Security System

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Offline by design

ZeroKeyUSB does not rely on the Internet, cloud storage, or companion apps. Everything — from encryption to key handling — happens inside the device, powered directly through USB. Your passwords never leave the hardware and cannot be accessed remotely, even by the manufacturer.

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Encryption architecture

All sensitive data is stored in the external EEPROM M24C64-WMN6TP, encrypted using AES-128 CBC (Cipher Block Chaining). The encryption key is derived from your Master PIN: the digits are hashed with SHA-256 and the first 16 bytes form the AES key. The original PIN is never stored or transmitted in plain text.

Element	Description
Encryption algorithm	AES-128 in CBC mode (industry-standard symmetric cipher).
Key source	Master PIN → SHA-256 → 128-bit AES key.
IV (Initialization Vector)	Generated from analog noise on first startup and stored securely in EEPROM.
Data blocks	Each credential uses three 32-byte encrypted pages (site, username, password).
Verification	Each write is confirmed via I²C acknowledgment to ensure completion.

If the EEPROM were removed from the board, its contents would appear as random ciphertext, with no recognizable structure or patterns.

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The Master PIN

Your PIN acts as your personal encryption key. It is never transmitted, stored in plain text, or shared with any external system. When you enter your PIN:

1. ZeroKeyUSB derives the AES-128 key.
2. This key decrypts an 8-byte signature block stored in EEPROM.
3. If the decrypted result matches the expected pattern, access is granted.
4. Otherwise, the failed-attempt counter increments and the device enforces a delay before retry.

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Intelligent lockout

Each incorrect PIN attempt increases the waiting time exponentially, and the count is stored persistently in EEPROM (0x0002):

Failed attempts	Wait time
1	5 seconds
2	10 seconds
3	20 seconds
4	40 seconds
5	80 seconds
…	up to 2560 seconds (≈ 43 minutes)

This mechanism prevents brute-force attempts and persists even after unplugging the device.

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Secure Initialization Vector (IV)

Every device generates a unique Initialization Vector on first startup. The IV is created using analog noise read from a floating pin on the SAMD21 and saved in the EEPROM. If the IV becomes corrupted or missing, it is regenerated automatically with fresh entropy. A single IV per device is used for all AES operations, ensuring every encryption chain is unique to that specific unit.

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Data segmentation

Each credential is divided into independent encrypted pages to minimize correlation:

• Page 1 → Site or domain
• Page 2 → Username
• Page 3 → Password
• Page 4 (optional) → TOTP secret

This segmented structure isolates corruption, avoids recognizable patterns, and extends memory endurance.

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Tamper protection

• The entire PCB is encapsulated in epoxy resin, making component access virtually impossible without destroying the device.
• No wireless interfaces (Wi-Fi, Bluetooth, NFC) are present.
• The firmware is static — flashed at the factory and cannot be updated remotely.
• All sensitive operations occur only in temporary RAM buffers, which are cleared immediately after use.

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Transparency, not dependence

ZeroKeyUSB’s firmware is fully open-source and available for public audit and verification. Anyone can review how encryption, IV handling, and access control are implemented to confirm that:

• There are no backdoors or remote commands.
• All cryptographic processes occur locally and deterministically.
• Security depends on design, not trust.

ZeroKeyUSB intentionally avoids OTA or wireless updates — physical access is required for reflashing if ever needed.

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