theory_of_operation.md

Theory of Operation

Block Diagram

Design Details

Memories

The OTBN processor core has access to two dedicated memories: an instruction memory (IMEM), and a data memory (DMEM). Each memory is 4 kiB in size.

The memory layout follows the Harvard architecture. Both memories are byte-addressed, with addresses starting at 0.

The instruction memory (IMEM) is 32b wide and provides the instruction stream to the OTBN processor. It cannot be read from or written to by user code through load or store instructions.

The data memory (DMEM) is 256b wide and read-write accessible from the base and big number instruction subsets of the OTBN processor core. There are four instructions that can access data memory. In the base instruction subset, there are {{#otbn-insn-ref LW}} (load word) and {{#otbn-insn-ref SW}} (store word). These access 32b-aligned 32b words. In the big number instruction subset, there are {{#otbn-insn-ref BN.LID}} (load indirect) and {{#otbn-insn-ref BN.SID}} (store indirect). These access 256b-aligned 256b words.

Both memories can be accessed through OTBN's register interface (DMEM and IMEM). All memory accesses through the register interface must be word-aligned 32b word accesses.

When OTBN is in any state other than idle, reads return zero and writes have no effect. Furthermore, a memory access when OTBN is neither idle nor locked will cause OTBN to generate a fatal error with code ILLEGAL_BUS_ACCESS. A host processor can check whether OTBN is busy by reading the STATUS register.

The underlying memories used to implement the IMEM and DMEM may not grant all access requests (see Memory Scrambling for details). A request won't be granted if new scrambling keys have been requested for the memory that aren't yet available. Functionally it should be impossible for either OTBN or a host processor to make a memory request whilst new scrambling keys are unavailable. OTBN is in the busy state whilst keys are requested so OTBN will not execute any programs and a host processor access will generated an ILLEGAL_BUS_ACCESS fatal error. Should a request not be granted due to a fault, a BAD_INTERNAL_STATE fatal error will be raised.

While DMEM is 4kiB, only the first 3kiB (at addresses 0x0 to 0xbff) is visible through the register interface. This is to allow OTBN applications to store sensitive information in the other 1kiB, making it harder for that information to leak back to Ibex.

Each memory write through the register interface updates a checksum. See the Memory Load Integrity section for more details.

Instruction Prefetch

OTBN employs an instruction prefetch stage to enable pre-decoding of instructions to enable the blanking SCA hardening measure. Its operation is entirely transparent to software. It does not speculate and will only prefetch where the next instruction address can be known. This results in a stall cycle for all conditional branches and jumps as the result is neither predicted nor known ahead of time. Instruction bits held in the prefetch buffer are unscrambled but use the integrity protection described in Data Integrity Protection.

Random Numbers

OTBN is connected to the Entropy Distribution Network (EDN) which can provide random numbers via the RND and URND CSRs and WSRs.

RND provides bits taken directly from the EDN connected via edn_rnd. The EDN interface provides 32b of entropy per transaction and comes from a different clock domain to the OTBN core. A FIFO is used to synchronize the incoming package to the OTBN clock domain. Synchronized packages are then set starting from bottom up to a single WLEN value of 256b. In order to service a single EDN request, a total of 8 transactions are required from EDN interface.

The RND CSR and WSR take their bits from the same source. A read from the RND CSR returns the bottom 32b; the other 192b are discarded. On a read from the RND CSR or WSR, OTBN will stall while it waits for data. It will resume execution on the cycle after it receives the final word of data from the EDN.

As an EDN request can take time, RND is backed by a single-entry cache containing the result of the most recent EDN request in OTBN core level. Writing any value to the RND_PREFETCH CSR initiates a prefetch. This requests data from the EDN, storing it in the cache, and can hide the EDN latency. Writes to RND_PREFETCH will be ignored whilst a prefetch is in progress or when the cache is already full. If the cache is full, a read from RND returns immediately with the contents of the cache, which is then emptied. If the cache is not full, a read from RND will block as described above until OTBN receives the final word of data from the EDN.

OTBN discards any data that is in the cache at the start of an operation. If there is still a pending prefetch when an OTBN operation starts, the results of the prefetch will also discarded.

URND provides bits from a local XoShiRo256++ PRNG within OTBN; reads from it never stall. This PRNG is seeded once from the EDN connected via edn_urnd when OTBN starts execution. Each new execution of OTBN will reseed the URND PRNG. The PRNG state is advanced every cycle when OTBN is running.

