INVITATION TO CYBERSECURITY 152 Table 7.4 Vigenère table for encoding all possible polyalphabetic substitutions. While this is a much stronger scheme than the monoalphabetic substitution cipher, it is still vulnerable to cryptanalysis because the key is recycled over and over again. Therefore, the underlying plaintext language pattern emerges in the ciphertext, except now it emerges at the periodicity of the key length—it just requires more ciphertext to reveal it. Charles Babbage, the first person to theorize a general purpose computer, wrote about how the Vigenère cipher could be broken through this type of cryptanalysis in 1854. The Vigenère cipher is a stronger cipher than the monoalphabetic substitution cipher, and more ciphertext is needed to recover the plaintext message using cryptanalysis—the longer the key, the more ciphertext is needed.
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