BCT

BCT (Boot Configuration Table) is a data structure present on Tegra based devices that supplies boot time configuration parameters.

The Switch's BCT is included in the firmware package titles (0100000000000819 and 010000000000081A) and is installed into eMMC storage's boot partition 0. A total of four BCT copies can be installed into the system: normal, normal backup, safe mode and safe mode backup.

By design, the BCT's data is only signed after offset 0x0510. Therefore, regions like customer_data can be freely modified without resigning. This is done by NS when injecting a new keyblob during a system update, for example.

During boot, the boot ROM parses the appropriate BCT from eMMC storage and stores a copy of it in IRAM at address 0x40000000.

Structure

Below is the BCT structure used by the Switch, which is a minimal variation of the Tegra 210 BCT format.

Offset	Size	Field	Description
0x0000	0x210	bad_block_table	Table containing information on bad blocks 0x0000: num_entries (0x200) 0x0004: virtual_block_size (0x0F) 0x0005: block_size (0x0E) 0x0006: bad_blocks 0x0206: reserved
0x0210	0x100	bct_key	BCT RSA key modulus
0x0310	0x110	bct_signature	BCT object signature 0x0310: hash (empty) 0x0320: rsa_pss_signature
0x0420	0x04	sec_provisioning_key_num_insecure	Used for Factory Secure Provisioning. Always 0.
0x0424	0x20	sec_provisioning_key	Used for Factory Secure Provisioning. Always empty.
0x0444	0xC4	customer_data	Data block available for the customer. Used in key generation. 0x0444: padding_0x0C 0x0450: keyblob_0xB0 0x0500: padding_0x08
0x0508	0x04	odm_data	Legacy field. Unused.
0x050C	0x04	reserved0	Legacy field. Unused.
0x0510	0x10	random_aes_block	Always empty.
0x0520	0x10	unique_chip_id	Always empty.
0x0530	0x04	boot_data_version	Set to 0x00210001 (BOOTDATA_VERSION_T210).
0x0534	0x04	block_size_log2	Always 0x0E.
0x0538	0x04	page_size_log2	Always 0x09.
0x053C	0x04	partition_size	Always 0x01000000.
0x0540	0x04	num_param_sets	Number of device parameter sets. Always 0x01.
0x0544	0x04	dev_type	Device type. Set to 0x04 (dev_type_sdmmc).
0x0548	0x40	dev_params	Device parameters 0x0548: sdmmc_clock_divider (0x09 == 24MHz) 0x054C: sdmmc_data_width (0x02 == sdmmc_data_width_8bit)
0x0588	0x04	num_sdram_sets	Number of SDRAM parameter sets. Always set to 0, but parameters are used despite this.
0x058C	0x768	sdram_params0	Default values filled in.
0x0CF4	0x768	sdram_params1	Default values filled in.
0x145C	0x768	sdram_params2	Default values filled in.
0x1BC4	0x768	sdram_params3	Default values filled in.
0x232C	0x04	num_bootloaders	Number of bootloaders installed. Always 0x02 (maximum is 0x04).
0x2330	0x12C	bootloader0_info	Configuration parameters for bootloader 0 (normal). 0x2330: version (variable) 0x2334: start_block (0x00000040) 0x2338: start_page (0x00000000) 0x233C: length (variable) 0x2340: load_addr (0x40010000) 0x2344: entry_point (0x40010020) 0x2348: attribute (0x00000000) 0x234C: bootloader0_hash (empty) 0x235C: bootloader0_rsa_pss_signature
0x245C	0x12C	bootloader1_info	Configuration parameters for bootloader 1 (safe mode). 0x245C: version (variable) 0x2460: start_block (0x00000050) 0x2464: start_page (0x00000000) 0x2468: length (variable) 0x246C: load_addr (0x40010000) 0x2470: entry_point (0x40010020) 0x2474: attribute (0x00000000) 0x2478: bootloader1_hash (empty) 0x2488: bootloader1_rsa_pss_signature
0x2588	0x258	reserved1	Reserved space for bootloaders 2 and 3 (currently unused).
0x27E0	0x01	enable_fail_back	Always 0.
0x27E1	0x04	secure_debug_control	Always 0.
0x27E5	0x04	sec_provisioning_key_num_secure	Used for Factory Secure Provisioning. Always 0.
0x27E9	0x12	reserved2	Always starts with 0x80000000 (NVBOOT padding pattern).
0x27FB	0x05	padding	Empty. Not part of BCT data.

customer_data

This data block is ignored by the boot ROM, therefore is available for the programmer to use freely. The Switch uses 0xB0 bytes of this area, at offset 0x0450, to store the active keyblob. All remaining bytes are zero.

The first bootloader validates and decrypts this block for further key generation. The decrypted keyblob payload is as follows.

Offset	Size	Description
0x0	0x80	Array of master static key encryption keys
0x80	0x10	PK11 key

bootloader0_info

The version field controls which keyblob is used, where 0x01 is the first one. See Cryptosystem for the keyblobs used by each system-version.