Let’s consider an encryption algorithm that is “unbreakable” by design. We code it and implement in on certain hardware (provided by 3rd party for example). The Side-Channel attack uses the unintentional leakage of secret information generated by that hardware. For instance, Paul Kocher realized that different plaintexts and keys cause different loads of system resources. Another example could be electro-magnetic radiation generated by the hardware we use. To prevent the first kind of leakage we can obscure the computation, therefore there won’t be any peaks or bottoms in consumption. The second one could be avoided by better EM shields.