Module java.base
Package javax.net.ssl

Class SSLEngine

java.lang.Object
javax.net.ssl.SSLEngine
public abstract class SSLEngine extends Object
A class which enables secure communications using protocols such as the Secure Sockets Layer (SSL) or IETF RFC 2246 "Transport Layer Security" (TLS) protocols, but is transport independent.

The secure communications modes include:

  • Integrity Protection. SSL/TLS/DTLS protects against modification of messages by an active wiretapper.
  • Authentication. In most modes, SSL/TLS/DTLS provides peer authentication. Servers are usually authenticated, and clients may be authenticated as requested by servers.
  • Confidentiality (Privacy Protection). In most modes, SSL/TLS/DTLS encrypts data being sent between client and server. This protects the confidentiality of data, so that passive wiretappers won't see sensitive data such as financial information or personal information of many kinds.
These kinds of protection are specified by a "cipher suite", which is a combination of cryptographic algorithms used by a given SSL connection. During the negotiation process, the two endpoints must agree on a cipher suite that is available in both environments. If there is no such suite in common, no SSL connection can be established, and no data can be exchanged.

The cipher suite used is established by a negotiation process called "handshaking". The goal of this process is to create or rejoin a "session", which may protect many connections over time. After handshaking has completed, you can access session attributes by using the getSession() method.

The SSLSocket class provides much of the same security functionality, but all the inbound and outbound data is automatically transported using the underlying Socket, which by design uses a blocking model. While this is appropriate for many applications, this model does not provide the scalability required by large servers.

The primary distinction of an SSLEngine is that it operates on inbound and outbound byte streams, independent of the transport mechanism. It is the responsibility of the SSLEngine user to arrange for reliable I/O transport to the peer. By separating the SSL/TLS/DTLS abstraction from the I/O transport mechanism, the SSLEngine can be used for a wide variety of I/O types, such as non-blocking I/O (polling), selectable non-blocking I/O, Socket and the traditional Input/OutputStreams, local ByteBuffers or byte arrays, future asynchronous I/O models , and so on.

At a high level, the SSLEngine appears thus: 

                   app data

                |           ^
                |     |     |
                v     |     |
           +----+-----|-----+----+
           |          |          |
           |       SSL|Engine    |
   wrap()  |          |          |  unwrap()
           | OUTBOUND | INBOUND  |
           |          |          |
           +----+-----|-----+----+
                |     |     ^
                |     |     |
                v           |

                   net data
Application data (also known as plaintext or cleartext) is data which is produced or consumed by an application. Its counterpart is network data, which consists of either handshaking and/or ciphertext (encrypted) data, and destined to be transported via an I/O mechanism. Inbound data is data which has been received from the peer, and outbound data is destined for the peer.

(In the context of an SSLEngine, the term "handshake data" is taken to mean any data exchanged to establish and control a secure connection. Handshake data includes the SSL/TLS/DTLS messages "alert", "change_cipher_spec," and "handshake.")

There are five distinct phases to an SSLEngine.

  1. Creation - The SSLEngine has been created and initialized, but has not yet been used. During this phase, an application may set any SSLEngine-specific settings (enabled cipher suites, whether the SSLEngine should handshake in client or server mode, and so on). Once handshaking has begun, though, any new settings (except client/server mode, see below) will be used for the next handshake.
  2. Initial Handshake - The initial handshake is a procedure by which the two peers exchange communication parameters until an SSLSession is established. Application data can not be sent during this phase.
  3. Application Data - Once the communication parameters have been established and the handshake is complete, application data may flow through the SSLEngine. Outbound application messages are encrypted and integrity protected, and inbound messages reverse the process.
  4. Rehandshaking - Either side may request a renegotiation of the session at any time during the Application Data phase. New handshaking data can be intermixed among the application data. Before starting the rehandshake phase, the application may reset the SSL/TLS/DTLS communication parameters such as the list of enabled ciphersuites and whether to use client authentication, but can not change between client/server modes. As before, once handshaking has begun, any new SSLEngine configuration settings will not be used until the next handshake.
  5. Closure - When the connection is no longer needed, the client and the server applications should each close both sides of their respective connections. For SSLEngine objects, an application should call closeOutbound() and send any remaining messages to the peer. Likewise, an application should receive any remaining messages from the peer before calling closeInbound(). The underlying transport mechanism can then be closed after both sides of the SSLEngine have been closed. If the connection is not closed in an orderly manner (for example closeInbound() is called before the peer's write closure notification has been received), exceptions will be raised to indicate that an error has occurred. Once an engine is closed, it is not reusable: a new SSLEngine must be created.
An SSLEngine is created by calling SSLContext.createSSLEngine() from an initialized SSLContext. Any configuration parameters should be set before making the first call to wrap(), unwrap(), or beginHandshake(). These methods all trigger the initial handshake.

