There are two types of keys in ASYMMETRIC encryption i.e., PUBLIC and private keys. There is a pair of private and public keys for each party. The public key, as its name implies, can be exchanged securely with communication PARTNERS, while the private key must remain confidential (secret). The private key is TYPICALLY used to decrypt data, while the public key is used to encrypt data.

To encrypt traffic between Jessica and Monica, as depicted in the above figure, the pair will exchange public keys.

• In order to encrypt Jessica' message to Monica, Jessica will USE Monica's public key. Monica will need to use his own private key to decrypt Jessica' message.
• When Monica replies to Jessica in the future, the same process will play out. Monica will use Jessica's public key to encrypt his reply message to Jessica.  Jessica will need to use his own private key to decrypt Monica's reply message.

Therefore, before any encrypted communication can take place, Jessica and Monica must exchange public keys.