Wireless sensor networks are often deployed in open and uncontrolled environments that make them more vulnerable to security attacks. Cryptographic algorithms can be used to protect the data collected by the sensors against an intruder. The cost in terms of energy to provide enough security can be quite large as these algorithms may be very complex. As communication is the main energy consumer, a way to save energy is to use data compression. We propose to measure the impact of the well-known DES algorithm on the energy consumption for various number of rounds and then, we show that energy-free security may be possible. We combine a cryptographic algorithm with a compression algorithm and show through a model that a node can provide security without consuming more energy. The only counterpart is the time for ciphering and compressing. We get some results from experiments on energy consumption of cryptographic and compression algorithms and establish the level of security that can be achieved in various cases, from a single node to a random network.