The integrated on-line system with enhanced data transfer protocol enables dissimilar computers to instantly interchange data and integrate applications over a transfer control protocol/internet protocol (TCP/IP) network. The system is based on a client-server architecture in which multiple clients exchange data through a data transfer server using the enhanced data transfer protocol. The server acts as a post office for transferring data and provides verification of the sender's identity, the receiver's identity, verification of data transfers, electronic mailbox facilities, an audit trail, and inquiry and reporting functions. The protocol implements a socket to socket data interchange protocol on a preassigned port. Communications require validation of a header and permit send and receive sessions, receive sessions, or and end to end session. Optionally, the system provides a dual key encryption system, including a header key and a data encryption key.