The PRNG has a long cycle length but has a fixed point: the sequence of numbers will get stuck if the state ever happens to become zero. This will never happen in normal operation. If a fault causes the state to become zero, OTBN raises a BAD_INTERNAL_STATE fatal error.

Operational States

OTBN can be in different operational states. After reset (init), OTBN performs a secure wipe of the internal state and then becomes idle. OTBN is busy for as long it is performing an operation. OTBN is locked if a fatal error was observed or after handling an RMA request.

The current operational state is reflected in the STATUS register.

• After reset, OTBN is busy with the internal secure wipe and the STATUS register is set to BUSY_SEC_WIPE_INT.
• If OTBN is idle, the STATUS register is set to IDLE.
• If OTBN is busy, the STATUS register is set to one of the values starting with BUSY_.
• If OTBN is locked, the STATUS register is set to LOCKED.

OTBN transitions into the busy state as result of host software issuing a command; OTBN is then said to perform an operation. OTBN transitions out of the busy state whenever the operation has completed. In the STATUS register the different BUSY_* values represent the operation that is currently being performed.

A transition out of the busy state is signaled by the done interrupt (INTR_STATE.done).

The locked state is a terminal state; transitioning out of it requires an OTBN reset.

Operations and Commands

OTBN understands a set of commands to perform certain operations. Commands are issued by writing to the CMD register.

The EXECUTE command starts the execution of the application contained in OTBN's instruction memory.

The SEC_WIPE_DMEM command securely wipes the data memory.

The SEC_WIPE_IMEM command securely wipes the instruction memory.

Software Execution

Software execution on OTBN is triggered by host software by issuing the EXECUTE command. The software then runs to completion, without the ability for host software to interrupt or inspect the execution.

• OTBN transitions into the busy state, and reflects this by setting STATUS to BUSY_EXECUTE.
• The internal randomness source, which provides random numbers to the URND CSR and WSR, is re-seeded from the EDN.
• The instruction at address zero is fetched and executed.
• From this point on, all subsequent instructions are executed according to their semantics until either an {{#otbn-insn-ref ECALL}} instruction is executed, or an error is detected.
• A secure wipe of internal state is performed.
• The ERR_BITS register is set to indicate either a successful execution (value 0), or to indicate the error that was observed (a non-zero value).
• OTBN transitions into the idle state (in case of a successful execution, or a recoverable error) or the locked state (in case of a fatal error). This transition is signaled by raising the done interrupt (INTR_STATE.done), and reflected in the STATUS register.

Errors

OTBN is able to detect a range of errors, which are classified as software errors or fatal errors. A software error is an error in the code that OTBN executes. In the absence of an attacker, these errors are due to a programmer's mistake. A fatal error is typically the violation of a security property. All errors and their classification are listed in the List of Errors.

Whenever an error is detected, OTBN reacts locally, and informs the OpenTitan system about it by raising an alert. OTBN generally does not try to recover from errors itself, and provides no error handling support to code that runs on it.

OTBN gives host software the option to recover from some errors by restarting the operation. All software errors are treated as recoverable, unless CTRL.software_errs_fatal is set, and are handled as described in the section Reaction to Recoverable Errors. When CTRL.software_errs_fatal is set, software errors become fatal errors.

Fatal errors are treated as described in the section Reaction to Fatal Errors.

Reaction to Recoverable Errors

Recoverable errors can be the result of a programming error in OTBN software. Recoverable errors can only occur during the execution of software on OTBN, and not in other situations in which OTBN might be busy.

The following actions are taken when OTBN detects a recoverable error:

1. The currently running operation is terminated, similar to the way an {{#otbn-insn-ref ECALL}} instruction is executed:
   • No more instructions are fetched or executed.
   • A secure wipe of internal state is performed.
   • The ERR_BITS register is set to a non-zero value that describes the error.
   • The current operation is marked as complete by setting INTR_STATE.done.
   • The STATUS register is set to IDLE.
2. A recoverable alert is raised.

The host software can start another operation on OTBN after a recoverable error was detected.

Reaction to Fatal Errors

Fatal errors are generally seen as a sign of an intrusion, resulting in more drastic measures to protect the secrets stored within OTBN. Fatal errors can occur at any time, even when an OTBN operation isn't in progress.

