Table of Contents

• Introduction
• About the Device
• Inter-Device Communication
• Reversing the Android App
• Reversing the Hardware
• Dumping the EEPROM
• Decapping Proprietary Chips
• SEM Imaging of Decapped Chips

Introduction

This past semester I’ve been working on a directed study at my university with Prof. Wil Robertson reverse engineering embedded devices.  After a couple of months looking at a passport scanner, one of my friends jokingly suggested I hack a Furby, the notoriously annoying toy of late 1990s fame.  Everyone laughed, and we all moved on with our lives.

However, the joke didn’t stop there.  Within two weeks, this same friend said they had a present for me.  And that’s how I started reverse engineering a Furby.

About the Device

A Furby is an evil robotic children’s toy wrapped in colored fur.  Besides speaking its own gibberish-like language called Furbish, a variety of sensors and buttons allow it to react to different kinds of stimuli.

Since its original debut in 1998, the Furby apparently received a number of upgrades and new features.  The specific model I looked at was from 2012, which supported communication between devices, sported LCD eyes, and even came with a mobile app.

Inter-Device Communication

As mentioned above, one feature of the 2012 version was the toy’s ability to communicate with other Furbys as well as the mobile app.  However, after some investigation I realized that it didn’t use Bluetooth, RF, or any other common wireless protocols.  Instead, a look at the official Hasbro Furby FAQ told a more interesting story:

Q. There is a high pitched tone coming from Furby and/or my iOS device.

A. The noise you are hearing is how Furby communicates with the mobile device and other Furbys. Some people may hear it, others will not. Some animals may also hear the noise. Don’t worry, the tone will not cause any harm to people or animals.

Digging into this lead, I learned that Furbys in fact perform inter-device communication with an audio protocol that encodes data into bursts of high-pitch frequencies.  That is, devices communicate with one another via high-pitch sound waves with a speaker and microphone.  #badBIOS anyone?

This was easily confirmed by use of the mobile app which emitted a modulated sound similar to the mosquito tone whenever an item or command was sent to the Furby.  The toy would also respond with a similar sound which was recorded by the phone’s microphone and decoded by the app.

Upon searching, I learned that other individuals had performed a bit of prior work in analyzing this protocol.  Notably, the GitHub project Hacksby appears to have successfully reverse engineered the packet specification, developed scripts to encode and decode data, and compiled a fairly complete database of events understood by the Furby.

Reversing the Android App

Since the open source database of events is not currently complete, I decided to spend a few minutes looking at the Android app to identify how it performed its audio decoding.

After grabbing the .apk via APK Downloader, it was simple work to get to the app’s juicy bits:

$ unzip -q com.hasbro.furby.apk
$ d2j-dex2jar.sh classes.dex
dex2jar classes.dex -> classes-dex2jar.jar
$

Using jd-gui, I then decompiled classes-dex2jar.jar into a set of .java source files.  I skimmed through the source files of a few app features that utilized the communication protocol (e.g., Deli, Pantry, Translator) and noticed a few calls to methods named sendComAirCmd().

Each method accepted an integer as input, which was spliced and passed to objects created from the generalplus.com.GPLib.ComAirWrapper class:

private void sendComAirCmd(int paramInt)
{    
  Logger.log(Deli.TAG, "sent command: " + paramInt);
  Integer localInteger1 = Integer.valueOf(paramInt);
  int i = 0x1F & localInteger1.intValue() >> 5;
  int j = 32 + (0x1F & localInteger1.intValue());
  ComAirWrapper.ComAirCommand[] arrayOfComAirCommand = new ComAirWrapper.ComAirCommand[2];
  ComAirWrapper localComAirWrapper1 = this.comairWrapper;
  localComAirWrapper1.getClass();
  arrayOfComAirCommand[0] = new ComAirWrapper.ComAirCommand(localComAirWrapper1, i, 0.5F);
  ComAirWrapper localComAirWrapper2 = this.comairWrapper;
  localComAirWrapper2.getClass();
  arrayOfComAirCommand[1] = new ComAirWrapper.ComAirCommand(localComAirWrapper2, j, 0.0F);

The name generalplus appears to identify the Taiwanese company General Plus, which “engage[s] in the research, development, design, testing and sales of high quality, high value-added consumer integrated circuits (ICs).”  I was unable to find any public information about the GPLib/ComAir library.  However, a thread on /g/ from 2012 appears to have made some steps towards identifying the General Plus chip, among others.

