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BKTR patch documentation
The two sections specified by the two BKTR entries are usually(?) at the very end of the section data(section_endoffset-{size of BKTR sections}).
The two sections specified by the two BKTR entries are usually(?) at the very end of the section data(section_endoffset-{size of BKTR sections}).
===RomFS Patching===
The BKTR section enables combining data from an update NCA with the RomFS from a base NCA to create a single patched RomFS image.
The first BKTR entry describes how to map regions of the two RomFS images to create the patched RomFS. It has the following format:
{| class="wikitable"
|-
! Start
! Length
! Description
|-
| 0x0
| 0x4004
| Padding/Unused?
|-
| 0x4004 || 0x4 || Number of Entries
|-
| 0x4008 || 0x8 || Patched RomFS Size
|-
| 0x4010 || 0x14*X || Relocation Entries
|}
Where relocation entries are as follows:
{| class="wikitable"
|-
! Start
! Length
! Description
|-
| 0x0 || 0x8 || Address in Patched RomFS
|-
| 0x8 || 0x8 || Address in Source RomFS
|-
| 0x10 || 0x4 || 1=Is from Patch RomFS, 0=Is from Base RomFS
|}
The second BKTR entry describes the subsections within the Patch RomFS. It has the following format:
{| class="wikitable"
|-
! Start
! Length
! Description
|-
| 0x0
| 0x4004
| Padding/Unused?
|-
| 0x4004 || 0x4 || Number of Entries
|-
| 0x4008 || 0x8 || Patch RomFS Size - BKTR entries
|-
| 0x4010 || 0x10*X || Subsection Entries
|}
Where subsection entries are as follows:
{| class="wikitable"
|-
! Start
! Length
! Description
|-
| 0x0 || 0x8 || Address in Patch RomFS
|-
| 0x8 || 0x4 || Padding/Unused?
|-
| 0xC || 0x4 || Value for subsection AES-CTR
|}
Official code assumes the relocation entries are sorted, and performs a binary search when determining where to read from. Each subsection in the Patch RomFS has its CTR calculated separately from the others based on the value in its entry (the BKTR entries use normal crypto). Thus decrypting a Patch RomFS requires decrypting and parsing the BKTR entries before anything else.
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