Method for authenticating a user to a service provider, among a plurality of service providers, by means of a single authentication device identified by a device identifier, comprising the steps of: - transmitting, from the authentication device to the service provider, an authentication request comprising at least said device identifier, - preparing, by the service provider, provider authentication data on the basis of pairing data shared by both said authentication device and said service provider, - sending said provider authentication data from the service provider to the authentication device, - authenticating at the authentication device said provider authentication data; in case of positive outcome, then - preparing device authentication data based on any of said pairing data by the authentication device, then sending said device authentication data to the service provider, - verifying the authenticity of the device authentication data by the service provider and in case of positive outcome, then validating the authentication of said user, characterized in that - said authentication device comprises a provider record for each of said service providers with which the user is registered by having a user account at each of said service providers, each provider record comprises a pairing key KP and first data, said pairing key KP and said first data being shared with the service provider to which said provider record refers, - said provider authentication data comprises a first cryptogram obtained by encrypting said first data with said pairing key KP, - authenticating said provider authentication data is performed at the authentication device by the steps of: - decrypting said first cryptogram by means of the pairing key KP stored in one of said provider records, then - comparing the result of this decryption with first data resulting from pairing data stored in said provider record, - if the comparison does not indicate a match, then processing again the previous decryption and comparison steps by using the pairing key KP of another provider record until each of said provider records stored in the authentication device has been processed. - transmitting, from the authentication device to the service provider, an authentication request comprising at least said device identifier, - preparing, by the service provider, provider authentication data on the basis of pairing data shared by both said authentication device and said service provider, - sending said provider authentication data from the service provider to the authentication device, - authenticating at the authentication device said provider authentication data; in case of positive outcome, then - preparing device authentication data based on any of said pairing data by the authentication device, then sending said device authentication data to the service provider, - verifying the authenticity of the device authentication data by the service provider and in case of positive outcome, then validating the authentication of said user, - said authentication device comprises a provider record for each of said service providers with which the user is registered by having a user account at each of said service providers, each provider record comprises a pairing key KP and first data, said pairing key KP and said first data being shared with the service provider to which said provider record refers, - said provider authentication data comprises a first cryptogram obtained by encrypting said first data with said pairing key KP, - authenticating said provider authentication data is performed at the authentication device by the steps of: - decrypting said first cryptogram by means of the pairing key KP stored in one of said provider records, then - comparing the result of this decryption with first data resulting from pairing data stored in said provider record, - if the comparison does not indicate a match, then processing again the previous decryption and comparison steps by using the pairing key KP of another provider record until each of said provider records stored in the authentication device has been processed.