Exam-style question
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A Caesar cipher uses a shift of 3. Encrypt the plaintext message CAT and then decrypt the resulting ciphertext. Use the alphabet cyclically, so that shifting beyond Z continues from A. Explain why this cipher is easily cracked.
Model answer
What a good answer should say
- Encrypting CAT with a shift of 3 gives FDW: C becomes F, A becomes D, and T becomes W.
- Decrypting FDW by shifting each letter back by 3 gives CAT.
- The cipher is easily cracked because it has only a small number of possible shifts.
- An attacker can try the possible shifts until a meaningful plaintext message is obtained.
Explanation
Why this works
Encryption applies the same shift to each letter. Decryption reverses the shift.
The limited number of possible keys makes exhaustive testing practical, so the Caesar cipher does not provide strong security.
Common mistake
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