A device is described for electronic signature of data which can be immediately applied in e-banking and e-commerce or in any system based on electronic signature, with an optical input for receiving data from any computer monitor and with a display for showing the data to be signed. In addition it has a keyboard for interaction with the user and a system for processing the electronic signature. The device is not re-programmable, so that it is immune to possible attacks from viruses or malicious software which could compromise the system security. Moreover, as the data to be signed are displayed the user knows what data he or she is actually signing.