C is correct. Digital envelope uses symmetric key to encrypt messages, and uses symmetric encryption technology, and symmetric key is encrypted by the public key of the receiver to ensure security.

A is incorrect. The content of digital envelope consists of message ciphertext and key ciphertext, and does not contain digital signature.

B that's not correct. Digital envelopes are used to transmit confidential information. It cannot confirm the identity of the sender. It is the digital signature that can confirm the identity, not the digital envelope. The digital envelope does not contain the identity of the sender.

D is incorrect. How can the sender have the private key of the receiver?