TOTP vs HOTP (Push notification)

Hi,

我想做Two factor authentication (2FA), 但係有問題:

1. 如果一定要有push notification (accept/reject), 請問TOTP / HOTP 邊隻support? 我睇過吓, 兩隻只係分別係counter. 基本上冇大分別, 其實係咪要support, 是mobile app的問題, 同選擇TOTP / HOTP冇關係? 或者只有HOTP support push?

2. 請問有咩FREE mobile app可以有六位數字及push notification? 最好係唔需要開個remote 美國account. 只需要register個QR code就得?

3. 如果要有device based (共用account時或不同手機時), 請問server side邊隻software做到? 我睇過multiotp, gen個 TOTP QR code都OK, 但就唔知點用device based.

Thanks.