title: “Universal 2nd-Factor Security Key”

When used as a security key, OpenTitan implements the Universal 2nd Factor (U2F) authentication standard, using a Universal Serial Bus (USB) 1.1 interface to communicate with host devices. U2F requires the implementation of a challenge-response authentication protocol based on public key cryptography. The security key is provisioned with a unique identity in the form of an asymmetric key, which may be self-endorsed by a certificate issued at manufacturing time.

When used as a security key, OpenTitan shall meet the FIDO Authenticator security goals and measures described in the FIDO Security Reference v1.2 specification. See Universal 2nd Factor (U2F) Overview v1.2 for more details on the functional requirements of this use case.

Certification Requirements

• BSI-PP-CC-0096-V3-2018 FIDO Universal Second Factor (U2F) Authenticator. The minimum assurance level for this Protection Profile (PP) is EAL4 augmented. This PP supports composite certification on top of the Security IC Platform Protection Profile with Augmentation Packages, BSI-CC-PP-0084-2014 (referred to as PP84).
• FIPS 140-2 L1 + L3 physical certification is required for some use cases.

Minimum Crypto Algorithm Requirements

The current target for all crypto is at least 128-bit security strength. This is subject to change based on the implementation timeline of any given instantiation of OpenTitan. It is expected that a future implementation may be required to target a minimum of 192-bit or 256-bit security strength.

• TRNG:
  • Entropy source for ECDSA keypair generation (seed and nonce).
  • (optional) Symmetric MAC key generation.
• Asymmetric Key Algorithms:
  • ECDSA: Signature and verification on NIST P-256 curve for identity and attestation keys.
  • RSA-3072: Secure boot signature verification. Used to verify the signature of the device's firmware.
• Symmetric Key Algorithms:
  • AES-CTR:
    • (optional) Used to wrap a user private key in a key handle. Implementation dependent.
  • HMAC-SHA256:
    • For application key handle generation.
• Hash Algorithms:
  • SHA-256:
    • Code and hardware measurements used in internal secure boot implementation.
    • (optional) For key handle generation. Implementation dependent.
    • (optional) Attestation cert generation, if generated on the fly.

Provisioning Requirements

OpenTitan used as a security key has the following provisioning requirements:

• Unique Global Identifier: Non-Cryptographic big integer value (up to 256b) used to facilitate tracking of the devices throughout their life cycle. The identifier is stored in One Time Programmable (OTP) storage during manufacturing.
• Attestation Key: Unique cryptographic identity used for attestation purposes.
• Self-Signed Attestation Certificate: Self signed certificate and extracted at manufacturing time for registration purposes. U2F backend servers can create an allow-list of certificates reported by the secure key manufacturer, and use them to perform authenticity checks as part of the registration flow.
• Factory Firmware: Baseline image with support for firmware update via USB, and the USB HID U2F command spec.

Additional Requirements

• Physical Presence GPIO: U2F requires physical user presence checks for registration and authentication flows. This is implemented either via a push button or capacitive touch sensor connected to an input GPIO pin.
  • At least 2 PWM peripherals can facilitate implementation of capacitive touch sensor IO operations.
• Status LEDs GPIO: The security key may use LEDs to provide feedback to the user. This requires up to 4 additional output GPIO pins.
• USB HID U2F Stack: The security key communicates with host devices via a USB HID protocol. OpenTitan shall meet the USB 1.1 connectivity and protocol requirements to interface with the host.

Relevant specs

https://fidoalliance.org/specifications/download/