Experimental Evaluation of Byte-Scale XOR Vigenère Transformation with a Deterministic Keystream

The classical Vigenère cipher is computationally simple but remains constrained by alphabet-oriented processing and repeating-key structures. This study experimentally evaluates a byte-scale exclusive-OR (XOR) Vigenère transformation with a deterministic keystream and compares it with Vigenère Modulo 26 and Extended Modulo 95. Experiments used Python 3.12.13 on Linux 6.6.122 x86_64 with an Intel Xeon environment. Evaluation covered byte-identical recovery, Index of Coincidence (IC), Shannon entropy, normalized distribution measures, repeated performance benchmarking, keystream-reuse testing, and a known-plaintext scenario. The XOR transformation recovered all byte values from 0 to 255 and seeded binary payloads of 1 KiB and 1 MiB without alteration. Its ciphertext produced an IC of 0.003770 and entropy of 7.7715 bits, with an IC-to-uniform ratio of 0.965 and normalized entropy of 97.14%. At 500,000 bytes, median encryption and decryption times were 0.047692 s and 0.049109 s over 30 repetitions. However, these results characterize reversibility, distribution, and implementation behavior rather than cryptographic security. With the same key and equal message length, the tested recurrence reused an identical keystream, while known plaintext exposed the initial key bytes. Accordingly, the construction is positioned as an experimental reversible byte-domain transformation rather than a production-secure cipher.