A unidirectional network (also referred to as a unidirectional security gateway or data diode) is a network appliance or device allowing data to travel only in one direction, used in guaranteeing information security. They are most commonly found in high security environments such as defense, where they serve as connections between two or more networks of differing security classifications. This technology can now be found at the Industrial Control level for such facilities as nuclear power plants, and electric power generation.
Benefits and limitations
The physical nature of unidirectional networks only allows data to pass from one side (referred to as the "low" side) of a network connection to another (referred to as the "high" side), and not the other way around. The benefits for the users of the high side network are that their data is kept confidential while they have access to data from the low side. Such functionality can be attractive if sensitive data is stored on a network which requires connectivity with the Internet. Traditionally the data would be vulnerable to intrusions from the Internet, however with a unidirectional network separating a high side with sensitive data, and a low side with Internet connectivity, one can achieve the best of both worlds. This holds true even if both the low and the high network are compromised, as the security guarantees are physical in nature.
The controlled interface that comprises the send and receive elements of a unidirectional network acts as a one-way "communications protocol break" between the 2 two-way network domains it connects. This DOES NOT preclude unidirectional network use in transferring protocols like TCP/IP, that require communications (including acknowledgments) between sender and receiver. By employing TCP/IP client-server proxies prior to, and after one-way transfer, data transported as TCP packet flows can gain the security value of unidirectional transfer.
It is true that a primary unidirectional network path cannot be used as a "backwards" path for acknowledgement of the receipt of data by the ultimate destination. However, a scheme for such receipt acknowledgement exists, as granted in the 2010 award of US Patent 7,675,867  for a "One-Way Data Transfer System with Built-in Data Verification Mechanism." This mechanism ensures that the original sender of data is notified of successful receipt (or any number of alternative conditions). The Primary unidirectional path and the networks it connects are not compromised.
The idea of unidirectional networks have been around since the 1960s. This was developed further in the 1990s by Australia's Defence Science and Technology Organisation (DSTO) in the 1990s on the data diode and the Interactive Link.
The most common form of a unidirectional network is a simple, modified, fiber-optic cable, with send and receive transceivers removed for one direction. Commercial products rely on this basic design, but add other software functionality.
Some commercial offerings use proprietary protocols that allow for data transfer from protocols that usually require bidirectional links.
The US Naval Research Laboratory (NRL) has developed its own unidirectional network called the Data Pump. This is in many ways similar to DSTO's work, except that it allows a limited backchannel going from the high side to the low side for the transmission of acknowledgments. This technology allows more protocols to be used over the network, but introduces a potential covert channel if both the high and low side are compromised through artificially delaying the timing of the acknowledgment.
There are two general models for using unidirectional network connections. In the classical model, the purpose of the data diode is to prevent export of classified data from a secure machine while allowing import of data from an insecure machine. In the alternative model, the diode is used to allow export of data from a protected machine while preventing attacks on that machine. These are described in more detail below.
One-way flow to more secure machines
In the Bell-LaPadula security model, users of a computer system can only create data at or above their own security level. This applies in contexts where there is a hierarchy of information classifications. Examples include the hierarchy that runs from unclassified at the low end through confidential and secret to top secret. If users at each security level share a machine dedicated to that level, and if the machines are connected by data diodes, the Bell-Lapadula constraints can be rigidly enforced.
The majority of unidirectional network applications in this category are in defense, and defense contractors. These organizations traditionally have applied air gaps to keep classified data physically separate from any Internet connection. With the introduction of unidirectional networks in some of these environments, a degree of connectivity can safely exist between a network with classified data, and a network with an Internet connection.
Examples of this use of unidirectional technology include:
One-way flow to less secure machines
The second broad application involves systems that must be secured against attack from public networks while publishing information to such networks. For example, an election management system used with electronic voting must make election results available to the public while at the same time it must be immune to attack. The conventional solution to this is to use an air gap between the public network and the election management system, with data export by "sneakernet." The alternative is to use a data diode on the export channel.
This model is applicable to a variety of critical infrastructure protection problems. For example, the public living downstream from a dam needs up-to-date information on the outflow, and the same information is a critical input to the control system for the floodgates. In such a situation, it is critical that the flow of information be from the secure control system to the public, and not vice versa.
Common deployments of Data Diode technology include:
- Secure printing from a less secure network to a high secure network (reducing print costs)
- Transferring application and operating system updates from a less secure network to a high secure network
- Monitoring multiple networks in a SOC
- Time synchronisation in high secure networks
- File transfer
- Providing a “you have mail” alert in a high secure network, on a less secure network
Data Diode Products
There are a number of commercial providers of Data Diode, including (alphabetical):
- AROW Data Diode
- BAE SYSTEMS - Detica
- Fox DataDiode
- Arbit Lightspeed Datadiode
- NEXOR Data Diode
- Owl Computing Technologies
- VADO Security
- Waterfall Security Data Diode Solutions
- Slay, J & Turnbull, B 2004, 'The Uses and Limitations of Unidirectional Network Bridges in a Secure Electronic Commerce Environment', paper presented at the INC 2004 Conference, Plymouth, UK, 6–9 July 2004
- United States Patent: 7675867
- Stevens, MW & Pope, M 1995, Data Diodes, DSTO Electronics and Surveillance Research Laboratory, Adelaide
- Stevens, MW 1999, An Implementation of an Optical Data Diode, DSTO Electronics and Surveillance Research Laboratory, Adelaide
- Anderson, M, North, C, Griffin, J, Milner, R, Yesberg, J & Yiu, K 1996, 'Starlight: Interactive Link', San Diego, CA, USA
- Myong, HK, Moskowitz, IS & Chincheck, S 2005, 'The Pump: A Decade of Covert Fun'
- Curt A. Nilsen, Method for Transferring Data from an Unsecured Computer to a Secured Computer, U.S. Patent 5,703,562, December 30, 1997.
- Australian Government Information Management Office 2003, Securing systems with Starlight, Department of Finance and Administration, viewed 14 April 2011, [dead link]
- Wordsworth, C 1998, Media Release: Minister Awards Pioneer In Computer Security, viewed 14 April 2011, 
- Douglas W. Jones and Tom C. Bowersox, Secure Data Export and Auditing Using Data Diodes, Proceedings of the 2006 USENIX/ACCURATE Electronic Voting Technology Workshop, August 1, 2006, Vancouver.
- "Waterfall Security Granted Patent for Secure Implementation of Network-Based Sensors".