@conference {uvigo:visible, title = {Visible Encryption: Using Paper as a Secure Channel}, booktitle = {Security and Watermarking of Multimedia Contents}, series = {Proc. of SPIE}, volume = {5020}, year = {2003}, address = {Santa Clara, USA}, keywords = {2-D-Bar-Codes, data-hiding, print-\&-scan, Two-dimensional-modulations, visible-encryption}, author = {Norberto Degara-Quintela and Fernando P{\'e}rez-Gonz{\'a}lez}, editor = {Ping W. Wong and Edward J. Delp} }