1. A system for digitally signing electronic documents, the system comprising a mobile device, an application server, and a database, the application server being connected to the mobile device and the database, the database being configured for storing electronic documents to be signed digitally, the mobile device storing a digital certificate of a signer, the digital certificate comprising a private key, the mobile device comprising a requesting module and a digest encrypting module, the application server comprising an obtaining module, a digest generating module and a merging module, wherein: the requesting module is configured for sending a request for a digital signature of an electronic document to the application server; the obtaining module is configured for obtaining the electronic document from the database according to the request; the digest generating module is configured for generating a digest of the electronic document using a hash algorithm, and sending the digest of the electronic document to the mobile device; the digest encrypting module is configured for encrypting the digest of the electronic document with the private key using a public key cryptogram algorithm thereby generating a first encrypted value, and sending the first encrypted value to the application server; the merging module is configured for merging the electronic document and the first encrypted value according to the PKCS #7 standard; the digest generating module is further configured for generating a digest of the merged document using the hash algorithm, and sending the digest of the merged document to the mobile device; the digest encrypting module is further configured for encrypting the digest of the merged document with the private key using the public key cryptogram algorithm thereby generating a second encrypted value, and sending the second encrypted value to the application server; and the merging module is further configured for merging the first encrypted value, the second encrypted value and the electronic document according to the PKCS #7 standard thereby generating a digitally-signed electronic document.
1. A system for digitally signing electronic documents, the system comprising a mobile device, an application server, and a database, the application server being connected to the mobile device and the database, the database being configured for storing electronic documents to be signed digitally, the mobile device storing a digital certificate of a signer, the digital certificate comprising a private key, the mobile device comprising a requesting module and a digest encrypting module, the application server comprising an obtaining module, a digest generating module and a merging module, wherein: the requesting module is configured for sending a request for a digital signature of an electronic document to the application server; the obtaining module is configured for obtaining the electronic document from the database according to the request; the digest generating module is configured for generating a digest of the electronic document using a hash algorithm, and sending the digest of the electronic document to the mobile device; the digest encrypting module is configured for encrypting the digest of the electronic document with the private key using a public key cryptogram algorithm thereby generating a first encrypted value, and sending the first encrypted value to the application server; the merging module is configured for merging the electronic document and the first encrypted value according to the PKCS #7 standard; the digest generating module is further configured for generating a digest of the merged document using the hash algorithm, and sending the digest of the merged document to the mobile device; the digest encrypting module is further configured for encrypting the digest of the merged document with the private key using the public key cryptogram algorithm thereby generating a second encrypted value, and sending the second encrypted value to the application server; and the merging module is further configured for merging the first encrypted value, the second encrypted value and the electronic document according to the PKCS #7 standard thereby generating a digitally-signed electronic document. 2. The system according to claim 1 , wherein the hash algorithm is the Secure Hash Algorithm 1 (SHA1).