2 * Copyright 2014 The Android Open Source Project
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
8 * http://www.apache.org/licenses/LICENSE-2.0
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
19 import com.android.org.conscrypt.PSKKeyManager;
20 import java.net.Socket;
21 import javax.crypto.SecretKey;
22 import javax.net.ssl.SSLEngine;
25 * Provider of key material for pre-shared key (PSK) key exchange used in TLS-PSK cipher suites.
27 * <h3>Overview of TLS-PSK</h3>
29 * <p>TLS-PSK is a set of TLS/SSL cipher suites which rely on a symmetric pre-shared key (PSK) to
30 * secure the TLS/SSL connection and mutually authenticate its peers. These cipher suites may be
31 * a more natural fit compared to conventional public key based cipher suites in some scenarios
32 * where communication between peers is bootstrapped via a separate step (for example, a pairing
33 * step) and requires both peers to authenticate each other. In such scenarios a symmetric key (PSK)
34 * can be exchanged during the bootstrapping step, removing the need to generate and exchange public
35 * key pairs and X.509 certificates.</p>
37 * <p>When a TLS-PSK cipher suite is used, both peers have to use the same key for the TLS/SSL
38 * handshake to succeed. Thus, both peers are implicitly authenticated by a successful handshake.
39 * This removes the need to use a {@code TrustManager} in conjunction with this {@code KeyManager}.
42 * <h3>Supporting multiple keys</h3>
44 * <p>A peer may have multiple keys to choose from. To help choose the right key, during the
45 * handshake the server can provide a <em>PSK identity hint</em> to the client, and the client can
46 * provide a <em>PSK identity</em> to the server. The contents of these two pieces of information
47 * are specific to application-level protocols.</p>
49 * <p><em>NOTE: Both the PSK identity hint and the PSK identity are transmitted in cleartext.
50 * Moreover, these data are received and processed prior to peer having been authenticated. Thus,
51 * they must not contain or leak key material or other sensitive information, and should be
52 * treated (e.g., parsed) with caution, as untrusted data.</em></p>
54 * <p>The high-level flow leading to peers choosing a key during TLS/SSL handshake is as follows:
56 * <li>Server receives a handshake request from client.
57 * <li>Server replies, optionally providing a PSK identity hint to client.</li>
58 * <li>Client chooses the key.</li>
59 * <li>Client provides a PSK identity of the chosen key to server.</li>
60 * <li>Server chooses the key.</li>
63 * <p>In the flow above, either peer can signal that they do not have a suitable key, in which case
64 * the the handshake will be aborted immediately. This may enable a network attacker who does not
65 * know the key to learn which PSK identity hints or PSK identities are supported. If this is a
66 * concern then a randomly generated key should be used in the scenario where no key is available.
67 * This will lead to the handshake aborting later, due to key mismatch -- same as in the scenario
68 * where a key is available -- making it appear to the attacker that all PSK identity hints and PSK
69 * identities are supported.</p>
71 * <h3>Maximum sizes</h3>
73 * <p>The maximum supported sizes are as follows:
75 * <li>256 bytes for keys (see {@link #MAX_KEY_LENGTH_BYTES}),</li>
76 * <li>128 bytes for PSK identity and PSK identity hint (in modified UTF-8 representation) (see
77 * {@link #MAX_IDENTITY_LENGTH_BYTES} and {@link #MAX_IDENTITY_HINT_LENGTH_BYTES}).</li>
80 * <h3>Subclassing</h3>
81 * Subclasses should normally provide their own implementation of {@code getKey} because the default
82 * implementation returns no key, which aborts the handshake.
