A method of registering a device with an authentication service, the method, being performed by a first device, comprises the steps of: establishing a secure connection between the first device and a second device (316, 318), in which the second device is registered with the authentication service and is allocated to a user, and in which the secure connection comprises one of: a wireless data connection over a local area network; and a wired data connection over a local area network; obtaining, over the secure connection from the second device, an identifier uniquely associated with the second device; providing, to the authentication service, a first credential known to the user, and a second credential derived from the identifier; requesting registration of the first device with the authentication service on the basis of the first and second credentials; characterised by the further steps of: establishing secure connections between the first device and a plurality of second devices (316, 318), in which each of the plurality of second devices is registered with the authentication service and is allocated to the user, and in which each of the secure connections comprises one of: a wireless data connection over a local area network; and a wired data connection over a local area network; obtaining from each of the plurality of second devices, an identifier uniquely associated with at least one of the plurality of second devices; providing to the authentication service a plurality of second credentials derived from the identifiers obtained from each of the plurality of second devices; and requesting registration on the basis of the first and the plurality of second credentials. establishing a secure connection between the first device and a second device (316, 318), in which the second device is registered with the authentication service and is allocated to a user, and in which the secure connection comprises one of: a wireless data connection over a local area network; and a wired data connection over a local area network; a wireless data connection over a local area network; and a wired data connection over a local area network; obtaining, over the secure connection from the second device, an identifier uniquely associated with the second device; providing, to the authentication service, a first credential known to the user, and a second credential derived from the identifier; requesting registration of the first device with the authentication service on the basis of the first and second credentials; establishing secure connections between the first device and a plurality of second devices (316, 318), in which each of the plurality of second devices is registered with the authentication service and is allocated to the user, and in which each of the secure connections comprises one of: a wireless data connection over a local area network; and a wired data connection over a local area network; a wireless data connection over a local area network; and a wired data connection over a local area network; obtaining from each of the plurality of second devices, an identifier uniquely associated with at least one of the plurality of second devices; providing to the authentication service a plurality of second credentials derived from the identifiers obtained from each of the plurality of second devices; and requesting registration on the basis of the first and the plurality of second credentials.