A certificate contains a pair of keys. According to relevant public information, each certificate contains a pair of keys, namely, a signed public key and a signed private key, an encrypted public key and an encrypted private key. When signing, sign with the private key of the signing certificate, and other users can verify the signature with the signing public key published by the external network. When encrypting, user B encrypts the information and transmits it to user A, and user A decrypts the encrypted information with his own encryption private key to obtain complete plaintext information.