A controller and an accessory controllable by the controller can communicate using secure read and write procedures. The procedures can include encrypting identifiers of accessory characteristics targeted by a read or write operation as well as any data being read or written. The procedures can also include the accessory returning a cryptographically signed response verifying receipt and execution of the read or write instruction. In some instances, a write procedure can be implemented as a timed write in which a first instruction containing the write data is sent separately from a second instruction to execute the write operation; the accessory can disregard the write data if the second instruction is not received within a timeout period after receiving the first instruction.