Method of operating a hearing aid fitting system and a hearing aid fitting system

A method of fitting a hearing aid system comprising the steps of:

• storing first data in a hearing aid memory;
• storing second data in an external memory, wherein the second data is a copy of the first data;
• using a hearing aid processor to generate a first hash key representing the first data,
• using a fitting device processor to generate a second hash key representing the second data;
• providing the first hash key to the fitting device;
• comparing the first hash key with the second hash key in order to determine whether the values of the first and the second hash key match;
• and
• triggering a specific action in response to the result of the comparison of the hash keys;
• and
• wherein the step of triggering a specific action comprises:
• using the second data as a basis for deriving third data to be stored in the hearing aid memory, if the values of the first and the second hash key match, and
• providing the first data from the hearing aid and to the fitting device and using the first data as a basis for deriving third data to be stored in the hearing aid memory, if the values of the first and the second hash key don't match.