Which security strategy requires using several, varying methods to protect IT systems against attacks?

Defense in depth applies several varied security controls - such as firewalls, intrusion detection systems, encryption, and access controls - at different layers of an IT environment, ensuring no single point of failure can expose the whole system. This layered approach means an attacker who bypasses one control is still confronted by additional independent barriers.

Covert channels are hidden communication paths used to exfiltrate data outside of normal monitored channels, not a protective multi-layer security strategy.

Exponential backoff is a network retry algorithm that progressively increases wait time between retransmission attempts after collisions or failures, unrelated to security layering.

The three-way handshake is the TCP connection establishment mechanism (SYN, SYN-ACK, ACK) and is a transport-layer protocol process, not a security protection strategy.