Publication Details
Abstract
Deoxyribonucleic acid (DNA) encoding combined with finite-state machines has become an attractive paradigm for lightweight symmetric encryption, because it offers a large key space, simple invertible transforms, and high diffusion at low computational cost. However, the overwhelming majority of existing DNA/automaton cryptosystems are validated only through descriptive arguments and a handful of statistical experiments; they provide no reduction-based (“provable”) security guarantee, and several of them embed content-dependent control schedules whose running time grows without bound in the plaintext. This paper revisits a recent multi-level DNA encoding scheme driven by a state-changing Mealy machine and reconstructs it as a provably secure stream construction. The redesigned scheme generates all keystream and control material from an HMAC–SHA-256 counter-mode pseudorandom function under a fresh per-message nonce, while the DNA-encoding and Mealy-transduction layers are proven to form a public, key-dependent bijection. We give a complete game-hopping proof that the construction is indistinguishable under chosen-plaintext attack (IND-CPA), with the concrete advantage bounded by 2 Adv_F^prf+q^2/2^128. The same design eliminates the exponential Fibonacci-of-ASCII-sum schedule of the baseline, reducing the per-block control cost to a constant-time keystream read and removing a timing side-channel. A reproducible Python implementation is evaluated with the NIST SP 800-22 battery (all ten applicable tests pass over 40 independent 2×10^5-bit sequences), Shannon entropy (7.9962 bit/byte and 2.0000 bit/base), adjacent-symbol correlation (0.0047), differential metrics (NPCR =100%, UACI =42.2%), key avalanche (50.06% at the bit level and 75.10% at the base level), and an empirical IND-CPA distinguishing experiment whose measured advantage, 0.0010±0.0179, is statistically indistinguishable from zero. The results confirm both the theoretical guarantee and the practical randomness quality of the proposed cryptosystem.