In described examples, a system on a chip (SoC) and method for sending messages in the SoC include determining locations of initiator-side firewall block and receiver-side firewall block memories using respective pointers to the firewall block memories stored in a single, contiguous memory. Addresses of the pointers within the single memory depend on respective unique firewall identifiers of the firewall blocks. An exclusive security configuration controller uses the pointers to configure the firewall blocks over a security bus which is electrically isolated from a system bus. The system bus is used to send messages from sending functional blocks to receiving functional blocks. The initiator-side firewall block adds a message identifier to messages. The message identifier depends on the initiator-side firewall block's configuration settings. The receiver-side firewall block controls permission for the receiving functional block to access the message, depending on the message identifier and the receiver-side firewall block's configuration settings.