Method and device for securely operating a field device

A method for securely operating a field device having at least one human-machine interface comprising a display device and a keyboard configured to operate the field device, and a communications interface configured to connect a local operating device, wherein the field device during use as intended does not have a secure connection to a network for process control, the method comprising:

• connecting the field device, at least logically, to the local operating device via a local connection;
• establishing a secure connection between the local operating device and a trusted server using authentication features of a local operator;
• generating a query key in the field device;
• transmitting the query key via the local operating device and a secure connection to the trusted server;
• forming a signed response key by signing the query key with a private key of the trusted server
• transmitting the signed response key via the secure connection between the trusted server and the local operating device to the local operating device, and further via the local connection to the field device;
• extracting, in the field device, using the public key, the response key from the signed response key;
• comparing the response key to the query key;
• and
• enabling a predetermined procedure for authenticated unlocking of a local operation of the field device when the response key matches the query key, while maintaining an existing configuration in the field device.