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

The policy file syntax is not described here; if you're interested, see Default Policy Implementation and Policy File Syntax on the public java.sun.com web site.

You don't need to know the syntax; you can always use the Policy Tool to create policy files, as shown in Quick Tour of Controlling Applets , 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.

• TerrysGame 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's the policy file:

keystore "kim.keystore";

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

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

Requiring that TerrysGame 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 TerrysGame 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 TerrysGame calls upon this class to update the user's high score, TerrysGame 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's 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 (i.e, 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.

Here's the required permission:

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

As before, the HighScorePermission itself needs to be signed by "chris", the person who actually implemented the permission.