The present invention relates to the use of ulvans, in particular extracted from green algae of the genus Ulva or Enteromorpha, or of ulvan-derived oligosaccharides, as activators of plant defense and resistance reactions against biotic or abiotic stresses. The microwave oven is capable of using a plurality of input powers with different frequencies. For each plain text block of the plurality of plain text blocks other than a first plain text block, the plain text block is multiplied by a preceding cipher text block over the finite field of even characteristic, modulo the primitive polynomial, to generate an intermediate block, wherein the preceding cipher text block was generated from a preceding plain text block.