Trail: Security Features in Java SE
Lesson: Implementing Your Own Permission

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A Sample Policy File

Below is a complete policy file for a user wanting to run ExampleGame.

The policy file syntax is not described here; if you are interested, see the Default Policy Implementation and Policy File Syntax page.

You don't need to know the syntax; you can always use the Policy Tool to create policy files, as shown in Creating a Policy File, Quick Tour of Controlling Applications, and Signing Code and Granting It Permissions lessons.

Below is the sample policy file, followed by a description of the individual entries. Assume that

The policy file is on Kim's computer, and Kim's keystore is named kim.keystore.
ExampleGame has been signed by the game creator Terry's private key, and the corresponding public key is in the keystore entry aliased by "terry".
The HighScore and HighScorePermissions classes were signed by the private key of the person who implemented them (Chris), and the corresponding public key is in the keystore entry aliased by "chris".

Here is the policy file: kim.policy

keystore "kim.keystore";

// Here is the permission ExampleGame needs.
// It grants code signed by "terry" the
// HighScorePermission, if the
// HighScorePermission was signed by "chris"
grant SignedBy "terry" {
  permission
    com.scoredev.scores.HighScorePermission
      "ExampleGame", signedBy "chris";
};

// Here is the set of permissions the HighScore
// class needs:
grant SignedBy "chris" {
  // The HighScore class needs permission to read
  // "user.home" to find the location of the
  // highscore file

  permission java.util.PropertyPermission
    "user.home", "read";

  // It needs permission to read and write the
  // high score file itself

  permission java.io.FilePermission
      "${user.home}${/}.highscore", "read,write";

  // It needs to get granted its own permission,
  // so it can call checkPermission
  // to see if its caller has permission.
  // Only grant it the permission
  // if the permission itself was signed by
  // "chris"

  permission
    com.scoredev.scores.HighScorePermission 
      "*", signedBy "chris";
};

The Keystore Entry

A keystore is a repository of keys and certificates, and is used to look up the public keys of the signers specified in the policy file ("terry" and "chris" in this example).

The keytool utility is used to create and administer keystores.

For this lesson, assume Kim would like to play ExampleGame. If Kim's keystore is named kim.keystore, then Kim's policy file needs the following line at the very beginning:

keystore "kim.keystore";

The ExampleGame Entry

A policy file entry specifies one or more permissions for code from a particular code source - either code from a particular location (URL), or code signed by a particular entity, or both.

Our policy file needs an entry for each game, granting code signed by a key from that game's creator a HighScorePermission whose name is the game name. That permission allows the game to call the HighScore methods to get or update the user's high score value for that particular game.

The entry required for ExampleGame is:

grant SignedBy "terry" {
    permission
        com.scoredev.scores.HighScorePermission 
            "ExampleGame", signedBy "chris";
};

Requiring that ExampleGame be signed by "terry" enables Kim to know that the game is the actual game that Terry developed. For this to work, Kim must have already stored Terry's public key certificate into kim.keystore using the alias "terry".

Notice that the HighScorePermission needs to be signed by "chris", the person who actually implemented that permission. This ensures that ExampleGame is granted the actual permission implemented by "chris", and not someone else. As before, for this to work Kim must have already stored Chris's public key certificate into kim.keystore using the alias "chris".

The HighScore Entry

The final entry in the policy file grants permissions to the HighScore class. More specifically, it grants permissions to code signed by "chris", who created and signed the class. Requiring the class to be signed by "chris" ensures that when ExampleGame calls upon this class to update the user's high score, ExampleGame knows for sure that it is using the original class implemented by "chris".

To update the user's high score value for any games that call upon it do so, the HighScore class requires three permissions:

1. Permission to read the `"user.home"` property value.

The HighScore class stores the user's high score values in a .highscore file in the user's home directory. Therefore this class needs a java.util.PropertyPermission that allows it to read the "user.home" property value to find out exactly where the user's home directory resides:

permission java.util.PropertyPermission 
    "user.home", "read";

2. Permission to read and write to the high score file itself.

This permission is needed so the HighScore getHighScore and setHighScore methods can access the user's .highscore file to get or set, respectively, the current high score for the current game.

Here is the required permission:

permission java.io.FilePermission
    "${user.home}${/}.highscore", "read,write";

Note: The notation ${propName} specifies the value of a property. Thus, ${user.home} will be replaced by the value of the "user.home" property. The notation ${/} is a platform-independent way of specifying a file separator.

3. All HighScorePermissions (that is, HighScorePermissions of any name).

This permission is needed so that the HighScore checks to ensure the calling game has been granted a HighScorePermission whose name is the game name will work. That is, the HighScore class must also be granted the permission, since a permission check requires that all code currently on the stack have the specified permission.