Data moves through the engine by calling wrap() or unwrap() on outbound or inbound data, respectively. Depending on the state of the SSLEngine, a wrap() call may consume application data from the source buffer and may produce network data in the destination buffer. The outbound data may contain application and/or handshake data. A call to unwrap() will examine the source buffer and may advance the handshake if the data is handshaking information, or may place application data in the destination buffer if the data is application. The state of the underlying SSL/TLS/DTLS algorithm will determine when data is consumed and produced.

Calls to wrap() and unwrap() return an SSLEngineResult which indicates the status of the operation, and (optionally) how to interact with the engine to make progress.

The SSLEngine produces/consumes complete SSL/TLS/DTLS packets only, and does not store application data internally between calls to wrap()/unwrap(). Thus input and output ByteBuffers must be sized appropriately to hold the maximum record that can be produced. Calls to SSLSession.getPacketBufferSize() and SSLSession.getApplicationBufferSize() should be used to determine the appropriate buffer sizes. The size of the outbound application data buffer generally does not matter. If buffer conditions do not allow for the proper consumption/production of data, the application must determine (via SSLEngineResult) and correct the problem, and then try the call again.

For example, unwrap() will return a SSLEngineResult.Status.BUFFER_OVERFLOW result if the engine determines that there is not enough destination buffer space available. Applications should call SSLSession.getApplicationBufferSize() and compare that value with the space available in the destination buffer, enlarging the buffer if necessary. Similarly, if unwrap() were to return a SSLEngineResult.Status.BUFFER_UNDERFLOW, the application should call SSLSession.getPacketBufferSize() to ensure that the source buffer has enough room to hold a record (enlarging if necessary), and then obtain more inbound data. 


   SSLEngineResult r = engine.unwrap(src, dst);
   switch (r.getStatus()) {
   case BUFFER_OVERFLOW:
       // Could attempt to drain the dst buffer of any already obtained
       // data, but we'll just increase it to the size needed.
       int appSize = engine.getSession().getApplicationBufferSize();
       ByteBuffer b = ByteBuffer.allocate(appSize + dst.position());
       dst.flip();
       b.put(dst);
       dst = b;
       // retry the operation.
       break;
   case BUFFER_UNDERFLOW:
       int netSize = engine.getSession().getPacketBufferSize();
       // Resize buffer if needed.
       if (netSize > src.capacity()) {
           ByteBuffer b = ByteBuffer.allocate(netSize);
           src.flip();
           b.put(src);
           src = b;
       }
       // Obtain more inbound network data for src,
       // then retry the operation.
       break;
   // other cases: CLOSED, OK.
   }

Unlike SSLSocket, all methods of SSLEngine are non-blocking. SSLEngine implementations may require the results of tasks that may take an extended period of time to complete, or may even block. For example, a TrustManager may need to connect to a remote certificate validation service, or a KeyManager might need to prompt a user to determine which certificate to use as part of client authentication. Additionally, creating cryptographic signatures and verifying them can be slow, seemingly blocking.

For any operation which may potentially block, the SSLEngine will create a Runnable delegated task. When SSLEngineResult indicates that a delegated task result is needed, the application must call getDelegatedTask() to obtain an outstanding delegated task and call its run() method (possibly using a different thread depending on the compute strategy). The application should continue obtaining delegated tasks until no more exist, and try the original operation again.

At the end of a communication session, applications should properly close the SSL/TLS/DTLS link. The SSL/TLS/DTLS protocols have closure handshake messages, and these messages should be communicated to the peer before releasing the SSLEngine and closing the underlying transport mechanism. A close can be initiated by one of: an SSLException, an inbound closure handshake message, or one of the close methods. In all cases, closure handshake messages are generated by the engine, and wrap() should be repeatedly called until the resulting SSLEngineResult's status returns "CLOSED", or isOutboundDone() returns true. All data obtained from the wrap() method should be sent to the peer.

closeOutbound() is used to signal the engine that the application will not be sending any more data.

A peer will signal its intent to close by sending its own closure handshake message. After this message has been received and processed by the local SSLEngine's unwrap() call, the application can detect the close by calling unwrap() and looking for a SSLEngineResult with status "CLOSED", or if isInboundDone() returns true. If for some reason the peer closes the communication link without sending the proper SSL/TLS/DTLS closure message, the application can detect the end-of-stream and can signal the engine via closeInbound() that there will no more inbound messages to process. Some applications might choose to require orderly shutdown messages from a peer, in which case they can check that the closure was generated by a handshake message and not by an end-of-stream condition.

