If you use substitution alone, what weakness is present in the resulting ciphertext?

The correct answer highlights that a substitution cipher retains letter and word frequency patterns from the plaintext to the ciphertext. This characteristic makes it vulnerable to various types of cryptanalysis, particularly frequency analysis. In languages like English, certain letters and combinations of letters occur with predictable frequencies. For instance, the letter 'E' is more common than 'Q.' When a substitution cipher is used, the frequency of each letter remains consistent in the encrypted message, which can be exploited by an adversary who studies the ciphertext.

In the context of cryptanalysis, if an attacker knows the language of the plaintext, they can analyze the frequency of characters in the ciphertext and match them to the expected frequencies of characters in that language. This can lead to successful deciphering of the message without needing the key. This vulnerability underscores the limitations of using substitution alone for secure encryption.

Other weaknesses mentioned in the options may relate to different types of encryption techniques or attack mechanisms but do not directly address the fundamental nature of substitution ciphers and their handling of letter and word frequencies.