Posts Tagged ‘Embedded Systems

Tamper Resistant Design: Countering Security Attacks

Tamper-resistant design techniques that has been proposed to strengthen embedded systems against the various attacks described in the previous section. In order to better understand and compare approaches to tamper-resistant design, we decompose the objective of tamper resistance into more specific, narrower objectives, as shown in Figure 1.

Figure 1: Specific objectives of tamper-resistant design approaches

  1. Attack prevention techniques make it more difficult to initiate an attack on the embedded system. These techniques can include physical protection mechanisms (e.g., packaging), hardware design (e.g., circuit implementations whose timing and power characteristics are data independent), and software design(e.g., software authentication before execution).
  2. In the event that an attack is launched despite any employed prevention techniques, attack detection techniques attempt to detect the attack as soon as possible. The elapsed time interval between the launch of an attack and its detection (the detection latency) represents a period of vulnerability, and needs to be kept as low as possible. An example of attack detection is the run-time detection of illegal memory accesses to secure data from an untrusted software application.
  3. Once an attack is detected, the embedded system needs to take appropriate action. Attack recovery refers to techniques used to ensure that the attack is countered, and that the system returns to secure operation. Attack recovery techniques could include locking up the system and rendering it useless for further operation,zeroing out sensitive data in memory, or displaying a security warning and rebooting the system. The design of attack recovery schemes involves tradeoffs between the level of security and the inconvenience caused to users in the usage of the system after an attack.
  4. In some cases, it may be desirable to preserve an irrefutable,persistent record of the attack in the embedded system, for inspection at a later time. Tamper evident design techniques target this objective. Analogies of physical tamper evident design mechanisms abound: seals that have to be broken, wires that have to be cut, or coatings that have to be removed. In all cases,tamper evidence requires a mechanism that cannot be reversed by malicious entities.

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Security a New Dimension in Embedded System Design

Embedded systems, which will be ubiquitously used to capture, store, manipulate, and access data of a sensitive nature, pose several unique and interesting security challenges. Security has been the subject of intensive research in the areas of cryptography, computing, and networking. However, security is often mis-construed by embedded system designers as the addition of features, such as specific cryptographic algorithms and security protocols, to the system. In reality, it is an entirely new metric that designers should consider throughout the design process, along with other metrics such as cost, performance, and power.security in one form or another is a requirement for an increasing number of embedded systems, ranging from low-end systems such as PDAs, wireless handsets, networked sensors, and smart cards, to high-end systems such as routers, gateways, firewalls, storage servers, and web servers. Technological advances that have spurred the development of these electronic systems have also ushered in seemingly parallel trends in the sophistication of security attacks. It has been observed that the cost of insecurity in electronic systems can be very high. For example, it was estimated that the “I Love You” virus caused nearly one billion dollars in lost revenues worldwide.
With an increasing proliferation of such attacks, it is not surprising that a large number of users in the mobile commerce world (nearly 52% of cell phone users and 47% of PDA users, according to a survey by Forrester Research) feel that security is the single largest concern preventing the successful deployment of next-generation mobile services. With the evolution of the Internet, information and communications security has gained significant attention. For example, various security protocols and standards such as IPSec, SSL, WEP, and WTLS, are used for secure communications. While security protocols and the cryptographic algorithms they contain address security considerations from a functional perspective, many embedded systems are constrained by the environments they operate in, and by the resources they possess. For such systems, there are several factors that are moving security considerations from a functioncentric perspective into a system architecture (hardware/software) design issue.

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Protecting Embedded-Systems Security

The key to preventing such disasters is to equip mobile devices with software that is secure by design. As the experience of the PC world shows, this is not easy — clearly, the standard of mobile device security must be much higher than what we areused to from the PC world. The problem is that the software driving mobile devices is becoming as complex as that of PCs, owing to the dramatic increase in functionality of such devices. Top-of-the line cell phones already run software that is composed of millions of lines of code (LOC), and top of-the-line cars contain in excess of a gigabyte of software.

