Design Principles for Tamper-Resistant Smartcard Processors

Oliver Kömmerling <ok@adsr.de>

Markus Kuhn <mgk25@cl.cam.ac.uk>

Abstract

We describe techniques for extracting protected software and data from smartcard processors. This includes manual microprobing, laser cutting, focused ion-beam manipulation, glitch attacks, and power analysis. Many of these methods have already been used to compromise widely-fielded conditional-access systems, and current smartcards offer little protection against them. We give examples of low-cost protection concepts that make such attacks considerably more difficult.

This paper will be published by the USENIX Association in the USENIX Workshop on Smartcard Technology Proceedings, Chicago, Illinois, USA, May 10-11, 1999.

1  Introduction

Smartcard piracy has become a common occurrence. Since around 1994, almost every type of smartcard processor used in European, and later also American and Asian, pay-TV conditional-access systems has been successfully reverse engineered. Compromised secrets have been sold in the form of illicit clone cards that decrypt TV channels without revenue for the broadcaster. The industry has had to update the security processor technology several times already and the race is far from over.

Smartcards promise numerous security benefits. They can participate in cryptographic protocols, and unlike magnetic stripe cards, the stored data can be protected against unauthorized access. However, the strength of this protection seems to be frequently overestimated.

In Section 2, we give a brief overview on the most important hardware techniques for breaking into smartcards. We aim to help software engineers without a background in modern VLSI test techniques in getting a realistic impression of how physical tampering works and what it costs. Based on our observations of what makes these attacks particularly easy, in Section 3 we discuss various ideas for countermeasures. Some of these we believe to be new, while others have already been implemented in products but are either not widely used or have design flaws that have allowed us to circumvent them.

2  Tampering Techniques

We can distinguish four major attack categories:

• Microprobing techniques can be used to access the chip surface directly, thus we can observe, manipulate, and interfere with the integrated circuit.
• Software attacks use the normal communication interface of the processor and exploit security vulnerabilities found in the protocols, cryptographic algorithms, or their implementation.
• Eavesdropping techniques monitor, with high time resolution, the analog characteristics of all supply and interface connections and any other electromagnetic radiation produced by the processor during normal operation.
• Fault generation techniques use abnormal environmental conditions to generate malfunctions in the processor that provide additional access.

All microprobing techniques are invasive attacks. They require hours or weeks in a specialized laboratory and in the process they destroy the packaging. The other three are non-invasive attacks. After we have prepared such an attack for a specific processor type and software version, we can usually reproduce it within seconds on another card of the same type. The attacked card is not physically harmed and the equipment used in the attack can usually be disguised as a normal smartcard reader.

Non-invasive attacks are particularly dangerous in some applications for two reasons. Firstly, the owner of the compromised card might not notice that the secret keys have been stolen, therefore it is unlikely that the validity of the compromised keys will be revoked before they are abused. Secondly, non-invasive attacks often scale well, as the necessary equipment (e.g., a small DSP board with special software) can usually be reproduced and updated at low cost.

The design of most non-invasive attacks requires detailed knowledge of both the processor and software. On the other hand, invasive microprobing attacks require very little initial knowledge and usually work with a similar set of techniques on a wide range of products. Attacks therefore often start with invasive reverse engineering, the results of which then help to develop cheaper and faster non-invasive attacks. We have seen this pattern numerous times on the conditional-access piracy market.

Non-invasive attacks are of particular concern in applications where the security processor is primarily required to provide tamper evidence, while invasive attacks violate the tamper-resistance characteristics of a card [fips]. Tamper evidence is of primary concern in applications such as banking and digital signatures, where the validity of keys can easily be revoked and where the owner of the card has already all the access that the keys provide anyway. Tamper resistance is of importance in applications such as copyright enforcement, intellectual property protection, and some electronic cash schemes, where the security of an entire system collapses as soon as a few cards are compromised.

To understand better which countermeasures are of practical value, we first of all have to understand the techniques that pirates have used so far to break practically all major smartcard processors on the market. In the next section, we give a short guided tour through a typical laboratory of a smartcard pirate.

2.1  Invasive Attacks

2.1.1  Depackaging of Smartcards

Invasive attacks start with the removal of the chip package. We heat the card plastic until it becomes flexible. This softens the glue and the chip module can then be removed easily by bending the card. We cover the chip module with 20-50 ml of fuming nitric acid heated to around 60 °C and wait for the black epoxy resin that encapsulates the silicon die to completely dissolve (Fig. 1). The procedure should preferably be carried out under very dry conditions, as the presence of water could corrode exposed aluminium interconnects. The chip is then washed with acetone in an ultrasonic bath, followed optionally by a short bath in deionized water and isopropanol. We remove the remaining bonding wires with tweezers, glue the die into a test package, and bond its pads manually to the pins (Fig. 2). Detailed descriptions of these and other preparation techniques are given in [Bec98] [LP93].

Figure 1: Hot fuming nitric acid (> 98% HNO₃) dissolves the package without affecting the chip.

