Dump the SPI flash by using the linux command line tool dd.
Command: dd if=/dev/mtd0 of=mtd0
This will dump the SPI flash to the file mtd0.
Device tree entry for the SPI
This is what the SPI flash device tree entry looks like.
The dump
The first 6128 bytes are the BCT, encrypted with your platform key. The loader for UEFI is located at 0x0100000.
How we can tell
To check it yourself,
To encrypt the BCT you got form nvflash use the following script, and replace <platform key> with your platform key.
Example usage: ./encrypt.sh surfacert.bct surfacert-encrypted.bct
To check if it is really the same, create a hexdump of your SPI flash dump and encrypted BCT. Your encrypted BCT should match the first 6128 bytes of the SPI flash dump.
Downloads
The files are encrypted and board specific.
You cant use them on your Surface RT
A dump from @Leander's Surface RT. Here is the platform key of the used Surface RT: 28a5d126adf421e6a39bfc8f7ff32308