Every mobile phone has a serial number, and each serial number can correspond to a mobile phone certificate. Just like an ID card, you can't use my ID card. As long as you provide the serial number of your mobile phone, you can make a proof on the last website of your mobile phone. Key is a document used when signing, which is useless, but signing is only for use. The key is the same. You can sign by making a certificate online. Hmm. How interesting