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    3DS CEC (StreetPass) Documentation
Overall Model
The CEC system module consists of two main components: mailbox manager and StreetPass communicator. Both work on the system save 00010026, which stores all mailbox information and message data. A simplified diagram of CEC communication model is
Application <=> Mailbox Manager <=> System Save (Mailbox) <=> StreetPass communicator <=> NWM/network <=> StreetPass communicator of another 3DS <=> ...

Many CEC service (cecd:u and cecd:s) functions are interfaces exposed from the mailbox manager, which read and write the system save. The mailbox manager is essentially a thick layer over direct file IO, which performances data initialization and verification on top of raw system data read/write.
The StreetPass communicator runs in the background, fetches data from the system save, sends and receives messages, and put new messages in the system save. It only exposed a few interfaces to control its activity, some of which are misc functions in cecd:u and cecd:s, and others are in cecd:ndm (which is ultimately exposed via NDM service ndm:u).
Mailbox Manager Interfaces
The mailbox manager exposes the following functions via cecd:u and cecd:s
[0x0001](Result, DataSize) Open(TitleID, CecDataPathType, FileOption, PID);
[0x0002](Result, DataSize) Read(DataSize, DataBuffer);
[0x0003](Result, DataSize) ReadMessage(TitleID, BoxType, MessageIDSize, DataSize, MessageIDBuffer, DataBuffer);
[0x0004](Result, DataSize) ReadMessageWithHmac(TitleID, BoxType, MessageIDSize, DataSize, MessageIDBuffer, HmacKeyBuffer, DataBuffer);
[0x0005](Result          ) Write(DataSize, DataBuffer);
[0x0006](Result          ) WriteMessage(TitleID, BoxType, MessageIDSize, DataSize, DataBuffer, MessageIDBuffer);
[0x0007](Result          ) WriteMessageWithHmac(TitleID, BoxType, MessageIDSize, DataSize, DataBuffer, HmacKeyBuffer, MessageIDBuffer);
[0x0008](Result          ) Delete(TitleID, CecDataPathType, BoxType, MessageIDSize, MessageIDBuffer);
[0x0011](Result          ) OpenAndWriteFile(DataSize, TitleID, CecDataPathType, FileOption, PID, DataBuffer);
[0x0012](Result, DataSize) OpenAndReadFile(DataSize, TitleID, CecDataPathType, FileOption, PID, DataBuffer);

Function signature convention:
[Function Main ID](Reply list) Name(Param list)

The reply list and param list are ordered according to their apperance in the IPC command buffer. Each param/reply takes one word, except for:

PID is the process ID descriptor and takes two words.
xxxBuffer is the mapped buffer descriptor and takes two words.

Some notes of different parameter types are listed below

TitleID is a 32-bit ID as a mailbox identifier of a specific game. It is usually the same as game title ID
CecDataPathType is an enum refering to a specific file / folder in the system save. See TODO for the meaning of each value
FileOption are bitfields (starting from bit0):

bit1: read
bit2: write
bit3: create folder
bit4: skip some checks?
bit30: ?


MessageIDSize is always 8

CEC System Save (00010026) Format
Folder and files                         `CecDataPathType`
[Root]
├── eventlog.dat
└── CEC                                  10
    ├── MacFilter___
    ├── MBoxList____                     1
    ├── <8-digit ID>                     11
    │   ├── MBoxInfo____                 2
    │   ├── MBoxData.001                 101
    │   ├── MBoxData.010                 110
    │   ├── MBoxData.050                 150
    │   ├── MBoxData.<3-digit number>    100 + <number>
    │   ├── ...
    │   ├── InBox___                     12
    │   │   ├── BoxInfo_____             3
    │   │   ├── _<12-char ID>            6
    │   │   └── ...
    │   └── OutBox__                     13
    │       ├── BoxInfo_____             4
    │       ├── OBIndex_____             5
    │       ├── _<12-char ID>            7
    │       └── ...
    ├── <8-digit ID>
    ...

File MBoxList____


Offset
Length
Description


0x00
2
Magic 0x6868


0x02
2
Padding


0x04
4
Version? always 1


0x08
4
Number of boxes


0x0C
16 * 24
List of box name


Each box name is 16-char long. However, due to the fact that box name is usually a 8-digit ID, the rest of 8 chars are always null characters. Unused box names are filled with null characters.


File MBoxInfo____


Offset
Length
Description


0x00
2
Magic 0x6363


0x02
2
Padding


0x04
4
Title ID (matches the box directory name)


0x08
4
Private ID?


