Enabling Secure Keyless Acoustic Communication for Smartphones

=== When you start the application, there are two buttons. First for sender and another for receiver.

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SenderActivity: Keep input value, encode that value.

encodeMessage: Encodes the message and plays the sound signal using FSK. 

ReceiverActivity: Start the receiver to receive the sound signal from mic. The data received is stored in a buffer and then will appear on the screen

AudioReceiver: Receive the data/message.

messageReceived: Check the message is received, if not then send last message.

###FSK Modulation/Demodulation: When app starts a thread object that continuously records sound.

ArduinoService: sound acquisition

AudioRecordingRun(): loop that records sound
write(int): encodes the integer and plays the sound signal.

FSKDecoder: Thread object that demodulates the sound signal.

FSKModule: Utility static methods related to FSK modulation/demodulation.

      encode(int): encodes the number into a sound signal
      decodeSound(double): decodes the sound signal into an integer

ErrorDetection: Before transmission the number(between 0 and 31) is added with a checksum. Then at the reception, the message is decoded and if checksum check:

      fails: an ARQ (Automatic Request Query) code is sent, then the sender repeats the last message sent.
      agrees: an ACK (Acknowledgment) code is sent (if not received the sender tries to the send the last message again). 

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In this process, we use two major techniques to implement transition, FSK and ECC.

Frequency shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. Normally, the transmitted audio alternates between two tones: one , the "mark", represents a binary one, the other, the "space", represents a binary zero.

Error correction code (ECC) is a systematic way of building codes that could detect and correct multiple random symbols. It viewed as cyclic BCH codes, where encoding symbols are derived from the coefficients of a polynomial constructed by multiplying p(x) with a cyclic generator polynomial, which gives rise to efficient decoding algorithms.

The applications of accelerometer and distance detector determine:

1. the receiver generates jamming signal;
2. sender sends information to receiver;
3. after receiver gets the mixed signal, it will remove the jamming signal.

==== Chen Si

Wang Jun

Tu Yujin