3rdKey Password Protection

What is a Diffie-Hellman secret key exchange?

Diffie-Hellman, "named after Whitfield Diffie and Martin Hellman is one of the earliest practical examples of public key exchange" --Wikipedia
There are numerous explanations, but I think this is the simplest

Alex and Brenda want to talk secretly, they need a secret key
Alex comes up with a random number that he keeps secret, it's his private key
Brenda does the same
Alex uses his private key in a function to get a public key
Brenda does the same with her private key
Alex tells Brenda his public key
Brenda tells Alex her public key
Alex feeds his private key and Brenda's public key into a function and gets a shared secret key
Brenda feeds her private key and Alex's public key into a function and gets a shared secret key
Alex and Brenda use the shared secret key for encrypting their messages
Anyone listening will know both public keys but not be able to find the shared secret key unless they have one of the private keys
The only way to break a Diffie-Hellman key exchange is to act as a Man In The Middle