Mii Maker lets you create Miis, and is the successor of the Wii's Mii Channel.
It can transfer Miis over local wireless from other systems running Mii Maker (3DS/Wii U), or receive, but not send, from Mii Channel.
Wii Mii Channel transfer protocol
The Wii beacons are similar to the usual multi-cart NDS beacons, except: beacon_type is zero, and payload size is 0x14. The payload data is just the Wii UTF-16 nickname, with some extra unused zero data. The usual multi-cast NDS protocol is used for sending the 3DS nick to the Wii. After many keep-alive frames, it eventually sends a bunch of frames, each containing the whole Mii. There's a 6-byte header, followed by Mii data. At the end of these frames like most NDS frames is the 0200 byte marker.
Mii QR Code format
3DS Mii QR is a standard 57x57 pixel Level 10 High ECC QR code with 'Mii' logo in center (refer to Denso-Wave Inc web site for QR Code format specifications). It contains 0x70-bytes of binary data. 3DS seems to have a fully implemented QR-code decoder, as it can interpret such Mii binary data being encoded even in the smallest possible for that data size Level 6 Low ECC QR code.
The data itself is encrypted with AES-CCM, xorpads can be determined from known plaintext here. The Mii Maker application uses the NS APT Wrap/Unwrap commands to encrypt/decrypt this Mii data. For the NS Unwrap command, the Mii Maker application uses nonceoffset=12, noncesize=10, and inputbuffer-size=0x60. Note that the actual nonce size is 8 bytes due to Wrap/Unwrap implementation, and the nonce data should be moved 12 bytes afterwards after decryption. The rest of the data at 0x8-0x5F is encrypted, and should be split into two parts after decryption, with the nonce data in the middle. (See Wrap and Unwrap for more information)
|0x0||0x4||Mii ID (big-endian 32-bit unsigned integer)|
The most significant 3 bits determine whether the Mii is Special, Foreign, or Normal 
time_offset = (mii_id & 0x0FFFFFFF) * 2;
time_offset is the time the Mii was created, represented as the number of seconds since 01/01/2010 00:00:00
|0x4||0x4||High 4 octets of MAC address |
|0x8||0x4||Mii ID, the encrypted data begins here.|
|0x14||0x2||Low 2 octets of MAC address|
|0x18||0x2||Bit-mapped: Birthday (4bit-day,5bit-month), Sex, Shirt, ??|
|0x1A||0x14||UTF-16 Mii Name (10 chars max)|
|0x2E||0x2||width & height|
|0x30||0x1||bit 0: disable sharing|
bit 1-4: face shape
bit 5-7: skin color
|0x31||0x1||bit 0-3: wrinkles|
bit 4-7: makeup
|0x33||0x1||bit 0-2: hair color|
bit 3: flip hair
|0x38||0x1||bit 0-4: eyebrow style|
bit 5-7: eyebrow color
|0x39||0x1||bit 0-3: eyebrow scale|
bit 4-6: eyebrow yscale
|0x3A||0x2||note that the bytes are swapped over (little-endian layout)|
bit 0-3: eyebrow rotation
bit 5-8: eyebrow x spacing
bit 9-13: eyebrow y position
|0x40||0x1||Mii Sharing Value|
|0x48||0x14||UTF-16 Author Name (10 chars max)|
- QR codes made from the same 3DS for the same Mii are use the same AES-CCM nonce (you can recreate the xorpad by xoring with known values from this table).
Created, received, or even met-in-multiplayer Miis are saved in CFL_DB.dat.
|0x0||0x4||"TIDE" header (EDIT byteswapped)|
|0xC||0x4 (?)||01000000 (constant?)|
|0x100||0x4||Number of scanned Special Mii QRs|
|0x104||-||Some data identifying each scanned Special Mii QRs, for the purpose of not making them scannable again. 8 or 12 byte each?|
The ExtData File System for Mii Maker is as follows:
root ├── icon ├── boss └── user └── ExBanner └── COMMON.bin
|icon||Duplicate from Application ExeFS. Always image 00000002.||0x36c0 Bytes||1.0.0-0||Download|
|COMMON.bin||Extended Banner for Home Menu. Always image 00000003.||0x20224 Bytes||1.0.0-0||Download|