Creating a Bespoke Data Diode for Air-Gapped Networks

The article provides a detailed technical walkthrough of the creation of a bespoke data diode designed for air-gapped networks. It begins by defining the critical security requirement: ensuring a physical, unidirectional data path that prevents any possibility of data flowing back from a secure network to an untrusted one. The author describes the selection of core hardware components, specifically focusing on the use of a Field-Programmable Gate Array (FPGA) to implement the core logic.

Further sections detail the firmware development process for the FPGA, which is designed to strictly enforce the one-way transfer of data packets. The article also covers the physical construction, including the enclosure and connector choices, and the rigorous testing methodology used to validate the device's integrity. The project serves as a case study in the challenges and considerations of building custom security hardware rather than relying on commercial products.

The project began with a clear set of security requirements for the data diode. The primary objective was to create a hardware-enforced barrier between two networks, ensuring that data could only move from a lower-security zone to a higher-security zone. This air-gap philosophy requires that no electrical path exists for data to return, effectively eliminating the risk of remote exploits traversing the boundary.

Key specifications for the device included:

• Support for high-speed data transfer rates to accommodate network traffic.
• A robust physical design to prevent tampering.
• Reliable operation without software intervention that could introduce vulnerabilities.

The decision to build a custom solution was driven by the need for specific performance metrics that were not met by existing commercial offerings.

The core of the bespoke diode relies on a Field-Programmable Gate Array (FPGA). Unlike a standard microprocessor that executes software instructions, the FPGA is configured to act as a fixed digital circuit. This hardware-based approach is critical for security, as it removes the layer of software that could potentially be compromised or contain bugs.

The physical design separates the input and output sides of the device. The author notes the importance of isolating the power supplies and clock sources for the two sides to prevent side-channel attacks. The selection of optical interfaces was also a key decision, as fiber optics naturally provide electrical isolation between networks.

Developing the firmware for the FPGA involved creating a logic design that strictly enforces unidirectional flow. The firmware acts as a gatekeeper, allowing data packets to pass through from the input interface to the output interface but blocking any signals attempting to travel in reverse. The author describes the implementation of a simple state machine to manage this process.

To ensure reliability, the design avoids complex buffering or processing that could introduce latency or errors. The logic is minimal and focused solely on the task of passing data one way. Validation of the firmware was performed using simulation tools before being loaded onto the physical hardware.

The physical assembly of the device required careful attention to detail to maintain the integrity of the air gap. The enclosure was designed to separate the input and output electronics completely. Connectors were mounted on opposite sides of the chassis to prevent accidental bridging or cabling errors.

Testing the completed diode involved several stages:

1. Visual Inspection: Verifying that no unintended physical connections exist between the secure and insecure sides.
2. Electrical Isolation Test: Using high-voltage testing to confirm the resistance between isolated grounds.
3. Traffic Verification: Passing known data patterns through the device to ensure data integrity and confirm that no data flows backward.

The article concludes that while building a custom diode is a complex undertaking, it provides a high degree of confidence in the security of the network boundary.