Key Manager HWIP Technical Specification

Overview

This document specifies the functionality of the OpenTitan key manager.

Features

• One-way key and identity (working) state hidden from software.
• Version controlled identity and key generation.
• Key generation for both software consumption and hardware sideload.
• Support for DICE open profile

Description

The key manager implements the hardware component of the identities and root keys strategy of OpenTitan.

It enables the system to shield critical assets from software directly and provides a simple model for software to use derived key and identity outputs.

Theory of Operation

Key manager behavior can be summarized by the functional model below.

In the diagram, the red boxes represent the working state and the associated internal key, the black ovals represent derivation functions, the green squares represent software inputs, and the remaining green / purple shapes represent outputs to both software and hardware.

In OpenTitan, the derivation method selected is KMAC. Each valid operation involves a KMAC invocation using the key manager working state as the “key” and other HW / SW supplied inputs as data. While KMAC can generate outputs of arbitrary length, this design fixes the size to 256b.

Effectively, the key manager behavior is divided into 3 classes of functions

• Key manager state advancement

  • The results are never visible to software and not directly usable by any software controlled hardware

• Output key generation

  • Results can be visible to software or consumed by hardware (sideload)

• Identity / seed generation

  • Results are always visible to software and used for asymmetric cryptography

In general, the key generation and seed generation functions are identical. They differ only in how software chooses to deploy the outputs.

Key Manager State

The key manager working state (red boxes in the functional model) represents both the current state of the key manager as well as its related secret material. Each valid state (Initialized / CreatorRootKey / OwnerIntermediateKey / OwnerRootKey), supplies its secret material as the “key” input to a KMAC operation. Invalid states, such as Reset / Disabled on the other hand, either do not honor operation requests, or supplies random data when invoked.

The data input is dependent on each state, see below.

Reset

The key manager working state is not directly reset to any value. This ensures there is no deterministic hamming delta upon reset. Instead at reset time, the state value is simply unknown - which is expected to be some biased value the registers settle to based on silicon corner and environment conditions.

To begin operation, the state must first transition to Initialize. The advancement from Reset to Initialized is irreversible during the current power cycle. Until the initialize command is invoked, the key manager rejects all other software commands.

Initialized

When transitioning from Reset to Initialized, random values obtained from the entropy source are used to populate the working state. This ensures that the hamming delta from both the previous value and the next value are both non-deterministic. The advancement from Initialized to CreatorRootKey is irreversible during the current power cycle.

CreatorRootKey

CreatorRootKey is the first operational state of the key manager. When transitioning from Initialized to this state, a KMAC operation is invoked using the RootKey as the key (from OTP), and the remaining inputs as data.

See below:

• DiversificationKey: Secret seed from flash
• HealthMeasurement: Current life cycle state
  • To avoid a state value corresponding to each life cycle state, the raw life cycle value is not used.
  • Instead, certain life cycle states diversify the same way.
  • Please see the life cycle controller for more details.
• DeviceIdentifier: Unique device identification.
• HardwareRevisionSecret: A global design time constant.

Other than the DiversificationKey, none of the values above are considered secret.

Once the CreatorRootKey is reached, software can request key manager to advance state, generate output key or generate output identity. The key used for all 3 functions is the CreatorRootKey.

The advancement from this state to the next is irreversible during the current power cycle.

OwnerIntermediateKey

This is the second operational state of the key manager. This state is reached through another invocation of the KMAC operation using the previous working state as the key, and other inputs as data. The output of the KMAC operation replaces the previous value of the working state, and the new value becomes the OwnerIntermediateKey.

The relevant data inputs are:

• OwnerRootSecret: Secret seed from flash.
• SoftwareBinding: A software programmed value representing the first owner code to be run.

Once the OwnerIntermediateKey is created, software can request key manager to advance state, generate output key or generate output identity. The key used for all 3 functions is the OwnerIntermediateKey.

The advancement from this state to the next is irreversible during the current power cycle.