There are two groups of cipher suites which you will need to know about when managing cipher suites:

  • Supported cipher suites: all the suites which are supported by the SSL implementation. This list is reported using getSupportedCipherSuites().
  • Enabled cipher suites, which may be fewer than the full set of supported suites. This group is set using the setEnabledCipherSuites(String[]) method, and queried using the getEnabledCipherSuites() method. Initially, a default set of cipher suites will be enabled on a new engine that represents the minimum suggested configuration.
Implementation defaults require that only cipher suites which authenticate servers and provide confidentiality be enabled by default. Only if both sides explicitly agree to unauthenticated and/or non-private (unencrypted) communications will such a cipher suite be selected.

Each SSL/TLS/DTLS connection must have one client and one server, thus each endpoint must decide which role to assume. This choice determines who begins the handshaking process as well as which type of messages should be sent by each party. The method setUseClientMode(boolean) configures the mode. Note that the default mode for a new SSLEngine is provider-specific. Applications should set the mode explicitly before invoking other methods of the SSLEngine. Once the initial handshaking has started, an SSLEngine can not switch between client and server modes, even when performing renegotiations.

The ApplicationProtocol String values returned by the methods in this class are in the network byte representation sent by the peer. The bytes could be directly compared, or converted to its Unicode String format for comparison. 

     String networkString = sslEngine.getHandshakeApplicationProtocol();
     byte[] bytes = networkString.getBytes(StandardCharsets.ISO_8859_1);

     //
     // Match using bytes:
     //
     //   "http/1.1"                       (7-bit ASCII values same in UTF-8)
     //   MEETEI MAYEK LETTERS "HUK UN I"  (Unicode 0xabcd->0xabcf)
     //
     String HTTP1_1 = "http/1.1";
     byte[] HTTP1_1_BYTES = HTTP1_1.getBytes(StandardCharsets.UTF_8);

     byte[] HUK_UN_I_BYTES = new byte[] {
         (byte) 0xab, (byte) 0xcd,
         (byte) 0xab, (byte) 0xce,
         (byte) 0xab, (byte) 0xcf};

     if ((Arrays.compare(bytes, HTTP1_1_BYTES) == 0 )
             || Arrays.compare(bytes, HUK_UN_I_BYTES) == 0) {
        ...
     }

     //
     // Alternatively match using string.equals() if we know the ALPN value
     // was encoded from a String using a certain character set,
     // for example UTF-8.  The ALPN value must first be properly
     // decoded to a Unicode String before use.
     //
     String unicodeString = new String(bytes, StandardCharsets.UTF_8);
     if (unicodeString.equals(HTTP1_1)
             || unicodeString.equals("\uabcd\uabce\uabcf")) {
         ...
     }

Applications might choose to process delegated tasks in different threads. When an SSLEngine is created, the current AccessControlContext is saved. All future delegated tasks will be processed using this context: that is, all access control decisions will be made using the context captured at engine creation.

Concurrency Notes: There are two concurrency issues to be aware of:
  1. The wrap() and unwrap() methods may execute concurrently of each other.
  2. The SSL/TLS/DTLS protocols employ ordered packets. Applications must take care to ensure that generated packets are delivered in sequence. If packets arrive out-of-order, unexpected or fatal results may occur.

    For example: 

                  synchronized (outboundLock) {
                  sslEngine.wrap(src, dst);
                  outboundQueue.put(dst);
              }
    As a corollary, two threads must not attempt to call the same method (either wrap() or unwrap()) concurrently, because there is no way to guarantee the eventual packet ordering.
Since:
1.5
See Also:

  • Constructor Details

    • SSLEngine

      protected SSLEngine()
      Constructor for an SSLEngine providing no hints for an internal session reuse strategy.
      See Also:

    • SSLEngine

      protected SSLEngine(String peerHost, int peerPort)
      Constructor for an SSLEngine.

      SSLEngine implementations may use the peerHost and peerPort parameters as hints for their internal session reuse strategy.

      Some cipher suites (such as Kerberos) require remote hostname information. Implementations of this class should use this constructor to use Kerberos.

      The parameters are not authenticated by the SSLEngine.

      Parameters:
      peerHost - the name of the peer host
      peerPort - the port number of the peer
      See Also:

  • Method Details

    • getPeerHost

      public String getPeerHost()
      Returns the host name of the peer.

      Note that the value is not authenticated, and should not be relied upon.

      Returns:
      the host name of the peer, or null if nothing is available.

