Crypto Keys and Key Exchange

In Crypto, encryption and decryption keys need to be well guided. In classical cryptography, A secret key is used. Key distribution is difficult.

British GCHQ (now NCSC) introduced the public key cryptography concept. A third party (CA) is responsible for key creation and distribution. This makes key exchange convenient. 

Fig. Public Key Cryptography

RSA public key encryption is based on two large prime numbers. Factor the product of those numbers seems simple but requires tremendous amount of time, or in many cases important. 

Since public key algorithms such as RSA require complex computations, it is slow. As a result, people usually do not use it directly for encrypting message (a large amount of data). Instead, use it to exchange short session keys for secret key algorithms (aka symmetric algorithms) such as AES. 

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