OwnerRootKey

This is the last operational state of the key manager. This state is reached through another invocation of the KMAC operation using the previous working state as the key, and other inputs as data. The output of the KMAC operation replaces the previous value of the working state, and the new value becomes the OwnerRootKey.

The relevant inputs are:

• SoftwareBinding - A software programmed value representing the owner kernel code.

Once the OwnerRootKey is created, software can request key manager to advance state, generate output key or generate output identity. An advance command invoked from OwnerRootKey state simply moves the state to Disabled.

The generate output and generate identity functions use OwnerRootKey as the KMAC key. The advancement from this state to the next is irreversible during the current power cycle.

Disabled

Disabled is a state where the key manager is no longer operational. Upon Disabled entry, the working state is updated with KMAC computed random values; however, sideload keys are preserved. This allows the software to keep the last valid sideload keys while preventing the system from further advancing the valid key.

When advance and generate calls are invoked from this state, the outputs and keys are indiscriminately updated with randomly computed values. Key manager enters disabled state based on direct invocation by software:

• Advance from OwnerRootKey
• Disable operation

Invalid

Invalid state is entered whenever key manager is disabled through the life cycle connection or when an operation encounters a fault . Upon Invalid entry, both the working state and the sideload keys are wiped with entropy directly. Note, this is different from Disabled state entry, which updates internal key with KMAC outputs but leaves sideload and software keys intact.

Invalid Entry Wiping

Since the life cycle controller can disable the key manager at any time, the key manager attempts to gracefully handle the wiping process. When the disable is seen, the key manager immediately begins wiping all keys (internal key, hardware sideload key, software key) with entropy. However, if an operation was already ongoing, the key manager waits for the transaction to complete gracefully before transitioning to invalid state.

While waiting for the transaction to complete, the key manager continuously wipes all keys with entropy.

Invalid and Disabled State

Note that Invalid and Disabled states are functionally equivalent. The main difference between the two is “how” the state was reached.

Disabled state is reached through intentional software commands. While Invalid state is reached through life cycle disable or operational faults.

This also means that only Invalid is a terminal state. If after entering Disabled life cycle is disabled or a fault is encountered, the same invalid entry procedure is followed to bring the system to a terminal Invalid state.

If ever multiple conditions collide (a fault is detected at the same time software issues disable command), the Invalid entry path always takes precedence.

Life Cycle Connection

The function of the key manager is directly tied to the life cycle controller. During specific life cycle states, the key manager is explicitly invalidated.

When invalidated, the following key manager behavior applies:

• If the key manager has not been initialized, it cannot be initialized until life cycle enables key manager.
• If the key manager has been initialized and is currently in a valid state (including Disabled), it immediately wipes its key contents with entropy (working state, sideload keys and software keys) and transitions to Invalid.
  • Note, unlike a normal software requested disable, this path does not gracefully interact with KMAC, instead the secret contents are forcibly wiped.
  • If there is an ongoing transaction with KMAC, the handshake with KMAC is still completed as usual, however the results are discarded and the value sent to KMAC are also not real.
• Once the system settles to Invalid state, the behavior is consistent with Disabled state.

Commands in Each State

During each state, there are 3 valid commands software can issue:

• Advance state
• Output generation
• Identity generation

The software is able to select a command and trigger the key manager FSM to process one of the commands. If a command is valid during the current working state, it is processed and acknowledged when complete.

If a command is invalid, the behavior depends on the current state. If the current state is Reset, the invalid command is immediately rejected as the key manager FSM has not yet been initialized. If the current state is any other state, the key manager FSM processes with random, dummy data, but does not update working state or relevant output registers. For each valid command, a set of inputs are selected and sequenced to the KMAC module.

During Disable and Invalid states, working state and output registers are updated based on the input commands as with normal states. There are however a few differences:

• Working state and output registers are updated regardless of any error status to ensure their values are further scrambled.
• Instead of normal input data, random data is selected for KMAC processing.
• All operations return an invalid operations error, in addition to any other error that might naturally occur.

