Key exchange
The key exchange problem is how to exchange whatever keys or other information needed so, in such a way, that no one else can obtain a copy. Usually, this required trusted secure channel. With the arrival of (public key / private key) cipher algorithms, the encrypting key (aka, the public key) could be made public, since (at least for "high quality" ciphers) no one without the decrypting key (aka, the private key) could decrypt the message.