A method provided herein includes the following steps: storing seal data of an electronic seal, a digital certificate, electronic signature program and a private key of a sealer in an external portable apparatus; performing a Hash conversion to a file to be sealed and the seal data of the electronic seal to generate a data digest, wherein the file to be sealed is a layout file; sealing, in the portable apparatus, the data digest using the private key of the sealer and the electronic signature program to generate an electronic signature result; and combining the file to be sealed, the seal data of the electronic seal, the digital certificate and the electronic signature result to generate a seal combination file.