What addition could Alice and Bob implement to avoid the "man in the middle" attack?

Implementing a digital signature is a highly effective way for Alice and Bob to protect their communication from a "man in the middle" attack. A digital signature provides a means to validate the authenticity and integrity of the messages being sent. When Alice sends a message to Bob, she can sign it with her private key. Bob, upon receiving the message, can then use Alice’s public key to verify that the signature is valid and that the message has not been altered during transmission. This dual assurance means that the message truly came from Alice and remains intact.

In comparison, while checksums can provide some verification of data integrity, they do not authenticate the sender. Thus, an attacker could modify both the message and the checksum to maintain a façade of integrity. Timestamps can help in avoiding replay attacks but don't offer authenticity of the message sender. An encryption key is crucial for keeping the communication confidential; however, it does not provide a mechanism to validate the sender's identity, which is fundamental in mitigating man in the middle threats.