Xiao Ai should write the verification code according to the following steps: 1. Generate a random verification code string. 2. Draw the verification code string in the square picture. 3. Add some interference lines and interference points to the verification code picture, making it difficult to be recognized by the machine. 4. Return the verification code picture to the user. After the user submits the verification code, Xiao Ai should compare the verification code submitted by the user with the verification code stored in the server to verify whether it is correct. At the same time, in order to prevent attacks, the effective time and the number of attempts of the verification code should be limited.