This document describes the composition of the Silicon Creator and Silicon Owner cryptographic identities and the Silicon Owner root key derivation scheme. This scheme is based on a symmetric key manager with support for software binding and key versioning.
This document also defines a non-cryptographic Device Identifier to facilitate silicon tracking during manufacturing flows. The Device Identifier is also mixed into the Creator Identity.
Further, this scheme is compatible with the Open DICE profile.
DICE compatible identity flow:
Boot stages:
ROM: Metal ROM, sometimes known as Boot ROM.ROM_EXT: ROM Extension. Stored in flash and signed by the Silicon Creator1.BL0: Bootloader. Signed by the Silicon Owner.Kernel: Signed by the Silicon Owner.
Key manager operations:
KM_DERIVE: Key manager one-way function used to derive a new symmetric key.
Memory state operations:
CLEAR_BEFORE_NEXT_BOOT_STAGE: Clear key material before moving to the next boot stage.CLEAR_AFTER_USE: Clear immediately after use.
The device identifier is a globally unique 256b value provisioned on each device's OTP memory in early manufacturing stages (e.g. wafer test). It is used to facilitate device tracking during manufacturing and provisioning. This value is also used as a component in the generation of the device's Silicon Creator Identity, a cryptographically unique identity.
The 256b value is split into two halves. The first contains hardware origin information following a global standard format, while the second one is defined by the device SKU provisioning requirements.
128b Hardware origin information
| No. Bits | Description |
| 16 | Silicon Creator identifier. Assigned by the OpenTitan project. |
| 16 | Product identifier. Assigned by the Silicon Creator. Used to identify a class of devices. |
| 64 | Individual device identification number. Assigned by the Silicon Creator. For example, the wafer lot number and die's X,Y coordinates may be encoded here to simplify manufacturing tracking. Another option is to use a non cryptographic hash function with collision checks to guarantee global uniqueness. |
| 32 | CRC-32 IEEE 802.3. covering the previous bytes. |
128b SKU specific device information
The device provisioner information varies for each provisioning use case. Each use case must have a specification defining the allocation of these bits. See the UICC EID Specification as an example.
The following sequence describes the creation of the CreatorRootKey. All
inputs into the key manager can be locked down during ROM execution.
The size of the inputs is dependent on the security strength and masking configuration of the implementation. The input concatenation function must be injective. This can be achieved by fixing the width of all the operands.
CreatorRootKey = KM_DERIVE(RootKey,
DiversificationKey | HealthStateMeasurement | DeviceIdentifier |
ROMExtSecurityDescriptor | HardwareRevisionSecret)
| Name | Encoding | Description |
| RootKey | OTP |
Device root key. Provisioned at manufacturing time by the Silicon Creator.
Hidden from software once personalization is complete. |
| DiversificationKey | Flash |
Additional diversification key stored in flash. Provisioned at
manufacturing time by the Silicon Creator.
Hidden from software once provisioned. |
| ROMExtSecurityDescriptor | SW register |
The implementation may choose one of the following options:
|
| DeviceIdentifier | OTP | Provisioned at manufacturing time. Readable from software and JTAG interface. |
| HardwareRevisionSecret | Gates | Encoded in gates. Provisioned by Silicon Creator before tapeout. Hidden from software. |
| Health State Measurement | Register (ROM stage) |
Comprises the following measurements:
Some values are read from the device life cycle controller. The device life cycle state should be consumed by the ROM stage. The debug mode shall be used as well if there are multiple debug configurations supported by a single life cycle state. |
The CreatorRootKey can be used to generate the Creator Identity key and the OwnerIntermediateKey described in the following sections.
The Creator Identity is an asymmetric key derived from the CreatorRootKey. It
is used as a cryptographic identifier bound to the device and the Silicon
Creator. It is used to attest to the authenticity of the physical device and the
ROM and ROM_EXT configuration.
The Creator Identity is generated as follows:
CreatorIdentitySeed =
KM_DERIVE(CreatorRootKey, IdentityDiversificationConstant)
// ASYM_KDF is a KDF function compliant to the Asymmetric Key
// requirements defined in the Attestation specification document.
CreatorIdentity_Private = ASYM_KDF(CreatorIdentitySeed)
CLEAR_BEFORE_NEXT_BOOT_STAGE(CreatorIdentitySeed, CreatorIdentity_Private)
| Name | Encoding | Description |
| IdentityDiversificationConstant | Gates |
A constant defined in gates. Used to derive the CreatorIdentitySeed from the
CreatorRootKey.
