1. A security document system comprising: a database for storing security document data including, for each of a number of security documents: identity data, the identity data being at least partially indicative of an identity of the security document; and a signature associated with each of the identities; a sensing device for sensing a coded data portion disposed on one of the security documents, each security document having disposed thereon a plurality of coded data portions, each coded data portion encoding the identity of the security document and a fragment of the signature associated with that identity of the security document, the sensing device being further for determining an identifier of the fragment; and a computer system to: receive, from the sensing device, indicating data indicative of the identity of the security document, the fragment and the identifier of the fragment; retrieve from the database the signature associated with the identity of the security document received in the indicating data; determine a further fragment of the signature retrieved from the database and using the identifier received in the indicating data; and authenticate the security document by comparing the fragment received in the indicating data and the further fragment.