How to generate a digital signature

To generate a digital signature it is necessary to use a pair of asymmetric digital keys uniquely attributed to an individual , called the owner. The private key known only by the holder is used to generate the digital signature to be affixed to the document, and the public key can be used by anyone who needs to verify the authenticity of the signature on it. This method is known as double key encryption and ensures complete security since the public key cannot be used to reconstruct the private key. Asymmetric digital keys can only be issued by a Qualified Trust Service Provider which is responsible for verifying the identity of the holder and issuing him a digital certificate of subscription.

The digital certificate can be contained in a signature device (USB key), or stored on the servers of the Qualified Trust Service Provider and recalled by the holder through specific software tools (remote signature).  Both methods allow the "private", unique and secret encryption key to be used to affix the digital signature; the related "public" key, on the other hand, made available online by the issuing Certification Body, allows any subsequent verification.