1. A method of signing a document by a plurality of contracting users, wherein said contracting users are pre-registered users and have associated electronic pen devices such that each contracting user among the contracting users has an associated electronic pen device among the electronic pen devices, the method comprising the following steps: a) receiving at a secure server, from a first user, an invitation request of signing the document for the purpose of approving content of the document by a plurality of contracting users; b) receiving, at the secure sever, user identification information associated with the contracting users; c) generating, by the secure server, a unique document identifier in connection with said document to be signed; d) transmitting, by the secure server, an invitation to sign the document to the contracting users through their associated electronic pen devices along with the unique document identifier such that each contracting user among the contracting users receives said invitation to sign the document at an electronic pen device associated therewith; wherein the electronic pen devices are configured for receiving signature requests from the contracting users for signing the document, for the purpose of approving the content of the document, the signature requests comprising first authentication information associated with said contracting users, and wherein the electronic pen devices are configured for transmitting the first authentication information to the secure server for authenticating the contracting users; e) authenticating by the secure server the contracting users using said first authentication information comprising inquiring an authentication database at the secure sever storing second authentication information in connection with the contracting users for determining if there is a match between the first authentication information and the second authentication information; f) rejecting, by the secure server, signature requests associated with unsuccessfully authenticated contracting users; g) for successfully authenticated contracting users, automatically transmitting by the secure server to electronic pen devices associated with said successfully authenticated contracting users notifications of successful authentication and said associated electronic pen devices accepting signature requests from their respective contracting users to sign the document, receiving by the associated electronic pen devices and transmitting to the secure server handwritten signatures of the authenticated contracting users approving content of the document, automatically storing by the secure server said handwritten signatures inside a signature registration database, automatically generating by said secure server a unique signature identifier and associating said unique signature identifier to said handwritten signatures, and automatically communicating said unique signature identifier to said associated electronic pen devices, and communicating said unique signature identifier by the associated electronic pen devices to the successfully authenticated contracting users for indication on the document in order to be used as a reference during future authentication of said document; and h) during authentication of the document, providing access to said signature registration database for enabling an authenticating party to authenticate said document, where said authentication of said document comprises obtaining the unique signature identifier as indicated on the document; querying the secure server using this unique signature identifier to retrieve the stored signature associated with the unique signature identifier and determining if there is an exact match between the signature stored in the server and the signature appearing on said document.