The word “backdoor” is not alien to everyone. It exists in a variety of social circles across industries, and in the design of chips, it has existed from the very beginning. The first public example comes from a film, “war game”, made in 1983, in which a young computer expert, whiz, used some kind of chip back door to hack into the united states nuclear arsenal。
In the real world, the back door of modern features was found in multics, which emerged in 1965, some 20 years before hollywood. At that time, the united states department of defense was attempting to develop a secure operating system, the product of which was multics。
“multics' first back door was discovered by the united states air force in the course of its assessment of the operating system,” mentioned by pankaj rohatgi, director of engineering of the password research department in rambus. The two pioneers, paul karger and roger shell, were both in the army at the time and were involved in the project and assigned to assess the security of multics. In doing so, they used a design language known as pn1 to design so-called “trap doors” to test systems and their security loopholes。
Karger and shell were hailed as fathers of modern computer security. Shell has created an orange book that provides the basis for computer security。
What's the back door?
The back door has existed for thousands of years. The trojan horse of the ancient greek war was the first example of a record, a name that is now being used for bad software. Another example of the back door is the ennig code code machine, which was encrypted and declassified in german military communications during the second world war. As of today, the term back door generally refers to any unauthorized access to electronic equipment. It can be a hidden entry point or an unrecorded form of access. In this discussion, the focus is on chips and their systems, as well as on some issues surrounding the back door of hardware。
The back door can be in many ways and not all motivated by evil. In fact, many of the issues known as the back door should be regarded as design defects that could be used to threaten the safety of the chip. Given the extreme complexity of today's large system-scale chips and the increasing use of third-party ips in many designs, it is also possible to construct a back door in additional circuits, allowing external control of a chip. That's the case with heartbleed。
“if i were an attacker, i would push for as many ip openings as possible,” mentor graphics’ vice president sergei leef said. “each ip has a test platform attached to it to prove that the ip is capable of carrying out what it says it can do. So if you have a usbip block, you can run it under a test platform alongside the ip, and you can prove if the ip is able to complete the usb controller. But what else can it do besides fulfilling its declared functions? If your verilog code exceeds 20,000 lines, basically no one can control the complexity of this level, so you can't judge whether it can perform functions other than those claimed. The open source design is the best place to add back doors, trap doors and wooden horses, which will never be discovered, regardless of size.”
Validation of bugs and contaminated ips may also not be fully tested. However, considering that even formal vips do not require all tests, validation ips usually do not identify any problems. Smart programmers can set up back doors in some boundaries and will not normally appear unless activated。
“some companies have tried to resolve backdoor problems by formalizing certification.” leef said. “we think this is futile, because you need to know what the backdoor is. If a back door is hidden somewhere, and you don't want the back door to be activated, there are many other ways to activate it. The only hope of finding a wooden horse is to monitor those functions that can identify different patterns of behaviour.”
He compared this approach to an analysis of traffic patterns on the highway, identifying which vehicles were moving too quickly, which were not in compliance with traffic rules and which were slower than normal traffic flows. Monitoring unusual behaviour is often easier than searching for back-door circuits, a strategy that has been applied for many years in the area of network security。
Good back door and bad back door
Not all back doors are demons. Some of the back doors were placed in the chip for reasons of reliability or ease of testing. “only every chip in the world has the capability to test,” richard newell, senior chief product architect at the soc product enterprise of america. “chip manufacturers need a method to test their chips to ensure their probabilities in order to sell high-quality products to customers.”
Test capability is an essential function for controlling product quality. An on-site upgrade capability to address internal errors in chips and prevent product obsolescence and upgrading is also essential. For all these reasons, it is quite necessary to leave some back doors behind. In the 1990s, pc manufacturers added some features to pc hardware so that they could still burn images of the company at the company's loading dock, thus saving the company's it sector thousands of hours。
However, the back door of the hardware can also be used to destroy the hardware or security mechanisms of the chip, thus manipulating the program and data. This is not the same thing as attacking programs at the software code level, which is what viruses and malware do. Of course, it is not that the hardware back door cannot be used to load viruses and malware, but rather that the hardware back door on the chip is usually used to change the solidware code within the chip。
“there are many types of back door”, rohatgi notes. “... Most notably, some unrelated processes are being run or additional codes are being inserted that are not part of normal programming. The more complex, the more difficult it is to detect. The best back door is binary. The back door in this form will never appear at the source code level. There is also the concept of putting the back door in as an encrypted realization.”
The back door, which is the encryption method, affects the output of the encryption chip and is not detected. This is the so-called kleptography, defined as “a science of safe and undetectable information theft”
In the kleptography attack, malicious developers used asymmetric encryption to encrypt the back door. In this way, encryption is used against encryption. No additional conduits are required to communicate with the world outside this problematic back door and encryption system and no additional data need be transmitted. Instead, such a back door is embedded directly into normal anticipated communications. Therefore, kleptogramy can be considered as a branch of encryption virology, applying encryption to malicious software。
Views and tactics are controversial
The back door allows remote access to equipment hardware. Manipulated codes allowing access to the back door are located within the solid area. However, the placement of back doors in chips has become a science and is highly controversial。
“it was definitely a bad idea to leave the back door,” ronald rivest, a research professor and password expert at the massachusetts institute of technology, asserted. “there is no support for such an act to leave behind.”
However, this in practice depends on who defines the back door。
It's not always true. Many people expect from time to time that, with or without internet connections, some can access their computers remotely to clear the virus or repair the start-up problem, or simply reset an device whose access data is at risk or forgotten. These requirements also exist for mobile devices, which is one of the main reasons for supporting the back door。
At the same time, low-cost manufacturing processes open up risk exposures for wrong back doors. There are hundreds of low-cost alternative factories around the world, and the ownership of many of them is unclear. This brings with it a demand throughout the industry to spend money to layer the chips produced from these proxy plants and to compare their network tables with those originally designed。
“some generation factories are trustworthy, others are not.” mentor's leef says. “many start-ups are attracted to the lowest possible prices. At the other end of the industry, large companies will send personnel to the workshop to look after crystal circle production.”
He added that, as new technologies developed, such risks could be minimized, for example, by filling their space in the form of duplicate circuits. Any rewriting of a false unit would alter the distribution of electrical properties and trigger alarms。
Even in the back door design, which is used for legitimate purposes, there are many poorly designed designs. “some access ports are poorly designed and manufacturers keep them open,” newell says. Other unsatisfactory designs include the use of a single factory code for all chips at all stages. “in such a case, once the factory code is calculated, the chip is no longer protected and all similar chips are then threatened.”
If a back door is to be designed inside the chip, it must be sufficiently secure to prevent access by unethical senior programmers. It's the golden rule of chip manufacturing. So if people keep their conduct and use the back door in a friendly manner, there is no problem. The use of the back door varies according to the chip visitor. Newell adds, “we lock the chip in encryption before it leaves the factory, and once it reaches the client, the client installs its own data on the chip, it should disable the plant key and use the user key for locking.” then the chip manufacturer can no longer access the chip。
However, that approach could also offer some concessions. For example, if chips are used in mission-critical applications such as some military hardware, failure analysis can no longer be performed once such access ports are locked. In such cases, one solution would be to allow the user to unlock the chip with its own key and restart the plant key. However, it is clear that this technology should be subject to very strict controls. If it falls into the wrong hands, it can have very dangerous consequences。