    • getPeerPort

      public int getPeerPort()
      Returns the port number of the peer.

      Note that the value is not authenticated, and should not be relied upon.

      Returns:
      the port number of the peer, or -1 if nothing is available.

    • wrap

      public SSLEngineResult wrap(ByteBuffer src, ByteBuffer dst) throws SSLException
      Attempts to encode a buffer of plaintext application data into SSL/TLS/DTLS network data.

      An invocation of this method behaves in exactly the same manner as the invocation: 

       engine.wrap(new ByteBuffer[] { src }, 0, 1, dst);
      Parameters:
      src - a ByteBuffer containing outbound application data
      dst - a ByteBuffer to hold outbound network data
      Returns:
      an SSLEngineResult describing the result of this operation.
      Throws:
      SSLException - A problem was encountered while processing the data that caused the SSLEngine to abort. See the class description for more information on engine closure.
      ReadOnlyBufferException - if the dst buffer is read-only.
      IllegalArgumentException - if either src or dst is null.
      IllegalStateException - if the client/server mode has not yet been set.
      See Also:

    • wrap

      public SSLEngineResult wrap(ByteBuffer[] srcs, ByteBuffer dst) throws SSLException
      Attempts to encode plaintext bytes from a sequence of data buffers into SSL/TLS/DTLS network data.

      An invocation of this method behaves in exactly the same manner as the invocation: 

       engine.wrap(srcs, 0, srcs.length, dst);
      Parameters:
      srcs - an array of ByteBuffers containing the outbound application data
      dst - a ByteBuffer to hold outbound network data
      Returns:
      an SSLEngineResult describing the result of this operation.
      Throws:
      SSLException - A problem was encountered while processing the data that caused the SSLEngine to abort. See the class description for more information on engine closure.
      ReadOnlyBufferException - if the dst buffer is read-only.
      IllegalArgumentException - if either srcs or dst is null, or if any element in srcs is null.
      IllegalStateException - if the client/server mode has not yet been set.
      See Also:

    • wrap

      public abstract SSLEngineResult wrap(ByteBuffer[] srcs, int offset, int length, ByteBuffer dst) throws SSLException
      Attempts to encode plaintext bytes from a subsequence of data buffers into SSL/TLS/DTLS network data. This "gathering" operation encodes, in a single invocation, a sequence of bytes from one or more of a given sequence of buffers. Gathering wraps are often useful when implementing network protocols or file formats that, for example, group data into segments consisting of one or more fixed-length headers followed by a variable-length body. See GatheringByteChannel for more information on gathering, and GatheringByteChannel.write(ByteBuffer[], int, int) for more information on the subsequence behavior.

      Depending on the state of the SSLEngine, this method may produce network data without consuming any application data (for example, it may generate handshake data.)

      The application is responsible for reliably transporting the network data to the peer, and for ensuring that data created by multiple calls to wrap() is transported in the same order in which it was generated. The application must properly synchronize multiple calls to this method.

      If this SSLEngine has not yet started its initial handshake, this method will automatically start the handshake.

      This method will attempt to produce SSL/TLS/DTLS records, and will consume as much source data as possible, but will never consume more than the sum of the bytes remaining in each buffer. Each ByteBuffer's position is updated to reflect the amount of data consumed or produced. The limits remain the same.

      The underlying memory used by the srcs and dst ByteBuffers must not be the same.

      See the class description for more information on engine closure.

      Parameters:
      srcs - an array of ByteBuffers containing the outbound application data
      offset - The offset within the buffer array of the first buffer from which bytes are to be retrieved; it must be non-negative and no larger than srcs.length
      length - The maximum number of buffers to be accessed; it must be non-negative and no larger than srcs.length - offset
      dst - a ByteBuffer to hold outbound network data
      Returns:
      an SSLEngineResult describing the result of this operation.
      Throws:
      SSLException - A problem was encountered while processing the data that caused the SSLEngine to abort. See the class description for more information on engine closure.
      IndexOutOfBoundsException - if the preconditions on the offset and length parameters do not hold.
      ReadOnlyBufferException - if the dst buffer is read-only.
      IllegalArgumentException - if either srcs or dst is null, or if any element in the srcs subsequence specified is null.
      IllegalStateException - if the client/server mode has not yet been set.
      See Also:

    • unwrap

      public SSLEngineResult unwrap(ByteBuffer src, ByteBuffer dst) throws SSLException
      Attempts to decode SSL/TLS/DTLS network data into a plaintext application data buffer.

