Advanced Placement (AP) Computer Science

What principle does frequency analysis rely on when decrypting ciphertext?

1. The randomness of letters

2. The specific patterns of language

3. The use of symbols instead of letters

4. The repetition of words

The correct answer is: The specific patterns of language

Frequency analysis relies on the understanding that certain letters and combinations of letters occur with varying frequencies in a given language. Every language has specific patterns that can be identified, such as the common usage of letters like 'E', 'T', 'A', and 'O' in English. By analyzing the frequency of letters or groups of letters in the ciphertext, one can make educated guesses about which letters might correspond to which characters in the plaintext. This principle is particularly effective for simple substitution ciphers, where each letter is replaced with another letter. Since the frequency of letters in any language is not random but follows particular patterns, evaluating the frequency of symbols in ciphertext allows decryptors to deduce which letters may have been substituted and ultimately reconstruct the original message. Other options provided do not align closely with the methodology of frequency analysis. While randomness (option A) suggests unpredictability, patterns of language (option B) are fundamental to effective decryption. The use of symbols (option C) would pertain more to symbolic representation than to frequency analysis, which focuses specifically on letters. Finally, the repetition of words (option D) might assist in understanding context but does not directly pertain to frequency analysis as it primarily focuses on individual letter frequencies and characteristics of the language