1. A method for decoding encoded data, comprising source data and redundancy data said redundancy data being obtained by the application to said source data, when encoding, of an error-correcting code which can be represented by a generating matrix comprising, in its systematic form, an identity matrix and an invertible matrix for switching from source data to redundancy data, wherein said method comprises: a step of decoding, which simultaneously implements said error-correcting code, called a primal error-correcting code, and a code that is the dual of the primal error-correcting code, called a dual error correcting code, wherein the step of decoding uses a modified Tanner graph, called a primal-dual graph, obtained by superimposing a Tanner graph corresponding to the primal error-correcting code, called a primal graph, and a Tanner graph corresponding to the dual error-correcting code, called a dual graph, said superimposition comprising a two-by-two connection of input variables of said primal graph with input variables of said dual graph corresponding to the same source data and a two-by-two connection of output variables of the primal graph with output variables of said dual graph corresponding to the same redundancy data.