Below is a representation of the UNDERGROUND Advanced Security Key (ASK) communication protocol. In this example UNDERGROUND Client A just logged in and is contacting UNDERGROUND Client B that is also logged in.

Usually private keys are kept secret, while the public keys may be widely distributed. Incoming messages that are encrypted with the recipient's public key can only be decrypted with his corresponding private key. The keys are related mathematically, but the private key cannot be practically derived from the public key.

All UNDERGROUND keys are kept secret and none of the keys are send over the Internet in an unencrypted form. Keys like the Shared Secret Key (SSK), the Temporary Secret Key (TSK) and the Current Session Key (CSK) are for both encryption and decryption. To use this kind of symmetric cryptography for communication, both the sender & receiver have to know the keys. Anything encrypted with such keys cannot be decrypted by anyone except the recipient possessing the same key. The SSK is derived from the Master Password and the Custom Version Number (CVN). Without this knowledge communication is not possible. This successfully eliminates the man in the middle attack.

In addition a new set of keys is used every time a new UNDERGROUND session starts, including the public and private keys. All keys are irrevocably lost as soon as the session ends.