Study of an Optical Chaotic Steganographic Free Space System Fabrizio Chiarello 9 June 2011, Padova Relatore: Prof. Marco Santagiustina Controrelatore: Ch.mo Prof. Gianfranco Pierobon Correlatore: Dott. Ing. Leonora Ursini (20 April 2010)
Introduction The aim is to increase the security of infrared free-space communications through optical chaotic steganography. Message? Eavesdropper (Eve) Transmitter (Alice) Channel Receiver (Bob) Optical chaotic steganography is a cheap and efficient alternative (or complement) to software cryptosystems for increasing privacy and security of communications. Recovered Message 1
Outline What is steganography? What is chaos? Steganography through optical chaos Indoor wireless infrared communications Main results Conclusions 2
What is steganography? steganography : from the Greek words stéganos (secret) and gráphein (writing) is the act (or the art) of hiding the very presence of a message. Examples: Herodotus (484 425 BCE) tells the story of a message tattooed on a slave's shaved head, hidden by the growth of his hair, and exposed by shaving his head again. The message allegedly carried a warning to Greece about Persian invasion plans. Invisible inks, used during World War II. Digital image steganography. PIN codes hidden in phone books 3
Deterministic Chaos Deterministic chaos is a peculiar behavior of nonlinear systems satisfying: aperiodicity exponential sensitivity to initial conditions long-term unpredictability apparent randomness ( noise-like ), but deterministic The Lorenz s attractor (the Butterfly ) Different evolutions for two starting points differing by 0.1% (named the Butterfly after Lorenz s paper Predictability: Does the Flap of a Butterfly s Wings in Brazil set off a Tornado in Texas?, 1972) 4
Steganography through Optical Chaos Chaotic optical carrier generated by laser with delayed opto-electronic feedback. Message superposition (<5%) on chaotic carrier at physical layer. Message Chaotic Emitter 5
Chaos synchronization L. Pecora and T. Carroll (1990): a chaotic system ( master ), can, under proper conditions, force another similar system ( slave ) to follow its same evolution. Master Chaotic Emitter Slave Chaotic System Identical master and slave, effect of internal noises Different master and slave 6
Steganography through Optical Chaos Message? Eavesdropper (Eve) Master Chaotic Laser Channel Slave Chaotic Laser Transmitter (Alice) The security resides in the impossibility of the eavesdropper to identify and to exactly reproduce the chaotic carrier. Receiver (Bob) Recovered Message 7
Steganography through Optical Chaos Two different methods of message superposition on the chaotic carrier: CIM Chaos Intensity Modulation ACM Additive Chaos Modulation (TX and RX simmetric) Master laser Message Eve Slave laser Recovered Message 8
Wireless Infrared Communications Advantages: Very large, free and unlicensed spectra (tens of Mb/s up to few Gb/s) Signal eavesdropping partially limited In indoor environments radiation is confined by walls Outdoor propagation requires high beam directivity Disadvantages: Signal leakage still possible High path loss TX -RX alignment requirements can be strict Environment interference Multipath reflections Ambient and artificial light noises 9
Indoor Free Space Channels The channel behaves like an infinite-components linear filter with additive noise Line of Sight (LOS) Hybrid-LOS conf. Multipath interference J. B. Carruthers and P. Kannan, Iterative site-based modeling for wireless infrared channels, IEEE Trans. Antennas Propag., vol. 50, no. 5, pp. 759 765, May 2002. 10
Example of masking and recovering Time Series Source signal Masked signal Eye Diagrams Q = 0.48dB Recovered signal Q = 10.78dB The Q factor is a measure of the eye opening: Q 1 0 1 0 Averages of the recovered message levels 1 and 0 Std. dev.s of the recovered message levels 1 and 0 11
Synchronization Performance The synchronization error SE measures the goodness of master-slave locking S E 1 C orr( P, P ) M S ACM Only chaos CIM Free-space attenuation must be carefully compensated. The message acts as a perturbation, worsening the synchronization. 12
Masking and Recovering Performance TX Ø = 5 RX Ø = 30 Recovering Q = 10.78dB TX Ø = 10 Masking Masking ACM Masking CIM Q = 0.48dB F. Chiarello, L. Ursini, M. Santagiustina, Securing Wireless Infrared Communications Through Optical Chaos, IEEE Photonics Technology Letters, no. 23, pp. 564 566, May 2011 13
Conclusions A steganographic system through optical chaos has been studied. Indoor wireless propagation has been considered including multipath interference and environmental noise. Good performance can be achieved at hundred of Mb/s However: Good synchronization requires good path loss compensation. There is no definitive proof about the level of security (steganalysis is an open field) 14
Thank you for your kind attention! Questions? PS: a message could be hidden in these slides through steganography... 15