The source code at generalplus/com/GPLib/ComAirWrapper.java defined a number of methods providing wrapper functionality around encoding and decoding data, though none of the functionality itself.  Continuing to dig, I found the file libGPLibComAir.so:

$ file lib/armeabi/libGPLibComAir.so 
lib/armeabi/libGPLibComAir.so: ELF 32-bit LSB shared object, ARM, version 1 (SYSV), dynamically linked, stripped

Quick analysis on the binary showed that this was likely the code I had been looking for:

$ nm -D lib/armeabi/libGPLibComAir.so | grep -i -e encode -e decode -e command
0000787d T ComAir_GetCommand
00004231 T Java_generalplus_com_GPLib_ComAirWrapper_Decode
000045e9 T Java_generalplus_com_GPLib_ComAirWrapper_GenerateComAirCommand
00004585 T Java_generalplus_com_GPLib_ComAirWrapper_GetComAirDecodeMode
000045c9 T Java_generalplus_com_GPLib_ComAirWrapper_GetComAirEncodeMode
00004561 T Java_generalplus_com_GPLib_ComAirWrapper_SetComAirDecodeMode
000045a5 T Java_generalplus_com_GPLib_ComAirWrapper_SetComAirEncodeMode
000041f1 T Java_generalplus_com_GPLib_ComAirWrapper_StartComAirDecode
00004211 T Java_generalplus_com_GPLib_ComAirWrapper_StopComAirDecode
00005af5 T _Z13DecodeRegCodePhP15tagCustomerInfo
000058cd T _Z13EncodeRegCodethPh
00004f3d T _ZN12C_ComAirCore12DecodeBufferEPsi
00004c41 T _ZN12C_ComAirCore13GetDecodeModeEv
00004ec9 T _ZN12C_ComAirCore13GetDecodeSizeEv
00004b69 T _ZN12C_ComAirCore13SetDecodeModeE16eAudioDecodeMode
000050a1 T _ZN12C_ComAirCore16SetPlaySoundBuffEP19S_ComAirCommand_Tag
00004e05 T _ZN12C_ComAirCore6DecodeEPsi
00005445 T _ZN15C_ComAirEncoder10SetPinCodeEs
00005411 T _ZN15C_ComAirEncoder11GetiDfValueEv
0000547d T _ZN15C_ComAirEncoder11PlayCommandEi
000053fd T _ZN15C_ComAirEncoder11SetiDfValueEi
00005465 T _ZN15C_ComAirEncoder12IsCmdPlayingEv
0000588d T _ZN15C_ComAirEncoder13GetComAirDataEPPcRi
000053c9 T _ZN15C_ComAirEncoder13GetEncodeModeEv
000053b5 T _ZN15C_ComAirEncoder13SetEncodeModeE16eAudioEncodeMode
000053ed T _ZN15C_ComAirEncoder14GetCentralFreqEv
00005379 T _ZN15C_ComAirEncoder14ReleasePlayersEv
000053d9 T _ZN15C_ComAirEncoder14SetCentralFreqEi
000056c1 T _ZN15C_ComAirEncoder15GenComAirBufferEiPiPs
00005435 T _ZN15C_ComAirEncoder15GetWaveFormTypeEv
000054bd T _ZN15C_ComAirEncoder15PlayCommandListEiP20tagComAirCommandList
00005421 T _ZN15C_ComAirEncoder15SetWaveFormTypeEi
00005645 T _ZN15C_ComAirEncoder17PlayComAirCommandEif
00005755 T _ZN15C_ComAirEncoder24FillWavInfoAndPlayBufferEiPsf
00005369 T _ZN15C_ComAirEncoder4InitEv
000051f9 T _ZN15C_ComAirEncoderC1Ev
000050b9 T _ZN15C_ComAirEncoderC2Ev
00005351 T _ZN15C_ComAirEncoderD1Ev
00005339 T _ZN15C_ComAirEncoderD2Ev