The following actions are taken when OTBN detects a fatal error:

1. A secure wipe of the data memory and a secure wipe of the instruction memory is initiated.
2. If OTBN is not idle, then the currently running operation is terminated, similarly to how an operation ends after an {{#otbn-insn-ref ECALL}} instruction is executed:
   • No more instructions are fetched or executed.
   • A secure wipe of internal state is performed.
   • The ERR_BITS register is set to a non-zero value that describes the error.
   • The current operation is marked as complete by setting INTR_STATE.done.
3. The STATUS register is set to LOCKED.
4. A fatal alert is raised.

Note that OTBN can detect some errors even when it isn't running. One example of this is an error caused by an integrity error when reading or writing OTBN's memories over the bus. In this case, the ERR_BITS register will not change. This avoids race conditions with the host processor's error handling software. However, every error that OTBN detects when it isn't running is fatal. This means that the cause will be reflected in FATAL_ALERT_CAUSE, as described below in Alerts. This way, no alert is generated without setting an error code somewhere.

List of Errors

Name	Class	Description
BAD_DATA_ADDR	software	A data memory access occurred with an out of bounds or unaligned access.
BAD_INSN_ADDR	software	An instruction memory access occurred with an out of bounds or unaligned access.
CALL_STACK	software	An instruction tried to pop from an empty call stack or push to a full call stack.
ILLEGAL_INSN	software	An illegal instruction was about to be executed.
LOOP	software	A loop stack-related error was detected.
KEY_INVALID	software	An attempt to read a `KEY_*` WSR was detected, but no key was provided by the key manager.
RND_REP_CHK_FAIL	recoverable	The random number obtained from the last read of the RND register failed the repetition check. The RND EDN interface returned identical random numbers on two subsequent entropy requests.
RND_FIPS_CHK_FAIL	recoverable	The random number obtained from the last read of the RND register has been generated from entropy that at least partially failed the FIPS health checks in the entropy source.
IMEM_INTG_VIOLATION	fatal	Data read from the instruction memory failed the integrity checks.
DMEM_INTG_VIOLATION	fatal	Data read from the data memory failed the integrity checks.
REG_INTG_VIOLATION	fatal	Data read from a GPR or WDR failed the integrity checks.
BUS_INTG_VIOLATION	fatal	An incoming bus transaction failed the integrity checks.
BAD_INTERNAL_STATE	fatal	The internal state of OTBN has become corrupt.
ILLEGAL_BUS_ACCESS	fatal	A bus-accessible register or memory was accessed when not allowed.
LIFECYCLE_ESCALATION	fatal	A life cycle escalation request was received.
FATAL_SOFTWARE	fatal	A software error was seen and [`CTRL.software_errs_fatal`](registers.md#ctrl) was set.

Alerts

An alert is a reaction to an error that OTBN detected. OTBN has two alerts, one recoverable and one fatal.

A recoverable alert is a one-time triggered alert caused by recoverable errors. The error that caused the alert can be determined by reading the ERR_BITS register.

A fatal alert is a continuously triggered alert caused by fatal errors. The error that caused the alert can be determined by reading the FATAL_ALERT_CAUSE register. If OTBN was running, this value will also be reflected in the ERR_BITS register. A fatal alert can only be cleared by resetting OTBN through the rst_ni line.

The host CPU can clear the ERR_BITS when OTBN is not running. Writing any value to ERR_BITS clears this register to zero. Write attempts while OTBN is running are ignored.

Reaction to Life Cycle Escalation Requests

OTBN receives and reacts to escalation signals from the life cycle controller. An incoming life cycle escalation is a fatal error of type lifecycle_escalation and treated as described in the section Fatal Errors.

Idle

OTBN exposes a single-bit idle_o signal, intended to be used by the clock manager to clock-gate the block when it is not in use. This signal is in the same clock domain as clk_i. The idle_o signal is high when OTBN is idle, and low otherwise.

OTBN also exposes another version of the idle signal as idle_otp_o. This works analogously, but is in the same clock domain as clk_otp_i.

TODO: Specify interactions between idle_o, idle_otp_o and the clock manager fully.

Data Integrity Protection

OTBN stores and operates on data (state) in its dedicated memories, register files, and internal registers. OTBN's data integrity protection is designed to protect all data stored and transmitted within OTBN from modifications through physical attacks.

During transmission, the integrity of data is protected with an integrity protection code. Data at rest in the instruction and data memories is additionally scrambled.

In the following, the Integrity Protection Code and the scrambling algorithm are discussed, followed by their application to individual storage elements.

Integrity Protection Code

OTBN uses the same integrity protection code everywhere to provide overarching data protection without regular re-encoding. The code is applied to 32b data words, and produces 39b of encoded data.

