Copyright 2003, Marchany Hiding Text in MP3 Files Randy Marchany VA Tech Computing Center Blacksburg, VA 24060

Copyright 2003, Marchany Introduction Hackers and Identity thieves are constantly coming up with ways to subvert computer & network defenses. Steganography (literal meaning: covered writing) is one way to accomplish this. Steganography is the art of passing information in a manner that the very existence of the message is unknown.

Copyright 2003, Marchany Audio Steganography Basics K. Weeks note that digital steganography is based on the fact that artifacts like bitmaps and audio files contain redundant information. Compression techniques like JPEG and MP3 eliminate part of the redundancy allowing the file to be compressed.

Copyright 2003, Marchany Audio Steganography Basics With DigSteg, you replace some of the redundant information with other data. The Human Auditory System (HAS) operates over a wide dynamic range of frequencies but it’s fairly poor with respect to its differential range. Loud sounds tend to mask out software sounds easily. This gives us a clue on possible candidates for DigSteg masking.

Copyright 2003, Marchany Ways to Hide Data in Audio Low-bit encoding replaces the LSB of information in each sampling point with a coded binary string. Can encode large amounts of hidden data in a given audio signal at the examples of introducing significant noise at the upper limits.

Copyright 2003, Marchany Ways to Hide Data in Audio Phase encoding substitutes the phase of an initial audio segment with a reference to the phase that represents the data. The phase of subsequent segments is adjusted in order to preserve the relative phase between segments. More complicated than low-bit.

Copyright 2003, Marchany Ways to Hide Data in Audio Spread Spectrum spreads the encoded data across as much of the frequency spectrum as possible. Makes it difficult for an adversary to find the data unless they have access to and or reconstruct the pseudo-random signal used to spread the data across the frequency range.

Copyright 2003, Marchany Ways to Hide Data in Audio Echo Data Hiding embeds data into a host signal by introducing an echo. The data are hidden by varying 3 parameters of the echo: Initial amplitude Decay rate Offset The echo blends as the offset between the original and the echo decreases. The echo is perceived as the original sound.

Copyright 2003, Marchany MP3 in a Nutshell MP3 = ISO-MPEG Audio Layer-3 standard Basic principle is the audio signal contains a significant portion of info that can be discarded w/o the average listener noticing the change. How telephones work.

Copyright 2003, Marchany MP3 in a Nutshell Layer-3 encoder uses 2 nested iteration loops. Inner loop is a rate loop. Outer loop is a noise control/distortion loop. MP3stego tool operates within the MP3 encoding process. It hides data during data compression.

Copyright 2003, Marchany MP3Stego Created by Fabien Petitcolas The secret data is compressed, encrypted and then hidden in the MP3 bit stream. Hiding process takes place in the inner loop

Copyright 2003, Marchany MP3Stego Encode –E secret_text.txt –P passphrase hells_bells.wav hells_bells.mp3 Decode –X –P passphrase hells_bells.mp3 Example compresses hells_bells.wav with secret_text.txt into hells_bells.mp3. To decrypt, you need the password “passphrase”.

Copyright 2003, Marchany Caveats The.wav file must be a mono 16 bit file encoded with pulse code modulation at Hz. MP3Stego can be used to insert a copyright marking system for MP3 files. Makes it easier to track illegal copies.

Copyright 2003, Marchany References “MP3Stego: Hiding Text in MP3 Files” by Mark Noto, SANS GIAC paper “On the limits of steganography” by Anderson, R. & Petticolas F.\ “Hiding in plain sight” by K. Weeks