I loaded the binary in IDA Pro and quickly confirmed my thought.  The method generalplus.com.GPLib.ComAirWrapper.Decode() decompiled to the following function:

unsigned int __fastcall Java_generalplus_com_GPLib_ComAirWrapper_Decode(int a1, int a2, int a3)
{
  int v3; // ST0C_4@1
  int v4; // ST04_4@1
  int v5; // ST1C_4@1
  const void *v6; // ST18_4@1
  unsigned int v7; // ST14_4@1

  v3 = a1;
  v4 = a3;
  v5 = _JNIEnv::GetArrayLength();
  v6 = (const void *)_JNIEnv::GetShortArrayElements(v3, v4, 0);
  v7 = C_ComAirCore::DecodeBuffer((int)&unk_10EB0, v6, v5);
  _JNIEnv::ReleaseShortArrayElements(v3);
  return v7;
}

Within C_ComAirCore::DecodeBuffer() resided a looping call to ComAir_DecFrameProc() which appeared to be referencing some table of phase coefficients:

int __fastcall ComAir_DecFrameProc(int a1, int a2)
{
  int v2; // r5@1
  signed int v3; // r4@1
  int v4; // r0@3
  int v5; // r3@5
  signed int v6; // r2@5

  v2 = a1;
  v3 = 0x40;
  if ( ComAir_Rate_Mode != 1 )
  {
    v3 = 0x80;
    if ( ComAir_Rate_Mode == 2 )
      v3 = 0x20;
  }
  v4 = (a2 << 0xC) / 0x64;   if ( v4 > (signed int)&PHASE_COEF[0x157F] )
    v4 = (int)&PHASE_COEF[0x157F];
  v5 = v2;
  v6 = 0;
  do
  {
    ++v6;
    *(_WORD *)v5 = (unsigned int)(*(_WORD *)v5 * v4) >> 0x10;
    v5 += 2;
  }
  while ( v3 > v6 );
  ComAirDec();
  return ComAir_GetCommand();
}

Near the end of the function was a call to the very large function ComAirDec(), which likely was decompiled with the incorrect number of arguments and performed the bulk of the audio decoding process.  Data was transformed and parsed, and a number of symbols apparently associated with frequency-shift keying were referenced.

Itching to continue onto reverse engineering the hardware, I began disassembling the device.

Reversing the Hardware

Actually disassembling the Furby itself proved more difficult than expected due to the form factor of the toy and number of hidden screws.  Since various tear-downs of the hardware are already available online, let’s just skip ahead to extracting juicy secrets from the device.

The heart of the Furby lies in the following two-piece circuit board:

Thanks to another friend, I also had access to a second Furby 2012 model, this time the French version.  Although the circuit boards of both devices were incredibly similar, differences did exist, most notably in the layout of the right-hand daughterboard. Additionally, the EEPROM chip (U2 on the board) was branded as Shenzen LIZE on the U.S. version, the French version was branded ATMEL:

The first feature I noticed about the boards was the fact that a number of chips were hidden by a thick blob of epoxy.  This is likely meant to thwart reverse engineers, as many of the important chips on the Furby are actually proprietary and designed (or at least contracted for development) by Hasbro.  This is a standard PCB assembly technique known as “chip-on-board” or “direct chip attachment,” though it proves harder to identify the chips due to the lack of markings.  However, one may still simply inspect the traces connected to the chip and infer its functionality from there.

For now, let’s start with something more accessible and dump the exposed EEPROM.

Dumping the EEPROM

The EEPROM chip on the French version Furby is fairly standard and may be easily identified by its form and markings:

By googling the markings, we find the datasheet and learn that it is a 24Cxx family EEPROM chip manufactured by ATMEL. This particular chip provides 2048 bits of memory (256 bytes), speaks I2C, and offers a write protect pin to prevent accidental data corruption.  The chip on the U.S. version Furby has similar specs but is marked L24C02B-SI and manufactured by Shenzen LIZE.

