A TR-31 key block can hold optional fields which are securely bound to the key block using the integrated MAC. The optional blocks may either contain information defined in the TR-31 standard, or they may contain proprietary data. A separate range of optional block identifiers is reserved for use with proprietary blocks. Applications can call the TR-31 Optional Data Read callable service to obtain lists of the optional block identifiers and optional block lengths, and to obtain the data for a particular optional block. This callable service is often used in conjunction with the TR-31 Parse Callable Service which can be used to determine the number of optional blocks in the TR-31 token.