Hardware trojan
A malicious circuit modification inserted during design or manufacturing.
A hardware trojan (HT) is a deliberate, malicious alteration made to an integrated circuit's internal circuitry. Its nature is defined by both its physical form and how it behaves. When triggered, the trojan's payload is everything it does in response. These trojans often aim to bypass or break a system's security, for instance by leaking secret data through radio signals, or they might disable, damage, or destroy the entire chip or parts of it.
Such trojans can be introduced as hidden backdoors during the chip design phase, through the use of pre-made ASIC IP cores bought from untrustworthy sources, or by a rogue employee acting alone or on behalf of special interest groups or state-sponsored espionage. A 2015 IEEE paper describes how a hardware design with a trojan could leak a cryptographic key over an antenna or network connection, but only when a specific "easter egg" trigger is applied. In high-security government IT departments, hardware trojans are a known risk when buying equipment like KVM switches, keyboards, mice, network cards, or other network gear, especially from non-reputable sources that might embed trojans to steal keyboard passwords or provide unauthorized remote access.
**Background**
Outsourcing production is common in a global economy to lower costs. Embedded hardware devices are not always made by the firms that design or sell them, nor in the same country where they are used. This raises doubts about whether the final product matches its original design. Anyone with access to the manufacturing process could, in theory, introduce changes. For complex products, small changes with large effects can be hard to detect. The threat of a serious, malicious design change is especially relevant to government agencies. Resolving doubts about hardware integrity helps reduce technology vulnerabilities in military, finance, energy, and political sectors. Since fabricating integrated circuits in untrustworthy factories is common, advanced detection techniques have emerged to find hidden components or sabotage.
**Characterization of Hardware Trojans**
An HT can be characterized by its physical representation, activation phase, and action phase. Other methods describe it by its trigger, payload, and stealth.
**Physical Characteristics**
One physical characteristic is the type: functional or parametric.
- Type
- functional or parametric
- Activation
- internally activated, externally activated, or always-on
- Trigger
- condition-based (sensors, internal logic states, input pattern, counter value) or sequential
- Payload
- modify function, change parametric properties, transmit confidential information, disable/destroy chip
- Detection methods
- physical inspection (SOM, SEM, PICA, VCI, LIVA, CIVA) and functional testing
Lore & Background
Hardware trojans are a well-known problem in high-security governmental IT departments when purchasing hardware such as KVM switches, keyboards, mice, network cards, or other network equipment, especially from non-reputable sources. Outsourced manufacturing raises doubt about product integrity, as anyone with access to the manufacturing process could introduce changes. For complex products, small changes with large effects can be difficult to detect. The threat is especially relevant to government agencies in military, finance, energy, and political sectors.
A hardware trojan can be characterized by its physical representation, activation phase, and action phase. Physical characteristics include type (functional—adding or deleting transistors/gates; or parametric—modifying circuitry such as thinning wires or weakening flip-flops), size (physical extension or number of components), and distribution (loose or tight). Activation can be internal (monitoring signals, countdown logic) or external (using antenna or sensors). Always-on trojans, such as a reduced wire, produce errors or failures when the wire is used intensely. Combinational trojans monitor for a specific condition; sequential trojans search for sequences.
One paper published by IEEE in 2015 explains how a hardware design containing a trojan could leak a cryptographic key over an antenna or network connection, provided the correct 'easter egg' trigger is applied. Extraction of secret keys without detection may require the trojan to use a random signal or cryptographic implementation; a physical unclonable function can avoid storing a key in the trojan itself. Peripheral device hardware trojans, such as a USB keyboard, can hide malicious processing cycles and exfiltrate data via unintended USB channels or wireless.
Reader's Guide
Hardware trojans represent a significant threat to the integrity of integrated circuits, particularly in contexts where manufacturing is outsourced to untrustworthy factories. The article notes that anyone with access to the manufacturing process could introduce a change, and for complex products, small changes with large effects can be difficult to detect. This is especially relevant to government agencies in military, finance, energy, and political sectors, where resolving doubt about hardware integrity reduces technology vulnerabilities.
The legacy of hardware trojans is that they have driven the development of advanced detection techniques, including physical inspection methods such as scanning optical microscopy, scanning electron microscopy, pico-second imaging circuit analysis, voltage contrast imaging, light induced voltage alteration, and charge induced voltage alteration. Functional testing stimulates input ports and monitors outputs to detect manufacturing faults, though traditional testing techniques are insufficient because malicious changes are well placed to avoid detection. Trojans are stealthy, often passive most of the time, and activated by rare events to avoid detection. The article emphasizes that hardware trojans can disable, damage, or destroy an entire chip or its components, leak confidential information (e.g., by radio emission), or tear down the security and safety of a device. The threat extends to peripheral devices like keyboards and network equipment, which can be used to leak passwords or provide remote unauthorized entry.
Did You Know?
- A hardware trojan is completely characterized by its physical representation and its behavior.
- Hardware trojans may be inserted by a rogue employee acting on their own or on behalf of state-sponsored spying and espionage.
- One IEEE paper from 2015 explains how a trojan could leak a cryptographic key over an antenna or network connection using an 'easter egg' trigger.
- A USB keyboard hardware trojan can hide malicious processing cycles and exfiltrate data using unintended USB channels.
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