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Hardware Interfaces

Referring to the Comportable guideline for peripheral device functionality, the module otbn has the following hardware interfaces defined

  • Primary Clock: clk_i
  • Other Clocks: clk_edn_i, clk_otp_i
  • Bus Device Interfaces (TL-UL): tl
  • Bus Host Interfaces (TL-UL): none
  • Peripheral Pins for Chip IO: none

Inter-Module Signals

Port Name Package::Struct Type Act Width Description
otbn_otp_key otp_ctrl_pkg::otbn_otp_key req_rsp req 1
edn_rnd edn_pkg::edn req_rsp req 1
edn_urnd edn_pkg::edn req_rsp req 1
idle prim_mubi_pkg::mubi4 uni req 1
ram_cfg prim_ram_1p_pkg::ram_1p_cfg uni rcv 1
lc_escalate_en lc_ctrl_pkg::lc_tx uni rcv 1
lc_rma_req lc_ctrl_pkg::lc_tx uni rcv 1
lc_rma_ack lc_ctrl_pkg::lc_tx uni req 1
keymgr_key keymgr_pkg::otbn_key_req uni rcv 1
tl tlul_pkg::tl req_rsp rsp 1

Interrupts

Interrupt Name Type Description
done Event OTBN has completed the operation.

Security Alerts

Alert Name Description
fatal A fatal error. Fatal alerts are non-recoverable and will be asserted until a hard reset.
recov A recoverable error. Just sent once (as the processor stops).

Security Countermeasures

Countermeasure ID Description
OTBN.MEM.SCRAMBLE Both the imem and dmem are scrambled by using prim_ram_1p_scr.
OTBN.DATA.MEM.INTEGRITY Dmem is protected with ECC integrity. This is carried through to OTBN's register file.
OTBN.INSTRUCTION.MEM.INTEGRITY Imem is protected with ECC integrity. This is carried through into OTBN's execute stage.
OTBN.BUS.INTEGRITY End-to-end bus integrity scheme.
OTBN.CONTROLLER.FSM.GLOBAL_ESC The controller FSM moves to a terminal error state upon global escalation.
OTBN.CONTROLLER.FSM.LOCAL_ESC The controller FSM moves to a terminal error state upon local escalation. Can be triggered by CONTROLLER.FSM.SPARSE, SCRAMBLE_CTRL.FSM.SPARSE, and START_STOP_CTRL.FSM.SPARSE.
OTBN.CONTROLLER.FSM.SPARSE The controller FSM uses a sparse state encoding.
OTBN.SCRAMBLE.KEY.SIDELOAD The scrambling key is sideloaded from OTP and thus unreadable by SW.
OTBN.SCRAMBLE_CTRL.FSM.LOCAL_ESC The scramble control FSM moves to a terminal error state upon local escalation. Can be triggered by SCRAMBLE_CTRL.FSM.SPARSE.
OTBN.SCRAMBLE_CTRL.FSM.SPARSE The scramble control FSM uses a sparse state encoding.
OTBN.START_STOP_CTRL.FSM.GLOBAL_ESC The start-stop control FSM moves to a terminal error state upon global escalation.
OTBN.START_STOP_CTRL.FSM.LOCAL_ESC The start-stop control FSM moves to a terminal error state upon local escalation. Can be triggered by START_STOP_CTRL.FSM.SPARSE.
OTBN.START_STOP_CTRL.FSM.SPARSE The start-stop control FSM uses a sparse state encoding.
OTBN.DATA_REG_SW.SCA Blanking of bignum data paths when unused by the executing instruction.
OTBN.CTRL.REDUN Check pre-decoded control matches separately decoded control from main decoder. This includes control signals used for blanking, pushing/popping the call stack, controlling loop and branch/jump instructions, as well as the actual branch target.
OTBN.PC.CTRL_FLOW.REDUN Check prefetch stage PC and execute stage PC match. The prefetch stage and execute stage store their PC's separately and have separate increment calculations.
OTBN.RND.BUS.CONSISTENCY Comparison on successive bus values received over the EDN RND interface.
OTBN.RND.RNG.DIGEST Checking that the random numbers received over the EDN RND interface have not been generated from entropy that failed the FIPS health checks in the entropy source.
OTBN.RF_BASE.DATA_REG_SW.INTEGRITY Register file is protected with ECC integrity.
OTBN.RF_BASE.DATA_REG_SW.GLITCH_DETECT This countermeasure checks for spurious write-enable signals on the register file by monitoring the one-hot0 property of the individual write-enable strobes.
OTBN.STACK_WR_PTR.CTR.REDUN The write pointer of the stack (used for calls and loops) is redundant. If the two instances of the counter mismatch, an error is emitted.
OTBN.RF_BIGNUM.DATA_REG_SW.INTEGRITY Register file is protected with ECC integrity.
OTBN.RF_BIGNUM.DATA_REG_SW.GLITCH_DETECT This countermeasure checks for spurious write-enable signals on the register file by monitoring the one-hot0 property of the individual write-enable strobes.
OTBN.LOOP_STACK.CTR.REDUN The iteration counter of each entry in the loop step uses cross counts via prim_count.
OTBN.LOOP_STACK.ADDR.INTEGRITY Loop start and end address on the loop stack are protected with ECC integrity.
OTBN.CALL_STACK.ADDR.INTEGRITY Call stack entries are protected with ECC integrity.
OTBN.START_STOP_CTRL.STATE.CONSISTENCY The secure wipe handshake between otbn_controller and otbn_start_stop_control uses a level-based req/ack interface. At the otbn_controller end, there is a check for unexpected acks. In otbn_start_stop_control, there is a check for secure wipe requests when we aren't in a state that allows it, and also a check for if the request drops at an unexpected time.
OTBN.DATA.MEM.SEC_WIPE Rotate the scrambling key, effectively wiping the dmem. Initiated on command, upon fatal errors and before RMA entry.
OTBN.INSTRUCTION.MEM.SEC_WIPE Rotate the scrambling key, effectively wiping the imem. Initiated on command, upon fatal errors and before RMA entry.
OTBN.DATA_REG_SW.SEC_WIPE Securely wipe programmer visible OTBN register (GPRs, WDRs, CSRs, WSRs) state with random data. Initiated after reset, at the end of any OTBN operation, upon recoverable and fatal errors, and before RMA entry.
OTBN.WRITE.MEM.INTEGRITY A software visible checksum is calculated for all dmem and imem writes
OTBN.CTRL_FLOW.COUNT A software visible count of instructions executed
OTBN.CTRL_FLOW.SCA OTBN architecture does not have any data dependent timing behaviour
OTBN.DATA.MEM.SW_NOACCESS A portion of DMEM is invisible to CPU software
OTBN.KEY.SIDELOAD Keys can be sideloaded without exposing them to the CPU
OTBN.TLUL_FIFO.CTR.REDUN The TL-UL response FIFO pointers are implemented with duplicate counters.

