# How RSA Signed Applet Verification Works in Java Plug-in

This section covers the following topics:

- Introduction
- Support for RSA Verification
- Support for Dynamic Trust Management
- `usePolicy` Permission

## Introduction

This page provides an overview of how RSA signed applet support is implemented in Java Plug-in. For an overview of how to use RSA signed applets, see How to Deploy RSA-Signed Applets in Java Plug-in.

## Support for RSA Verification

So that Java Plug-in can verify RSA signatures in a browser-independent way, Java Plug-in includes a Cryptographic Service Provider (CSP). The CSP supports the "MD2withRSA", "MD5withRSA", and "SHA1withRSA" digital signature algorithms. It is automatically registered with the Java Cryptographic Architecture framework as part of the static initializer of the `PluginClassLoader`.

## Support for Dynamic Trust Management

Java 2 SDK, Standard Edition v 1.3 introduced fine-grained access control based on "codesource" and "protection domain," as described below:

Every class that is loaded from a JAR file has a codesource, which encapsulates two pieces of information:

1. The location (URL) where the class came from;
2. The list of principals who signed it (its certificates).

Each signer principal in the codesource is represented by its X.509 public-key certificate and supporting certificate chain.

In addition, every class that is loaded by a classloader belongs to one and only one protection domain, based on its codesource (i.e., based on where the class came from and who signed it). Every protection domain has a set of permissions associated with it, based on the configured security policy. This means that a protection domain encapsulates two things:

1. A codesource;
2. The set of permissions granted to it.

A certificate chain is a list of hierarchically ordered public-key certificates, starting at the signer's public-key certificate and ending at the certificate of a Root Certification Authority ("Root CA"). The public key of one certificate in the chain is used to verify the signature on the previous certificate in the chain. The Root CA certificate is self-signed. The assumption is that the Root CA is trusted because it is well known and widely published.

The `PluginClassLoader` checks the configured security policy to determine which permissions to grant to a given codesource. The codesource and the set of permissions granted to it then form a protection domain. This behaviour is common to all secure classloaders (i.e., instances of `java.security.SecureClassLoader`). (Note: `sun.plugin.security.PluginClassLoader` extends `sun.applet.AppletClassLoader`, which is a subclass of `java.net.URLClassLoader`, which in turn extends `java.security.SecureClassLoader`.)

The `PluginClassLoader` does additional work: If the applet is signed, and the permissions granted to it do not include the special `usePolicy` permission, the `PluginClassLoader` extracts the signers (and their supporting certificate chains) from the applet's codesource and attempts to verify them.

If Plug-in can verify the certificate chain all the way up to its Root CA certificate, it checks if that Root CA certificate is contained in the database of trusted Root CA certificates. If so, Plug-in will display the certificate chain of the authenticated signer and ask the user whether or not to grant `AllPermission` to code signed by that principal. Java code that is assigned the `AllPermission` permission is treated the same as system code, meaning it has all the privileges that system code has. The user can then choose whether or not to grant `AllPermission` to code signed by that principal, and whether such permission should be granted to any code signed by that principal for all subsequent sessions or for the current session only.

(For the case of unsigned applets, or signed applets whose permissions include the `usePolicy`, see Overview—Applet Security Basics.)

## `usePolicy` Permission

A permission named `usePolicy` (introduced with version 1.3) allows system administrators to turn off the `PluginClassLoader` behaviour. If the `usePolicy` permission is granted to the codesource by the configured security policy, no user prompting will take place; only the permissions specified in the security policy will be granted to the codesource.