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The following is wild conjecture based on LordLandon's sendpkm.py.
Communication with the GTS is done over regular HTTP with http://gamestats2.gs.nintendowifi.net/. The same protocol is used for all five Gen IV games.
The games don't seem to care about these at all. The GTS sends back a bunch of boilerplate response headers, but the game happily accepts a response with only a Content-Length.
All requests to the server are GET requests of the form <code><var>page</var>.asp?pid=<var>pid</var>&hash=<var>hash</var>&data=<var>data</var></code>. However devices prior to the DSi also send requests without the hash (followed by a requests with hash) to which you should also return proper values or else the DS will assume communication broke off.
The pid is an unsigned 32-bit integer that appears to uniquely identify a game cartridge. Although this has not been confirmed entirely: Your PID is generated when you get your friendcode for the first time and is set to: friendcode & 0x7fffffff When you change the device and are forced to change your friendcode with it, your PID doesn't change but you get a new friendcode (at this point there's no friendcode->pid mapping anymore).
For the mathematically inclined: Eevee's Platinum pid is 192615460 (0x0b7b1424) and his Pal Pad code is 0904 2026 4621.
Before each "real" request, the game sends a request of the form <code><var>page</var>.asp?pid=<var>pid</var></code> and the server responds with a 32-byte challenge token. The game computes <code>sha1("sAdeqWo3voLeC5r16DYv" + token)</code> and uses that as the <var>hash</var> value which it sends to the server. The <var>data</var> parameter is encrypted, as explained further down.
The process is mostly the same, except that the response hash is <code>sha1("HZEdGCzcGGLvguqUEKQN" + token)</code>
That is, each request looks like the following:
- Game requests <code>GET /pokemondpds/<var>page</var>.asp?pid=<var>pid</var></code>
- Server responds with <var>token</var>
- Game requests <code>GET /pokemondpds/<var>page</var>.asp?pid=<var>pid</var>&hash=<var>sha1(...)</var>&data=<var>data</var></code>
- Server responds with payload
The exact details of the game's requests are not currently known, but we are working on it!
A 32-bit checksum of the data is used as the key for the encryption. The checksum is simply the sum of every byte of the data.
The first 4 bytes of the sent data are the checksum, <code>xor</code>ed with <code>0x4a3b2c1d</code>, and sent in network byte order (big-endian).
This encryption algorithm, like others used in the game, uses a Linear Congruential Generator (not a very strong choice). The multiplicative constant is <code>0x45</code>, and the additive constant is <code>0x1111</code>. It appears that the game uses a signed dword to store the seed, which doesn't really matter; it shouldn't affect anything. (It means that the modulus is effectively <code>0x80000000</code>.)
GRNG[n+1] = (GRNG[n] * 0x45 + 0x1111) & 0x7fffffff
The GRNG is seeded with the <var>checksum</var>, like so:
GRNG = <var>checksum</var> | (<var>checksum</var> << 16)
The keystream is composed of the lower byte of the high word of successive GRNG values.
keystream[n] = (GRNG[n] >> 16) & 0xff
<code>Xor</code> the keystream with the plaintext to get the ciphertext. <code>Xor</code> the keystream with the ciphertext to get the plaintext.
The first request the game makes is to <code>/pokemondpds/worldexchange/info.asp</code>. The server responds with 0x0001.
Platinum, Heart Gold, and Soul Silver will then make a request to <code>/pokemondpds/common/setProfile.asp</code>. The server responds with eight NULs (0x00000000 0x00000000).
After the above step(s) or performing any of the tasks below other than searching, the game makes a request to <code>/pokemondpds/worldexchange/result.asp</code>. If the game has had a Pokémon sent to it via a trade, the server responds with the entire encrypted Pokémon save struct. Otherwise, if there is a Pokémon deposited in the GTS, it responds with 0x0004; if not, it responds with 0x0005.
Receiving a traded Pokémon
If the game receives a Pokémon from a successful trade as a response from <code>result.asp</code>, it next requests <code>/pokemondpds/worldexchange/delete.asp</code>. The server responds with 0x0001.
A note on sendpkm.py
After doing the above, some Platinum, Heart Gold, and Soul Silver games will report a communication error and dump the player back to the title screen. The Pokémon is still successfully received. At least one person with HG/SS has received a Pokémon from a fake server without getting the error, and Diamond/Pearl have never been reported to have the problem.
Depositing a Pokémon
Pokémon are offered on the GTS by requesting <code>/pokemondpds/worldexchange/post.asp</code>. The sent data is 300 bytes long, and includes the Pokémon struct. If the Pokémon is rejected by the server, the response is 0x000c; otherwise, if the deposit is successful, 0x0001.
The game then saves. After the save is complete, it issues a request to <code>/pokemondpds/worldexchange/post_finish.asp</code>.
Retrieving the deposited Pokémon
Checking on the deposited Pokémon is apparently done by <code>/pokemondpds/worldexchange/get.asp</code>. The response appears to be a Pokémon save struct.
Retrieving the deposited Pokémon is done by <code>/pokemondpds/worldexchange/return.asp</code>. The response is merely 0x0001; the actual Pokémon data is taken from the <code>get.asp</code> request.
Searching is done through <code>/pokemondpds/worldexchange/search.asp</code>. The sent data is either 15 or 16 bytes long.
The server responds with a full 292-byte Pokémon struct for each result. If there are <var>n</var> results, the response will be 292 * <var>n</var> bytes long. If there are no results, the server will give an empty response (0 bytes).
The Pokémon data for the GTS is 292 bytes—56 bytes larger than a party Pokémon struct. The extra 56 bytes are GTS-specific data, such as the player's name & country, and what Pokémon they are requesting.
They are as follows:
<pre> 0x00-0x01 Current Pokémon 0x02 Gender 1=male; 2=female; 3=either/neither 0x03 Level 0x04-0x05 Requested Pokémon 0x06 Gender 0x07 Min level 0x08 Max level 0x09 always 0? 0x0A Trainer Gender 0=male; 1=female 0x0B always 0? 0x0C-0x13 Timestamp - Deposited time 0x14-0x1B Timestamp - Time traded away? 0x1c-0x1F pid - also 0x4c in the sav 0x20-0x2F Trainer Name 0x30-0x31 Trainer ID 0x32 Country 0x33 City 0x34 Trainer Sprite 0x35 Exchanged flag 0x36 Version 0x0A=Diamond; 0x0B=Pearl; ... 0x37 Language 1=jp; 2=en; 3=fr; ... </pre>
The timestamps are set by the server, and are always PST (UTC-8). Timestamp format: <pre> 0x00-0x01 Year 0x02 Month 0x03 Day 0x04 Hour 0x05 Minute 0x06 Second 0x07 always 0? </pre>