We received the following ciphertext which was encoded with a shift cipher:JGNNQKPFKCPerform an attack against the cipher based on a letter frequency count. What is the cleartext?Letter Frequency Letter FrequencyA 0.0817 N 0.0675B 0.0150 O 0.0751C 0.0278 P 0.0193D 0.0425 Q 0.0010E 0.1270 R 0.0599F 0.0223 S 0.0633G 0.0202 T 0.0906H 0.0609 U 0.0276I 0.0697 V 0.0098J 0.0015 W 0.0236K 0.0077 X 0.0015L 0.0403 Y 0.0197M 0.0241 Z 0.0007
Question
We received the following ciphertext which was encoded with a shift cipher:JGNNQKPFKCPerform an attack against the cipher based on a letter frequency count. What is the cleartext?Letter Frequency Letter FrequencyA 0.0817 N 0.0675B 0.0150 O 0.0751C 0.0278 P 0.0193D 0.0425 Q 0.0010E 0.1270 R 0.0599F 0.0223 S 0.0633G 0.0202 T 0.0906H 0.0609 U 0.0276I 0.0697 V 0.0098J 0.0015 W 0.0236K 0.0077 X 0.0015L 0.0403 Y 0.0197M 0.0241 Z 0.0007
Solution
The text you provided is a ciphertext encoded with a shift cipher, also known as a Caesar cipher. This is a type of substitution cipher where each letter in the plaintext is 'shifted' a certain number of places down the alphabet.
To perform a frequency analysis attack, we first need to identify the most common letter in the ciphertext. In English, the most common letter is 'E' with a frequency of 0.1270.
Looking at the ciphertext "JGNNQKPFKC", the most frequent letter is 'N'.
In the English alphabet, 'N' is the 14th letter and 'E' is the 5th. The difference between these two is 9. This suggests that the shift used in the cipher is 9.
To decode the message, we shift each letter in the ciphertext 9 places back in the alphabet.
Here's how it works:
J -> A G -> Z N -> E N -> E Q -> H K -> B P -> G F -> W K -> B C -> T
So, the decoded message is "AZEEHBGWBT".
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