One of the main problem of symmetric key encryption is key exchange. A research propose combine public key and symmetric key will solve this problem. Assuming Alice and Bob want to talk to each other and they decide to use public key to exchange a common key. Please explain how to proceed by:

1. List the formula using E as Encrytopn, D as Decryption and Key_Pub_Alice, and Key_Pub_Bob for public keys, Key_Priv_Alice and Key_Priv_Bob for private key. Key_Sym as the common key.

2. Explain why it works.

3. Why they do not need to remember the common key after the communication.(thus why key exchange)