Figure 2: The depackaged smartcard processor is glued into a test package, whose pins are then connected to the contact pads of the chip with fine aluminium wires in a manual bonding machine.

2.1.2  Layout Reconstruction

The next step in an invasive attack on a new processor is to create a map of it. We use an optical microscope with a CCD camera to produce several meter large mosaics of high-resolution photographs of the chip surface. Basic architectural structures, such as data and address bus lines, can be identified quite quickly by studying connectivity patterns and by tracing metal lines that cross clearly visible module boundaries (ROM, RAM, EEPROM, ALU, instruction decoder, etc.). All processing modules are usually connected to the main bus via easily recognizable latches and bus drivers. The attacker obviously has to be well familiar with CMOS VLSI design techniques and microcontroller architectures, but the necessary knowledge is easily available from numerous textbooks [WE93] [KL96] [Car96] [Sze85].

Figure 3: Left: CMOS AND gate imaged by a confocal microscope. Right: same gate after removal of metal layer (HF wet etching). Polysilicon interconnects and diffusion areas are now fully visible.

Photographs of the chip surface show the top metal layer, which is not transparent and therefore obscures the view on many structures below. Unless the oxide layers have been planarized, lower layers can still be recognized through the height variations that they cause in the covering layers. Deeper layers can only be recognized in a second series of photographs after the metal layers have been stripped off, which we achieve by submerging the chip for a few seconds in hydrofluoric acid (HF) in an ultrasonic bath [Bec98]. HF quickly dissolves the silicon oxide around the metal tracks and detaches them from the chip surface. HF is an extremely dangerous substance and safety precautions have to be followed carefully when handling it.

Figure 3 demonstrates an optical layout reconstruction of a NAND gate followed by an inverter. These images were taken with a confocal microscope (Zeiss Axiotron-2 CSM), which assigns different colors to different focal planes (e.g., metal=blue, polysilicon=green) and thus preserves depth information [CK96]. Multilayer images like those shown in Fig. 3 can be read with some experience almost as easily as circuit diagrams. These photographs help us in understanding those parts of the circuitry that are relevant for the planned attack.

If the processor has a commonly accessible standard architecture, then we have to reconstruct the layout only until we have identified those bus lines and functional modules that we have to manipulate to access all memory values. More recently, designers of conditional-access smartcards have started to add proprietary cryptographic hardware functions that forced the attackers to reconstruct more complex circuitry involving several thousand transistors before the system was fully compromised. However, the use of standard-cell ASIC designs allows us to easily identify logic gates from their diffusion area layout, which makes the task significantly easier than the reconstruction of a transistor-level netlist.

Some manufacturers use non-standard instruction sets and bus-scrambling techniques in their security processors. In this case, the entire path from the EEPROM memory cells to the instruction decoder and ALU has to be examined carefully before a successful disassembly of extracted machine code becomes possible. However, the attempts of bus scrambling that we encountered so far in smartcard processors were mostly only simple permutations of lines that can be spotted easily (Fig. 4).

Figure 4: The vias in this structure found in a ST16F48A form a permutation matrix between the memory readout column lines and the 16:1 demultiplexer. The applied mapping remains clearly visible.

Any good microscope can be used in optical VLSI layout reconstruction, but confocal microscopes have a number of properties that make them particularly suited for this task. While normal microscopes produce a blurred image of any plane that is out of focus, in confocal scanning optical microscopes, everything outside the focal plane just becomes dark [CK96]. Confocal microscopes also provide better resolution and contrast. A chromatic lens in the system can make the location of the focal plane wavelength dependent, such that under white light different layers of the chip will appear simultaneously, but in different colors.

Automatic layout reconstruction has been demonstrated with scanning electron microscopy [Bly93]. We consider confocal microscopy to be an attractive alternative, because we do not need a vacuum environment, the depth information is preserved, and the option of oil immersion allows the hiding of unevenly removed oxide layers. With UV microscopy, even chip structures down to 0.1 µm can be resolved.

With semiautomatic image-processing methods, significant portions of a processor can be reverse engineered within a few days. The resulting polygon data can then be used to automatically generate transistor and gate-level netlists for circuit simulations.

Figure 5: The data of this NOR ROM becomes clearly visible when the covering metal and polysilicon access lines plus the surrounding field oxide have been removed (HF wet etching). The image shows 16×10 bits in an ST16xyz. Every bit is represented by either a present or missing diffusion layer connection.

Figure 6: The implant-mask layout of a NAND ROM can be made visible by a dopant-selective crystallographic etch (Dash etchand [Bec98]). This image shows 16×14 bits plus parts of the row selector of a ROM found on an MC68HC05SC2x CPU. The threshold voltage of 0-bit p-channel transistors (stained dark here) was brought below 0 V through ion implantation.

Optical reconstruction techniques can also be used to read ROM directly. The ROM bit pattern is stored in the diffusion layer, which leaves hardly any optical indication of the data on the chip surface. We have to remove all covering layers using HF wet etching, after which we can easily recognize the rims of the diffusion regions that reveal the stored bit pattern (Fig. 5).

