BCT

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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.