Hardware Interface Requirements

OTBN connects to other components in an OpenTitan system. This section lists requirements on those interfaces that go beyond the physical connectivity.

Entropy Distribution Network (EDN)

OTBN has two EDN connections: edn_urnd and edn_rnd. What kind of randomness is provided on the EDN connections is configurable at runtime, but unknown to OTBN. To maintain its security properties, OTBN requires the following configuration for the two EDN connections:

  • OTBN has no specific requirements on the randomness drawn from edn_urnd. For performance reasons, requests on this EDN connection should be answered quickly.
  • edn_rnd must provide AIS31-compliant class PTG.3 random numbers. The randomness from this interface is made available through the RND WSR and intended to be used for key generation.

Life Cycle Controller (LC_CTRL)

OTBN has three LC_CTRL connections: one for triggering life cycle escalation requests (lc_escalate_en) and two for handling RMA entry (lc_rma_req/ack).

As LC_CTRL might sit in a different clock domain and since all these connections are using multi-bit signals, OTBN might observe staggered signal transitions due to the clock domain crossings. To avoid spurious life cycle escalations and to enable reliable RMA entry, it should be ensured that:

  • The lc_escalate_en and lc_rma_req inputs are stably driven to lc_ctrl_pkg::Off before releasing the reset of OTBN.
  • When triggering RMA entry, the lc_rma_req input switches from lc_ctrl_pkg::Off to lc_ctrl_pkg::On exactly once, and then remains On until OTBN signals completion of the secure wipe operation with the lc_rma_ack output switching to lc_ctrl_pkg::On.