What technique should be combined with substitution to enhance security against frequency analysis attacks?

To enhance security against frequency analysis attacks, combining substitution with transposition is an effective strategy. Substitution ciphers replace each letter or group of letters in plaintext with another letter or group, which can be vulnerable to analysis because the frequency of the letters remains the same in the ciphertext. Frequency analysis relies on the predictable nature of language, where certain letters appear with varying frequency.

By incorporating transposition, the positions of the letters in the ciphertext are altered without changing the actual letters themselves. This means that even if the substitution makes certain characters predictable, the transposition scramble can obscure those relationships, making it significantly harder for an attacker to perform frequency analysis successfully. This technique relies on the principle that altering the arrangement of characters can thwart attempts to detect patterns in the encrypted message.

In contrast, other techniques like diffusion or block ciphering address different aspects of encryption and may not specifically counteract the direct vulnerabilities of frequency analysis when used solely with substitution. Thus, the combination of substitution and transposition effectively fortifies the cipher against analysis by breaking the visual and statistical patterns that attackers rely on.