For the sake of demonstration, let Bob choose 7 and 11 as p and q and calculate n = 7 × 11 = 77. The value of φ(n) = (7 − 1)(11 − 1), or 60. If he chooses e to be 13, then d is 37. Note that e × d mod 60 = 1. Now imagine that Alice wants to send the plaintext 5 to Bob. She uses the public exponent 13 to encrypt 5. This system is not safe because p and q are small.