Using the same technique as on my Withings WS-30 project, I used a heat gun to desolder the chip from the board.  Note that this MUST be done in a well-ventilated area. Intense, direct heat will likely scorch the board and release horrible chemicals into the air.

Unlike my Withings WS-30 project, however, I no longer had access to an ISP programmer and would need to wire the EEPROM manually.  I chose to use my Arduino Duemilanove since it provides an I2C interface and accompanying libraries for easy development.

Referencing the datasheet, we find that there are eight total pins to deal with.  Pins 1-3 (A0, A1, A2) are device address input pins and are used to assign a unique identifier to the chip. Since multiple EEPROM chips may be wired in parallel, a method must be used to identify which chip a controller wishes to speak with.  By pulling the A0, A1, and A2 pins high or low, a 3-bit number is formed that uniquely identifies the chip.  Since we only have one EEPROM, we can simply tie all three to ground.  Likewise, pin 4 (GND) is also connected to ground.

Pins 5 and 6 (SDA, SCL) designate the data and clock pins on the chip, respectively. These pins are what give “Two Wire Interface” (TWI) its name, as full communication may be achieved with just these two lines.  SDA provides bi-directional serial data transfer, while SCL provides a clock signal.

Pin 7 (WP) is the write protect pin and provides a means to place the chip in read-only mode.  Since we have no intention of writing to the chip (we only want to read the chip without corrupting its contents), we can pull this pin high (5 volts).  Note that some chips provide a “negative” WP pin; that is, connecting it to ground will enable write protection and pulling it high will disable it.  Pin 8 (VCC) is also connected to the same positive power source.

After some time learning the Wire library and looking at example code online, I used the following Arduino sketch to successfully dump 256 bytes of data from the French version Furby EEPROM chip:

#include <Wire.h>

#define disk1 0x50    // Address of eeprom chip

byte i2c_eeprom_read_byte( int deviceaddress, unsigned int eeaddress ) {
    byte rdata = 0x11;
    Wire.beginTransmission(deviceaddress);
//    Wire.write((int)(eeaddress >> 8)); // MSB
    Wire.write((int)(eeaddress & 0xFF)); // LSB
    Wire.endTransmission();
    Wire.requestFrom(deviceaddress,1);
    if (Wire.available()) rdata = Wire.read();
    return rdata;

}

void setup(void)
{
  Serial.begin(9600);
  Wire.begin();  

  unsigned int i, j;
  unsigned char b;

  for ( i = 0; i < 16; i++ )
  {
    for ( j = 0; j < 16; j++ )
    {
      b = i2c_eeprom_read_byte(disk1, (i * 16) + j);
      if ( (b & 0xf0) == 0 )
        Serial.print("0");
      Serial.print(b, HEX);
      Serial.print(" ");
    }
    Serial.println();
  }

}

void loop(){}

Note that unlike most code examples online, the “MSB” line of code within i2c_eeprom_read_byte() is commented out.  Since our EEPROM chip is only 256 bytes large, we are only using 8-bit memory addressing, hence using a single byte. Larger memory capacities require use of larger address spaces (9 bits, 10 bits, so on) which require two bytes to accompany all necessary address bits.

Upon running the sketch, we are presented with the following output:

2F 64 00 00 00 00 5A EB 2F 64 00 00 00 00 5A EB 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
05 00 00 04 00 00 02 18 05 00 00 04 00 00 02 18 
0F 00 00 00 00 00 18 18 0F 00 00 00 00 00 18 18 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 F8

Unfortunately, without much guidance or further analysis of the hardware (perhaps at runtime), it is difficult to make sense of this data.  By watching the contents change over time or in response to specific events, it may be possible to gain a better understanding of these few bytes.

Decapping Proprietary Chips

With few other interesting chips freely available to probe, I turned my focus to the proprietary chips hidden by epoxy.  Having seen a number of online resources showcase the fun that is chip decapping, I had the urge to try it myself.  Additionally, the use of corrosive acid might just solve the issue of the epoxy in itself.

