Extended-Chacha20 Stream Cipher With Enhanced Quarter Round Function

Abstract

Chacha20 is a widely used stream cipher known for using permutation functions to enhance resistance against cryptanalysis. Although the existing literature highlights its strengths, it is worth further exploring its potential susceptibility to differential attacks. This paper proposes an Extended Chacha20 (EChacha20) stream cipher, which offers a slight improvement of Chacha20. It incorporates enhanced Quarter Round Functions $QR-F$ with 32-bit input words and $Add $ , $Rotate $ , and $XOR (ARX)$ operations on 16, 12, 8, 7, 4, and 2 constants. Using these improved $QR-Fs$ , we expect EChacha20 to be more secure and effective against attacks than Chacha20. The threat model leveraged in this paper considers attacker assumptions based on the Bellare-Rogaway Model (B-RM) and the Chosen Plaintext Attack (CPA) to assess the potential security weaknesses. Then, the study analyzes the EChacha20 cipher using the NIST Statistical Test Suite (NSTS) and demonstrates its effectiveness against differential cryptanalysis. A differential attack addresses this challenge, where the study comprehensively analyses the differences between original and flipped bits. The NSTS has been used to statistically analyze the outcome for uniformity and evaluate the randomness of generating sequences of tests considering 1000 tests based on a range of $[{0,1}]$ . Uniformity is evaluated based on the $p-values$ test against a battery of passing sequences, and 100% is achieved from $Runs$ and $Serial (2): Test 1$ , respectively. The performance evaluation metrics leveraged include encryption speed, decryption speed, and memory usage. Based on the test conducted, it has been observed that with increased $QR-F$ , EChacha20 maintains a good balance in speed although slightly higher than Chacha20; however, with also slightly high memory usage compared to Chacha20. Despite that, a comparative study has been conducted against state-of-the-art studies, and the outcome has been reported to show the significance of the current study. Ultimately, the outcome indicates that the EChacha20 cipher has improved $QR-F$ and security properties compared to Chacha20 and may provide a more robust encryption solution for various applications.