1. In each sector, the last block (blocks number 3, 7, 11..) is called the sector trailer. This block stores the 2 keys or passwords and controls the access to the rest of the blocks in the sector (e.g. block number 7 controls block 4, 5 and 6). The first 6 bytes (0..5) are the key A, and the last 5 bytes (10..15) are the key B. As we will see, before reading or writing in any block, we must first authenticate us to that sector by providing one of the two keys (normally the A key). The bytes number 6, 7 and 8 store the control bytes, which control the way the the blocks may be accessed (read/write). Byte number 9 is not defined in all sector trailers.