DACS: A Plaintext-Dependent Dynamic Affine–Chaotic Stream Cipher for Efficient Symmetric Encryption with Resistance to Linear and Statistical Cryptanalysis

Abstract

The affine cipher is fast and almost free to run, but it breaks the moment an attacker sees two plaintext–ciphertext pairs: its parameters never change, so recovering them recovers the whole message. This paper asks what happens if those parameters stop being constant. This paper introduces DACS, a stream cipher that keeps the affine map as its core operation yet generates a new pair  together with a keystream byte  for each symbol; all three values are drawn from a two-dimensional coupled Logistic–Tent chaotic system. The seed of that system is bound to the message itself via a keyed synthetic-IV (HMAC–SHA-256) schedule combined with a ciphertext-feedback chain, which makes the keystream a function of the plaintext and not of the key alone. On the standard 512×512 cameraman test image, DACS attains an information entropy of 7.9992 bits, while the adjacent-pixel correlations fall to |-0.0047|, |-0.0053|, and 0.0128 in the horizontal, vertical, and diagonal directions; the cipher histogram is statistically flat as well ( , under the 293.25 threshold). Flipping one bit of the plaintext changes 99.6342% of the output (NPCR) with an average intensity change of 33.5117% (UACI); a one-bit change in the key changes 99.5979% of the output. The key-only keystream clears every NIST SP 800-22 test we ran. Most tellingly, the classical two-pair attack that defeats an ordinary affine cipher recovers nothing here—0.0% of subsequent bytes, consistent with blind guessing—because there is no longer a single key to solve for. Taken together, these measured results put DACS alongside the strongest 2024–2025 chaos-based ciphers while doing what none of them set out to do: remove the affine map’s linear weakness at its source.