We analyze duality in code design between encryption and data hiding problems. First we point out dual operations of the encoder/decoder pairs of the two problems, and then, we apply the developed duality principle to practical image communication. Using the same image DCT coefficients as side information for both problems, practical code designs based on trellis quantization and LDPC codes are then provided. Under the same simulation condition, the performance of the two practical designs show consistency with each other, hence validating the coding duality between the two problems.