1. Alice sends a message to Bob, which is intercepted by Mallory:
Alice "Hi Bob, it's Alice. Give me your key"--> Mallory Bob
2. Mallory relays this message to Bob; Bob cannot tell it is not really from Alice:
Alice Mallory "Hi Bob, it's Alice. Give me your key"--> Bob
3. Bob responds with his encryption key:
Alice Mallory <--[Bob's_key] Bob
4. Mallory replaces Bob's key with her own, and relays this to Alice, claiming that it is Bob's key:
Alice <--[Mallory's_key] Mallory Bob
5. Alice encrypts a message with what she believes to be Bob's key, thinking that only Bob can read it:
Alice "Meet me at the bus stop!"[encrypted with Mallory's key]--> Mallory Bob
6. However, because it was actually encrypted with Mallory's key, Mallory can decrypt it, read it, modify it (if desired), re-encrypt with Bob's key, and forward it to Bob:
Alice Mallory "Meet me in the windowless van at 22nd Ave!"[encrypted with Bob's key]--> Bob
7. Bob thinks that this message is a secure communication from Alice.