# Access Control Abstractions

### Last Modified: 20 October 1997

# java.security.acl

## Introduction

An Access Control List (ACL) is a data structure that guards access to resources. The java.security.acl package provides the interface to such a data structure and the sun.security.acl package provides a default implementation of the interfaces specified in the java.security.acl package.

Note: This API is not used internally for JDK system security. It was introduced in JDK 1.1, and remains for backwards compatibility. It may undergo substantial revision and extension or even replacement in future releases. This document describes the current interfaces.

An ACL can be thought of as a data structure with multiple ACL entries. Each ACL entry, of interface type AclEntry, contains a set of permissions associated with a particular principal. (A principal represents an entity such as an individual user or a group). Additionally, each ACL entry is specified as being either positive or negative. If positive, the permissions are to be granted to the associated principal. If negative, the permissions are to be denied.

An access control list is independent of the authentication scheme used to verify the validity of the principal. It is also independent of the encryption scheme used to transmit the data across the network. The ACL is consulted after the authentication phase. After the principal is verified to be an authenticated user in the system, the principal might access resources. For each such resource, the principal might or might not be granted access depending on the permissions that are granted to the principal in the ACL that guards the resource. The ACL itself is independent of the resource that it guards. The ACL can be consulted to find the list of permissions a particular principal has or to find out whether or not a principal is granted a particular permission.

## ACL structure

An ACL is an object that implements the java.security.acl.Acl interface. Each Acl is a list of AclEntry objects. Each AclEntry associates a Principal or a Group object to a list of Permission objects. (Note: Group is a subclass of Principal.) Each AclEntry can also be designated as a positive entry or a negative entry. A positive entry grants the list of permissions in the entry to the principal or group and a negative entry denies the list of permissions to the principal or group.

## Calculation of granted permissions

When calculating the net permissions a principal is granted, the following rules are used.

1. Each principal or group can have at most one positive ACL entry and one negative entry; that is, multiple positive or negative ACL entries are not allowed for any principal or group.
2. If there is no entry for a particular principal or a group, then the principal or the group has the null permission set.
3. The net group positive permission set for a principal is the union of all the positive permissions of each group that the principal belongs to.
4. The net group negative permission set for a principal is the union of all the negative permissions of each group that the principal belongs to.
5. If there is a positive entry that grants a principal or a group a particular permission and a negative entry that denies the principal or group the same permission, then that permission is removed from both the positive permissions set and the negative permissions set.
6. Individual permissions (permissions granted or denied to a specific principal) always override the Group permissions. Specifically, individual negative permissions (specific denial of permissions) override the group's positive permissions. And individual positive permissions override the group's negative permissions.
7. Assume that the positive permission set of all the groups that the principal belongs to is g1 and the negative permission set of all the groups that the principal belongs to is g2. Also assume that the individual positive permission set for the principal is p1 and the individual negative permission set for the principal is p2. Then the resulting permissions that the principal is granted are (p1 + (g1 - p2)) - (p2 + (g2 - p1)).

## Example permission calculations

Assume that a principal P belongs to groups G1 and G2. The table below shows 5 columns using some examples of permissions given to G1, G2 and P. The resulting permissions granted to P are shown in the last column.

| Group G1 Permissions | Group G2 Permissions | Union (G1, G2) perms | Individual Permissions | Resulting Permissions |
Positive | A | B | A+B | C | A+B+C
Negative | null set | null set | null set | null set
Positive | A | B | B | C | B+C
Negative | -C | -A | -C | null set
Positive | A | B | A+B | C | B+C
Negative | null set | null set | null set | -A
Positive | A | C | A | B | B
Negative | -C | -B | -B | -A

## Example usage

    /* Note: This sample program is meant just as an example
     * of the types of things that can be done with an
     * implementation of the java.security.acl interfaces.
     * This example uses the implementation supplied by the
     * sun.security.acl package. Please note that sun.* classes
     * are unsupported and subject to change.
    */

    import java.security.Principal;
    import java.security.acl.*;
    import sun.security.acl.*;
    import java.util.Enumeration;

    public class AclEx {

      public static void main(String argv[])
        throws Exception
      {

        Principal p1 = new PrincipalImpl("user1");
        Principal p2 = new PrincipalImpl("user2");
        Principal owner = new PrincipalImpl("owner");

        Permission read = new PermissionImpl("READ");
        Permission write = new PermissionImpl("WRITE");

        System.out.println("Creating a new group with two members: user1 and
    user2");
        Group g = new GroupImpl("group1");
        g.addMember(p1);
        g.addMember(p2);

        //
        // create a new acl with the name "exampleAcl"
        //
        System.out.println("Creating a new Acl named 'exampleAcl'");
        Acl acl = new AclImpl(owner, "exampleAcl");

        //
        // Allow group all permissions
        //
        System.out.println("Creating a new Acl Entry in exampleAcl for the
    group, ");
        System.out.println("  with read & write permissions");
        AclEntry entry1 = new AclEntryImpl(g);
        entry1.addPermission(read);
        entry1.addPermission(write);
        acl.addEntry(owner, entry1);

        //
        // Take away WRITE permissions for
        // user1. All others in groups still have
        // WRITE privileges.
        //
        System.out.println("Creating a new Acl Entry in exampleAcl for user1");
        System.out.println("  without write permission");
        AclEntry entry2 = new AclEntryImpl(p1);
        entry2.addPermission(write);
        entry2.setNegativePermissions();
        acl.addEntry(owner, entry2);

        //
        // This enumeration is an enumeration of
        // Permission interfaces. It should return
        // only "READ" permission.
        Enumeration e1 = acl.getPermissions(p1);
        System.out.println("Permissions for user1 are:");
        while (e1.hasMoreElements()) {
    	System.out.println("  " + e1.nextElement());
    	};

        //
        // This enumeration should have "READ" and"WRITE"
        // permissions.
        Enumeration e2 = acl.getPermissions(p2);
        System.out.println("Permissions for user2 are:");
        while (e2.hasMoreElements()) {
    	System.out.println("  " + e2.nextElement());
    	};

        // This should return false.
        boolean b1 = acl.checkPermission(p1, write);
        System.out.println("user1 has write permission: " + b1);

        // This should all return true;
        boolean b2 = acl.checkPermission(p1, read);
        boolean b3 = acl.checkPermission(p2, read);
        boolean b4 = acl.checkPermission(p2, write);
        System.out.println("user1 has read permission: " + b2);
        System.out.println("user2 has read permission: " + b3);
        System.out.println("user2 has write permission: " + b4);
      }
    }

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