Generating Output Key

The generate output command is composed of 2 options

• Generate output key for software, referred to as generate-output-sw
• Generate output key for hardware, referred to as generate-output-hw

The hardware option is meant specifically for symmetric side load use cases. When this option is issued, the output of the KMAC invocation is not stored in software visible registers, but instead in hardware registers that directly output to symmetric primitives such as AES, HMAC and KMAC.

KMAC Operations

All invoked KMAC operations expect the key in two shares. This means the working states, even though functionally 256b, are maintained as 512b.

For advance-state and generate-output commands, the KMAC emitted output are also in 2-shares. Software is responsible for determining if the key should be preserved in shares or combined, depending on the use case.

Errors, Faults and Alerts

The key manager has two overall categories of errors:

• Recoverable errors
• Fatal errors

Recoverable errors are those likely to have been introduced by software and not fatal to the key manager or the system. Fatal errors are logically impossible errors that have a high likelihood of being a fault and thus fatal.

Each category of error can be further divided into two:

• Synchronous errors
• Asynchronous errors

Synchronous errors happen only during a key manager transaction. Asynchronous errors can happen at any time.

Given the above, we have 4 total categories of errors:

• Synchronous recoverable errors
• Asynchronous recoverable errors
• Synchronous fatal errors
• Asynchronous fatal errors

All recoverable errors (synchronous and asynchronous) are captured in ERR_CODE. All fatal errors (synchronous and asynchronous) are captured in FAULT_STATUS.

Recoverable errors cause a recoverable alert to be sent from the key manager. Fatal errors cause a fatal alert to be sent from the key manager.

Below, the behavior of each category and its constituent errors are described in detail.

Synchronous Recoverable Errors

These errors can only happen when a key manager transaction is invoked and are typically associated with incorrect software programming. At the end of the transaction, key manager reports whether there was an error in ERR_CODE and sends a recoverable alert.

• ERR_CODE.INVALID_OP Software issued an invalid operation given the current key manager state.
• ERR_CODE.INVALID_KMAC_INPUT Software supplied invalid input (for example a key greater than the max version) for a key manager operation.

Asynchronous Recoverable Errors

These errors can happen at any time regardless of whether there is a key manager operation. The error is reported in ERR_CODE and the key manager sends a recoverable alert.

• ERR_CODE.INVALID_SHADOW_UPDATE Software performed an invalid sequence while trying to update a key manager shadow register.

Synchronous Fatal Errors

These errors can only happen when a key manager transaction is invoked and receives malformed transaction results that are not logically possible. At the end of the transaction, key manager reports whether there was an error in FAULT_STATUS and continuously sends fatal alerts .

• ERR_CODE.KMAC_OP KMAC reports a transaction error, this is not possible given current definitions.
• ERR_CODE.KMAC_OUT KMAC returns all 0’s or all 1’s as a result, this is not possible given current definitions.

Note, these errors are synchronous from the perspective of the key manager, but they may be asynchronous from the perspective of another module.

Asynchronous Fatal Errors

These errors can happen at any time regardless of whether there is a key manager operation. The error is reported in FAULT_STATUS and the key manager continuously sends fatal alerts.

• ERR_CODE.CMD KMAC control’s command lines are displaying non-one hot values.
• ERR_CODE.KMAC_FSM KMAC control’s FSM is in an invalid state.
• ERR_CODE.REGFILE_INTG The key manager’s regfile reports an integrity error.
• ERR_CODE.SHADOW The key manager’s regfile reports a shadow storage error.
• ERR_CODE.CTRL_FSM_INTG The key manager’s main control FSM is in an invalid state.
• ERR_CODE.CTRL_FSM_CNT The key manager’s main control count exhibits an incorrect value.

Faults and Operational Faults

Since fatal errors (faults) can happen at any time, their impact on the key manager depends on transaction timing.

If the fault happens while a transaction is ongoing, key manager transitions to the Invalid state.

If the fault happens while there is no transaction, an alert is first sent to the alert handler. If before the alert handler escalates an operation is run, the key manager again transitions to Invalid state. If the alert handler escalates and disables the key manager, then the key manager will also transition to Invalid state if it is not already there.

