In this system, there are two matching keys, one is the public key and the other is the private key. If a message is encrypted with my public key, it can only be decrypted with my private key, which means that many people can send me encrypted messages, but only I can decrypt them, thus realizing the confidentiality of data. In addition, if the message is encrypted with my private key, it can be decrypted with my public key. If there is an encrypted message decrypted by someone else with my public key, then this message must be sent by me, because if it is sent by someone else, it cannot be decrypted by my public key. If it can be decrypted by my public key, it must be data encrypted by my private key, because the private key is only mine, so I can't deny that this data is mine, thus realizing undeniable.