Some ROM technologies store bits not in the shape of the active area but by modifying transistor threshold voltages. In this case, additional dopant-selective staining techniques have to be applied to make the bits visible (Fig. 6). Together with an understanding of the (sometimes slightly scrambled, see Fig. 4) memory-cell addressing, we obtain disassembler listings of the entire ROM content. Again, automated processing techniques can be used to extract the data from photos, but we also know cases where an enthusiastic smartcard hacker has reconstructed several kilobytes of ROM manually.

While the ROM usually does not contain any cryptographic key material, it does often contain enough I/O, access control, and cryptographic routines to be of use in the design of a non-invasive attack.

2.1.3  Manual Microprobing

The most important tool for invasive attacks is a microprobing workstation. Its major component is a special optical microscope (e.g., Mitutoyo FS-60) with a working distance of at least 8 mm between the chip surface and the objective lens. On a stable platform around a socket for the test package, we install several micropositioners (e.g., from Karl Suss, Micromanipulator, or Wentworth Labs), which allow us to move a probe arm with submicrometer precision over a chip surface. On this arm, we install a ``cat whisker'' probe (e.g., Picoprobe T-4-10). This is a metal shaft that holds a 10 µm diameter and 5 mm long tungsten-hair, which has been sharpened at the end into a <0.1 µm tip. These elastic probe hairs allow us to establish electrical contact with on-chip bus lines without damaging them. We connect them via an amplifier to a digital signal processor card that records or overrides processor signals and also provides the power, clock, reset, and I/O signals needed to operate the processor via the pins of the test package.

On the depackaged chip, the top-layer aluminium interconnect lines are still covered by a passivation layer (usually silicon oxide or nitride), which protects the chip from the environment and ion migration. On top of this, we might also find a polyimide layer that was not entirely removed by HNO₃ but which can be dissolved with ethylendiamine. We have to remove the passivation layer before the probes can establish contact. The most convenient depassivation technique is the use of a laser cutter (e.g., from New Wave Research).

The UV or green laser is mounted on the camera port of the microscope and fires laser pulses through the microscope onto rectangular areas of the chip with micrometer precision. Carefully dosed laser flashes remove patches of the passivation layer. The resulting hole in the passivation layer can be made so small that only a single bus line is exposed (Fig. 7). This prevents accidental contacts with neighbouring lines and the hole also stabilizes the position of the probe and makes it less sensitive to vibrations and temperature changes.

Figure 7: This image shows 9 horizontal bus lines on a depackaged smartcard processor. A UV laser (355 nm, 5 ns) was used to remove small patches of the passivation layer over the eight data-bus lines to provide for microprobing access.

Complete microprobing workstations cost tens of thousands of dollars, with the more luxurious versions reaching over a hundred thousand US$. The cost of a new laser cutter is roughly in the same region.

Low-budget attackers are likely to get a cheaper solution on the second-hand market for semiconductor test equipment. With patience and skill it should not be too difficult to assemble all the required tools for even under ten thousand US$ by buying a second-hand microscope and using self-designed micropositioners. The laser is not essential for first results, because vibrations in the probing needle can also be used to break holes into the passivation.

2.1.4  Memory Read-out Techniques

It is usually not practical to read the information stored on a security processor directly out of each single memory cell, except for ROM. The stored data has to be accessed via the memory bus where all data is available at a single location. Microprobing is used to observe the entire bus and record the values in memory as they are accessed.

It is difficult to observe all (usually over 20) data and address bus lines at the same time. Various techniques can be used to get around this problem. For instance we can repeat the same transaction many times and use only two to four probes to observe various subsets of the bus lines. As long as the processor performs the same sequence of memory accesses each time, we can combine the recorded bus subset signals into a complete bus trace. Overlapping bus lines in the various recordings help us to synchronize them before they are combined.

In applications such as pay-TV, attackers can easily replay some authentic protocol exchange with the card during a microprobing examination. These applications cannot implement strong replay protections in their protocols, because the transaction counters required to do this would cause an NVRAM write access per transaction. Some conditional-access cards have to perform over a thousand protocol exchanges per hour and EEPROM technology allows only 10⁴-10⁶ write cycles during the lifetime of a storage cell. An NVRAM transaction counter would damage the memory cells, and a RAM counter can be reset by the attacker easily by removing power. Newer memory technologies such as FERAM allow over 10⁹ write cycles, which should solve this problem.

Just replaying transactions might not suffice to make the processor access all critical memory locations. For instance, some banking cards read critical keys from memory only after authenticating that they are indeed talking to an ATM. Pay-TV card designers have started to implement many different encryption keys and variations of encryption algorithms in every card, and they switch between these every few weeks. The memory locations of algorithm and key variations are not accessed by the processor before these variations have been activated by a signed message from the broadcaster, so that passive monitoring of bus lines will not reveal these secrets to an attacker early.

