In this paper we show how interval analysis can be used to calculate rigorously valid enclosures of transversal homoclinic points in discrete dynamical systems. The existence of such points guarantees that a Smale horseshoe is embedded in the mapping which therefore is chaotic. We take special interest in the 2-dimensional case and describe a method to obtain a computer-assisted proof of chaos for these systems. Numerical results are presented for the standard map.