Sign: Sender用自己的private key签名后，连同自己的public key，发送出去。Receiver用sender的public key解密
Encrypt: Sender用Receiver的public key加密，发送出去。
Because the private key in a key pair belongs to only one party, any time that it is shown that the private key has been used, it can be concluded that only the owner of that key has used it.
Plaintext would have to be encrypted to ciphertext using a private key and then decrypted back to plaintext using the corresponding public key.
Combining the message with the private key is accomplished by encrypting the hash value with the sender's private key, which creates the actual digital signature.
Because only one private key can correspond to a public key, and only the owner of the public key could use it to encrypt the hash value successfully, decrypting the hash with the public key shows that the private key owner encrypted the hash value.