A Modified Key Generation Scheme of Vigenère Cipher Algorithm using Pseudo-Random Number and Alphabet Extension

In recent years, many modifications have been done to combat the weaknesses of the Vigenère Cipher Algorithm. Several studies have been carried out to rectify the flaw of the algorithm’s repeating key nature by increasing the key length equal to that of the plain text. However, some characters cannot be encrypted due to the limited set of characters in the key. This paper modified the algorithm’s key generation process using a Pseudo-Random Number Generator to improve the algorithm’s security and expanded the table of characters to up to 190 characters. The results show that based on Monobit examination and frequency analysis, the repeating nature of the key is non-existent, and the generated key can be used to encrypt a larger set of characters. The ciphertext has a low IC value of 0.030, which is similar to a random string and polyalphabetic cipher with an IC value of 0.038 but not equal to a monoalphabetic cipher with an IC value of 0.065. Results show that the modified version of the algorithm performs better than some of the recent studies conducted on it