The number of devices connected through the Internet of Things (IoT) will significantly grow in the next few years while security of their interconnections is going to be a major challenge. For many devices in IoT scenarios, the necessary resources to send and receive bytes are extremely high and when such devices are powered with battery the amount of exchanged bytes directly impacts their life time. As a result, compression of existing protocols is a widely accepted technique to make IoT benefit from the protocols developed over the last decades. This paper presents ESP Header Compression (EHC), a framework that enables compression of packets protected with Encapsulating Security Payload (ESP). EHC is composed of EHC Rules, targeting the compression of a specific field and organized according to EHC Strategies. Further, the paper presents Diet-ESP, an EHC Strategy that highly reduces the networking overhead of ESP packets to address the IoT security and bandwidth requirements. Diet-ESP results in sending fewer bytes over the network which in turn reduces the number of required radio frames and thus battery consumption. For example, 10 byte application data on IEEE 802.15.4 radio networks secured with the standard ESP requires sending an additional frame, resulting into a 95% energy overhead compared to the unprotected data. In contrast, Diet-ESP results only in a 3% overhead compared to unprotected data. Diet-ESP remains fully security compliant to ESP and performs better than any other compression framework as far as ESP is considered. However, as some compression needs to be performed inside the ESP stack the implementation of EHC requires modifying the ESP protocol