Inspiration
The idea of building a secret network between phones using hypersonic frequencies to avoid human detection.
What it does
ShadowSound, the program, provides a disguised channel using any audible sound by subtly modifying any audio played through any system's speakers. A bitstream can be placed on these modifications, allowing other nearby speaker systems to receive transmissions. This can be used to exfiltrate data without using any networks. The audio that is broadcasted is not considered suspicious, as it hijacks user played audio in a way in which the modification remains nearly unnoticeable.
How we built it
The clients are built in Python. The fundamental mechanism that we used removed a insignificant section of the audio being sent using a bandpass filter. The human ear doesn't notice the modified gap while a nearby mic with ShadowSound can pick up and decode the the message using the missing sound. The presence of this gap is oscillated to encode bits. The baud rate is relatively low allowing the encoding of characters using the five bit Baudot encoding rather than the eight bit ASCII. This made the data transmission 37.5% more efficient. The transmitting side sits in the audio driver position, intercepts audio sent to the speakers, and redirects it into ShadowSound. ShadowSound then modifies the audio,encoding the next bit of any given message to finally be outputted through the speakers. On the receiving end, the program takes the Fast Fourier Transform of a chunk of the audio and checks for whether a band around, 500hz, has been removed.
Challenges we ran into
There were two major challenges we ran into. The first was finding a way to modify audio in a way which is not obvious to listeners. The second was dealing with the poor frequency responses of consumer laptop microphones and speakers.
Accomplishments that we're proud of
The team is most proud of bringing the idea of ShadowSound to a working reality within a 36 hour hackathon.
What we learned
Three fourths of the team has never worked audio or audio processing. Together with the knowledge of the fourth teammate the team was able to overcome the challenges faced while creating ShadowSound. Initially the team approached the project attacking the same problem at the same time. About 8 hours into the project we realized that we would not finish if we worked on the same part of the project. The work was then divided and conquered.
What's next for ShadowSound
The team plans to make the transmissions much more discrete. The team would also like to optimize the rate at which bits can be transmitted. As the transmission of data through an audible, publicly projected, source unnoticeably is the purpose of ShadowSound, making it harder to find, and faster is its main goals.
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