Method for authenticating a User by a Manager, wherein said User is associated to an electronic equipment in the form of a mobile phone terminal on which a software program is loaded, said software program being adapted to store inside said mobile phone terminal a cryptographic key encrypted via a PIN, wherein said Manager is associated to an electronic equipment, wherein the method provides an Intermediary associated to an electronic equipment adapted to communicate with said User electronic equipment and said Manager electronic equipment by means of text messages and to store a mobile telephone number of the User, and provides the following steps: A) the Manager electronic equipment sends to the Intermediary electronic equipment a text message containing at least an identity code of the User, B) the Intermediary electronic equipment identifies the User and the User mobile telephone number and sends to the User electronic equipment a text message containing at least an authentication key, said text message being encrypted, C) the User electronic equipment receives said text message from the Intermediary electronic equipment, decrypts it via said cryptographic key after having obtained said PIN from a person using the User mobile phone terminal, and sends another encrypted text message to the Intermediary electronic equipment containing at least said authentication key, D) the Intermediary electronic equipment receives said other encrypted text message from the User electronic equipment, decrypts it, and performs a comparison between the authentication key sent to the User electronic equipment and the authentication key received from the User electronic equipment, and E) the Intermediary electronic equipment sends a text message containing at least the outcome of said comparison or information deriving therefrom to at least the Manager electronic equipment; wherein said PIN is used by said mobile phone terminal only for internal encryption and decryption of said cryptographic key after having received it from a person using the User mobile phone terminal in each case; whereby the Manager authenticates the User based on said received outcome or information deriving therefrom. A) the Manager electronic equipment sends to the Intermediary electronic equipment a text message containing at least an identity code of the User, B) the Intermediary electronic equipment identifies the User and the User mobile telephone number and sends to the User electronic equipment a text message containing at least an authentication key, said text message being encrypted, C) the User electronic equipment receives said text message from the Intermediary electronic equipment, decrypts it via said cryptographic key after having obtained said PIN from a person using the User mobile phone terminal, and sends another encrypted text message to the Intermediary electronic equipment containing at least said authentication key, D) the Intermediary electronic equipment receives said other encrypted text message from the User electronic equipment, decrypts it, and performs a comparison between the authentication key sent to the User electronic equipment and the authentication key received from the User electronic equipment, and E) the Intermediary electronic equipment sends a text message containing at least the outcome of said comparison or information deriving therefrom to at least the Manager electronic equipment; wherein said PIN is used by said mobile phone terminal only for internal encryption and decryption of said cryptographic key after having received it from a person using the User mobile phone terminal in each case; whereby the Manager authenticates the User based on said received outcome or information deriving therefrom.