The 16 byte RC4 key used to decrypt parts of the EEPROM.
The key is passed to kernel from from 2BL. It is only initialized on cold-boot.
If an XBE is loaded which is not allowed to run in the manufacturing region, the Microsoft Kernels will set the EEPROM key to all zeros. As the key is not re-initialized, this means running the first non-manufacturing XBE (such as a game or Dashboard[FIXME]), will make the key unretrievable until a reboot.
The Chihiro EEPROM key is
7B 35 A8 B7 27 ED 43 7A A0 BA FB 8F A4 38 61 80