Such large systems are impossible to make fault free. Experience shows that even well-engineered software averages at least one fault every few thousand lines of code (and well-engineered software is rare). This is made worse by the traditional approach to embedded-systems software, which tends to be built on top of a real-time executive without memory protection. In such a system every bug in any part of the system can cause a security violation.

In security terms, the part of a system that can circumvent security policies (and must therefore befully trusted) is called the trusted computing base (TCB). In a system without memory protection, the TCB is the complete system (of potentially millions of lines of code). Clearly, such a large TCB cannot be made trustworthy.

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Memory Management Unit

A memory mangement unit’s (MMU) main responsibility is to define and enforce the boundaries that separate different tasks or processes. To understand the effect of separating tasks, consider two different office environments. In the first one (the formal environment), there is an individual office for each employee. Each employee has a key to the door of his or her office and is considered the owner of that space. The second environment (the informal environment) uses a bullpen setup. There are no walls between employees, and everyone is on the same floor in the same air space. Each one of these configurations has its particular advantages and disadvantages. Inthe formal environment, you don’t have to worry about one employee bothering another employee because there are walls between them. No matter how noisy one employee gets, the neighboring worker does not hear the disturbance. This is not the case for an informal setup. If an employee is talking loudly on the phone in the open environment, this inconsiderate individual distracts the other workers. If, on the other hand, the employees are considerate, two employees can quickly communicate with each other without needing to leave their seats. Similarly, if one employee needs to borrow another employee’s stapler, it’s just a toss away. In the formal environment,each time communication needs to take place, the communicating employee needs to go through some series of steps, like making a phone call or walking to the other office.

Each employee can be compared to a block of code (or task) that is designed to perform some specified job in an embedded systems program. The air space equates to the memory space of the target system, and the noise generated by a misbehaving employee can be equated to a bug that corrupts the memory space used by someother task. The MMU in hardware, if configured properly, can be compared to the walls of the formal office environment. Each task is bounded by the limitations placed on it by the MMU, which means that a task that somehow has a bad pointer cannot corrupt the memory space of any other task. However, it also means that when one process wants to talk to another process, more overhead is required.

A few years ago, the full use of an MMU in an embedded system was rare. Even today, most embedded systems don’t fully use the capabilities of the MMU; however, as embedded systems become more and more complex and CPU speeds continue to rise, the use of an MMU to provide walls between processes is becoming more common.

 

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Hardware Attacks on Embedded Systems

The main goal of hardware attacks depends on the wish of the attacker. Two main opportunities can be targeted. The first one is trying to get secret information like cipher keys.The second one is to attack the system to turn it out of order (i.e. denial of service attack). Below attacks which aimto catch secrets are presented, then denial of service attacks are detailed. Some attacks are difficult to classify, hardware modification of the main memory is one of them. The goal of this attack is to insert a malicious program. A similar attack targets FPGAs through bit stream alteration.

When the attacker wants to decrypt information, he needs to have the cipher key. A solution to get cipher keys is to listen to side channels. This kind of attack is called side channel attack and is declined in several forms. The most known relies on the power signature of the algorithm. By analyzing the algorithm signature it is possible to infer the round of the algorithm. Moreover, a differential analysis combined with a statistic study of the power signature can lead to an extraction of the cipher key. However it is necessary to make assumptions on the value of the key to obtain a correct result. These two methods are called SAP: Simple Power Analysis and DPA: Differential Power Analysis. Similar solutions also work with electromagnetic emissions (Differential Electromagnetic Analysis). Instead of analyzing the power signature, the electromagnetic signature of the chip is analyzed. A significant remark concerns the cost of such attacks. It is especially cheaper than reverse engineering attack which needs an electronic microscope to study the structure.

