6. The method according to claim 4 , wherein the public key cryptogram algorithm is the RSA algorithm.
4. A computer-based method for digitally signing electronic documents, the method comprising: sending a request for a digital signature of an electronic document to an application server through a mobile device, the mobile device storing a digital certificate of a signer, the digital certificate including a private key; obtaining the electronic document by the application server from a database according to the request, generating a digest of the electronic document using a hash algorithm, and sending the digest of the electronic document to the mobile device; encrypting the digest of the electronic document with the private key using a public key cryptogram algorithm through the mobile device thereby generating a first encrypted value, and sending the first encrypted value to the application server; merging the electronic document and the first encrypted value according to the PKCS #7 standard through the application server, generating a digest of the merged document using the hash algorithm, and sending the digest of the merged document to the mobile device; encrypting the digest of the merged document with the private key using the public key cryptogram algorithm through the mobile device thereby generating a second encrypted value, and sending the second encrypted value to the application server; and merging the first encrypted value, the second encrypted value and the electronic document according to the PKCS #7 standard through the application server thereby generating a digitally-signed electronic document.