In terms of a private key and a public key? cryptography, what, specifically, is the distinction between

What is the difference between a block cipher and a stream cipher? What is the difference between an unconditionally secure cipher and a computationally secure cipher?   What is the difference between a monoalphabetic cipher and a polyalphabetic cipher?  Why is it important to study Feistel ciphers?

A government agency comes to the conclusion that it must use encryption in order to safeguard the communication that takes place between itself and its overseas counterparts. It is determined to design a proprietary cryptographic algorithm for this reason during a meeting with its counterparts, and this will be done in order to: Is there a justification for this choice. What risks are being taken?

When putting into practice cryptography that is reliant on public keys, what is the absolute bare minimum amount of keys that are need to be used?