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PskKeyManager.javaAPI DocAndroid 5.1 API9614Thu Mar 12 22:22:10 GMT 2015android.net

PskKeyManager

public abstract class PskKeyManager extends Object implements com.android.org.conscrypt.PSKKeyManager
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:

  1. Server receives a handshake request from client.
  2. Server replies, optionally providing a PSK identity hint to client.
  3. Client chooses the key.
  4. Client provides a PSK identity of the chosen key to server.
  5. 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}).

Subclassing

Subclasses should normally provide their own implementation of {@code getKey} because the default implementation returns no key, which aborts the handshake.

Known issues

The implementation of {@code ECDHE_PSK} cipher suites in API Level 21 contains a bug which breaks compatibility with other implementations. {@code ECDHE_PSK} cipher suites are enabled by default on platforms with API Level 21 when an {@code SSLContext} is initialized with a {@code PskKeyManager}. A workaround is to disable {@code ECDHE_PSK} cipher suites on platforms with API Level 21.

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 pskKeyManager = ...;

SSLContext sslContext = SSLContext.getInstance("TLS");
sslContext.init(
new KeyManager[] {pskKeyManager},
new TrustManager[0], // No TrustManagers needed for TLS-PSK
null // Use the default source of entropy
);

SSLSocket sslSocket = (SSLSocket) sslContext.getSocketFactory().createSocket(...);
}

Fields Summary
public static final int
MAX_IDENTITY_HINT_LENGTH_BYTES
Maximum supported length (in bytes) for PSK identity hint (in modified UTF-8 representation).
public static final int
MAX_IDENTITY_LENGTH_BYTES
Maximum supported length (in bytes) for PSK identity (in modified UTF-8 representation).
public static final int
MAX_KEY_LENGTH_BYTES
Maximum supported length (in bytes) for PSK.
Constructors Summary
Methods Summary
public java.lang.StringchooseClientKeyIdentity(java.lang.String identityHint, java.net.Socket socket)
Gets the PSK identity to report to the server to help agree on the PSK for the provided socket.

The default implementation returns an empty string.

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.

        return "";
    
public java.lang.StringchooseClientKeyIdentity(java.lang.String identityHint, javax.net.ssl.SSLEngine engine)
Gets the PSK identity to report to the server to help agree on the PSK for the provided engine.

The default implementation returns an empty string.

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.

        return "";
    
public java.lang.StringchooseServerKeyIdentityHint(java.net.Socket socket)
Gets the PSK identity hint to report to the client to help agree on the PSK for the provided socket.

The default implementation returns {@code null}.

return
PSK identity hint to be provided to the client or {@code null} to provide no hint.


                                                     
    
        
        return null;
    
public java.lang.StringchooseServerKeyIdentityHint(javax.net.ssl.SSLEngine engine)
Gets the PSK identity hint to report to the client to help agree on the PSK for the provided engine.

The default implementation returns {@code null}.

return
PSK identity hint to be provided to the client or {@code null} to provide no hint.

        return null;
    
public javax.crypto.SecretKeygetKey(java.lang.String identityHint, java.lang.String identity, java.net.Socket socket)
Gets the PSK to use for the provided socket.

The default implementation returns {@code null}.

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.

        return null;
    
public javax.crypto.SecretKeygetKey(java.lang.String identityHint, java.lang.String identity, javax.net.ssl.SSLEngine engine)
Gets the PSK to use for the provided engine.

The default implementation returns {@code null}.

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.

        return null;