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ECEN 426

Power Analysis

The following notes are from the book “Power Analysis” by Stefan Mangard

Smart Cards

  • most popular kind of cryptographic devices
  • most of them are equipped with a microcontroller
  • In 2003 - 1 billion smart cards
    2006 - 2 billion
    2018 - ?

#### Most of them are used in security- sensitive applications such as…

  • Telecommunications
  • Financial Services
  • Government Services
  • Pay TV
  • IoT (Internet of Things) ?

They are usually eqipped with software or hardware performing cryptographic algorithms

  • A common feature in RFID and USB tokens

Some Thoughts

  • Crypto devices are hardware that store keys and perfom crypto operations
  • Security of system security of cyrpto algorithm
    • one needs to consider the security of crypto devices
  • Attackers are going to steal the key in an unauthorized way.
  • Security of a crypto device should not reply on the secrecy of the implementation
    -> Security through obscurity

Attacks on cyrpto devices- categorization

  • Passive attacks
    • operated largely or entirely within the specification
    • secret key is revealed by observing physical properties (eg. power consumption, timing)
  • Active attacks
    • i/ps and operating environments are manipulated (tampered device)
    • device is operated abnormally
    • exploit the secret key to detect abnormal behavior

Another classification is based on the interface (logic + physical)

Some of them can be accessed easily or need special equipment

  • Invasive Attack
    • practically no limit on what you can do (can be destructive)
    • typically depackage the device, access the devoce through a probe station, laser cutters
    • very powerful!
  • Semi-Invasive Attack
    • may involve depackaging, but no electrical contact
    • typically used to read-out memory contents of the device without probing
    • goal is to induce faults using x-rays, electromagnetic fields, light, sound (!)
    • may not need expensive equipment
    • but relatively more effort than invasive because finding the right location to attack takes effort
  • Non-Invasive Attacks
    • only directly accessible interfaces are touched
    • no permanent alteration to the device -> no evidence of an attack
    • requires relatively inexpensive equipment
    • three major classes
      1. Power (This class!)
      2. Timing
      3. EM

Power Analysis Attacks

 - started in 1999 (check out Kocher's famous paper)
 - one of the most famous attacks
 - companies have specific positions

Power analysis attacks exploits the fact that instantaenous power depends upon the data being processed and operation being performed

Book logo

  • μc is executing AES (software version)
  • encrypts plaintext from PC and send back the ciphertext
  • 1 Ω resistor to measure the current (through the voltage drop)

Book logo

  • First .35 ms, quite uniform
    -> μc is waiting for plaintext
  • After getting plaintest 9 fullround AES is performed
    -> each round is approximately .4 ms
    -> notice the negative peak
  • 10 round is shortened
  • 4.1 ms powertrace becomes uniform

power3

Power consumption (zoomed view)

  • Power trace in 6 clock cycles
  • Each peak -> 1 clock cycle
  • Peaks are different
    -> depends on operation
    -> depends on data

Simple Power Analysis (SPA)
-> one powertrace is good enough to leak the secret

power4

  • Peaks reveal the moments of time
    -> depends on the data
    -> depends on the operation
  • Highest peak is at 132 μs
    -> this is when the byte containing the MSB is loaded

  • Power for d = 0 and d = 1 are almost the same except for 3 clock cycles
    -> That’s the moment of interest
    -> This is whe d or data related to d is processed
    Can we exploit this? -> power analysis

    Example with key:

    • Plaintext (p) is first XORed with roundkey (RO); let’s say Ro = k
    • Substitution box is then performed
    • Power consumption at some point

    power5