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.

Use the TR-31 Optional Data Build callable service to construct the optional block data structure for a TR-31 key block. It builds the structure by adding one optional block with each call, until your entire set of optional blocks have been added.

With each call, the application program provides a single optional block by specifying its ID, its length, and its data in parameters opt_block_id, opt_block_length, and opt_block_data respectively. Each subsequent call appends the current optional block to any preexisting blocks in the opt_blocks parameter. On the first call to the callable service, opt_blocks is typically empty.

The callable service name for AMODE(64) is CSNET31O.