Abstract: According to the present invention there is provided a method of determining identity data in respect of a user of an electronic device, the method comprising the steps of: the electronic device receiving a second sound signal resulting from a first sound signal interacting with a part of the head of the user; deriving a signature from at least the second sound signal, the signature being characteristic of a topography of a part of the head of the user, determining identity data in dependence on the signature. In one embodiment, the electronic device produces the first sound signal which is substantially undetectable by the human ear or unobtrusive to the user. In another embodiment, the first sound signal is produced by the user.