Secure site-to-site transactional communication between at least two
network servers coupled to a data communication network, including secure
registration by an authentication server associated with a multi-site
user authentication system. A network server receives a request via a
browser f of a client computer. In response, the network server initiates
a transaction with the authentication server and defines a data
structure, such as a query string, associated with the transaction. The
network server also generates a digital signature of the data structure
and then adds it to the data structure before directing the client
computer from the network server to the authentication server with the
data structure and the added digital signature. The network server also
adds an index to the data structure. The index is associated with the
transaction and unique, per transaction, to the network server initiating
the transaction.