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Below is a complete policy file for a user wanting to runTerrysGame
.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
Here's the policy file:
- 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
andHighScorePermissions
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"
.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"; };
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 namedkim.keystore
, then Kim's policy file needs the following line at the very beginning:keystore "kim.keystore";
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 theHighScore
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 intokim.keystore
using the alias"terry"
.Notice that the
HighScorePermission
needs to be signed by"chris"
, the person who actually implemented that permission. This ensures thatTerrysGame
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 intokim.keystore
using the alias"chris"
.
The final entry in the policy file grants permissions to theHighScore
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 whenTerrysGame
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 ajava.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
andsetHighScore
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:
Note: The notationpermission java.io.FilePermission "${user.home}${/}.highscore", "read,write";${propName}
specifies the value of a property. Thus,${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 aHighScorePermission
whose name is the game name will work. That is, theHighScore
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:
As before, thepermission com.scoredev.scores.HighScorePermission "*", signedBy "chris";HighScorePermission
itself needs to be signed by"chris"
, the person who actually implemented the permission.
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