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/*
 * Copyright 2014 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.conscrypt;

import java.net.Socket;
import javax.crypto.SecretKey;
import javax.net.ssl.KeyManager;
import javax.net.ssl.SSLEngine;

/**
 * Provider of key material for pre-shared key (PSK) key exchange used in TLS-PSK cipher suites.
 *
 * Overview of TLS-PSK
 *
 * TLS-PSK is a set of TLS/SSL cipher suites which rely on a symmetric pre-shared key (PSK) to
 * secure the TLS/SSL connection and mutually authenticate its peers. These cipher suites may be
 * a more natural fit compared to conventional public key based cipher suites in some scenarios
 * where communication between peers is bootstrapped via a separate step (for example, a pairing
 * step) and requires both peers to authenticate each other. In such scenarios a symmetric key (PSK)
 * can be exchanged during the bootstrapping step, removing the need to generate and exchange public
 * key pairs and X.509 certificates.
 *
 * When a TLS-PSK cipher suite is used, both peers have to use the same key for the TLS/SSL
 * handshake to succeed. Thus, both peers are implicitly authenticated by a successful handshake.
 * This removes the need to use a {@code TrustManager} in conjunction with this {@code KeyManager}.
 * 
 *
 * Supporting multiple keys
 *
 * A peer may have multiple keys to choose from. To help choose the right key, during the
 * handshake the server can provide a PSK identity hint to the client, and the client can
 * provide a PSK identity to the server. The contents of these two pieces of information
 * are specific to application-level protocols.
 *
 * NOTE: Both the PSK identity hint and the PSK identity are transmitted in cleartext.
 * Moreover, these data are received and processed prior to peer having been authenticated. Thus,
 * they must not contain or leak key material or other sensitive information, and should be
 * treated (e.g., parsed) with caution, as untrusted data.
 *
 * The high-level flow leading to peers choosing a key during TLS/SSL handshake is as follows:
 * 

 * Server receives a handshake request from client.
 * 
Server replies, optionally providing a PSK identity hint to client.
 * Client chooses the key.
 * Client provides a PSK identity of the chosen key to server.
 * Server chooses the key.
 * 

 *
 * In the flow above, either peer can signal that they do not have a suitable key, in which case
 * the the handshake will be aborted immediately. This may enable a network attacker who does not
 * know the key to learn which PSK identity hints or PSK identities are supported. If this is a
 * concern then a randomly generated key should be used in the scenario where no key is available.
 * This will lead to the handshake aborting later, due to key mismatch -- same as in the scenario
 * where a key is available -- making it appear to the attacker that all PSK identity hints and PSK
 * identities are supported.
 *
 * Maximum sizes
 *
 * The maximum supported sizes are as follows:
 * 

 * 256 bytes for keys (see {@link #MAX_KEY_LENGTH_BYTES}),
 * 128 bytes for PSK identity and PSK identity hint (in modified UTF-8 representation) (see
 * {@link #MAX_IDENTITY_LENGTH_BYTES} and {@link #MAX_IDENTITY_HINT_LENGTH_BYTES}).
 * 

 *
 * Example
 * The following example illustrates how to create an {@code SSLContext} which enables the use of
 * TLS-PSK in {@code SSLSocket}, {@code SSLServerSocket} and {@code SSLEngine} instances obtained
 * from it.
 *  {@code
 * PSKKeyManager myPskKeyManager = ...;
 *
 * SSLContext sslContext = SSLContext.getInstance("TLS");
 * sslContext.init(
 *         new KeyManager[] {myPskKeyManager},
 *         new TrustManager[0], // No TrustManagers needed for TLS-PSK
 *         null // Use the default source of entropy
 *         );
 *
 * SSLSocket sslSocket = (SSLSocket) sslContext.getSocketFactory().createSocket(...);
 * }
 *
 * @deprecated This abstraction is deprecated because it does not work with TLS 1.3.
 * @hide
 */
@Deprecated
@Internal
public interface PSKKeyManager extends KeyManager {

    /**
     * Maximum supported length (in bytes) for PSK identity hint (in modified UTF-8 representation).
     */
    int MAX_IDENTITY_HINT_LENGTH_BYTES = 128;

    /** Maximum supported length (in bytes) for PSK identity (in modified UTF-8 representation). */
    int MAX_IDENTITY_LENGTH_BYTES = 128;

    /** Maximum supported length (in bytes) for PSK key. */
    int MAX_KEY_LENGTH_BYTES = 256;

    /**
     * Gets the PSK identity hint to report to the client to help agree on the PSK for the provided
     * socket.
     *
     * @return PSK identity hint to be provided to the client or {@code null} to provide no hint.
     */
    String chooseServerKeyIdentityHint(Socket socket);

    /**
     * Gets the PSK identity hint to report to the client to help agree on the PSK for the provided
     * engine.
     *
     * @return PSK identity hint to be provided to the client or {@code null} to provide no hint.
     */
    String chooseServerKeyIdentityHint(SSLEngine engine);

    /**
     * Gets the PSK identity to report to the server to help agree on the PSK for the provided
     * socket.
     *
     * @param identityHint identity hint provided by the server or {@code null} if none provided.
     *
     * @return PSK identity to provide to the server. {@code null} is permitted but will be
     *         converted into an empty string.
     */
    String chooseClientKeyIdentity(String identityHint, Socket socket);

    /**
     * Gets the PSK identity to report to the server to help agree on the PSK for the provided
     * engine.
     *
     * @param identityHint identity hint provided by the server or {@code null} if none provided.
     *
     * @return PSK identity to provide to the server. {@code null} is permitted but will be
     *         converted into an empty string.
     */
    String chooseClientKeyIdentity(String identityHint, SSLEngine engine);

    /**
     * Gets the PSK to use for the provided socket.
     *
     * @param identityHint identity hint provided by the server to help select the key or
     *        {@code null} if none provided.
     * @param identity identity provided by the client to help select the key.
     *
     * @return key or {@code null} to signal to peer that no suitable key is available and to abort
     *         the handshake.
     */
    SecretKey getKey(String identityHint, String identity, Socket socket);

    /**
     * Gets the PSK to use for the provided engine.
     *
     * @param identityHint identity hint provided by the server to help select the key or
     *        {@code null} if none provided.
     * @param identity identity provided by the client to help select the key.
     *
     * @return key or {@code null} to signal to peer that no suitable key is available and to abort
     *         the handshake.
     */
    SecretKey getKey(String identityHint, String identity, SSLEngine engine);
}