0x0C
1
Flags?


0x0D
1
Flags?


0x0E
2
Padding


0x10
32
HMAC Key


0x30
4
Zero?


0x34
12
Timestamp when last accessed


0x40
1
Flag?


0x41
1
Flag?


0x42
1
Flag?


0x43
1
Flag?


0x44
12
Timestamp when last received


0x50
16
Zero?


Note:

Private ID seems to be a number chosen by application arbitrarily, possibly for verification. Magic numbers such as 0x00000000, 0x00000001, 0xFFFFFFFF, 0xAABBCCDD and other random numbers have been observed here.

File MBoxData.<3-digit number>
File MBoxData.001
Icon
File MBoxData.010
This is the game title in null-terminated UTF-16 string.
File MBoxData.050
This is 8-byte file containing the title ID.
File BoxInfo_____
This file consists of a 0x20-byte header, and an array of 0x70-byte entry. Each entry is a copy of the message header. See the next section for the message header format. The box info header format is


Offset
Length
Description


0x00
2
Magic 0x6262


0x02
2
Padding


0x04
4
Size of this file


0x08
4
Maximum box size


0x0C
4
Current box size


0x10
4
Maximum message count


0x14
4
Current message count / the size of the following array


0x18
4
Maximum batch size


0x1C
4
Maximum message size


File _<12-char ID>
Each such file is a message. The ID in the file name is the message ID encoded in base-64. A message file consists of a 0x70-byte header, several extra header, a message body, and a 0x20-byte HMAC(?).
The header format is


Offset
Length
Description


0x00
2
Magic 0x6060


0x02
2
Padding


0x04
4
Message size


0x08
4
Header + extra headers size


0x0C
4
Body size


0x10
4
Title ID


0x14
4
Title ID 2?


0x18
4
Batch ID


0x1C
4
? ID


0x20
8
Message ID


0x28
4
Message version?


0x2C
8
Message ID 2?


0x34
1
Flags


0x35
1
Send method


0x36
1
Is unopen


0x37
1
Is new


0x38
8
Sender ID


0x40
8
Sender ID 2?


0x48
12
Timestamp when sent


0x54
12
Timestamp when received


0x60
12
Timestamp when created


0x6C
1
Send count


0x6D
1
Forward count


0x6E
2
User data


Each extra header has a format of


Offset
Length
Description


0x00
4
Header type


0x04
4
Data size


0x08
Data size
Header data


The header type can be one of the follows:


Value
Description


1
?


2
Icon


3
Game name


4
Info text


5
Region?
Offset	Length	Description
0x00	2	Magic 0x6868
0x02	2	Padding
0x04	4	Version? always 1
0x08	4	Number of boxes
0x0C	16 * 24	List of box name
Each box name is 16-char long. However, due to the fact that box name is usually a 8-digit ID, the rest of 8 chars are always null characters. Unused box names are filled with null characters.
Offset	Length	Description
0x00	2	Magic 0x6363
0x02	2	Padding
0x04	4	Title ID (matches the box directory name)
0x08	4	Private ID?
0x0C	1	Flags?
0x0D	1	Flags?
0x0E	2	Padding
0x10	32	HMAC Key
0x30	4	Zero?
0x34	12	Timestamp when last accessed
0x40	1	Flag?
0x41	1	Flag?
0x42	1	Flag?
0x43	1	Flag?
0x44	12	Timestamp when last received
0x50	16	Zero?
Offset	Length	Description
0x00	2	Magic 0x6262
0x02	2	Padding
0x04	4	Size of this file
0x08	4	Maximum box size
0x0C	4	Current box size
0x10	4	Maximum message count
0x14	4	Current message count / the size of the following array
0x18	4	Maximum batch size
0x1C	4	Maximum message size
Offset	Length	Description
0x00	2	Magic 0x6060
0x02	2	Padding
0x04	4	Message size
0x08	4	Header + extra headers size
0x0C	4	Body size
0x10	4	Title ID
0x14	4	Title ID 2?
0x18	4	Batch ID
0x1C	4	? ID
0x20	8	Message ID
0x28	4	Message version?
0x2C	8	Message ID 2?
0x34	1	Flags
0x35	1	Send method
0x36	1	Is unopen
0x37	1	Is new
0x38	8	Sender ID
0x40	8	Sender ID 2?
0x48	12	Timestamp when sent
0x54	12	Timestamp when received
0x60	12	Timestamp when created
0x6C	1	Send count
0x6D	1	Forward count
0x6E	2	User data