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20 June 2003 Visible encryption: using paper as a secure channel
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Proceedings Volume 5020, Security and Watermarking of Multimedia Contents V; (2003)
Event: Electronic Imaging 2003, 2003, Santa Clara, CA, United States
A novel technique allowing secure transmission/storage of electronic documents in printed form is described. First, given a document to protect, an error resilient "visibly encrypted" version is printed. Later, when the original document is to be recovered, the system scans the "visibly encrypted" document and decrypts it after asking for a secret key. Unfortunately, one faces the problem that when a document is printed and scanned, the rescanned document may look similar to the original, but will be distorted during the process. Therefore, to ensure reliable and high rate transmission over the print-and-scan channel it is essential a judicious theoretical model for characterizing the problem and providing reliable communications schemes. The proposed method is based on Pulse Amplitude Modulation (PAM), using small square-shaped pulses and a Maximum Likelihood (ML) detector that is derived after estimating the distortions introduced by the print-and-scan channel. Furthermore, it is essential to employ synchronization techniques to correctly demodulate the printed pulses. In our case, we use an adaptive scheme that resembles the well-known phase locked loops (PLL's). Finally, we will discuss schemes that can make the bit stream resilient to transmissions errors and how to combine them with cryptographic algorithms in order to produce a secure system.
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Norberto Degara-Quintela and Fernando Perez-Gonzalez "Visible encryption: using paper as a secure channel", Proc. SPIE 5020, Security and Watermarking of Multimedia Contents V, (20 June 2003);

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