Game files

Game

All available version of the game itself are listed on

 http://s3.amazonaws.com/Minecraft.Download/versions/versions.json

The jar file for a specific version is located at

 http://s3.amazonaws.com/Minecraft.Download/versions/<version>/<version>.jar

The server .jar file and its .exe equivalent for a specific version are located at

 http://s3.amazonaws.com/Minecraft.Download/versions/<version>/minecraft_server.<version>.jar
 http://s3.amazonaws.com/Minecraft.Download/versions/<version>/minecraft_server.<version>.exe

Some information on how to properly run said jar is available at

 http://s3.amazonaws.com/Minecraft.Download/versions/<version>/<version>.json

As the Minecraft launcher updates the format of the JSON or the requirements to run the client may change. The minimumLauncherVersion in the <version>.json should be used to detect changes instead of just failing on updates

Libraries

The libraries which can be used for all minecraft versions and are downloaded by the launcher are located at

 https://libraries.minecraft.net/

The json file downloaded above is used in order to determine which libraries to download. Inside the file is a key, libraries which contains information about each library required. Example:

{
  "name": "org.lwjgl.lwjgl:lwjgl-platform:2.9.1-nightly-20130708-debug3",
  "natives": {
    "linux": "natives-linux",
    "windows": "natives-windows",
    "osx": "natives-osx"
  },
  "extract": {
    "exclude": [
      "META-INF/"
    ]
  },
  "rules": [
    {
      "action": "allow",
      "os": {
        "name": "osx",
        "version": "^10\\.5\\.\\d$"
      }
    }
  ]
}

The name key is used to determine the url from which to download the library from. The format of this string is

<package>:<name>:<version>

The corresponding URL would look like this:

https://libraries.minecraft.net/<package>/name>/<version>/<name>-<version>.jar

where path is the result of replacing all the periods in package with slashes.

It also download a hash, in sha1, the url is:

https://libraries.minecraft.net/<package>/<name>/<version>/<name>-<version>.jar.sha1

http://libraries.minecraft.net/ will NOT work - you must use https

The natives key provides a string for each specific platform to be inserted between the .jar and the <version> of the filename, resulting in a filename with the format:

<name>-<version>-<native-string>.jar

This is applied to any file with the natives key. The natives string may contain ${arch}, which should be replaced with 32 or 64.

The extract key provides rules for the extraction of the file.

The rules key is used to determine which platforms to download the file to. When the action is allow, the file will be downloaded to the platform stated in os. When the action is disallow, the file will not be downloaded to the platform stated in os. If there is no os key, the rule is default for non-specified platforms.

In the example above, on an OS X platform, the full download path would be

https://libraries.minecraft.net/org/lwjgl/lwjgl/lwjgl-platform/2.9.1-nightly-20130708-debug3/lwjgl-platform-2.9.1-nightly-20130708-debug3-natives-osx.jar

The file would be extracted but the META-INF file would not be. Additionally, the file would only be downloaded if the computer was running OS X 10.5.

Assets

The game itself downloads further resources from

http://resources.download.minecraft.net/

An index is available at

https://s3.amazonaws.com/Minecraft.Download/indexes/<index>.json 

where index is obtained from the 'assets' field in the versions.json, if this field is missing it defaults to 'legacy'. Each resource in the file has a name (the field name of the resource object), a SHA1 hash, and a file size. The client currently downloads all resources in the index from

http://resources.download.minecraft.net/<first 2 hex letters of hash>/<whole hash>

and stores them in

.minecraft/assets/objects/<first 2 hex letters of hash>/<whole hash>

and a copy is stored in

.minecraft/assets/virtual/legacy/ 

in the old format for versions that don't support the new system (1.7.2 and below)