In which scenario is a multi-alphabet cipher more secure than a monoalphabetic cipher?

A multi-alphabet cipher is considered more secure than a monoalphabetic cipher because it employs different ciphertext alphabets throughout the encryption process. This means that the same plaintext character can be encrypted to different ciphertext characters at different positions or instances within the text.

The use of multiple alphabets complicates the frequency analysis technique, which is commonly used to break monoalphabetic ciphers. In a monoalphabetic cipher, there is a one-to-one mapping of characters, which allows attackers to analyze the frequency of letters in the ciphertext and make educated guesses about the underlying plaintext. Multi-alphabet ciphers, however, disrupt this pattern, as the variability in character substitutions can obscure the frequency distribution, making it far more challenging for an attacker to decode the message without knowledge of the substitution rules used.

In contrast, maintaining the same character frequency and using a fixed substitution pattern are characteristics of monoalphabetic ciphers, reducing their security. Simplicity is not a criterion associated with increased security; rather, more complex algorithms tend to offer stronger protection against cryptanalysis. Thus, employing different ciphertext alphabets inherently enhances security significantly over a monoalphabetic approach.