The code used is an (39,32) Hsiao "single error correction, double error detection" (SECDED) error correction code (ECC) [CHEN08]. It has a minimum Hamming distance of four, resulting in the ability to detect at least three errors in a 32 bit word. The code is used for error detection only; no error correction is performed.

Memory Scrambling

Contents of OTBN's instruction and data memories are scrambled while at rest. The data is bound to the address and scrambled before being stored in memory. The addresses are randomly remapped.

Note that data stored in other temporary memories within OTBN, including the register files, is not scrambled.

Scrambling is used to obfuscate the memory contents and to diffuse the data. Obfuscation makes passive probing more difficult, while diffusion makes active fault injection attacks more difficult.

The scrambling mechanism is described in detail in the section "Scrambling Primitive" of the SRAM Controller Technical Specification.

When OTBN comes out of reset, its memories have default scrambling keys. The host processor can request new keys for each memory by issuing a secure wipe of DMEM and a secure wipe of IMEM.

Actions on Integrity Errors

A fatal error is raised whenever a data integrity violation is detected, which results in an immediate stop of all processing and the issuing of a fatal alert. The section Error Handling and Reporting describes the error handling in more detail.

Register File Integrity Protection

OTBN contains two register files: the 32b GPRs and the 256b WDRs. The data stored in both register files is protected with the Integrity Protection Code. Neither the register file contents nor register addresses are scrambled.

The GPRs x2 to x31 store a 32b data word together with the Integrity Protection Code, resulting in 39b of stored data. (x0, the zero register, and x1, the call stack, require special treatment.)

Each 256b Wide Data Register (WDR) stores a 256b data word together with the Integrity Protection Code, resulting in 312b of stored data. The integrity protection is done separately for each of the eight 32b sub-words within a 256b word.

The register files can consume data protected with the Integrity Protection Code, or add it on demand. Whenever possible the Integrity Protection Code is preserved from its source and written directly to the register files without recalculation, in particular in the following cases:

• Data coming from the data memory (DMEM) through the load-store unit to a GPR or WDR.
• Data copied between WDRs using the {{#otbn-insn-ref BN.MOV}} or {{#otbn-insn-ref BN.MOVR}} instructions.
• Data conditionally copied between WDRs using the {{#otbn-insn-ref BN.SEL}} instruction.
• Data copied between the ACC and MOD WSRs and a WDR.
• Data copied between any of the MOD0 to MOD7 CSRs and a GPR. (TODO: Not yet implemented.)

In all other cases the register files add the Integrity Protection Code to the incoming data before storing the data word.

The integrity protection bits are checked on every read from the register files, even if the integrity protection is not removed from the data.

Detected integrity violations in a register file raise a fatal reg_error.

Data Memory (DMEM) Integrity Protection

OTBN's data memory is 256b wide, but allows for 32b word accesses. To facilitate such accesses, all integrity protection in the data memory is done on a 32b word granularity.

All data entering or leaving the data memory block is protected with the Integrity Protection Code; this code is not re-computed within the memory block.

Before being stored in SRAM, the data word with the attached Integrity Protection Code, as well as the address are scrambled according to the memory scrambling algorithm. The scrambling is reversed on a read.

The ephemeral memory scrambling key and the nonce are provided by the OTP block. They are set once when OTBN block is reset, and changed whenever a secure wipe of the data memory is performed.

The Integrity Protection Code is checked on every memory read, even though the code remains attached to the data. A further check must be performed when the data is consumed. Detected integrity violations in the data memory raise a fatal dmem_error.

Instruction Memory (IMEM) Integrity Protection

All data entering or leaving the instruction memory block is protected with the Integrity Protection Code; this code is not re-computed within the memory block.

Before being stored in SRAM, the instruction word with the attached Integrity Protection Code, as well as the address are scrambled according to the memory scrambling algorithm. The scrambling is reversed on a read.

The ephemeral memory scrambling key and the nonce are provided by the OTP block. They are set once when OTBN block is reset, and changed whenever a secure wipe of the instruction memory is performed.

The Integrity Protection Code is checked on every memory read, even though the code remains attached to the data. A further check must be performed when the data is consumed. Detected integrity violations in the data memory raise a fatal imem_error.

Memory Load Integrity

As well as the integrity protection discussed above for the memories and bus interface, OTBN has a second layer of integrity checking to allow a host processor to ensure that a program has been loaded correctly. This is visible through the LOAD_CHECKSUM register. The register exposes a cumulative CRC checksum which is updated on every write to either memory.