Hidden from software. |
| CreatorIdentitySeed | SW Register Output | Seed used to generate the CreatorIdentity asymmetric key. |
The CreatorIdentitySeed and the private portion of the Creator Identity shall
be cleared before the ROM Extension hands over execution to the Silicon Owner
first boot stage.
The OwnerIntermediateKey is used as a root component of the Silicon Owner key
hierarchy. It is used to establish a cryptographic link to the root secrets
provisioned at manufacturing time.
Visibility
The OwnerIntermediateKey shall be kept hidden from software to mitigate owner
impersonation attacks.
The OwnerIntermediateKey is generated as follows:
OwnerIntermediateKey =
KM_DERIVE(CreatorRootKey, OwnerRootSecret | SoftwareBindingValue)
| Name | Encoding | Description |
| OwnerRootSecret | Flash |
Used as a diversification constant with acceptable entropy. Provisioned at
Ownership Transfer time by the Silicon Creator.
The OwnerRootSecret has different visibility options depending on the level of isolation provided in hardware:
|
| SoftwareBindingValue | SW Register Lockable Input |
Software binding value configured during secure boot. See Software Binding for more details. |
The Owner Identity is used as a cryptographic identifier bound to the device and the Silicon Owner. It is used in Attestation flows. The Owner Identity is not expected to change during the lifetime of the device ownership assignment.
OwnerIdentitySeed =
KM_DERIVE(OwnerIntermediateKey, OwnerRootIdentityKey)
// ASYM_KDF is a KDF function compliant to the Asymmetric Key
// requirements defined in the Attestation specification document.
OwnerIdentity_Private = ASYM_KDF(OwnerIdentitySeed)
CLEAR_BEFORE_NEXT_BOOT_STAGE(OwnerIdentitySeed, OwnerIdentity_Private)
| Name | Encoding | Description |
| Owner Root Identity Key | Gates |
The OwnerRootIdentityKey is a diversification constant with acceptable entropy
provisioned in gates.
Visibility: Hidden from software. |
| Owner Identity Seed | SW Register Output | Seed used to generate the OwnerIdentity asymmetric key. |
The OwnerIdentitySeed and the private portion of the Owner Identity shall be
cleared before the bootloader (BL0) hands over execution to the kernel.
The key manager supports the generation of versioned keys with lineage to the
OwnerRootKey for software consumption and sideload operations.
OwnerRootKey =
KM_DERIVE(OwnerIntermediateKey, SoftwareBindingValue)
VersionedKey = KM_DERIVE(OwnerRootKey,
KeyVersion | KeyID | Salt | SoftwareExportConstant)
The concatenation function must be injective. This can be achieved by fixing the width of all the operands.
| Name | Encoding | Description |
| OwnerRootKey | Internal RAM |
Owner Root Key bound to the software stack.
Visibility: Hidden from software. |
| SoftwareBindingValue | SW Register Lockable Input |
Software binding value configured during secure boot. See Software Binding for more details. |
| KeyVersion | SW Register Input |
Key version. The value provided by software may be mixed with a gate constant
before key derivation steps.
The value should also pass the version comparison criteria configured during secure boot. See Key Versioning for more details. |
| KeyID | SW Register Input | Key identifier. Used to derive a VersionedKey from OwnerRootKey. Processing of this field should provide countermeasures against key recovery attacks. |
| SoftwareExportConstant | Gates |
The SoftwareExportConstant is a diversification constant with acceptable entropy
provisioned in gates. It is used to mitigate key recovery attacks on software
inputs.
Visibility: Hidden from software. |
| Salt | SW Register Input | Salt input controlled by software. |
| VersionedKey | SW Register Output | Output key derived from OwnerRootKey and KeyID. Support for sideloading may require additional support in the key manager, otherwise the software will be in charge of enforcing isolation. |
Software binding is used to ensure that the key derivation scheme is only reproducible for a trusted software configuration. This is achieved by having the secure boot implementation configure runtime-irrevocable binding tags in the key derivation scheme. Such tags are usually delivered inside the signed manifest of each code partition.
OpenTitan shall support software binding for at least two Silicon Owner code stages (e.g. bootloader and kernel). It is expected that the kernel will implement binding with the application layer exclusively in software.
Each key manager binding stage shall provide at least 128b of data.
