A communication system includes a first communication system and a second communication terminal. The first communication terminal generates a first shared key, and the second communication terminal generates a second shared key. During an exchange operation, the first communication terminal stores the second shared key of the second communication terminal, and the second communication terminal stores the first shared key of the first communication terminal. During a challenge operation, the first communication terminal sends a challenge string to the second communication terminal, the second communication terminal generates a response string by performing reversible encryption operations to the challenge string with the first shared key and the second shared key, the second communication terminal sends the response string to the first communication terminal, and the first communication terminal verifies the response string.