Ashley Madison is one of the first online dating websites started from $2001.$ They used MD$5$ to store customers$$ pwd$.$ In $2015,11$million users' passwords are cracked in $10$ days.

$1)$ Please first explain why in the server side, the cipher text encrypted by symmetric cryptography should NOT be used to store customers' passwords.

$2)$ Ashley Madison used MD$5$ hashing at the beginning. Please explain which type attack happens to its MD$5$ hash.

$3)$ Suggest which method can be used to defend against above attack happened in MD$5-$based pwd$.$