Known-plaintext attack
Alice sends Bob message using block size of 20 digits.
Eve intercepts, and she knows message starts with “Dear Bob” => knows both plaintext and ciphertext of first block.
cyph = plain + key mod 1020
=> key = cyph - plain mod 1020
Also works if Eve knows of some number of ways Alice’s message likely to begin. Tries each one => set of possible keys => tries each key on entire message.
Using prior knowledge about message -- in English