In this level, you can see an example of Bob sending Alice an encryption key in order to protect their messages. However, their simple protocol is vulnerable to a man-in-the-middle attack.
To beat this level, you need to trick Alice into encrypting her messages using a key Eve knows: 31337. You must also trick Bob into thinking he is having a conversation with Alice as normal! See if you can figure out how to trick them both without them discovering your presence between them.
Hint: there is an example of how the encryption protocol works at the top of the field. Use the button to see what the values of the packet headers are in the protocol.