      An invocation of this method behaves in exactly the same manner as the invocation: 

       engine.unwrap(src, new ByteBuffer[] { dst }, 0, 1);
      Parameters:
      src - a ByteBuffer containing inbound network data.
      dst - a ByteBuffer to hold inbound application data.
      Returns:
      an SSLEngineResult describing the result of this operation.
      Throws:
      SSLException - A problem was encountered while processing the data that caused the SSLEngine to abort. See the class description for more information on engine closure.
      ReadOnlyBufferException - if the dst buffer is read-only.
      IllegalArgumentException - if either src or dst is null.
      IllegalStateException - if the client/server mode has not yet been set.
      See Also:

    • unwrap

      public SSLEngineResult unwrap(ByteBuffer src, ByteBuffer[] dsts) throws SSLException
      Attempts to decode SSL/TLS/DTLS network data into a sequence of plaintext application data buffers.

      An invocation of this method behaves in exactly the same manner as the invocation: 

       engine.unwrap(src, dsts, 0, dsts.length);
      Parameters:
      src - a ByteBuffer containing inbound network data.
      dsts - an array of ByteBuffers to hold inbound application data.
      Returns:
      an SSLEngineResult describing the result of this operation.
      Throws:
      SSLException - A problem was encountered while processing the data that caused the SSLEngine to abort. See the class description for more information on engine closure.
      ReadOnlyBufferException - if any of the dst buffers are read-only.
      IllegalArgumentException - if either src or dsts is null, or if any element in dsts is null.
      IllegalStateException - if the client/server mode has not yet been set.
      See Also:

    • unwrap

      public abstract SSLEngineResult unwrap(ByteBuffer src, ByteBuffer[] dsts, int offset, int length) throws SSLException
      Attempts to decode SSL/TLS/DTLS network data into a subsequence of plaintext application data buffers. This "scattering" operation decodes, in a single invocation, a sequence of bytes into one or more of a given sequence of buffers. Scattering unwraps are often useful when implementing network protocols or file formats that, for example, group data into segments consisting of one or more fixed-length headers followed by a variable-length body. See ScatteringByteChannel for more information on scattering, and ScatteringByteChannel.read(ByteBuffer[], int, int) for more information on the subsequence behavior.

      Depending on the state of the SSLEngine, this method may consume network data without producing any application data (for example, it may consume handshake data.)

      The application is responsible for reliably obtaining the network data from the peer, and for invoking unwrap() on the data in the order it was received. The application must properly synchronize multiple calls to this method.

      If this SSLEngine has not yet started its initial handshake, this method will automatically start the handshake.

      This method will attempt to consume one complete SSL/TLS/DTLS network packet, but will never consume more than the sum of the bytes remaining in the buffers. Each ByteBuffer's position is updated to reflect the amount of data consumed or produced. The limits remain the same.

      The underlying memory used by the src and dsts ByteBuffers must not be the same.

      The inbound network buffer, src, may be modified as a result of this call: therefore if the network data packet is required for some secondary purpose, the data should be duplicated before calling this method. Note: the network data will not be useful to a second SSLEngine, as each SSLEngine contains unique random state which influences the SSL/TLS/DTLS messages.

      See the class description for more information on engine closure.

      Parameters:
      src - a ByteBuffer containing inbound network data.
      dsts - an array of ByteBuffers to hold inbound application data.
      offset - The offset within the buffer array of the first buffer from which bytes are to be transferred; it must be non-negative and no larger than dsts.length.
      length - The maximum number of buffers to be accessed; it must be non-negative and no larger than dsts.length - offset.
      Returns:
      an SSLEngineResult describing the result of this operation.
      Throws:
      SSLException - A problem was encountered while processing the data that caused the SSLEngine to abort. See the class description for more information on engine closure.
      IndexOutOfBoundsException - If the preconditions on the offset and length parameters do not hold.
      ReadOnlyBufferException - if any of the dst buffers are read-only.
      IllegalArgumentException - if either src or dsts is null, or if any element in the dsts subsequence specified is null.
      IllegalStateException - if the client/server mode has not yet been set.
      See Also:

    • getDelegatedTask

      public abstract Runnable getDelegatedTask()
      Returns a delegated Runnable task for this SSLEngine.

      SSLEngine operations may require the results of operations that block, or may take an extended period of time to complete. This method is used to obtain an outstanding Runnable operation (task). Each task must be assigned a thread (possibly the current) to perform the run operation. Once the run method returns, the Runnable object is no longer needed and may be discarded.

      Delegated tasks run in the AccessControlContext in place when this operatioUsibly ts="col-ss > - if the client/se ofipeerHost, int peerPort)

      Constructor for an SSLEngine.