The ISC International Journal of Information Security

Abstract In the quantum computing era, classical encryption faces unprecedented vulnerabilities, while Quantum Key Distribution (QKD) alone remains insufficient for top-secret data transmission due to practical hardware flaws. In this paper, a novel dual-layered framework that integrates steganography with QKD is proposed to enhance security and concealment. The proposed protocol embeds encrypted messages within QKD keys during post-processing, leveraging existing infrastructure without requiring hardware modifications. The message is first compressed, encoded, and encrypted using a pre-shared QKD key via one-time-pad encryption. A block-based search mechanism then hides message bits within the sifted key while preserving statistical randomness. Crucially, this approach provides two-layer security: information-theoretic encryption via QKD and undetectable message existence. Evaluations confirm ultra-low failure probabilities of embedding (below 10−12 for 1000-bit messages) and minimal deviations in sifted key length (under 1% for typical blocks). The solution enables eavesdropper detection, maintaining full compatibility with standard QKD post-processing. By unifying steganographic stealth with QKD’s theoretical security, this work establishes a practical solution for transmitting top-secret data against evolving quantum threats.

Abstract Impossible differential cryptanalysis, the extension of differential cryptanalysis, is one of the most efficient attacks against block ciphers. This cryptanalysis method has been applied to most of the block ciphers and has shown significant results. Using structures, key schedule considerations, early abort, and pre-computation are some common methods to reduce complexities of this attack. In this paper, we present a new method for decreasing the time complexity of impossible differential cryptanalysis through breaking down the target key space into subspaces, and extending the results on subspaces to the main target key space. The main advantage of this method is that there is no need to consider the effects of changes in the values of independent key bits on each other. Using the 14-round impossible differential characteristic observed by Boura et al. at ASIACRYPT 2014, we implement this method on 23-round LBlock and demonstrate that it can reduce the time complexity of the previous attacks to 2^71.8 23-round encryptions using 2⁵⁹ chosen plaintexts and 2 ⁷³ blocks of memory.