Swapdoodle

Swapdoodle is the successor of the Swapnote application that allows you to send doodles to friends using SpotPass.

Credits for Miles (with the BPK1 structures) and MrNbaYoh (with the custom CRC32 checksum) for providing information!

SpotPass usage

Currently, only these SpotPass endpoints are known to exist:

• https://npdl.cdn.nintendowifi.net/p01/nsa/<regionID>/RNG_LS1/dstsetting LZ11 compressed BPK1 structure
• https://npdl.cdn.nintendowifi.net/p01/nsa/<regionID>/RNG_EC1/x.dlp BPK1 structure with all game lessons (x = 1, 2 or 3)
• https://npdl.cdn.nintendowifi.net/p01/nsa/<regionID>/RNG_MD1/dstdatList.bin BPK1 structure with game metadata
• https://npdl.cdn.nintendowifi.net/p01/nsa/<regionID>/RNG_NTX/<language>/ntx For game notifications (x = 1 or 2)

Where regionID goes the value from the below table.

Region	ID
USA	0hFlOFo7pNTU2dyE
EUR	3vveLadT8H6xKkQH

Also, the game uses Hpp[1] servers for posting doodles and making status updates.

BPK1

The BPK1 structure is used in almost all game data files. Some of the files that use this structure are either LZ11 or LZ10 compressed (like the ones used for doodles and stationery).

The structure starts with a BPK1 header, followed by all of its block headers, which can share names between each other. The headers are the followed by each block's data, which can hold any data (including another BPK1 structure).

All of the structure entries are stored as little-endian data.

Offset	Size	Description
0x0	0x40	BPK1 header
0x40		Block header
		Additional block headers
		Block data
		Additional blocks data

BPK1 header

Offset	Size	Description
0x0	0x4	Header magic (BPK1)
0x4	0x4	Number of blocks
0x8	0x4	Maximum block name length?
0xC	0x4	Structure length
0x10	0x4	Header length
0x14	0x2C	Padding

Block header

Offset	Size	Description
0x0	0x4	Block data offset
0x4	0x4	Block size
0x8	0x4	Checksum of block data
0x10	0x8	Block name (if the name is smaller than 0x8, the rest is filled with NULL bytes)

Block checksum

The checksum of each block is calculated as a custom CRC-32 checksum of the block's data:

• Polynomial: 4C11DB7
• Initial value: 4C11DB7
• Output XOR: 0

Stationery blocks

The following blocks are used in the stationery files.

STAHED1

This block holds the stationery file name and 0x79 byte padding.

THUMB2

This block holds the thumbnail of the stationery as a JPEG image.

STBARD1

There are two of these blocks. The first one holds a JPEG of the 2D part of the stationery, and the second one holds the 3D part.

STMASK1

This block holds a 256x256 L4 data of the 3D part of the stationery.