In this course, we will study security and trust from the hardware perspective. Upon completing the course, students will understand the vulnerabilities in current digital system design flow and the physical attacks to these systems. They will learn that security starts from hardware design and be familiar with the tools and skills to build secure and trusted hardware.
Introduction to Side Channel Attacks
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En provenance du cours de University of Maryland, College Park
Hardware Security
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À partir de la leçon
Side Channel Attacks and Countermeasures
This week, we focus on side channel attacks (SCA). We will study in-depth the following SCAs: cache attacks, power analysis, timing attacks, scan chain attacks. We will also learn the available countermeasures from software, hardware, and algorithm design.
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Gang QuAssociate Professor
Electrical and Computer Engineering
Side channel attacks are perhaps the most successful attacks to modern
cryptographic systems for two main reasons.
First, they target the weakness of the implementation of the crypto-algorithms,
not the algorithms themselves.
Therefore, the mathematically sound algorithms,
can become vulnerable against side channel attacks.
Second, these attacks are non invasive.
Passive and to most of time will not leave any trace of attack.
They use the signals leaked from side channels during system's normal execution,
to reveal the secret information.
So it is hard to detect and catch such, such attacks.
This week, we will start a section of attacks.
We will start with what are set channels,
why they may mix information, how attackers can take advantage of this vuln,
vulnerabilities to launch side channel attacks.
We will discuss in details on four representative attacks.
Cache attacks, power analysis, including both sink power analysis,
and a differential power analysis.
Timing attacks, and scan chain attacks.
Of course, we will also talk about the counter measures from software,
hardware and algorithms.
Some background on electrical engineering will be helpful to understand
the mechanisms of information leak through different types of side channels.
To follow the example we're going to use, you need a good understanding of the con,
concept of modular exponentiation and
Montgomery reduction that we have to, learned last week.
Some basics of assembling programming will be helpful for a couple of examples, and
some entry level knowledge about the cache and memory structure are needed, too.
I'll try to cover this background as needed during the lecture.
Side channel attacks, normally have two phases, a marrying phase and
a data analysis phase.
During the first phase, the attacker will measure, will monitor
the systems physical characteristics when the system is running at the normal mode.
This physical, physical features can be power consumption, current, timing or
delay, electro magnetic ignition, acoustic information, optical information etc.
Then during the second phase the attacker will
perform data analysis on the collected side channel
data to determine the on-chip secret information of interest.
Normally, further measurement will be collected to conf,
confirm the deduced secret information.
Side channel attacks, are non
invasive because they do not require to open up the chip.
Some of the attacks do not need to have physical access to the chip either.
For example, the EM attacks and acoustic attacks.
Side channel attacks are also passive as the attacker will be
monitoring the normal operation of the chip.
However, there is the trained of combining side channel attack, with active attacks,
to improved efficiency and effectiveness of the attack.
For example, by doing input control, the attacker can put the chip in the normal
operation with his imputed data, and the attacker can use faulty
ejection techniques to force the system to run at some of normal conditions.
This will help the attacker to collect more valuable section information that
are not necess, are not easily available through a passive monitoring process.
There are many set channels that can be utilized by the attackers.
The power consumption and execution time during the normal operation.
The electromagnetic information, the optical, and
acoustic emission, as well as the system's output signals.
Next, we'll briefly discuss each of this.
First, the power and the current side channel is perhaps, that the, the side
channel that has been started extensively in the past twenty years or so.
Chips power consumption mainly comes from three sources.
The dynamic power, power that is needed to charge and a discharge the capacitors.
The leakage current that will occur even when the system is idle.
And short circuits and others.
Information may leak from dynamic power consumption, because it is
proportional to the effective capacitors of the switching activity.
A switching happens, when we change a logical 0 to a logical 1 or
logical 1 to a 0.
This will incur higher power consumption than the case when the logical value
remains at zero or one.
On the other hand, leakage current may leak information about the system because
the leakage current of a logical device is related to the input value to this device.
For example, for this two input nano gate
the leakage current on both input signals are one is about 454 nanoamp,
about 12 times higher than the leakage when the input vector is 00.
Naturally, timing or delay is related to the execution time of an operation.
The execution time of different operations may be different.
The execution time of the same operation with different operants under different
conditions may also be different.
This will give attackers some insight, about the system's internal information.
