Reverse engineering Spacehuhn's NFC monsters for 39C3

This is also a rust program that can decode parts of it. Planned features include a full decode and create of monster payloads.

Technical information

• Monsters are saved on an Ntag216 -> Payload is 858 bytes

offset	length	name	description
0x00	2	Magic Number	Always contains 39C3
0x02	1	Palette	Defines the color palette (see below for more info)
0x03	6	Unknown	Currently unknown
0x09	722	Image Data	Half-byte pixel encoding (see below for more)
0x2DB	18	Name	6-bit encoded fixed-length string in base63
0x2ED	12	Trait 1	6-bit encoded fixed-length string in base63
0x2F9	12	Trait 2	6-bit encoded fixed-length string in base63
0x305	12	Trait 3	6-bit encoded fixed-length string in base63
0x311	24	Secret	fixed-length ASCII string
0x329	48	Unknown	Currently Unknown
0x359	1	Unused	Unused Byte

If you want a deeper inspection of the file, monsters.hexproj is my ImHex Workspace that contains everything I know up to this point

What can be improved

• If you encode secret as base63 aswell you save 9 bytes
• Instead of fixxed length strings save a null terminator or prefix with length

0300 89 A6 F1 C7 02 E2 D6 95 23 EA A6 AD F8 B1 00 00
0310 00 42 75 62 61 74 7A 00 00 00 00 00 00 00 00 00
0320 00 00 00 00 00 00 00 00 00 <- Trailing null bytes 

0300 89 A6 F1 C7 02 E2 D6 95 23 EA A6 AD F8 B1 00 00
0310 00 42 75 62 61 74 7A 00 <- null byte as indicator of end of string