STEGANOGRAPHY Question Definition. Historical Cases of Steganography. Can we send hidden information?

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1 Clemens H. Cap Universität Rostock clemens.cap (at) uni rostock (dot) de STEGANOGRAPHY Question Definition Can we send hidden information? Steganography is the art of transmitting information but hiding the information you really want to transmit Historical Cases of Steganography Hair steganography Tattoo message onto the scalp of a slave Let hair grow over the message Secret ink steganography Write message with lemon juice Let it dry difficult to read Heat it near the fire turns black 1

2 A harmless little poem Fishing freshwater bends and saltwater coasts reward anyone feeling stressed. Resourceful anglers usually find masterful leapers fun andadmitadmit swordfish rank overwhelming anyway. "really" is a secret message Fishing freshwater bends and saltwater coasts reward anyone feeling stressed. Resourceful anglers usually find masterful leapers fun and admit swordfish rank overwhelming anyway. Common idea Use the "permissible noise" in a medium to encode a hidden message inside of a cover message. 2

3 Media Used for Steganography Text Still Images and Video Audio Binaries Protocols & Behaviour Text Steganography This is just a simple message consisting of some text. This is just a simple message consisting of some text. Positions 2, 5, 7 and 8 have an additional blank This is Just a siemple message consisting off sam text. Words 3, 5, 8 and 9 have a writing error First Problem in Steganography Is there really a secret message? This is just a simple message consisting of some text. Words 3, 5 and 10 have special emphasis. Problem: Does the emphasis code something hidden? Or is it just the emphasis, that this is JUST are SIMPLE message consisting of some TEXT (and not of a secret) 3

4 Second Problem in Steganography Where is the secret message? This is the example of some text to be used for practicing. Assume we know this sentence contains a secret. How to find it? Problem may be: Maybe: Secret is heavily encrypted. First letter of every word Titeosttbufp and some decryption Possibly: Second letter of every word Hshxfoeoesor and some decryption How to distinguish if we do not know the password? General Method Optional Crypto key Cover Stego Stego Medium Method Medium Secret Message Cover Image 4

5 Cover Image and Embedded Message 8 bit grey scale image MSB 5

6 Bit 4 LSB LSB Embedding Change the least significant bits to value of secret Works with many media types (audio, video, still) Practically no effect on the human perception Danger: Can be detected 6

7 Is this really noise? Steganalysis Can we detect, if an original media file has been tampered? Depends on Amount of hidden information If too much is embedded: Yes Employed steganographic algorithm Type of medium Video: 3 dimensions of correlation Still image: 2 dimensions of correlation Audio: 1 dimension of correlation Pevny et al.: IEEE Transactions on Information Forensics 5, 2 (2010) 7

8 Steganography in Audio Streams (1) LSB encoding Adapt the LSB of the audio signal Produces 1 bit stego per 8 bit or 16 bit audio Approx 10% of data stream Typical audio stream 44 kbps Provides some 2 4 kbps stego channel Echo encoding Audio streams have an echo Human ear is bad in detecting changes in echo Modulate amplitude, decay rate and delay time of echo Steganography in Audio Streams (2) Phase Coding Break sound stream into short segments and do FFT Calculate the phase of each segment Setthe the phase offirstsegment first segment (this is the secret) and adjust the phase of the subsequent segments Do this repeatedly Stereo Coding Use subtle differences in left and right channel Eg: Change amplitude and phase in the channels Eg: Modulate the position of the speaker Steganography in Audio Streams (3) Direct Sequence Spread Spectrum DSSS Take high frequency random noise sequence Use it inverted or normal as additional noise signal Very robust against filtering and processing Adds some noise to the signal Code

9 Kessler: An Overview of Steganography for the Computer Forensics Examiner. Contained hidden secret Kessler: An Overview of Steganography for the Computer Forensics Examiner. Problems of audio steganography Robustness against common processing on transport Resampling Recoding & Compression Eg: MP3compression; different audio formats and codecs De Noising 9

10 Text Big variety of possibilities depending on what text "is". Shift lines, shift words Different fonts or attributes Spelling mistakes, punctuation, spacing Old or new, american or british writing style Semantic variations Begin every enumeration with longest or shortest word Eg: First sentence on every page begins with a latin word or not Usually sufficient to encode a serial number into the file Binaries Uses synonym coding Equivalent instructions: Leftshift by 1 or multiply by 2 Collect a set of equivalent instructions Use specific versions to embed information Coding rate: 1 in 110 for average Intel processor Program remains functionally the same Applications: Embedding of serial numbers Tracing of program copying Obscuring the storage of keys Protocols and Behaviour Counter specification behavior Eg: Fields which should be zero are not zero Choice of means: Use different techniques for same goal Timing: Issue request on full second X0or X.0 on X.5 X5 Sequence of doing things Eg: ftp get downloads files in different permutations Modulate Ressource Usage in Time Eg: Makes some request appear to be serviced slower 10

11 Two Relatives: Watermarking and Steganography Steganography Want to encode a hidden message Goal: Efficient: Embed large hidden message Goal: Stay secret: Cannot be detected by third party Watermark Want to encode copyright message or id of buyer Goal: Robust: Cannot remove easily Goal: Robust: Remains readable during all possible typical transformations of the cover media Non goal: Does not (always) have to stay secret 11