A basic idea of this system is to separate ordered delivery data and unordered delivery data in a security protocol running on top of a reliable transport protocol, and perform a first type of security processing for ordered delivery data and a second different type of security processing for unordered delivery data in the security protocol. Preferably, data messages using ordered delivery and data messages using unordered delivery within a secure data stream are separated into two message sequence spaces on the security protocol layer, and data security processing is then performed differently in these two spaces. This system is particularly suitable for a reliable transport protocol such as SCTP (Stream Control Transmission Protocol). The security protocol running on top of the transport protocol is preferably based on the TLS (Transport Layer Security) or a TLS-like protocol with a security processing extension for unordered delivery.