Sometimes, hostile bus observers are lucky and encounter a card where the programmer believed that by calculating and verifying some memory checksum after every reset the tamper-resistance could somehow be increased. This gives the attacker of course easy immediate access to all memory locations on the bus and simplifies completing the read-out operation considerably. Surprisingly, such memory integrity checks were even suggested in the smartcard security literature [Mah97], in order to defeat a proposed memory rewrite attack technique [AK97]. This demonstrates the importance of training the designers of security processors and applications in performing a wide range of attacks before they start to design countermeasures. Otherwise, measures against one attack can far too easily backfire and simplify other approaches in unexpected ways.

In order to read out all memory cells without the help of the card software, we have to abuse a CPU component as an address counter to access all memory cells for us. The program counter is already incremented automatically during every instruction cycle and used to read the next address, which makes it perfectly suited to serve us as an address sequence generator [AK96]. We only have to prevent the processor from executing jump, call, or return instructions, which would disturb the program counter in its normal read sequence. Tiny modifications of the instruction decoder or program counter circuit, which can easily be performed by opening the right metal interconnect with a laser, often have the desired effect.

2.1.5  Particle Beam Techniques

Most currently available smartcard processors have feature sizes of 0.5-1 µm and only two metal layers. These can be reverse-engineered and observed with the manual and optical techniques described in the previous sections. For future card generations with more metal layers and features below the wavelength of visible light, more expensive tools additionally might have to be used.

A focused ion beam (FIB) workstation consists of a vacuum chamber with a particle gun, comparable to a scanning electron microscope (SEM). Gallium ions are accelerated and focused from a liquid metal cathode with 30 kV into a beam of down to 5-10 nm diameter, with beam currents ranging from 1 pA to 10 nA. FIBs can image samples from secondary particles similar to a SEM with down to 5 nm resolution. By increasing the beam current, chip material can be removed with the same resolution at a rate of around 0.25 µm³nA^-1s^-1 [DMW98]. Better etch rates can be achieved by injecting a gas like iodine via a needle that is brought to within a few hundred micrometers from the beam target. Gas molecules settle down on the chip surface and react with removed material to form a volatile compound that can be pumped away and is not redeposited. Using this gas-assisted etch technique, holes that are up to 12 times deeper than wide can be created at arbitrary angles to get access to deep metal layers without damaging nearby structures. By injecting a platinum-based organo-metallic gas that is broken down on the chip surface by the ion beam, platinum can be deposited to establish new contacts. With other gas chemistries, even insulators can be deposited to establish surface contacts to deep metal without contacting any covering layers.

Using laser interferometer stages, a FIB operator can navigate blindly on a chip surface with 0.15 µm precision, even if the chip has been planarized and has no recognizable surface structures. Chips can also be polished from the back side down to a thickness of just a few tens of micrometers. Using laser-interferometer navigation or infrared laser imaging, it is then possible to locate individual transistors and contact them through the silicon substrate by FIB editing a suitable hole. This rear-access technique has probably not yet been used by pirates so far, but the technique is about to become much more commonly available and therefore has to be taken into account by designers of new security chips.

FIBs are used by attackers today primarily to simplify manual probing of deep metal and polysilicon lines. A hole is drilled to the signal line of interest, filled with platinum to bring the signal to the surface, where a several micrometer large probing pad or cross is created to allow easy access (Fig. 11). Modern FIB workstations (for example the FIB 200xP from FEI) cost less than half a million US$ and are available in over hundred organizations. Processing time can be rented from numerous companies all over the world for a few hundred dollars per hour.

Another useful particle beam tool are electron-beam testers (EBT) [Feu82]. These are SEMs with a voltage-contrast function. Typical acceleration voltages and beam currents for the primary electrons are 2.5 kV and 5 nA. The number and energy of secondary electrons are an indication of the local electric field on the chip surface and signal lines can be observed with submicrometer resolution. The signal generated during e-beam testing is essentially the low-pass filtered product of the beam current multiplied with a function of the signal voltage, plus noise. EBTs can measure waveforms with a bandwidth of several gigahertz, but only with periodic signals where stroboscopic techniques and periodic averaging can be used. If we use real-time voltage-contrast mode, where the beam is continuously directed to a single spot and the blurred and noisy stream of secondary electrons is recorded, then the signal bandwidth is limited to a few megahertz [Feu82]. While such a bandwidth might just be sufficient for observing a single signal line in a 3.5 MHz smartcard, it is too low to observe an entire bus with a sample frequency of several megahertz for each line.

EBTs are very convenient attack tools if the clock frequency of the observed processor can be reduced below 100 kHz to allow real-time recording of all bus lines or if the processor can be forced to generate periodic signals by continuously repeating the same transaction during the measurement.