84 * <h3>Known issues</h3>
85 * The implementation of {@code ECDHE_PSK} cipher suites in API Level 21 contains a bug which breaks
86 * compatibility with other implementations. {@code ECDHE_PSK} cipher suites are enabled by default
87 * on platforms with API Level 21 when an {@code SSLContext} is initialized with a
88 * {@code PskKeyManager}. A workaround is to disable {@code ECDHE_PSK} cipher suites on platforms
92 * The following example illustrates how to create an {@code SSLContext} which enables the use of
93 * TLS-PSK in {@code SSLSocket}, {@code SSLServerSocket} and {@code SSLEngine} instances obtained
96 * PskKeyManager pskKeyManager = ...;
98 * SSLContext sslContext = SSLContext.getInstance("TLS");
100 * new KeyManager[] { pskKeyManager },
101 * new TrustManager[0], // No TrustManagers needed for TLS-PSK
102 * null // Use the default source of entropy
105 * SSLSocket sslSocket = (SSLSocket) sslContext.getSocketFactory().createSocket(...);
108 public abstract class PskKeyManager implements PSKKeyManager {
109 // IMPLEMENTATION DETAILS: This class exists only because the default implemenetation of the
110 // TLS/SSL JSSE provider (currently Conscrypt) cannot depend on Android framework classes.
111 // As a result, this framework class simply extends the PSKKeyManager interface from Conscrypt
112 // without adding any new methods or fields. Moreover, for technical reasons (Conscrypt classes
113 // are "hidden") this class replaces the Javadoc of Conscrypt's PSKKeyManager.
116 * Maximum supported length (in bytes) for PSK identity hint (in modified UTF-8 representation).
118 public static final int MAX_IDENTITY_HINT_LENGTH_BYTES =
119 PSKKeyManager.MAX_IDENTITY_HINT_LENGTH_BYTES;
121 /** Maximum supported length (in bytes) for PSK identity (in modified UTF-8 representation). */
122 public static final int MAX_IDENTITY_LENGTH_BYTES = PSKKeyManager.MAX_IDENTITY_LENGTH_BYTES;
124 /** Maximum supported length (in bytes) for PSK. */
125 public static final int MAX_KEY_LENGTH_BYTES = PSKKeyManager.MAX_KEY_LENGTH_BYTES;
128 * Gets the PSK identity hint to report to the client to help agree on the PSK for the provided
132 * The default implementation returns {@code null}.
134 * @return PSK identity hint to be provided to the client or {@code null} to provide no hint.
137 public String chooseServerKeyIdentityHint(Socket socket) {
142 * Gets the PSK identity hint to report to the client to help agree on the PSK for the provided
146 * The default implementation returns {@code null}.
148 * @return PSK identity hint to be provided to the client or {@code null} to provide no hint.
151 public String chooseServerKeyIdentityHint(SSLEngine engine) {
156 * Gets the PSK identity to report to the server to help agree on the PSK for the provided
160 * The default implementation returns an empty string.
162 * @param identityHint identity hint provided by the server or {@code null} if none provided.
164 * @return PSK identity to provide to the server. {@code null} is permitted but will be
165 * converted into an empty string.
168 public String chooseClientKeyIdentity(String identityHint, Socket socket) {
173 * Gets the PSK identity to report to the server to help agree on the PSK for the provided
177 * The default implementation returns an empty string.
179 * @param identityHint identity hint provided by the server or {@code null} if none provided.
181 * @return PSK identity to provide to the server. {@code null} is permitted but will be
182 * converted into an empty string.
185 public String chooseClientKeyIdentity(String identityHint, SSLEngine engine) {
190 * Gets the PSK to use for the provided socket.
193 * The default implementation returns {@code null}.
195 * @param identityHint identity hint provided by the server to help select the key or
196 * {@code null} if none provided.
197 * @param identity identity provided by the client to help select the key.
199 * @return key or {@code null} to signal to peer that no suitable key is available and to abort
203 public SecretKey getKey(String identityHint, String identity, Socket socket) {
208 * Gets the PSK to use for the provided engine.
211 * The default implementation returns {@code null}.
213 * @param identityHint identity hint provided by the server to help select the key or
214 * {@code null} if none provided.
215 * @param identity identity provided by the client to help select the key.
217 * @return key or {@code null} to signal to peer that no suitable key is available and to abort
221 public SecretKey getKey(String identityHint, String identity, SSLEngine engine) {