On the Security of Caesar Cipher, Linear Cipher and Enhanced Linear Cipher in Hiragana Messages

Cryptography, as both an art and a science, employs specific terminology to secure messages, including plaintext, ciphertext, encryption, decryption, and keys. The encryption process transforms plaintext into ciphertext using a cryptographic key, while decryption reverses this operation. Among classical cipher techniques, the Caesar cipher operates by replacing each Hiragana character with a fixed alternative from the syllabary, while the Linear cipher applies mathematical transformations to convert numerical representations of Hiragana characters into ciphertext. Although these ciphers were originally designed for English letters and remain vulnerable to frequency attacks, their application to Hiragana presents unique vulnerabilities: the Caesar cipher can be compromised through frequency analysis of Hiragana’s distinctive phonetic patterns, and the Linear cipher remains susceptible to known plaintext attacks that leverage Hiragana’s predictable character distribution. While previous research has not systematically examined these ciphers’ application to Hiragana messages, this study develops comprehensive encryption and decryption frameworks for both the Caesar and Linear ciphers adapted to Hiragana, followed by a comparative security analysis. Furthermore, we propose an enhanced Linear cipher variant designed to resist frequency cryptanalysis. Our security evaluations demonstrate that this improved Linear cipher achieves significantly stronger protection compared to both the traditional Caesar cipher and the original Linear cipher implementation when processing Hiragana messages.