2.2  Non-invasive Attacks

A processor is essentially a set of a few hundred flipflops (registers, latches, and SRAM cells) that define its current state, plus combinatorial logic that calculates from the current state the next state during every clock cycle. Many analog effects in such a system can be used in non-invasive attacks. Some examples are:

• Every transistor and interconnection have a capacitance and resistance that, together with factors such as the temperature and supply voltage, determine the signal propagation delays. Due to production process fluctuations, these values can vary significantly within a single chip and between chips of the same type.
• A flipflop samples its input during a short time interval and compares it with a threshold voltage derived from its power supply voltage. The time of this sampling interval is fixed relative to the clock edge, but can vary between individual flipflops.
• The flipflops can accept the correct new state only after the outputs of the combinatorial logic have stabilized on the prior state.
• During every change in a CMOS gate, both the p- and n-transistors are open for a short time, creating a brief short circuit of the power supply lines [Vee84]. Without a change, the supply current remains extremely small.
• Power supply current is also needed to charge or discharge the load capacitances when an output changes.
• A normal flipflop consists of two inverters and two transmission gates (8 transistors). SRAM cells use only two inverters and two transistors to ground one of the outputs during a write operation. This saves some space but causes a significant short-circuit during every change of a bit.

There are numerous other effects. During careful security reviews of processor designs it is often necessary to perform detailed analog simulations and tests and it is not sufficient to just study a digital abstraction.

Smartcard processors are particularly vulnerable to non-invasive attacks, because the attacker has full control over the power and clock supply lines. Larger security modules can be equipped with backup batteries, electromagnetic shielding, low-pass filters, and autonomous clock signal generators to reduce many of the risks to which smartcard processors are particularly exposed.

2.2.1  Glitch Attacks

In a glitch attack, we deliberately generate a malfunction that causes one or more flipflops to adopt the wrong state. The aim is usually to replace a single critical machine instruction with an almost arbitrary other one. Glitches can also aim to corrupt data values as they are transferred between registers and memory. Of the many fault-induction attack techniques on smartcards that have been discussed in the recent literature [AK97] [AK96] [BDL97] [B+97] [JQBD97], it has been our experience that glitch attacks are the ones most useful in practical attacks.

We are currently aware of three techniques for creating fairly reliable malfunctions that affect only a very small number of machine cycles in smartcard processors: clock signal transients, power supply transients, and external electrical field transients.

Particularly interesting instructions that an attacker might want to replace with glitches are conditional jumps or the test instructions preceding them. They create a window of vulnerability in the processing stages of many security applications that often allows us to bypass sophisticated cryptographic barriers by simply preventing the execution of the code that detects that an authentication attempt was unsuccessful. Instruction glitches can also be used to extend the runtime of loops, for instance in serial port output routines to see more of the memory after the output buffer [AK96], or also to reduce the runtime of loops, for instance to transform an iterated cipher function into an easy to break single-round variant [AK97].

Clock-signal glitches are currently the simplest and most practical ones. They temporarily increase the clock frequency for one or more half cycles, such that some flipflops sample their input before the new state has reached them. Although many manufacturers claim to implement high-frequency detectors in their clock-signal processing logic, these circuits are often only simple-minded filters that do not detect single too short half-cycles. They can be circumvented by carefully selecting the duty cycles of the clock signal during the glitch.

In some designs, a clock-frequency sensor that is perfectly secure under normal operating voltage ignores clock glitches if they coincide with a carefully designed power fluctuation. We have identified clock and power waveform combinations for some widely used processors that reliably increment the program counter by one without altering any other processor state. An arbitrary subsequence of the instructions found in the card can be executed by the attacker this way, which leaves very little opportunity for the program designer to implement effective countermeasures in software alone.

Power fluctuations can shift the threshold voltages of gate inputs and anti-tampering sensors relative to the unchanged potential of connected capacitances, especially if this occurs close to the sampling time of the flipflops. Smartcard chips do not provide much space for large buffer capacitors, and voltage threshold sensors often do not react to very fast transients.

In a potential alternative glitch technique that we have yet to explore fully, we place two metal needles on the card surface, only a few hundred micrometers away from the processor. We then apply spikes of a few hundred volts for less than a microsecond on these needles to generate electrical fields in the silicon substrate of sufficient strength to temporarily shift the threshold voltages of nearby transistors.

2.2.2  Current Analysis

Using a 10-15 Ohm resistor in the power supply, we can measure with an analog/digital converter the fluctuations in the current consumed by the card. Preferably, the recording should be made with at least 12-bit resolution and the sampling frequency should be an integer multiple of the card clock frequency.

Drivers on the address and data bus often consist of up to a dozen parallel inverters per bit, each driving a large capacitive load. They cause a significant power-supply short circuit during any transition. Changing a single bus line from 0 to 1 or vice versa can contribute in the order of 0.5-1 mA to the total current at the right time after the clock edge, such that a 12-bit ADC is sufficient to estimate the number of bus bits that change at a time. SRAM write operations often generate the strongest signals. By averaging the current measurements of many repeated identical transactions, we can even identify smaller signals that are not transmitted over the bus. Signals such as carry bit states are of special interest, because many cryptographic key scheduling algorithms use shift operations that single out individual key bits in the carry flag. Even if the status-bit changes cannot be measured directly, they often cause changes in the instruction sequencer or microcode execution, which then cause a clear change in the power consumption.

