Analyzing SPI (Serial Peripheral Interface)

1. SPI protocol internals

  • Designed for synchronous serial communication between the master and slave devices
  • Master controls the clock for all the slaves
    • fastest serial clock speed in SPI is half the speed of the master clock
  • SPI (Serial Peripheral Interface) implementation standard is not strict
    • manufacturers design as their wish
  • Need to look up the datasheet to understand SPI implementation protocol

  • SPI advantages
    • Speed is not limited (has faster data transmission rates compared to I2C)
    • Full-duplex (both read and write data happens at the same time)
  • SPI downside
    • Single master protocol
    • Consists of 3 pins for data transfer and one pin for Chip/Slave select
      • increases the overall requirement of space compared to I2C
      • when there is no data to read, when there is a write request,
        • the slave should send dummy data to make the connection established

2. Pentest objectives

  • Get console access
    • custom bootloaders are designed to prevent from entering single user mode
    • but that can also be bypassed

3. Tools

  • SPI Converter/Adapter (any supported FTDI based hardware)
    • USB-TTL (or)
    • BusPriate (or)
    • Attify Badge

4. Identify the SPI Flash pinouts

  • Open the device
  • Visually identify the SPI flash chip on the PCB
  • Note the component number printed on the SPI chip
  • Look up online for the datasheet to figure out the pinouts

  • SPI pins

    SCK Serial Clock
    MOSI Master Out Slave In
    MISO Master In Slave Out
    SS—Slave Select (Active Low, output from master)
  • SCK, MISO, and MOSI pins are shared by slaves
  • Each SPI slave will have its own unique SS pin
  • Master have several SS pins (SS1, SS2, …)
  • Mater-slave connection table

    SPI Master SPI Slave
    SS1 SS
  • Master configures the clock frequency according to the slave’s clock frequency
  • Master selects the slave device with a logic level 0 on the SS pin
  • Master initiates the communication by sending a bit on the MOSI line
    • which is read by the slave
    • whereas the slave sends a bit on the MISO line which is read by the master

5. Connecting EEPROM

  • Method 1: Hooking
    • hook mini probes to the SPI flash
  • Method 2: SOIC clip
    • use a SOIC clip
  • Method 3: Desoldering
    • remove SPI flash the from the PCB by de-soldering
    • solder it on a EEPROM adapter (or reader)
      • use an adapter which is suitable for EEPROM packaging type
  • Adapter (Attify Badge) pin layout

    D0 SCK Serial Clock
    D1 MISO Master In Slave Out
    D2 MOSI Master Out Slave In
    D3 CS Chip Select
  • Connect the SPI flash pins to the adapter (Attify Badge)

    SPI flash Adapter
    CLK TCK (D0)
    MOSI/DO TDI (D1)
    MISO/DI TDO (D2)
    CS TMS (D3)
    WP, HOLD, and Vcc 3.3V
  • Connect the adapter to the analysis PC via USB

6. Reading data

  • Read data from the device using an SPI flash
  • Use the script

      make install -s 5120000 --read=new.bin
  • Dump the contents to a file

7. Writing data

  • Write data to the SPI flash
  • Use the script -s 5120000 -w new.bin