This is intended as a light-weight way to implement a more efficient "write and read back" check. It isn't a cryptographically secure MAC, so cannot spot an attacker who can completely control the bus. However, in this case the attacker would be equally able to control responses from OTBN, so any such check could be subverted.

The CRC used is the 32-bit CRC-32-IEEE checksum. This standard choice of generating polynomial makes it compatible with other tooling and libraries, such as the crc32 function in the python 'binascii' module and the crc instructions in the RISC-V bitmanip specification [SYMBIOTIC21]. The stream over which the checksum is computed is the stream of writes that have been seen since the last write to LOAD_CHECKSUM. Each write is treated as a 48b value, {imem, idx, wdata}. Here, imem is a single bit flag which is one for writes to IMEM and zero for writes to DMEM. The idx value is the index of the word within the memory, zero extended from 10b to 15b. Finally, wdata is the 32b word that was written. Writes that are less than 32b or not aligned on a 32b boundary are ignored and not factored into the CRC calculation.

The host processor can also write to the register. Typically, this will be to clear the value to 32'h00000000, the traditional starting value for a 32-bit CRC. Note the internal representation of the CRC is inverted from the register visible version. This is done to maintain compatibility with existing CRC-32-IEEE tooling and libraries.

To use this functionality, the host processor should set LOAD_CHECKSUM to a known value (traditionally, 32'h00000000). Next, it should write the program to be loaded to OTBN's IMEM and DMEM over the bus. Finally, it should read back the value of LOAD_CHECKSUM and compare it with an expected value.

Secure Wipe

Applications running on OTBN may store sensitive data in the internal registers or the memory. In order to prevent an untrusted application from reading any leftover data, OTBN provides the secure wipe operation. This operation can be applied to:

• Data memory
• Instruction memory
• Internal state

The three forms of secure wipe can be triggered in different ways.

A secure wipe of either the instruction or the data memory can be triggered from host software by issuing a SEC_WIPE_DMEM or SEC_WIPE_IMEM command.

A secure wipe of instruction memory, data memory, and all internal state is performed automatically when handling a fatal error. In addition, it can be triggered by the Life Cycle Controller before RMA entry using the lc_rma_req/ack interface. In both cases OTBN enters the locked state afterwards and needs to be reset.

A secure wipe of the internal state only is triggered automatically after reset and when OTBN ends the software execution, either successfully, or unsuccessfully due to a recoverable error.

If OTBN cannot complete a secure wipe of the internal state (e.g., due to failing to obtain the required randomness), it immediately becomes locked. In this case, OTBN must be reset and will then retry the secure wipe. The secure wipe after reset must succeed before OTBN can be used.

Data Memory (DMEM) Secure Wipe

The wiping is performed by securely replacing the memory scrambling key, making all data stored in the memory unusable. The key replacement is a two-step process:

• Overwrite the 128b key of the memory scrambling primitive with randomness from URND. This action takes a single cycle.
• Request new scrambling parameters from OTP. The request takes multiple cycles to complete.

Host software can initiate a data memory secure wipe by issuing the SEC_WIPE_DMEM command.

Instruction Memory (IMEM) Secure Wipe

The wiping is performed by securely replacing the memory scrambling key, making all instructions stored in the memory unusable. The key replacement is a two-step process:

• Overwrite the 128b key of the memory scrambling primitive with randomness from URND. This action takes a single cycle.
• Request new scrambling parameters from OTP. The request takes multiple cycles to complete.

Host software can initiate a data memory secure wipe by issuing the SEC_WIPE_IMEM command.

Internal State Secure Wipe

OTBN provides a mechanism to securely wipe all internal state, excluding the instruction and data memories.

The following state is wiped:

• Register files: GPRs and WDRs
• The accumulator register (also accessible through the ACC WSR)
• Flags (accessible through the FG0, FG1, and FLAGS CSRs)
• The modulus (accessible through the MOD0 to MOD7 CSRs and the MOD WSR)

The wiping procedure is a two-step process:

• Overwrite the state with randomness from URND and request a reseed of URND.
• Overwrite the state with randomness from reseeded URND.

Note that after internal secure wipe, the state of registers is undefined. In order to prevent mismatches between ISS and RTL, software needs to initialise a register with a full-word write before using its value.

Loop and call stack pointers are reset.

Host software cannot explicitly trigger an internal secure wipe; it is performed automatically after reset and at the end of an EXECUTE operation.