The various instructions cause different levels of activity in the instruction decoder and arithmetic units and can often be quite clearly distinguished, such that parts of algorithms can be reconstructed. Various units of the processor have their switching transients at different times relative to the clock edges and can be separated in high-frequency measurements.

3  Countermeasures

3.1  Randomized Clock Signal

Many non-invasive techniques require the attacker to predict the time at which a certain instruction is executed. A strictly deterministic processor that executes the same instruction c clock cycles after each reset - if provided with the same input at every cycle - makes this easy. Predictable processor behaviour also simplifies the use of protocol reaction times as a covert channel.

The obvious countermeasure is to insert random-time delays between any observable reaction and critical operations that might be subject to an attack. If the serial port were the only observable channel, then a few random delay routine calls controlled by a hardware noise source would seem sufficient. However, since attackers can use cross-correlation techniques to determine in real-time from the current fluctuations the currently executed instruction sequence, almost every instruction becomes an observable reaction, and a few localized delays will not suffice.

We therefore strongly recommend introducing timing randomness at the clock-cycle level. A random bit-sequence generator that is operated with the external clock signal should be used to generate an internal clock signal. This will effectively reduce the clock frequency by a factor of four, but most smartcards anyway reduce internally the 3.5 MHz provided for contact cards and the 13 MHz provided for contact-less cards.

Hardware random bit generators (usually the amplified thermal noise of transistors) are not always good at producing uniform output statistics at high bit rates, therefore their output should be smoothed with an additional simple pseudo-random bit generator.

The probability that n clock cycles have been executed by a card with a randomized clock signal after c clock cycles have been applied can be described as a binomial distribution:

So for instance after we have sent 1000 clock cycles to the smartcard, we can be fairly sure (probability >1-10^-9) that between 200 and 300 of them have been executed. This distribution can be used to verify that safety margins for timing-critical algorithms - such as the timely delivery of a pay-TV control word - are met with sufficiently high probability.

Only the clock signals of circuitry such as the serial port and timer need to be supplied directly with the external clock signal, all other processor parts can be driven from the randomized clock.

A lack of switching transients during the inactive periods of the random clock could allow the attacker to reconstruct the internal clock signal from the consumed current. It is therefore essential that the processor shows a characteristic current activity even during the delay phases of the random clock. This can be accomplished by driving the bus with random values or by causing the microcode to perform a write access to an unused RAM location while the processor is inactive.

3.2  Randomized Multithreading

To introduce even more non-determinism into the execution of algorithms, it is conceivable to design a multithreaded processor architecture [Moo96] that schedules the processor by hardware between two or more threads of execution randomly at a per-instruction level. Such a processor would have multiple copies of all registers (accumulator, program counter, instruction register, etc.), and the combinatorial logic would be used in a randomly alternating way to progress the execution state of the threads represented by these respective register sets.

The simple 8-bit microcontrollers of smartcards do not feature pipelines and caches and the entire state is defined only by a very small number of registers that can relatively easily be duplicated. The only other necessary addition would be new machine instructions to fork off the other thread(s) and to synchronize and terminate them. Multithreaded applications could interleave some of the many independent cryptographic operations needed in security protocols. For the remaining time, the auxiliary threads could just perform random encryptions in order to generate an realistic current pattern during the delay periods of the main application.

3.3  Robust Low-frequency Sensor

Bus-observation by e-beam testing becomes much easier when the processor can be clocked with only a few kilohertz, and therefore a low-frequency alarm is commonly found on smartcard processors. However, simple high-pass or low-pass RC elements are not sufficient, because by carefully varying the duty cycle of the clock signal, we can often prevent the activation of such detectors. A good low-frequency sensor must trigger if no clock edge has been seen for longer than some specified time limit (e.g., 0.5 µs). In this case, the processor must not only be reset immediately, but all bus lines and registers also have to be grounded quickly, as otherwise the values on them would remain visible sufficiently long for a voltage-contrast scan.

Even such carefully designed low-frequency detectors can quite easily be disabled by laser cutting or FIB editing the RC element. To prevent such simple tampering, we suggest that an intrinsic self-test be built into the detector. Any attempt to tamper with the sensor should result in the malfunction of the entire processor. We have designed such a circuit that tests the sensor during a required step in the normal reset sequence. External resets are not directly forwarded to the internal reset lines, but only cause an additional frequency divider to reduce the clock signal. This then activates the low-frequency detector, which then activates the internal reset lines, which finally deactivate the divider. The processor has now passed the sensor test and can start normal operation. The processor is designed such that it will not run after a power up without a proper internal reset. A large number of FIB edits would be necessary to make the processor operational without the frequency sensor being active.

Other sensor defenses against invasive attacks should equally be embedded into the normal operation of the processor, or they will easily be circumvented by merely destroying their signal or power supply connections.

