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); 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 the derivative key for a current EE.