$1.(25$ points$)$ Blockchain and Cryptography

$($a$)(5$ points$)$ What is public key cryptography? Outline its advantages against to symmetric key cryptography.

$($b$)(15$ points$)$ Work out the algorithm of Diffie$-$Hellman Key exchange in pseudocode.

$($c$)(5$ points$)$ Analysis the risk exposure of the Diffie$-$Hellman Key exchange through employing any attack.