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title: “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.

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 secret value, 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]({{< relref “hw/ip/kmac/doc” >}}). 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 state, when valid, supplies its secret material as the “key” input to a KMAC operation. 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.
• RomExtSecurityDescriptor: A software digest of the ROM_ext stage.

Other than the DiversificationKey, none of the values above are considered secret. The RomExtSecurityDescriptor is a 256b value calculated by ROM and loaded into key manager registers.

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 terminal state where the key manager is no longer operational. When entering Disabled random values are used to compute a new random value.

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 disabled.

When disabled, 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, it immediately wipes its key contents (working state, sideload keys and software keys) and transitions to Disabled.
  • 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 Disabled state, the behavior is consistent with normal behavior.

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 state, working state and output registers are updated as usual. The only difference during Disabled state is that random data is used and invalid operations are reported.

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, Interrupts and Alerts

An error code register is maintained {{< regref ERR_CODE >}} to check issues that might rise while using the key manager. There are two categories of errors

• Hardware fault errors - These errors indicate something fundamental has gone wrong and are errors that could not have been caused by software.

  • Invalid command - A non-one-hot command was issued from the key manager controller to the KMAC data interface. This is not possible by software and indicates a hardware fault. This error can also happen if the KMCA data fsm gets into an invalid state.
  • Invalid output - The data return from KMAC is all 0‘s or all 1’s. This is not possible given the set of KMAC data interface inputs.

• Software operation errors - These errors could have been caused by user errors and is a sign that software should examine its usage of key manager.

  • Invalid operation - An invalid operation (for example generate while in Idle) was invoked.
  • Invalid input - Invalid software input was supplied (for example a greater key version than allowed in {{< regref MAX_OWNER_KEY_VER >}}.

Whenever the error code changes from its previous value to a non-zero value, an interrupt is generated. Two separate alerts are also generated, one corresponding to each category above.

Invalid Command

When these errors occur, random data is fed to the KMAC for processing. At the completion of KMAC processing, the working state is over-written with KMAC's output and the state is transitioned to Disabled. An error interrupt and fault alert are also generated. Note the KMAC module itself may error during the derivation process. When this happens, the error is also registered as an invalid command and the same mechanisms apply.

Invalid Output

When these errors occur, an error interrupt and fault alert are generated. Neither the working state nor the relevant output registers are updated.

Invalid Input

When these errors occur, random data is fed to the KMAC for processing. Neither the working state nor the relevant output registers are updated. An error interrupt and operation alert are also generated.

Invalid Operation

When these errors occur, an error interrupt and fault are generated.

The effects of an invalid operation vary depending on state. 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. Note that even though Init is not a legal operation in most states, it is treated as an ignored command rather as an explicit error.

Current State	Legal Operations	Outcome
Reset	Advance	Advance to Initialized state upon completion.
Reset	Disable / Advance / Generate	Invalid operation error triggered with no other side effects.
Initialized	Disable / Advance	Advance to next Disabled state or CreatorRootKey.
Initialized	Generate	Invalid operation error triggered with random data, output registers are updated.
CreatorRootKey	Disable / Advance / Generate	Behave as expected.
OwnerIntKey	Disable / Advance / Generate	Behave as expected.
OwnerKey	Disable / Advance / Generate	Behave as expected.
Disabled	Disable / Advance / Generate	Invalid operation error triggered with random data, both working state and output registers are updated depending on command.

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.

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. This software binding exists for every stage of key manager except for OwnerKey. 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. In order to save on storage and not have a duplicate copy per stage, the software binding registers {{< regref SOFTWARE_BINDING >}} are shared between key manager stages.

Software sets the appropriate values and locks it by clearing {{< regref SOFT_BINDING_EN >}}. When later a successful advance call is made, the key manager then unlocks by setting {{< regref 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

{{< hwcfg “hw/ip/keymgr/data/keymgr.hjson” >}}

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 secret value

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

{{< registers “hw/ip/keymgr/data/keymgr.hjson” >}}