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ZeroKeyUSB encrypts each credential block using AES-128 in CBC mode. The encryption key is derived directly from the Master PIN, ensuring that only the owner can decrypt stored data.

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Key material

  • The Master PIN (4–16 digits) is zero-padded or truncated to 16 bytes.
  • Before use, it passes through a single round of SHA-256; the first 16 bytes become the AES key.
  • The derived key never leaves SRAM and is cleared immediately after authentication.
This lightweight derivation protects against weak numeric PINs while keeping unlocking quick on the Cortex-M0+ core.

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CBC chaining

  1. The firmware loads the persistent Initialization Vector (IV) from EEPROM.
  2. Each 32-byte field (site, username, password, TOTP) is split into two 16-byte blocks.
  3. Blocks are XORed with the previous ciphertext (or IV for the first block) and encrypted.
  4. The resulting ciphertext is written back to EEPROM with a CRC-16 checksum.
When decrypting, the process runs in reverse: decrypt, XOR with previous ciphertext, and verify the CRC before exposing plaintext.

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AES implementation

  • Based on the open-source tiny-AES-c library, optimized for minimal RAM usage.
  • Uses precomputed S-box tables stored in flash.
  • Encrypt/decrypt routines are constant-time with respect to plaintext values.
  • All 16-byte buffers are allocated on the stack to reduce persistent traces in memory.
The total encryption time for a full credential slot is under 2 ms.

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Handling padding

  • Plaintext shorter than 32 bytes is padded with null bytes prior to encryption.
  • During decryption, trailing nulls are stripped but preserved internally so auto-typing retains the original length.
  • The firmware rejects UTF-8 sequences that would overflow the 32-byte limit to avoid partial block writes.

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Security considerations

  • Because the Master PIN is the key, changing the PIN re-encrypts every slot with the new key.
  • A factory reset erases the IV and all ciphertext, preventing offline brute-force attempts on desoldered EEPROMs.
  • There is no key escrow or recovery mechanism; forgetting the PIN renders stored data unrecoverable.
Together with physical tamper resistance, AES-128 CBC provides strong protection even if the EEPROM is copied or probed.
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