Luckily, with the assistance and guidance of my Chemistry professor Dr. Geoffrey Davies, I was able to utilize the lab resources of my university and decap chips in a proper and safe manner.

First, I isolated the three chips I wanted to decap (henceforth referenced as tiny, medium, and large) by desoldering their individual boards from the main circuit board. Since the large chip was directly connected to the underside of the board, I simply took a pair of sheers and cut around it.

Each chip was placed in its own beaker of 70% nitric acid (HNO₃) on a hot plate at 68°C. Great care was taken to ensure that absolutely no amount of HNO₃ came in contact with skin or was accidentally consumed.  The entire experiment took place in a fume hood which ensured that the toxic nitrogen dioxide (NO₂) gas produced by the reaction was safely evacuated and not breathed in.

Each sample took a different amount of time to fully decompose the epoxy, circuit board, and chip casing depending on its size.  Since I was working with a lower concentration nitric acid than professionals typically use (red/white fuming nitric acid is generally preferred), the overall process took between 1-3 hours.

After each chip had been fully exposed and any leftover debris removed, I removed the beakers from the hot plate, let cool, and decanted the remaining nitric acid into a waste collection beaker, leaving the decapped chips behind. A small amount of distilled water was then added to each beaker and the entirety of it poured onto filter paper.  After rinsing each sample one or two more times with distilled water, the sample was then rinsed with acetone two or three times.

The large chip took the longest to finish simply due to the size of the attached circuit board fragment.  About 2.5 hours in, the underside of the chip had been exposed, though the epoxy blob had still not been entirely decomposed.  At this point, the bonding wires for the chip (guessed to be a microcontroller) were still visible and intact:

About thirty minutes later and with the addition of more nitric acid, all three samples were cleaned and ready for imaging:

SEM Imaging of Decapped Chips

The final step was to take high resolution images of each chip to learn more about its design and identify any potential manufacturer markings.  Once again, I leveraged university resources and was able to make use of a Hitachi S-4800 scanning electron microscope (SEM) with great thanks to Dr. William Fowle.

Each decapped chip was placed on a double-sided adhesive attached to a sample viewing plate.  A few initial experimental SEM images were taken; however a number of artifacts were present that severely affected the image quality.

To counter this, a small amount of colloidal graphite paint was added around the edges of each chip to provide a pathway to ground for the electrons.  Additionally, the viewing plate was treated in a sputter coater machine where each chip was coated with 4.5nm of palladium to create a more conductive surface.

After treatment, the samples were placed back in the SEM and imaged with greater success.  Each chip was imaged in pieces, and each individual image was stitched together to form a single large, high resolution picture.  The small and large chip overview images were shot at 5.0kV at 150x magnification, while the medium chip overview image was shot at 5.0kV at 30x magnification:

Unfortunately, as can be seen in the image above, the medium chip did not appear to have cleaned completely in its nitric acid bath.  Although it is believed to be a memory storage device of some sort (by looking at optical images), it is impossible to discern any finer details from the SEM image.

A number of interesting features were found during the imaging process.  The marking “GHG554” may be clearly seen directly west on the small chip.  Additionally, in similar font face, the marking “GFI392” may be seen on the south-east corner of the large chip:

Higher zoom images were also taken of generally interesting topology on the chips.  For instance, the following two images show what looks like a “cheese grater” feature on both the small and large chips:

“Tiny”

“Large”

If you are familiar with of these chips or any their features, feedback would be greatly appreciated.

EDIT: According to cpldcpu, thebobfoster, and Thilo, the “cheese grater” structures are likely bond pads.

Additional images taken throughout this project are available at: http://www.flickr.com/photos/mncoppola/

Tremendous thanks go out to the following people for their guidance and donation of time and resources towards this project:

• Prof. Wil Robertson – College of Computer and Information Science @ NEU
• Dr. Geoffrey Davies – Dept. of Chemistry & Chemical Biology @ NEU
• Dr. William Fowle – Nanomaterials Instrumentation Facility @ NEU
• Molly White
• Kaylie DeHart