多项选择题Which two statements about using the CHAP authentication mechanism in a PPP link are true?()
A. CHAP uses a two-way handshake
B. CHAP authentication periodically occurs after link establishment
C. CHAP has no protection from playback attacks
D. CHAP authentication is performed only upon link establishment
E. CHAP uses a three-way handshake
F. CHAP authentication passwords are sent in plaintext