Proxy Re-Encryption provides the ability to transform a ciphertext encrypted under some key to the one under another key. This seems interesting. But, something seems strange.
Suppose Alice has a key pair (sk_a, pk_a) and Bob also has a key pair (sk_b, pk_b). If Alice and Bob generates a proxy re-encryption key to transform a ciphertext under pk_a to the one under pk_b and vice versa, they can send message without revealing their secret keys.
At this point, what about the following scenario? Bob securely transmits his public key pk_b to Alice. Then, Alice encrypts her message vy using pk_b. There is no need to transform the ciphertext. Also, Alice can send Bob her public key pk_a when she wants to receive the message from Bob.
What is the advantage of proxy re-encryption? I think there is no potential difference between standard public key encryption scheme and proxy re-encryption.