hotp.go
func (otp HOTP) Increment() {
	for i := ctrSize - 1; i >= 0; i-- {
		if otp.counter[i]++; otp.counter[i] != 0 {
			return
		}
	}
}
func (otp HOTP) Increment() {
	for i := ctrSize - 1; i >= 0; i-- {
		if otp.counter[i]++; otp.counter[i] != 0 {
			return
		}
	}
}
// Increment will increment an HOTP source's counter. This is useful
// for providers like the Google Authenticator app, which immediately
// increments the counter and uses the 0 counter value as an integrity
// check.