3.4  Destruction of Test Circuitry

Microcontroller production has a yield of typically around 95%, so each chip has to be thoroughly tested after production. Test engineers - like microprobing attackers - have to get full access to a complex circuit with a small number of probing needles. They add special test circuitry to each chip, which is usually a parallel/serial converter for direct access to many bus and control lines. This test logic is accessible via small probing pads or multiplexed via the normal I/O pads. On normal microcontrollers, the test circuitry remains fully intact after the test. In smartcard processors, it is common practice to blow polysilicon fuses that disable access to these test circuits (Fig. 8). However, attackers have been able to reconnect these with microprobes or FIB editing, and then simply used the test logic to dump the entire memory content.

Figure 8: The interrupted white line at the bottom of the cavity in this FIB secondary-electron image is a blown polysilicon fuse next to a test pad (MC68HC05SC2x processor).

Therefore, it is essential that any test circuitry is not only slightly disabled but structurally destroyed by the manufacturer. One approach is to place the test interface for chip n onto the area of chip n+1 on the wafer, such that cutting the wafer into dies severs all its parallel connections. A wafer saw usually removes a 80-200 µm wide area that often only contains a few process control transistors. Locating essential parts of the test logic in these cut areas would eliminate any possibility that even substantial FIB edits could reactivate it.

3.5  Restricted Program Counter

Abusing the program counter as an address pattern generator significantly simplifies reading out the entire memory via microprobing or e-beam testing.

Separate watchdog counters that reset the processor if no jump, call, or return instruction is executed for a number of cycles would either require many transistors or are too easily disabled.

Instead, we recommend simply not providing a program counter that can run over the entire address space. A 16-bit program counter can easily be replaced with the combination of a say 7-bit offset counter O and a 16-bit segment register S, such that the accessed address is S+O. Instead of overflowing, the offset counter resets the processor after reaching its maximum value. Every jump, call, or return instruction writes the destination address into S and resets O to zero. The processor will now be completely unable to execute more than 127 bytes of machine code without a jump, and no simple FIB edit will change this. A simple machine-code postprocessor must be used by the programmer to insert jumps to the next address wherever unconditional branches are more than 127 bytes apart.

With the program counter now being unavailable, attackers will next try to increase the number of iterations in software loops that read data arrays from memory to get access to all bytes. This can for instance be achieved with a microprobe that performs a glitch attack directly on a bus-line. Programmers who want to use 16-bit counters in loops should keep this in mind.

3.6  Top-layer Sensor Meshes

Additional metallization layers that form a sensor mesh above the actual circuit and that do not carry any critical signals remain one of the more effective annoyances to microprobing attackers. They are found in a few smartcard CPUs such as the ST16SF48A or in some battery-buffered SRAM security processors such as the DS5002FPM and DS1954.

A sensor mesh in which all paths are continuously monitored for interruptions and short-circuits while power is available prevents laser cutter or selective etching access to the bus lines. Mesh alarms should immediately trigger a countermeasure such as zeroizing the non-volatile memory. In addition, such meshes make the preparation of lower layers more difficult, because since the etch progresses unevenly through them, their pattern remains visible in the layers below and therefore they complicate automatic layout reconstruction. Finally, a mesh on top of a polished oxide layer hides lower layers, which makes navigation on the chip surface for probing and FIB editing more tedious.

Figure 9: Escape route for imprisoned crypto bits: The ST16SF48A designers generously added this redundant extension of the data bus several micrometers beyond the protected mesh area, providing easy probing access.

Figure 10: Every second line is connected to VCC or GND at one end and open at the other. Not all are used to supply lower layers and therefore some can safely be opened with a laser for probing access to the bus lines below.

The implementations of sensor meshes in fielded products however show a number of quite surprising design flaws that significantly reduce the protection (Fig. 9 and 10). The most significant flaw is that a mesh breach will only set a flag in a status register and that zeroization of the memory is left completely to the application software. We noted in Section 2.1.4 that a common read-out technique involves severely disabling the instruction decoder, therefore software checks for invasive attacks are of little use.

Figure 11: A FIB was used here to drill a fine hole to a bus line through the gap between two sensor mesh lines, refill it with metal, and place a metal cross on top for easy microprobing access.

A well-designed mesh can make attacks by manual microprobing alone rather difficult, and more sophisticated FIB editing procedures will be required to bypass it. Several techniques can be applied here. The resolution of FIB drilling is much smaller than the mesh line spacings, therefore it is no problem to establish contact through three or more metal layers and make deeply buried signals accessible for microprobing via a platinum or tungsten pad on top of the passivation layer (Fig. 11). Alternatively, it is also possible to etch a larger window into the mesh and then reconnect the loose ends with FIB metal deposits around it.

4  Conclusion

We have presented a basis for understanding the mechanisms that make microcontrollers particularly easy to penetrate. With the restricted program counter, the randomized clock signal, and the tamper-resistant low-frequency sensor, we have shown some selected examples of low-cost countermeasures that we consider to be quite effective against a range of attacks.

