When a Bitstream is requested for authentication, Bootgen divides the Bitstream into blocks of 8MB each and assigns an authentication certificate for each block.
If the size of a Bitstream is not in multiples of 8 MB, the last block contains the remaining Bitstream data.
Figure 1. Bitstream Blocks
When both authentication and encryption are enabled, encryption is first done on the Bitstream. Bootgen then divides the encrypted data into blocks and assigns an Authentication certificate for each block.