Key versioning is the mechanism by which software implements key rotation triggered by security updates. Since there may be more than one updateable code partition in the system, the key versioning scheme has to implement at least 8 32b version comparators with lockable controls, which will be configured as part of the secure boot process.
The next figure shows an example on how to configure the maximum key version constraints in the key manager. The ROM_EXT software verifies the BL0 manifest, and configures one of the maximum key version registers with the maximum allowable version stored in the BL0 manifest. In the same way, the BL0 software verifies the Kernel manifest and configures a separate key version register. The software implementation is free to allocate more than one maximum key version register per boot stage.
|
|
Figure: Maximum allowable versions configured as part of the secure boot
Note: The diagram is overly simplified and does not take into account security hardening. |
Secrets wrapped with versioned keys shall have additional metadata including Key Version, Key ID and salt information.
The versioned key generation is gated on the version comparison check enforced by the key manager implementation. The following set of operations will only succeed if the key version set by software is valid.
|
|
Figure: Key version is set during key configuration
Note: The diagram is overly simplified and does not take into account security hardening. |
The hardware may opt to implement a software interface with higher level one-way step functions to advance the internal state of the key manager. The following are the minimum set of steps required:
|
| Figure: Minimum set of high level one-way step functions. |
Instantiations of the key manager can be conditioned to start at the current
internal state of the key manager, for example, kernel-level instantiations may
always start at the OwnerRootKey level, assuming the previous boot stages
advanced the state of the key manager.
The following high level software interface may be supported by hardware to generate versioned keys. The hardware may opt to implement versioned key functionality at each of the high level key manager states.
/**
* Generate versioned key for a given |key_id|.
*
* Generates a versioned key rooted in the current state of the
* key manager. Requires the key manager to be in one of the
* following states:
* CreatorRootKey, OwnerIntermediateKey, OwnerRootKey
*
* @param kmgr Key Manager state data.
* @param key_version Key version. Each version 32b word shall
* be less than its associated max version value. Requires the
* maximum version registers to be configured before calling this
* function
* @param key_id Key identifier.
* @param versioned_key Key output.
* @return true if the function was successful, false otherwise.
*/
bool keymgr_generate_vk(const kmgr_t *kmgr,
const uint32_t key_version[8],
const uint32_t key_id[8]
uint32_t *versioned_key[8]);
The Key Manager derive function should support at least 256b of security strength.
Note on standards: The key derivation function (KM_DERIVE), when instantiated
with a Pseudorandom Function (PRF), shall be compatible with
NIST SP 800-133r22
section 6.3: Symmetric Keys Produced by Combining (Multiple) Keys and Other
Data. This imposes provisioning requirements for the root keys, which are
covered in the Provisioning specification.
Security strength for the KM_DERIVE function based on a Pseudorandom Function
(PRF)3:
| PRF | Security Strength | Notes |
| KMAC256 | 256 | 1. NIST 800-108, which focuses on PRF key derivation functions, lists KMAC as a primitive. |
The hardware shall support countermeasures against key recovery attacks for all software controlled inputs.
The hardware shall support at least 8 32b write-lockable version comparison registers to provide key versioning functionality.
The hardware shall support at least 256b of software write-lockable registers to implement software-hardware key manager binding as part of the secure boot implementation.
The hardware shall support versioned key derivations for software provided key IDs. A key ID is defined as a 256b value used as a key handle.
The hardware shall isolate the RootKey and other provisioned secrets from
software after completion of personalization at manufacturing time.
Later boot stages shall have no access to secrets maintained by previous boot stages.
The software shall configure runtime lockable inputs as part of the secure boot implementation to fix the construction of identities and root keys in the key manager.
Hardware secrets stored in OTP and flash shall be scrambled to increase the difficulty of physical attacks.
ROM Extension updates may invalidate the Silicon Owner and Silicon Creator identities as well as the root keys.
The implementation should consider a software based backup mechanism to mitigate security and/or certification issues with the main implementation. The backup mechanism shall not rely on secrets from the main implementation.
Footnotes
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The Silicon Creator is the logical entity, or logical collection of entities that manufactures, packages, tests and provisions the chip with its first identity. ↩
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Recommendation for Cryptographic Key Generation (NIST SP 800-133r2). ↩
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Security strengths for PRF functions as documented in: Recommendation for Key-Derivation Methods in Key-Establishment Schemes (NIST 800-56Cr1) ↩