Example 1: Fault During Operation

The key manager is running a generate operation and a non-onehot command was observed by the kmac interface. Since the non-onehot condition is a fault, it will be reflected in FAULT_STATUS. Since an operation was ongoing when this fault was seen, it will also be reflected in ERR_CODE.INVALID_OP. This is considered an operational fault and begins transition to Invalid.

Example 2: Fault During Idle

The key manager is NOT running an operation and is idle. During this time, a fault was observed on the regfile (shadow storage error) and FSM (control FSM integrity error). The faults will be reflected in FAULT_STATUS.

This is not considered an operational fault and the key manager will remain in its current state until an operation is invoked or the alert handler escalates.

Example 3: Operation after Fault Detection

Continuing from the example above, assume now the key manager begins an operation. Since the key manager has previous encountered a fault, any operation now is considered an operational fault and begins transition to the Invalid state.

Additional Details on Invalid Input

What is considered invalid input changes based on current state and operation.

When an advance operation is invoked:

• The working state key is checked for all 0’s and all 1’s.
• During Initialized state, creator seed, device ID and health state data is checked for all 0’s and all 1’s.
• During CreatorRootKey state, the owner seed is checked for all 0’s and all 1’s.
• During all other states, nothing is explicitly checked.

When a generate output key operation is invoked:

• The working state key is checked for all 0’s and all 1’s.
• The key version is less than or equal to the max key version.

When a generate output identity is invoked:

• The working state key is checked for all 0’s and all 1’s.

Invalid Operation

The table below enumerates the legal operations in a given state. When an illegal operation is supplied, the error code is updated and the operation is flagged as done with error.

• All operations invoked during Invalid and Disabled states lead to invalid operation error.

Error Response

In addition to alerts and interrupts, key manager may also update the working state key and relevant outputs based on current state. See the tables below for an enumeration.

• During Reset state, the KMAC module is never invoked, thus certain errors are not possible.
• During Initialized, CreatorRootKey, OwnerIntermediateKey and OwnerRootKey states, a fault error causes the relevant key / outputs to be updated; however an operational error does not.
• During Invalid and Disabled states, the relevant key / outputs are updated regardless of the error.
• Only the relevant collateral is updated -> ie, advance / disable command leads to working key update, and generate command leads to software or sideload key update.
• During Disabled state, if life cycle is disabled or an operational fault is encountered, the key manager transitions to Invalid state, see here

DICE Support

The key manager supports DICE open profile. Specifically, the open profile has two compound device identifiers.

• Attestation CDI
• Sealing CDI

The attestation CDI is used to attest hardware and software configuration and is thus expected to change between updates. The sealing CDI on the other hand, is used to attest the authority of the hardware and softawre configuration. The sealing version is thus expected to remain stable across software updates.

To support these features, the key manager maintains two versions of the working state and associated internal key. There is one version for attestation and one version for sealing.

The main difference between the two CDIs is the different usage of SW_BINDING. For the Sealing CDI, the SEALING_SW_BINDING is used, all other inputs are the same. For the Attestation CDI, the ATTEST_SW_BINDING is used, all other inputs are the same.

When invoking an advance operation, both versions are advanced, one after the other. There are thus two kmac transactions. The first trasnaction uses the Sealing CDI internal key, SEALING_SW_BINDING and other common inputs. The second transaction uses the Attestation CDI internal key, ATTEST_SW_BINDING and other common inputs.

When invoking a generate operation, the software must specify which CDI to use as the source key. This is done through CONTROL.CDI_SEL. Unlike the advance operation, there is only 1 transaction since we pick a specific CDI to operate.

When disabling, both versions are disabled together.

Block Diagram

The following is a high level block diagram of the key manager.

Design Details

Key manager is primarily composed of two components:

• keymgr_ctrl
• keymgr_kmac_if

Key Manager Control

The key manager control block manages the working state, sideload key updates, as well as what commands are valid in each state. It also handles the life cycle keymgr_en input, which helps disable the entire key manager function in the event of an escalation.

KMAC Interface Control

The KMAC interface control represents the bulk of key manager logic. Based on input from key manager control, this module selects the inputs for each given command and sequences the data to KMAC.

