Trezor Bridge Protocol

Secure Connection | Trezor.io/Start®

INFRASTRUCTURE LAYER

Trezor Bridge: The Secure Communication Conduit

The **Trezor Bridge** is a lightweight background application essential for creating a reliable communication link between your hardware wallet and the Trezor Suite (browser or desktop application). It is the dedicated messenger that allows your Trezor to interact securely with the digital world.

I The Necessity of the Bridge: USB Abstraction

**Device Abstraction Layer:** Modern web browsers (like Chrome or Firefox) have security restrictions that prevent them from directly accessing local USB devices for low-level communication. The Trezor Bridge solves this by acting as an **intermediary service**. It listens for USB signals from your Trezor device and translates them into secure communication protocols (like WebSockets) that the browser-based Trezor Suite can understand.

  • **Key Role:** The Bridge bypasses standard browser USB limitations, enabling smooth device recognition and transaction signing.
  • **Protocol:** It ensures the data exchange maintains integrity and cryptographic isolation between the computer's OS and the Trezor Suite environment.
  • **Alternative:** While the Trezor Suite desktop app natively handles communication, the Bridge is mandatory for using the web version of Trezor Suite or third-party wallet interfaces.

II Bridge Installation and System Integration

**Seamless Integration:** Installation is typically automated and initiated when you first visit `trezor.io/start` or the web version of the Trezor Suite. The Bridge is platform-agnostic, available for Windows, macOS, and Linux, and requires administrator privileges only during installation.

**Prerequisites Check:** Ensure that any previous Trezor-related client software, particularly older versions of the Bridge or browser extensions that might conflict, are completely uninstalled before deploying the current version. The Bridge runs silently in the background and starts automatically when your operating system boots.

  • **Verification:** After installation, verify the Bridge is running by checking your OS process manager or system tray icon (if available).
  • **Web Access:** The Bridge serves communication over `localhost`, ensuring that the data connection remains entirely local and does not traverse the public internet.

III The Cryptographic Role in Transaction Signing

**The Secure Handshake:** It's crucial to understand what the Trezor Bridge *doesn't* do. The Bridge does **not** handle or process your private keys, PIN, or Recovery Seed. Its function is strictly to relay encrypted messages. The private key never leaves the secure chip within the Trezor device.

🛑 DATA ISOLATION

When you initiate a transaction in Trezor Suite, the Bridge transmits the *unsigned transaction data* to the Trezor device. The Trezor signs the transaction internally and returns the *signed transaction* data back through the Bridge to the Suite for broadcast. The Bridge is a messenger, not a processor of sensitive credentials.

  • **Input:** Unsigned transaction data from the software interface.
  • **Output:** Signed, broadcast-ready transaction data from the device.
  • **Security Model:** This separation of duties (transaction presentation in the Suite, signing on the device) is fundamental to cold storage security.

IV Trust Model and Open-Source Transparency

**Auditable Codebase:** The Trezor Bridge software is **open-source**, meaning its code is publicly available for security researchers and the community to inspect. This transparency ensures that the code does exactly what it claims—and nothing more. This open-audit process is a cornerstone of the Trezor trust model.

**Certificate Pinning:** The Bridge uses advanced security mechanisms like certificate pinning to ensure that only the official Trezor Suite can communicate with it, preventing malicious local applications from intercepting the USB traffic intended for your Trezor.

  • **Minimal Privilege:** The Bridge requires minimal operating system privileges to function, reducing its attack surface compared to full-fledged wallet applications.
  • **Regular Updates:** Regular updates not only improve compatibility with new operating systems and browsers but also patch any newly discovered vulnerabilities, emphasizing active security maintenance.

Connection Troubleshooting and Diagnostics

  • **Bridge Restart:** If the device is not detected, the first step is always to manually restart the Trezor Bridge service via your computer's task manager or services panel.
  • **Browser Cache:** Clear your browser cache and cookies, as cached connection data can sometimes interfere with the Bridge's WebSocket connection establishment.
  • **Firewall Check:** Ensure that your computer's local firewall is not blocking the Bridge's local communication on port `21325`. This port must be open for internal (`localhost`) traffic.
  • **Conflicting Services:** Temporarily disable other security or VPN software, as they may mistakenly flag the Bridge's local networking as suspicious and disrupt the connection.

Frequently Asked Questions (5 Bridge-Specific Inquiries)

The Trezor Bridge is **mandatory** if you are accessing Trezor Suite via your **web browser**. This is because web browsers cannot directly access USB devices due to security policies. If you use the standalone, installable **Trezor Suite Desktop** application, the Bridge service is integrated into the application itself and is managed automatically, so you don't need a separate installation.

No, it is highly secure. The Bridge's source code is **open-source and auditable**. Crucially, it only handles **unsigned data** and acts as a pipe; it never has access to your private keys, PIN, or Recovery Seed. The final transaction signing happens *inside* the Trezor device, completely isolated from the Bridge and your computer's operating system.

Operating System updates (especially major ones) can sometimes reset USB driver permissions or firewall settings. The most common fix is to **re-download and reinstall the latest version of the Trezor Bridge**. This ensures all necessary drivers and background services are correctly registered with the updated OS environment. Also, check if the OS update enabled a stricter firewall that needs manual allowance for the Bridge.

The Trezor Bridge primarily uses **port 21325** for WebSocket communication between the running Bridge application and the Trezor Suite (or other browser-based wallets). This communication is strictly local (`localhost`), meaning the data never leaves your computer. If you encounter firewall issues, ensure that your firewall permits local traffic on this specific port.

Yes, a single installation of Trezor Bridge can manage communication for multiple Trezor devices connected to the same computer. The Trezor Suite application handles the connection multiplexing, ensuring that commands are routed to the correct device, which is selected by the user when initiating a transaction or unlocking a specific wallet.