What is the explanation for the main features of secret-key cryptography?

Answer 1

Suppose that we have two people, Person A and Person B. Person A wants to encrypt a message to send to B so that nobody else can read it.

Secret key cryptography works on the main principle that Person A would devise an encryption algorithm and create a key that can decypher it. Person A would then share the key with Person B. Thus when Person A sends B a message, Person B can use the key to decypher it. Since nobody else would know the key only Person B can decypher the message.

The obvious problem with this method that makes it impossible is that if Person A had a way to sharing the key with Person B without anyone else evesdropping then there would not be a need for encryption at all because Person A can simply deliver his message this way.

Therefore a far more practical approach involves using a Public-key system.