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System Design

Security Features

Secret Storage

  • TOTP secrets are encrypted at rest using AES-256-GCM, ensuring data confidentiality
  • Encryption keys derived from vault password using PBKDF2, device has no knowledge of decryption keys without vault password
  • The keys are never stored on the device (ie. the device cannot decrypt any secrets without the user providing the vault password)
  • Encrypted TOTP secrets are stored in NVS (flash) on the same die as the microcontroller, making it difficult to sniff/extract them
  • Secrets are zeroed from memory immediately after use

    System Diagram

System Firmware

  • ESP32-C3 firmware has secure boot enabled with RSA 3072 signature verification, ensuring only trusted firmware can run
  • Firmware is encrypted using hardware-accelerated AES, providing an additional layer of security and mitigating TOCTOU attacks on secure boot
  • Firmware is written in Rust with minimal unsafe regions
  • Device can be attested with a challenge to prove itself
  • ESP32-C3 hardware random number generator is used for cryptographic operations

    HW RNG Design

Hardware Design

  • TOTP secrets are encrypted and stored in on-chip flash memory, substantially complicating attacks to dump or snoop secrets

  • USB interface is delegated to a separate chip (CH343P) from the main microcontroller (ESP32-C3) in order to isolate any USB-level exploits from the main uC

    USB Isolation

  • JTAG interfaces are disabled and eFuses locked down

Security Limitations

  • Communication between the ESP32 <-> USB/UART Chip <-> Host is unencrypted. After the vault has been unlocked, it is possible for an attacker to sniff these buses for plain-text TOTP codes and TOTP secrets (but only if the sniffing occurs while adding a new credential). It is impossible to extract a TOTP secret once it is stored.