For example, the control flow will leak
data when different branches execute very different instructions.
Take this if-else statement, for example.
It will take longer time to perform the operation x equal to c minus
d in the false branch, than the operation x equal to 8 in the true branch.
If the attacker can manage to measure the execution of this portion of the code,
whether a and b have the same value can be deduced.
The data dependency nature of many operations can also leak information.
Consider a very simple multiplication operation, x times y and
assign the product to x.
The execution time when y equal to a number such as 190, will be
very different, in particular will be much longer than the case when y equal to zero,
y equal to one, or y equal to 64.
In the last case, because 64 is 2 to the power 6, so this multiplication
normally is implemented by a logical shift which takes much shorter time.
In addition monitoring execution time can also
reveal whether cache misses and pipeline stores have happened.
This may also leak the cache memory content,
in particularly, the cache memory content.
EM emissions originate from the acceleration of
charges near a conductor or an, antennas.
In the near field range, which is about two wavelengths from the antenna,
the electric and magnetic fields are dominated by EM waves.
However, their intensity drops quickly as the distance gets further away.
Why data may leak from EM emissions?
This is because they can modulate other on-chip signals through
near-field inductive and capacitive couplings.
This makes it possible to use the EM tracers as set
channel signals to these cover operations inside the chip.
Optical emissions come from the movement of hard
carriers including both electrons and holes.
When they move in a fat channel, they can cause a visible or
infrared light emission.
This can be captured by a charge-coupled device cameras and
used to help IC testing and the debugging.
If this information about that circuit can be used for testing and the debugging.
It is also possible for
an attacker to use the same information to extract information about the system.
This type of attack can be used to detect the data from all kinds hardware devices,
from microcontrollers and smartcards to FPGAs and ASICs.
Acoustic information has been used to attack hardware systems for
several decades.
When a key is entered from the keyboard, or
when certain system components are running, sound might be captured.
For example, in the 1950s, British intelligence agents determined,
the starting position of the Egyptian encryption machine by
listening to the resetting of the key wells.
In old spy movies we have seen spy movies we have phone numbers leaked from
the dial tone.
Text printed on certain dot matrix printers can also
be detected from the acoustic side channel information.
The reason for
information leak from acoustic channel is fairly straightforward.
The acoustic traces of different key being keyed on the computer are different.
Microphones can capture this and
other sound information through the system's components during execution.
For instance, it was reported that on a home-built PC with
RSA encryption implemented, the mic, a micro,
when a microphone is placed close by, it can capture the acoustic
information to tell when the RSA encryption algorithm is running.
Furthermore the runs of the RSA with different key
values give different acoustic traces.
It is speculated that,
the acoustic emission is the result of the piezoelectric properties of
the ceramic capacitors used on the mother board of this system.
This is related to the power and the current draw on the chip.
Finally, we talk about the scan chain sidechannel.
This is another perfect example to show how design
features that are introduced for good intention can make the system vulnerable.
This figure shows a core and a test, and
the scan channel built to facilitate the testing.
The original five D flip flops, have been modified and
connected by this chain through the Q port, of the,
of flip-flop to the input of the next flip-flop.
Testing value, testing control signal,
which is called the TC here can choose the input to the D flip-flop.
When TC equal to zero the signal coming from core and
the test, will be used to keep the system running at a normal mode.
However, when we change the TC value to one, the system changes to the testing
mode, and the testing input vector will come from the scanning port and
forge to each of the flip flops through this scan chain.
For example, if we want to tested the system with stage information zero, zero,
one, one, one from left to right, we can push the input vector one,
one, one zero zero, one bit at each clock cycle from the the scan in input report.
And this, at the end, these,
these values will go through the scan chain to the designated flip flops.
For example, the first bit one will travel through the first
scan flip flop to the second on the second clock cycle, and third one and
the fourth one and eventually reach the last flip flop.
After that, we can change the test control signal to zero to let the system
run at a normal mode, and then the result will be written back to the flip flop.
Now we change the test control signal back to, to one for testing mode and
then we can get, the results from each flip-flop from the scan out signal.
This is one of the most important inventions in the test.
It allows the tester to test any system stage at any time.
However, this scan flip-flops can be turned into side
channel that will leak the system stage information directly, from the sent out
signals when the attacker set the TC value to be one for the tasking mode.
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