1 /* 2 * Copyright 2014 The Android Open Source Project 3 * 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 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 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. 15 */ 16 17 package android.net; 18 19 import com.android.org.conscrypt.PSKKeyManager; 20 import java.net.Socket; 21 import javax.crypto.SecretKey; 22 import javax.net.ssl.SSLEngine; 23 24 /** 25 * Provider of key material for pre-shared key (PSK) key exchange used in TLS-PSK cipher suites. 26 * 27 * <h3>Overview of TLS-PSK</h3> 28 * 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> 36 * 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}. 40 * </p> 41 * 42 * <h3>Supporting multiple keys</h3> 43 * 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> 48 * 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> 53 * 54 * <p>The high-level flow leading to peers choosing a key during TLS/SSL handshake is as follows: 55 * <ol> 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> 61 * </ol></p> 62 * 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> 70 * 71 * <h3>Maximum sizes</h3> 72 * 73 * <p>The maximum supported sizes are as follows: 74 * <ul> 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> 78 * </ul></p> 79 * 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. 83 * 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 89 * with API Level 21. 90 * 91 * <h3>Example</h3> 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 94 * from it. 95 * <pre> {@code 96 * PskKeyManager pskKeyManager = ...; 97 * 98 * SSLContext sslContext = SSLContext.getInstance("TLS"); 99 * sslContext.init( 100 * new KeyManager[] { pskKeyManager }, 101 * new TrustManager[0], // No TrustManagers needed for TLS-PSK 102 * null // Use the default source of entropy 103 * ); 104 * 105 * SSLSocket sslSocket = (SSLSocket) sslContext.getSocketFactory().createSocket(...); 106 * }</pre> 107 */ 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. 114 115 /** 116 * Maximum supported length (in bytes) for PSK identity hint (in modified UTF-8 representation). 117 */ 118 public static final int MAX_IDENTITY_HINT_LENGTH_BYTES = 119 PSKKeyManager.MAX_IDENTITY_HINT_LENGTH_BYTES; 120 121 /** 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; 123 124 /** Maximum supported length (in bytes) for PSK. */ 125 public static final int MAX_KEY_LENGTH_BYTES = PSKKeyManager.MAX_KEY_LENGTH_BYTES; 126 127 /** 128 * Gets the PSK identity hint to report to the client to help agree on the PSK for the provided 129 * socket. 130 * 131 * <p> 132 * The default implementation returns {@code null}. 133 * 134 * @return PSK identity hint to be provided to the client or {@code null} to provide no hint. 135 */ 136 @Override 137 public String chooseServerKeyIdentityHint(Socket socket) { 138 return null; 139 } 140 141 /** 142 * Gets the PSK identity hint to report to the client to help agree on the PSK for the provided 143 * engine. 144 * 145 * <p> 146 * The default implementation returns {@code null}. 147 * 148 * @return PSK identity hint to be provided to the client or {@code null} to provide no hint. 149 */ 150 @Override 151 public String chooseServerKeyIdentityHint(SSLEngine engine) { 152 return null; 153 } 154 155 /** 156 * Gets the PSK identity to report to the server to help agree on the PSK for the provided 157 * socket. 158 * 159 * <p> 160 * The default implementation returns an empty string. 161 * 162 * @param identityHint identity hint provided by the server or {@code null} if none provided. 163 * 164 * @return PSK identity to provide to the server. {@code null} is permitted but will be 165 * converted into an empty string. 166 */ 167 @Override 168 public String chooseClientKeyIdentity(String identityHint, Socket socket) { 169 return ""; 170 } 171 172 /** 173 * Gets the PSK identity to report to the server to help agree on the PSK for the provided 174 * engine. 175 * 176 * <p> 177 * The default implementation returns an empty string. 178 * 179 * @param identityHint identity hint provided by the server or {@code null} if none provided. 180 * 181 * @return PSK identity to provide to the server. {@code null} is permitted but will be 182 * converted into an empty string. 183 */ 184 @Override 185 public String chooseClientKeyIdentity(String identityHint, SSLEngine engine) { 186 return ""; 187 } 188 189 /** 190 * Gets the PSK to use for the provided socket. 191 * 192 * <p> 193 * The default implementation returns {@code null}. 194 * 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. 198 * 199 * @return key or {@code null} to signal to peer that no suitable key is available and to abort 200 * the handshake. 201 */ 202 @Override 203 public SecretKey getKey(String identityHint, String identity, Socket socket) { 204 return null; 205 } 206 207 /** 208 * Gets the PSK to use for the provided engine. 209 * 210 * <p> 211 * The default implementation returns {@code null}. 212 * 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. 216 * 217 * @return key or {@code null} to signal to peer that no suitable key is available and to abort 218 * the handshake. 219 */ 220 @Override 221 public SecretKey getKey(String identityHint, String identity, SSLEngine engine) { 222 return null; 223 } 224 } 225
