In recent years various DNA based symmetric cryptographic algorithms have been suggested to develop secure image encryption techniques but secrecy of key and key space is often compromised due to small size of key. In this regard, this paper proposes a new method of adaptive key length based image encryption using DNA approach. The proposed algorithm uses 72-bit external key to generate the initial condition for chaotic sequence and for expansion of key to the desired length using DNA replication and complement rule. The proposed algorithm has a huge key space generated using key expansion method although the original key sequence is small. The original image is then encrypted using DNA addition and DNA complementary rule guided by the chaotic sequence. Experimental results and security analysis show that the proposed algorithm has achieved the satisfactory computing-security level.
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