1. A method of generating a derivative key for an execution environment (EE) comprising: obtaining a first input value by a key derivation circuit; obtaining a second input value by the key derivation circuit, the second input value being from a one-time writable bit register (OWBR), the OWBR being configured such that each bit of the OWBR can only be written to one time per power cycle and the contents of the OWBR are lost when the OWBR is powered down or reset; and generating an output value by the key derivation circuit based on the first input value and on the second input value from the OWBR, the output value corresponding to a derivative key for a current EE.