System Architecture Design

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pSHIELDSystem Architecture DesignPUintensifies in the case of embedded systems, which can have many design choices and constraints thatare not easily comprehensible.In the late 1990s, side-channel attacks were introduced. Side-channels attacks are a method ofcryptanalysis that focuses on the implementation characteristics of a cryptographic algorithm in order toderive its secret keys. This advancement bridged the gap between embedded systems, a common targetof such attacks, and cryptographers. Vendors became aware and concerned by this new form of attacks,while cryptographers focused on the specifics of the implementations, in order to advance theircryptanalysis techniques.4.2.3.1 Physical SecuritySecrecy is always a desirable property. In the case of cryptographic algorithms, the secret keys of thealgorithm must be stored, accessed, used and destroyed in a secure manner, in order to provide therequired security functions. This statement is often overlooked and design or implementation flaws resultto insecure cryptographic implementations. It is well-known that general purpose computing systems andoperating systems cannot provide enough protection mechanisms for cryptographic keys.Embedded systems are commonly used for implementing security functions. Since they are completesystems, they can perform the necessary cryptographic operations in a sealed and controlledenvironment. Tamper resistance refers to the ability of a system to resist to tampering attacks, i.e.,attempts to bypass its attack prevention mechanisms. Smart cards are a well-known example of tamperresistant embedded systems that are used for financial transactions and subscription-based serviceprovision.In many cases, embedded systems used for security-critical operations do not implement any tamperresistance mechanisms. Rather, a thin layer of obscurity is preferred, both for simplicity and performanceissues. However, as users become more interested in bypassing the security mechanisms of the system,the thin layer of obscurity is easily broken and the cryptographic keys are publicly exposed.Finally, an often neglected issue is a lifecycle-wide management of cryptographic systems. While a devicemay be withdrawn from operation, the data it has stored or processed over time may still need to beprotected. The security of keys that relies on the fact that only authorized personnel has access to thesystem may not be sufficient for the recycled device.4.2.3.2 Side-channel cryptanalysisUntil the middle 1990s, academic research on cryptography focused on the mathematical properties of thecryptographic algorithms. Paul Kocher was the first to present cryptanalysis attacks on implementations ofcryptographic algorithms, which were based on the implementation properties of a system. Kocherobserved that a cryptographic implementation of the RSA algorithm required varying amounts of time toencrypt a block of data depending on the secret key used. Careful analysis of the timing differences,allowed him to derive the secret key and he extended this method to other algorithms as well. This resultcame as a surprise, since the RSA algorithm has withstood years of mathematical cryptanalysis and wasconsidered secure. These findings revealed a new class of attacks on cryptographic algorithms. The termside-channel attacks (SCA), has been widely used to refer to this type of cryptanalysis, while the termsfault-based cryptanalysis, implementation cryptanalysis, active/passive hardware attacks, leakage attacksand others have been used also. Cryptographic algorithms acquired a new security dimension, that oftheir exact implementation. Cryptographers had previously focused on understanding the underlyingmathematical problems and prove or conjecture for the security of a cryptographic algorithm based on theabstract mathematical symbols. Now, in spite of the hard underlying mathematical problems to be solved,an implementation may be vulnerable and allow the extraction of secret keys or other sensitive material.Implementation vulnerabilities are of course not a new security concept. The new concept of SCA is thateven cryptographic algorithms that are otherwise considered secure can be also vulnerable to such faults.PUD2.3.2Issue 5 Page 56 of 122
pSHIELDSystem Architecture DesignPUThis observation is of significant importance, since cryptography is widely used as a major building blockfor security; if cryptographic algorithms can be driven insecure, the whole construction collapses.In the following, we present the classes of side-channel attacks and countermeasures that have beendeveloped. Embedded system vendors must study the attacks carefully, evaluate the associated risks fortheir environment, and ensure that appropriate countermeasures are implemented in their systems;furthermore, they must be prepared to adapt promptly to new techniques for deriving secrets from theirsystems.4.2.3.3 Side channel implementationsA side channel is any physical channel that can carry information from the operation of a device whileimplementing a cryptographic operation; such channels are not captured by the existing abstractmathematical models. The definition is quite broad and the inventiveness of attackers is noticeable.Timing differences, power consumption, electromagnetic emissions, acoustic noise and faults have beencurrently exploited for leaking information out of cryptographic systems. The channel realization can becategorized in three broad classes: physical or probing attacks, fault-induction or glitch attacks andemission attacks, like TEMPEST. We shortly review the first two classes:The side channels may seem unavoidable and a frightening threat. However, it should be stronglyemphasized that in most cases, reported attacks, both theoretical and practical, rely for their success onthe detailed knowledge of the platform under attack and the specific implementation of the cryptographicalgorithm.4.2.3.3.1 Fault induction techniquesDevices are always susceptible to erroneous computations or other kinds of faults for several reasons.Faulty computations are a known issue from space systems, because, in deep space, devices areexposed to radiation which can cause temporary or permanent bit flips, gate destruction, or otherproblems. Incomplete testing during manufacturing may allow imperfect designs from reaching the marketor in the case of device operation in conditions out of their specifications. Careful manipulation of thepower supply or the clock oscillator can also cause glitches in code execution by tricking the processor forexample to execute unknown instructions or bypass a control statement. Some researchers havequestioned the feasibility of fault-injection attacks on real systems. While fault injection may seem as anapproach that requires expensive and specialized equipment, there have been reports that fault injectioncan be achieved with low cost and readily available equipment.The combined time-space isolation problem is of significant importance in fault-induction attacks. Thespace isolation problem refers to isolation of the appropriate space (area) of the chip in which to introducethe fault. The space isolation problem has four parameters:• Macroscopic: the part of the chip where the fault can be injected. Possible answers can be one ormore of the following: main memory, address bus, system bus, register file• Bandwidth: the number of bits that can be affected. It may be possible to change just one bit ormultiple bits at once. The exact number of changed bits can be controllable (e.g., one) or follow arandom distribution• Granularity: the area where can the error occur. The attacker may drive the fault injection positionat a bit level or a wider area, such as a byte or a multi-byte area. The fault injected area can becovered by a single error or by multiple errors. How are these errors distributed with respect to thearea? They may focus around the mark or evenly distributed• Lifetime: the time duration of the fault. It may be a transient fault or a permanent fault. Forexample, a power glitch may cause a transient fault at a memory location, since the next time thelocation will be written, a new value will be correctly written. In contrast, a cell or gate destructionwill result in a permanent error, since the output bit will be stuck at 0 or 1, independently of theinputPUD2.3.2Issue 5 Page 57 of 122
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System Architecture Design

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