Question

7. (16 points) Caesar cipher is the earliest known substitution cipher. Please answer the following questions. (a) Encrypt the following message "SECURITY" by translating the letters into numbers, applying the encryption function given, and then translating the numbers back into letters

ABCDEYCH IJ K L M N O P Q R S T U V W XYL 0 12 14 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
f(p)=(p+4) mod 26 (b) Suppose the encrypted message is "JEQYVEXXPIVW", what is the plain text (decrypted message)?

Answer 1

The encryption of "SECURITY" using the Caesar cipher is "WHGXUQMID". The decryption of "JEQYVEXXPIVW" using the inverse function reveals the plain text as "FAMOUSCIPHER".

Step-by-step explanation:

The Caesar cipher is a substitution cipher where each letter in the plaintext is shifted a certain number of places down or up the alphabet. For encryption, each letter of the plaintext is translated into a number, the encryption function f(p)=(p+4) mod 26 is applied, and then it is translated back into a letter.

To encrypt the message "SECURITY", we would first convert each letter to its respective numerical value: S=18, E=4, C=2, U=20, R=17, I=8, T=19, Y=24. Then, applying the encryption function, we get 22, 8, 6, 24, 21, 12, 23, 2. Converting these back to letters, we get "WHGXUQMID".

For decryption of the message "JEQYVEXXPIVW", we reverse the process. Each letter is converted to a number, and the inverse function f-1(c)=(c-4) mod 26 is applied. Hence, J=9, E=4, Q=16, Y=24 and so on. After applying the inverse function, we get 5, 0, 12, 20 secret message "FAMOUSCIPHER".