(第4空)Symmetric, or private-key, encryption is based on a secret key that is shared by both communcating parties. The ( ) party uses the secret key as part of the mathematical operation to encrypt ( ) text to cipher text. The receiving party uses the same secret key to decrypt the cipher text to plain text. Asymmetric, or public-key, encryption uses two different keys for each user: one is a ( ) key known only to this one user; the other is a corresponding public key, which is accessible to anyone. The private and public keys are mathematically related by the encryption algorithm. One key is used for encyption and the other for decryption, depending on the nature of the communication service being implemented. In addition, public key encryption technoligies allow digital ( ) to be placed on messages. A digital signature uses the sender's private key to encrypt some portion of the message. When the message is received, the receiver uses the sender's ( ) key to decipher the digital signature to verify the sender's identity.