A processor includes a processor core. A register of the core is to store: a bit range for a number of address bits of physical memory addresses used for key identifiers (IDs), and a first key ID to identify a boundary between non-restricted key IDs and restricted key IDs of the key identifiers. A memory controller is to: determine, via access to bit range and the first key ID in the register, a key ID range of the restricted key IDs within the physical memory addresses; access a processor state that a first logical processor of the processor core executes in an untrusted domain mode; receive a memory transaction, from the first logical processor, including an address associated with a second key ID; and generate a fault in response to a determination that the second key ID is within a key ID range of the restricted key IDs.