as evidence solely on the grounds that it is in an electronic form or that it does not meet the requirements for qualified electronic signatures. However, it also establishes that a qualified electronic signature shall have the equivalent legal effect of a handwritten signature.

Thus, the question is no longer simply knowing whether a document was "digitally signed". It is necessary to understand which type of signature was used, what guarantees it offers, and what legal effects are associated with it.

Decree-Law No. 12/2021 establishes specific rules regarding the form and evidentiary value of electronic documents. In particular, the affixing of a qualified electronic signature is equivalent to a handwritten signature on paper-based documents and creates presumptions regarding the identity of the signatory or the existence of representation powers, the intent to sign, and the integrity of the document.

This does not mean, however, that other electronic signatures are legally irrelevant. Unless special provisions state otherwise, the evidentiary value of electronic documents not associated with qualified trust services is assessed under general legal principles. In practice, the choice of signature mechanism should therefore consider not only convenience or ease of use, but also the level of legal certainty required by the transaction.

However, it must be noted that the possibility of signing electronically does not mean that any act can be performed through any digital mechanism. eIDAS does not prejudice national rules regarding the conclusion and validity of contracts, nor specific requirements regarding their form. Thus, the question "can we sign electronically?" must be accompanied by a second one: "What formality does the law require for this specific act?".

Electronic signatures also do not, by themselves, solve issues related to representation powers. A signature can be technically valid, and it may still be necessary to verify whether the signatory had the authority to bind the person or company in question.

The evolution of the electronic signature represents, therefore, more than replacing a pen with a click. On paper, trust was associated with the handwritten signature and the physical medium itself. In the digital environment, that trust results from a combination of identity, certificates, authentication mechanisms, document integrity, and legally recognized trust services.

The document may no longer feature a signature with a traditional appearance. But technology now allows us to demonstrate, with varying levels of security, who signed, under what conditions, and whether the document was subsequently altered.

The true transformation lies, therefore, in the shift from a trust based mainly on paper to a trust based on digital identity and information integrity. The dematerialization of documents did not, after all, mean the dematerialization of trust. As paper lost its central role, the Law developed new mechanisms to ensure what signatures have always sought to guarantee: authorship, the manifestation of intent, and the integrity of the declaration.

The signature format has changed. Its legal function remains.