The KMAC interafce works on a simple valid / ready protocol. When there is data to send, the KMAC interface sends out a valid and keeps it active. When the destination accepts the transaction, the ready is asserted. Note just like with any bus interface, the ready may already be asserted when valid asserts, or it may assert some time later, there are no restrictions. Since the data to be sent is always pre-buffered in key manager, the valid once asserts, never de-asserts until the entire transaction is complete.

The data interface itself is 64b wide. However, there may not always be 64b multiple aligned data to be sent. In these situations, the last transfer beat sent to KMAC has a byte mask / strobe attached. The byte mask indicates on the last beat which bytes are actually valid, and which are not. Not beats prior to the last always have fully asserted byte masks.

Once KMAC receives all the required data and the last indication, it begins processing the data into a digest. This process may take an arbitrary number of cycles. When this process is complete, a done indication pulse is sent back with the digest. Note, the acceptance of done has no back-pressure and keymgr must accept it within one cycle.

See diagram below for an example transfer:

Side Load Keys

There are three sideload keys. One for AES, one for HMAC, and one for KMAC. When a sideload key is generated successfully through the generate-output-hw command, the derived data is loaded into key storage registers. There is a set of storage registers for each destination.

The KMAC key however is further overloaded as it is the main derivation mechanism for key manager internal stage. The KMAC key thus has two possible outputs, one is the sideload key, and the other is internal state key.

When a valid operation is called, the internal state key is sent over the KMAC key. During all other times, the sideloaded value is presented. Note, there may not be a valid key in the sideload register if it has been cleared or never generated. The sideload key can be overwritten with another generate command, or cleared with entropy through SIDELOAD_CLEAR.

The clearing can be done one slot at a time, or all at once.

The following diagram illustrates an example when there is no valid key in the KMAC sideload registers and an operation is called. During the duration of the operation, the key is valid and shows the internal key state. Once the operation is complete, it falls back to the sideload key state, which is invalid in this case.

The following diagram illustrates an example when there is a valid key in the KMAC sideload registers and an operation is called. During the duration of the operation, the key is valid and shows the internal key state. Once the operation is complete, it falls back to the sideload key state, which is valid and contains a different value.

Software Binding

The identities flow employs an idea called software binding to ensure that a particular key derivation scheme is only reproducible for a given software configuration. The binding is created through the secure boot flow, where each stage sets the binding used for the next verified stage before advancing to it. The software binding is used during the following state transitions only:

• Initialized to CreatorRootKey
• CreatorRootKey to OwnerIntermedaiteKey
• OwnerIntermediateKey to OwnerRootKey

In order to save on storage and not have a duplicate copy per stage, the software binding registers SOFTWARE_BINDING are shared between key manager stages.

Software sets the appropriate values and locks it by clearing SOFT_BINDING_EN. When later a successful advance call is made, the key manager then unlocks by setting SOFT_BINDING_EN to 1. An unsuccessful advance call (errors) does not unlock the binding. This allows the next stage of software to re-use the binding registers.

Hardware Interfaces

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

Primary Clock: clk_i

Other Clocks: clk_edn_i

Bus Device Interfaces (TL-UL): tl

Bus Host Interfaces (TL-UL): none

Peripheral Pins for Chip IO: none

Interrupts:

Security Alerts:

Programmers Guide

Initialize

Advance or Generate

Software selects a command and triggers a “start”. If the command is valid and successful, key manager indicates done and no errors. If the command is invalid or unsuccessful, key manager indicates done with error. Regardless of the validity of the command, the hardware sequences are triggered to avoid leaking timing information.

The software is able to read the current state of key manager, however it never has access to the associated internal key.

When issuing the generate-output-hw command, software must select a destination primitive (aes, hmac or kmac). At the conclusion of the command, key and valid signals are forwarded by the key manager to the selected destination primitive. The key and valid signals remain asserted to the selected destination until software explicitly disables the output via another command, or issues another generate-output-hw command with a different destination primitive.

More details to come.

Register Table