Compare and contrast methods to detect and defend against malicious actors who could try to compromise or perform man-in-the-middle attacks on systems with E2EE.

Detecting and defending against malicious actors attempting to compromise or perform man-in-the-middle (MitM) attacks on end-to-end encrypted (E2EE) systems requires a multi-faceted approach. These attacks can range from subtle manipulations of key exchange to more overt attempts to eavesdrop on communications. The methods used to detect and defend against them can be categorized into proactive measures implemented within the E2EE protocol itself, and reactive measures, which include monitoring systems and educating users. One of the most critical proactive measures is the use of authenticated key exchange protocols. Standard Diffie-Hellman (DH) key exchange, while mathematically secure, is vulnerable to MitM attacks if not properly authenticated. In these attacks, an adversary intercepts the key exchange and replaces the legitimate public keys with their own, thus establishing separate encryption sessions with each communicating party. To prevent this, modern E2EE systems use protocols like Elliptic Curve Diffie-Hellman (ECDH) with digital signatures or the more advanced Extended Triple Diffie-Hellman (X3DH) used in the Signal Protocol. In X3DH, each party signs their public key using their long-term private key, which allows the other party to verify the authenticity of the key. This prevents the MitM attack because the attacker cannot forge the signature. This method ensures that the exchanged public keys are from the claimed party and not an imposter. Another proactive defense is the use of short-lived ephemeral keys for key exchange and encryption. Instead of relying on long-term keys alone, systems with Perfect Forward Secrecy (PFS) use ephemeral keys that are generated at the beginning of each session. If a long-term key is compromised, only that session is at risk, and other previous or future sessions ....