Tamperproofing is a methodology used to hinder, deter or detect unauthorised access to a device or circumvention of a security system. Since any device or system can be foiled by a person with sufficient knowledge, equipment, and time, the term "tamperproof" is a misnomer unless some limitations on the tampering party's resources is explicit or assumed.
Tamper resistance is resistance to intentional malfunction or sabotage by either the normal users of a product, package, or system or others with physical access to it. Tamper resistance ranges from simple features like screws with special drives and tamper-evident seals to more complex devices that render themselves inoperable or encrypt all data transmissions between individual chips, use of materials needing special tools and knowledge. Tamper-resistant devices or features are common on packages to deter package or product tampering or enable its detection. Anti-tamper devices have one or more components: tamper resistance, tamper detection, tamper response, and tamper evidence. In some applications, devices are only tamper-evident rather than tamper-resistant.
Tampering Tampering involves the deliberate altering or adulteration of a product, package, or system. Solutions may involve all phases of product production, packaging, distribution, logistics, sale, and use. No single solution can be considered as "tamper-proof". Often multiple levels of security need to be addressed to reduce the risk of tampering.
Some considerations might include:
Identify who a potential tamperer might be: average user, child, person under medical care, misguided joker, prisoner, saboteur, organized criminals, terrorists, corrupt government. What level of knowledge, materials, tools, etc. might they have? Identify all feasible methods of unauthorized access into a product, package, or system. In addition to the primary means of entry, also consider secondary or "back door" methods. Control or limit access to products or systems of interest. Improve the tamper resistance to make tampering more difficult, time-consuming, etc. Add tamper-evident features to help indicate the existence of tampering. Educate people to watch for evidence of tampering.
Methods
Mechanical Some devices contain non-standard screws or bolts in an attempt to deter access. Examples are telephone switching cabinets (which have triangular bolt heads that a hex socket fits), or bolts with 5-sided heads used to secure doors to outdoor electrical distribution transformers. A standard Torx screw head can be made in a tamper-resistant form with a pin in the center, which excludes standard Torx drivers. Various other security screw heads have been devised to discourage casual access to the interior of such devices as consumer electronics.
Electrical This style of tamper resistance is most commonly found in burglar alarms. Most trip devices (e.g. pressure pads, passive infrared sensors (motion detectors), door switches) use two signal wires that, depending on configuration, are normally open or normally closed. The sensors sometimes need power, so to simplify cable runs, multi-core cable is used. While 4 cores is normally enough for devices that require power (leaving two spare for those that don't), cable with additional cores can be used. These additional cores can be wired into a special so-called "tamper circuit" in the alarm system. Tamper circuits are monitored by the system to give an alarm if a disturbance to devices or wiring is detected. Enclosures for devices and control panels may be fitted with anti-tamper switches. Would-be intruders run the risk of triggering the alarm by attempting to circumvent a given device. Sensors such as movement detectors, tilt detectors, air-pressure sensors, light sensors, etc., which might be employed in some burglar alarms, might also be used in a bomb to hinder defusing.
Safety Nearly all appliances and accessories can only be opened with the use of a tool. This is intended to prevent casual or accidental access to energized or hot parts, or damage to the equipment. Manufacturers may use tamper-resistant screws, which cannot be unfastened with common tools. Tamper-resistant screws are used on electrical fittings in many public buildings to reduce tampering or vandalism that may cause a danger to others.
Warranties and support
A user who breaks equipment by modifying it in a way not intended by the manufacturer might deny they did it, in order to claim the warranty or (mainly in the case of PCs) call the helpdesk for help in fixing it. Tamper-evident seals may be enough to deal with this. However, they cannot easily be checked remotely, and many countries have statutory warranty terms that mean manufacturers may still have to service the equipment. Tamper proof screws will stop most casual users from tampering in the first place. In the US, the Magnuson-Moss Warranty Act prevents manufacturers from voiding warranties solely due to tampering. A warranty may be dishonored only if the tampering actually affected the part that has failed, and could have caused the failure.
Chips Tamper-resistant microprocessors are used to store and process private or sensitive information, such as private keys or electronic money credit. To prevent an attacker from retrieving or modifying the information, the chips are designed so that the information is not accessible through external means and can be accessed only by the embedded software, which should contain the appropriate security measures. Examples of tamper-resistant chips include all secure cryptoprocessors, such as the IBM 4758 and chips used in smartcards, as well as the Clipper chip. It has been argued that it is very difficult to make simple electronic devices secure against tampering, because numerous attacks are possible, including:
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