/*
 * Copyright 2006 Sun Microsystems, Inc. All rights reserved.
 */
/*
 * $Id: KeySelector.java,v 1.6 2005/05/10 15:47:42 mullan Exp $
 */
package javax.xml.crypto;

import java.security.Key;
import javax.xml.crypto.dsig.keyinfo.KeyInfo;
import javax.xml.crypto.dsig.keyinfo.RetrievalMethod;

/**
 * A selector that finds and returns a key using the data contained in a
 * {@link KeyInfo} object. An example of an implementation of
 * this class is one that searchs a {@link java.security.KeyStore} for 
 * trusted keys that match information contained in a <code>KeyInfo</code>.
 *
 * <p>Whether or not the returned key is trusted and the mechanisms 
 * used to determine that is implementation-specific.
 *
 * @author Sean Mullan
 * @author JSR 105 Expert Group
 * @since 1.6
 */
public abstract class KeySelector {

    /**
     * The purpose of the key that is to be selected.
     */
    public static class Purpose {

	private final String name;

	private Purpose(String name) 	{ this.name = name; }

	/**
	 * Returns a string representation of this purpose ("sign",
	 * "verify", "encrypt", or "decrypt").
	 *
	 * @return a string representation of this purpose
	 */
	public String toString()	{ return name; } 	

	/**
	 * A key for signing.
	 */
        public static final Purpose SIGN = new Purpose("sign");
	/**
	 * A key for verifying.
	 */
        public static final Purpose VERIFY = new Purpose("verify");
	/**
	 * A key for encrypting.
	 */
        public static final Purpose ENCRYPT = new Purpose("encrypt");
	/**
	 * A key for decrypting.
	 */
        public static final Purpose DECRYPT = new Purpose("decrypt");
    }

    /**
     * Default no-args constructor; intended for invocation by subclasses only.
     */
    protected KeySelector() {}

    /**
     * Attempts to find a key that satisfies the specified constraints.
     *
     * @param keyInfo a <code>KeyInfo</code> (may be <code>null</code>)
     * @param purpose the key's purpose ({@link Purpose#SIGN}, 
     *    {@link Purpose#VERIFY}, {@link Purpose#ENCRYPT}, or 
     *    {@link Purpose#DECRYPT})
     * @param method the algorithm method that this key is to be used for.
     *    Only keys that are compatible with the algorithm and meet the 
     *    constraints of the specified algorithm should be returned.
     * @param context an <code>XMLCryptoContext</code> that may contain
     *    useful information for finding an appropriate key. If this key 
     *    selector supports resolving {@link RetrievalMethod} types, the 
     *    context's <code>baseURI</code> and <code>dereferencer</code> 
     *    parameters (if specified) should be used by the selector to 
     *    resolve and dereference the URI.
     * @return the result of the key selector
     * @throws KeySelectorException if an exceptional condition occurs while 
     *    attempting to find a key. Note that an inability to find a key is not 
     *    considered an exception (<code>null</code> should be
     *    returned in that case). However, an error condition (ex: network 
     *    communications failure) that prevented the <code>KeySelector</code>
     *    from finding a potential key should be considered an exception.
     * @throws ClassCastException if the data type of <code>method</code> 
     *    is not supported by this key selector
     */
    public abstract KeySelectorResult select(KeyInfo keyInfo, Purpose purpose, 
	AlgorithmMethod method, XMLCryptoContext context) 
	throws KeySelectorException;

    /**
     * Returns a <code>KeySelector</code> that always selects the specified
     * key, regardless of the <code>KeyInfo</code> passed to it.
     *
     * @param key the sole key to be stored in the key selector
     * @return a key selector that always selects the specified key
     * @throws NullPointerException if <code>key</code> is <code>null</code>
     */
    public static KeySelector singletonKeySelector(Key key) {
        return new SingletonKeySelector(key);
    }

    private static class SingletonKeySelector extends KeySelector {
        private final Key key;

        SingletonKeySelector(Key key) {
            if (key == null) {
                throw new NullPointerException();
            }
            this.key = key;
        }

        public KeySelectorResult select(KeyInfo keyInfo, Purpose purpose,
	    AlgorithmMethod method, XMLCryptoContext context) 
	    throws KeySelectorException {

            return new KeySelectorResult() {
		public Key getKey() {
		    return key;
		}
	    };
        }
    }
}