The present invention relates to a method for symmetrical encryption or decryption of a data block from a secret key (K), the method comprising steps of: permutation (100) of at least one portion of the secret key (K) by means of a first permutation table (PC1′) so as to produce initial data, execution of several iterations, an iteration comprising steps of: rotation (102) of data dependent on the initial data so as to produce shifted data, permutation (104) of the shifted data by means of a second permutation table (PC2′) so as to produce a round key, execution of a plurality of encryption rounds (200) from the data block, an encryption round (200) using one of the round keys, generation of at least one of the permutation tables (PC1′, PC2′), the generation comprising determination of at least one function (F, G) variable from one encryption or decryption to another, composition of said function (F, G) with a predetermined permutation table (PC1, PC2), application of the inverse of said function (F, G) to data dependent on the data produced by the permutation step (100, 104) using the generated permutation table (PC1′, PC2′).