Encoding of identifiers of persistent keys. - Applications may freely choose key identifiers in the range #PSA_KEY_ID_USER_MIN to #PSA_KEY_ID_USER_MAX. - The implementation may define additional key identifiers in the range #PSA_KEY_ID_VENDOR_MIN to #PSA_KEY_ID_VENDOR_MAX. - 0 is reserved as an invalid key identifier. - Key identifiers outside these ranges are reserved for future use.