A computer-implemented method for secure online authentication, the method comprising: determining, via a processor of a device for secure data transmission, a connection being established between a browser application installed on a computer system and a protected website, wherein the computer system is distinct from the device for secure data transmission; obtaining, at the device for secure data transmission, information relating to the protected website in response to a plugin of the browser determining that the computer system has obtained a request for authentication from the protected website; establishing a protected data transmission channel between the device for secure data transmission and the protected website to receive, at the device for secure data transmission, at least one certificate of the protected website; receiving a complete tree of certificates, except a root certificate, associated with the protected website from the plugin of the browser; verifying validity of the complete tree of certificates based on a list of root certificates stored on the device for secure data transmission; when the validity of the complete tree of certificates is not verified, disconnecting the protected data transmission channel; responsive to the complete tree of certificates being verified, performing authentication and transmitting, from the device for secure data transmission, authentication data stored on the device for secure data transmission to the protected website; and in response to an indication of a successful authentication from the protected website, transmitting a new session identifier from the device for secure data transmission to the plugin of the browser application for enabling access to the protected website. determining, via a processor of a device for secure data transmission, a connection being established between a browser application installed on a computer system and a protected website, wherein the computer system is distinct from the device for secure data transmission; obtaining, at the device for secure data transmission, information relating to the protected website in response to a plugin of the browser determining that the computer system has obtained a request for authentication from the protected website; establishing a protected data transmission channel between the device for secure data transmission and the protected website to receive, at the device for secure data transmission, at least one certificate of the protected website; receiving a complete tree of certificates, except a root certificate, associated with the protected website from the plugin of the browser; verifying validity of the complete tree of certificates based on a list of root certificates stored on the device for secure data transmission; when the validity of the complete tree of certificates is not verified, disconnecting the protected data transmission channel; responsive to the complete tree of certificates being verified, performing authentication and transmitting, from the device for secure data transmission, authentication data stored on the device for secure data transmission to the protected website; and in response to an indication of a successful authentication from the protected website, transmitting a new session identifier from the device for secure data transmission to the plugin of the browser application for enabling access to the protected website.