Created How to convert base32 to base16 info-hashes (markdown)

How-to-convert-base32-to-base16-info-hashes.md

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+### Using info-hashes as downloads handles.
+
+When interfacing qBittorrent through the WebUI api, there are cases when you add a download via a magnet URI and then you want to track its state (for instance, to perform certain file operation once it's finished). The problem here is that the /command/download method does not return a handle to the download for you to track it. On the other hand, webUI api methods refer to every download through its info-hash, so it's possible to use that string as a handle. As every magnet URI will contain, al least, the info-hash (as stated in [this](http://www.bittorrent.org/beps/bep_0009.html) document at BitTorrent.org) you can parse the hash string from the uri itself. These hashes are base16 encoded, for a total of 40 characters.  
+
+### Special case: base32 encoded info-hashes.
+
+For compatibility reasons, the same document states that clients should also support the 32 character base32 encoded info-hash. When adding such a magnet link, qBittorrent will automatically convert it to a base16 hash, and you end up with a mismatching handle (the original base32 encoded info-hash and the base16 one used by qBittorrent). In this situation it's necesary to previously re-encode the hash yourself. Basically you need to decode the base32 hash and then re-encode it to base16, this way you´ll have the same string as qBittorrent.
+
+### Example:
+Supose you have the following magnet link:
+
+    magnet:?xt=urn:btih:WRN7ZT6NKMA6SSXYKAFRUGDDIFJUNKI2
+
+if it´s added to qBittorrent and then you issue a /query/torrents command, you´ll get a JSON object like this (fields other than "hash" are ommited):
+  
+    [{...,"hash":"b45bfccfcd5301e94af8500b1a1863415346a91a",...},...]
+
+which shows how qBittorrent converted our base32 hash to its base16 representation.
+
+### How to implement the base conversion:
+
+If you're into python, this base conversion can be accomplished as follows:
+    
+    >>> import base64
+    >>> b32Hash = "WRN7ZT6NKMA6SSXYKAFRUGDDIFJUNKI2"
+    >>> b16Hash = base64.b16encode(base64.b32decode(b32Hash))
+    >>> b16Hash = b16Hash.lower()
+    >>> print (b16Hash)
+
+which will output: b'b45bfccfcd5301e94af8500b1a1863415346a91a'
+
+In Delphi, you can make use of the excellent SZCodeBaseX library by Sasa Zeman (available for D4 to D2007, support seems to be dropped by now, but it's still available [here](http://www.torry.net/vcl/internet/coding/SZCodeBaseX.zip). This is a simple console application just to show how to perform the base conversion:
+
+    program Project1;
+
+    {$APPTYPE CONSOLE}
+
+    uses
+      SysUtils,
+      SZCodeBaseX in '<path to the library>';
+
+    var
+      b32Hash: AnsiString;
+      b16Hash: AnsiString;
+
+    begin
+      b32Hash:= 'WRN7ZT6NKMA6SSXYKAFRUGDDIFJUNKI2';
+      b16Hash:= SZEncodeBase16(SZDecodeBase32(b32Hash));
+      b16Hash:= LowerCase(b16Hash);
+      Writeln(b16Hash);
+    end.
+
+If you compile and run Project1.exe, it´ll output the converted hash string.
+
+Notice in both cases you need to perform a lowercase conversion of the resulting string.