Processes are disclosed in which an index value is generated for locating a security association in a security association database, such as an inbound SAD associated with the IPsec set of protocols. The index value is specified for insertion into a Security Parameter Index (SPI) field of a packet header, such as an IPsec header. For packets that are to be secured according to the policies and protocols identified in the header and the security association and that are transmitted to a particular network device or host, the index value is inserted into the SPI field of the packets by a packet sender. The packet is then parsed by a packet receiver, the index value determined from the SPI and used to identify the security association in the SAD, based solely on the index value determined from the SPI. Hence, a simple and efficient mechanism is provided for fast inbound security association lookups.