There are of course numerous other more obvious countermeasures against some of the commonly used attack techniques which we cannot cover in detail in this overview. Examples are current regulators and noisy loads against current analysis attacks and loosely coupled PLLs and edge barriers against clock glitch attacks. A combination of these together with e-field sensors and randomized clocks or perhaps even multithreading hardware in new processor designs will hopefully make high-speed non-invasive attacks considerably less likely to succeed. Other countermeasures in fielded processors such as light and depassivation sensors have turned out to be of little use as they can be easily bypassed.

We currently see no really effective short-term protection against carefully planned invasive tampering involving focused ion-beam tools. Zeroization mechanisms for erasing secrets when tampering is detected require a continuous power supply that the credit-card form factor does not allow. The attacker can thus safely disable the zeroization mechanism before powering up the processor. Zeroization remains a highly effective tampering protection for larger security modules that can afford to store secrets in battery-backed SRAM (e.g., DS1954 or IBM 4758), but this is not yet feasible for the smartcard package.

5  Acknowledgements

The authors would like to thank Ross Anderson, Simon Moore, Steven Weingart, Matthias Brunner, Gareth Evans and others for useful and highly interesting discussions.

References

[fips]   FIPS PUB 140-1: Security Requirements for Cryptographic Modules. National Institute of Standards and Technology, U.S. Department of Commerce, 11 January 1994.

[Bec98]   F. Beck: Integrated Circuit Failure Analysis - A Guide to Preparation Techniques. John Wiley & Sons, 1998.

[LP93]   T.W. Lee, S.V. Pabbisetty (eds.): Microelectronic Failure Analysis, Desk Reference. 3rd edition, ASM International, Ohio, 1993, ISBN 0-87170-479-X.

[WE93]   N.H.E. Weste, K. Eshraghian: Principles of CMOS VLSI Design. Addison-Wesley, 1993.

[KL96]   S.-M. Kang, Y. Leblebici: CMOS Digital Integrated Circuits: Analysis and Design. McGraw-Hill, 1996.

[Car96]   J. Carter: Microprocessor Architecture and Microprogramming - A State-Machine Approach. Prentice-Hall, 1996.

[Sze85]   S.M. Sze: Semiconductor Devices - Physics and Technology. John Wiley & Sons, 1985.

[CK96]   T.R. Corle, G.S. Kino: Confocal Scanning Optical Microscopy and Related Imaging Systems. Academic Press, 1996.

[Bly93]   S. Blythe, et al.: Layout Reconstruction of Complex Silicon Chips. IEEE Journal of Solid-State Circuits, 28(2):138-145, February 1993.

[Mah97]   D.P. Maher: Fault Induction Attacks, Tamper Resistance, and Hostile Reverse Engineering in Perspective. In R. Hirschfeld (ed.): Financial Cryptography, FC '97, Proceedings, LNCS 1318, pp. 109-121, Springer-Verlag, 1997.

[AK97]   R.J. Anderson, M.G. Kuhn: Low Cost Attacks on Tamper Resistant Devices. In M. Lomas, et al. (eds.), Security Protocols, 5th International Workshop, LNCS 1361, pp. 125-136, Springer-Verlag, 1997

[AK96]   R.J. Anderson, M.G. Kuhn: Tamper Resistance - a Cautionary Note. In The Second USENIX Workshop on Electronic Commerce Proceedings, pp. 1-11, Oakland, California, 18-21 November 1996.

[DMW98]   J.H. Daniel, D.F. Moore, J.F. Walker: Focused Ion Beams for Microfabrication. Engineering Science and Education Journal, pp. 53-56, April 1998.

[Feu82]   H.P. Feuerbaum: Electron Beam Testing: Methods and Applications. Scanning, 5(1):14-24, 1982.

[Vee84]   H.J.M. Veendrick: Short-Circuit Dissipation of Static CMOS Circuitry and Its Impact on the Design of Buffer Circuits. IEEE Journal of Solid-State Circuits, 19(4):468-473, August 1984.

[BDL97]   D. Boneh, R.A. DeMillo, R.J. Lipton: On the Importance of Checking Cryptographic Protocols for Faults. In Advances in Cryptology - EUROCRYPT '97, LNCS 1233, pp. 37-51, Springer-Verlag, 1997.

[B+97]   F. Bao, et al.: Breaking Public Key Cryptosystems on Tamper Resistant Devices in the Presence of Transient Faults. In M. Lomas, et al. (eds.), Security Protocols, 5th International Workshop, LNCS 1361, pp. 115-124, Springer-Verlag, 1997.

[JQBD97]   M. Joye, J.-J. Quisquater, F. Bao, R.H. Deng: RSA-type Signatures in the Presence of Transient Faults. In Cryptography and Coding, LNCS 1355, pp. 155-160, Springer-Verlag, 1997.

[Moo96]   S.W. Moore: Multithreaded Processor Design. Kluwer Academic Publishers, 1996.