Secure Boot Key
NVIDIA Tegra 2 systems have an optional signing process using a Secure Boot Key (SBK). A SBK is a shared-secret AES128 encryption key. The SBK is permanently burned into ROM within the device, and is readable only by a hardware AES engine.
Devices employing the SBK mechanism require nvflash commands to be encrypted with the SBK; nvflash performs the encryption, given the SBK value. If this is not done, or the given SBK is incorrect, nvflash will exit with status 0x4.
- http://www.cyber.com.au/~twb/doc/tf101.txt - main source of this article