Problem 4. Aoife has just intercepted a long ciphertext which she knows to be an encryption of English plaintext using a simple substutition cipher. She finds that the letters in the ciphertext occur with these frequencies: A 6.4% B 0.1% N 0 1.5% 4.6 % C 6.4% D 6.4% E 1.5% F 7.7% G 6.6% H I 0.2% 0.1% J 2.7% P Q R S T U V W 1.8 % 8.3 % 12.6% 3.2% 0.7% 9.3% 0.0% 2.0% K 2.2% X 2.6% Which ciphertext letter most likely corresponds to the plaintext letter E? Explain briefly. L 1.0% Y 2.4% M 2.7% Z 7.3%
Cipher frequency analysis.
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4. Aoife has just intercepted a long ciphertext which she knows to be an ancryption of English plaintext
using a simple substutition cipher. She finds that the letters in the ciphertext occur with these frequencies
C D E L 6.4% 0.1% 6.4 6.4 1.3% 7.7% 5.6% 9.2% 0.1% 2.7% 2.2% 1.0% 2.7% R STU V W X Y Z 4.6% 1. 8. 12.6 1.2% 9.7% 9,7% 0.0% 2.0% 2.6% 2.4% 7.38
Which ciphertext letter most likely corresponds to the plaintext letter? Explain briefly
To determine which ciphertext letter corresponds to the plaintext letter, we can compare the frequency distribution of the ciphertext letters to the frequency distribution of English letters. In English, the most common letters are E, T, A, O, and I, while the least common letters are J, Q, X, and Z.
Looking at the frequency distribution of the ciphertext letters, we see that the letters C, E, and L are the most common, while the letters V, W, Y, and Z are the least common. This suggests that the most common ciphertext letter is likely to correspond to the plaintext letter E, while the least common ciphertext letter is likely to correspond to the plaintext letter Z.
Therefore, based on the frequency distribution of the ciphertext letters, it is most likely that the ciphertext letter with a frequency of 6.4% corresponds to the plaintext letter E.
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