Exam-style question
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Use the following simple Vernam-cipher convention: represent A to Z as 0 to 25; encryption is c = (p + k) mod 26; decryption is p = (c - k) mod 26. Encrypt the plaintext DOG using the key QHF, then decrypt the ciphertext to recover DOG. Explain why the Vernam cipher is considered to have perfect security and state the rule about reusing the key.
Model answer
What a good answer should say
- Using A = 0, B = 1, and so on: D is 3 and Q is 16, so (3 + 16) mod 26 = 19, which is T.
- O is 14 and H is 7, so (14 + 7) mod 26 = 21, which is V.
- G is 6 and F is 5, so (6 + 5) mod 26 = 11, which is L.
- The ciphertext is TVL.
Explanation
Why this works
The same key values are added during encryption and subtracted during decryption, using modulo 26 to keep values within the alphabet. Perfect security depends on the key being uniformly random and never reused.
This distinguishes the Vernam cipher from ciphers whose security is based on computational difficulty.
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