Introduction to Cryptography with Coding Theory, 3rd Edition

2.8 Exercises

1. Caesar wants to arrange a secret meeting with Marc Antony, either at the Tiber (the river) or at the Coliseum (the arena). He sends the ciphertext $EVIRE$. However, Antony does not know the key, so he tries all possibilities. Where will he meet Caesar? (Hint: This is a trick question.)

2. Show that each of the ciphertexts $ZOMCIH$ and $ZKNGZR$, which were obtained by shift ciphers from one-word plaintexts, has two different decryptions.

3. The ciphertext $UCR$ was encrypted using the affine function $9x+2$ mod 26. Find the plaintext.

4. The ciphertext $JLH$ was obtained by affine encryption with the function $9x+1$ mod 26. Find the plaintext.

5. Encrypt howareyou using the affine function $5x+7(\text{mod}26)$. What is the decryption function? Check that it ...