Research papers in lightweight cryptography for the MQTT (Message Queuing Telemetry Transport) protocol focus on providing secure communication in IoT environments while minimizing computational, memory, and energy overhead on resource-constrained devices. MQTT’s lightweight, publish-subscribe architecture over TCP/IP or TCP/UDP makes it suitable for low-bandwidth and high-latency networks, but standard cryptographic protocols such as TLS can be too heavy for constrained nodes. To address this, researchers have explored lightweight symmetric-key algorithms like AES-CCM, ChaCha20-Poly1305, and SPECK for encryption and message authentication, which offer strong security with low computational cost. Elliptic Curve Cryptography (ECC), including ECDH for key exchange and ECDSA for digital signatures, is widely studied for providing secure authentication and integrity with smaller key sizes compared to RSA, reducing bandwidth and energy consumption. Identity-based and certificate-less cryptography schemes have also been proposed to simplify key management in MQTT networks, particularly in large-scale IoT deployments. Integration of lightweight cryptography with protocol-level optimizations, such as MQTT-S (Secure MQTT), compressed headers, and session resumption techniques, has been investigated to reduce handshake latency and resource usage. Recent research explores hybrid approaches combining lightweight cryptography with trust-based mechanisms, intrusion detection, or blockchain-assisted key management to enhance security while maintaining efficiency. Despite advancements, open challenges remain in achieving ultra-low overhead, scalability in dense networks, and resistance against evolving attacks such as replay, spoofing, and collusion. Overall, the literature highlights that lightweight cryptography is essential for securing MQTT-based IoT systems, balancing strong protection with the severe constraints of low-power, low-memory devices.