Key exchange
If two parties wish to exchange encrypted messages, each needs to know how to decrypt received messages and to encrypt sent messages. The information they require to do so depends on the encryption technique they might use. If they use a code, both will require a copy of the same codebook. If they use a cipher, they will need appropriate keys. If the cipher is a symmetric key cipher, both will need a copy of the same key. If an asymmetric key cipher with the public/private key property, both will need the other's public key.