Temporal analysis or timing attack is another way to catch cipher keys. Temporal reaction of the system leaks information which enables the extraction of cipher key or password. Like with the DPA, it is necessary to make assumptions concerning the information to be extracted. The knowledge of the algorithm, so the branch instructions in the program can also help to find a secret since a timing model of the algorithm can be established. Indeed, timing hypotheses can be done as the program running on the target is often known. Thus, thanks to statistic studies, information can be extracted. Fault injection is the last way to obtain secrets through side channel. However, like reverse engineering, the need of material is more important than previous attacks. The injectionof a fault into a system through a memory corresponds to a modification of a bit (laser or electromagnetic waves). The knowledge of the implementation of  the algorithm isan important point to determine a secret. In most cases the injection of a fault is done in the last round of an algorithm. The reason is that the mark of the fault is more visiblein the ciphered result.

The goal of the hardware attacks presented above, is to get secret information from the chip. Denial of service attacks are different and aim to put the system out of order. In autonomous embedded systems, power is an essential concern. It is one of the most important constraints on the system. As an example with a cell phone or a PDA, the attacker can perform a large number of requests which aim to activate the battery and to reduce the system lifetime. In wireless communication systems, another attack leads to solicit the transmitter antenna in order to have the same result as previously (lifetime reduction). Increasing the workload of a processor is also anissue to consume more battery. Indeed the workload is related to power consumption, so an assailant may try to force the processor to work harder. As a consequence the lifetime will be affected. Other ways can be used to put a system out of order. Taking the control of the temperature regulation system is a solution. Through the control of the regulation it is possible to increase the temperature and then to activate the overheat security mechanisms.The panel of attacks against a system is important and depends according to several parameters: goal, budget and nature of the system. Hardware attacks represent an important threat against embedded systems but software attacks are also becoming critical.

 

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XOM, a secure architecture in Embedded Systems

XOM is the acronym of eXecute Only Memory. XOM wishes to completely secure an architecture. Moreover XOM is supposed to be sure and claimed that hardware solutions are more efficient than the software ones. So XOM mostly relies on hardware mechanisms to ensure security. New primitives are provided within the OS in orderto handle key and signature manipulation. The name of the OS extension is XOMOS. The main features of XOM are : memory ciphering and hashing, data and program partitioning, interruption and context switching protection.

Each partition of the memory is associated with a secret key to decrypt its content (the session key in Figure 3). The session key is obtained with the XOM key table which establishes the connection between the session key and the secret keyof a specific partition of the memory. The secret key is also encrypted with an asymmetric encryption. The key (private keyin Figure 3) required for the asymmetric decryption is stored in the secure zone of the architecture. The use of the hash solution is classic. The signature result of the hash algorithm is compared with the original one to validate the integrity of the hashed message. In addition the data stored in cache memory is associated with an identifier. When a task wants to use adata, the identifier of the task must be the same as the data one, in that case it means the task is allowed to read and modify the data. This feature protects the system from malicious program which tries to get illegal information. XOM proposes hardware security primitives to protect cipher keys and hash signatures which are essential to guarantee the architecture durability.

Fig. 3. XOM architecture

The last point of the XOM solution concerns the preemption within the OS which has similarities with the management ofthe interruptions. The context must be saved. It is essential to store and to protect the context in order to fend off an attack who aims to change some register values. XOM ciphers and hashes the switching context which is interesting for a solution with an OS. XOMOS can be seen as an extension of a non-secure OS which brings new security primitives (ciphering and hashing).

All the protections added by the solution have a cost. The first one concerns the implementation of XOM in an existing OS. A work is necessary on the kernel to add the instructions which help for the use of the security primitives. All this work is invisible for the user of the kernel. A real overhead appears inthe cache management. The number of cache miss raises from 10 to 40%. It depends on the kernel operation. This raise is due to the information added in the cache to secure the data. Data are associated with the identifier of the task. It means some parts of the cache are used to store the identifier. The protection of the context switching also brings an increase of the number of cycles to store the context and to protect it.

 

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