https://lastpass.com/how-it-works 裡面有寫: "All sensitive data is encrypted and decrypted locally before syncing with LastPass. Your key never leaves your device, and is never shared with LastPass. Your data stays accessible only to you."
可以幫忙填密碼是因為 Chrome 有 plugin 幫你記住你的 master password,這樣比較方便 (當然是用安全性換來的,就看怎麼取捨),server 上存的資料都是用你的 master password 加密過的