Add a setting for the used flush mode when compressing data

Fixes SMACK-593

smack-compression-jzlib/src/main/java/org/jivesoftware/smack/compression/jzlib/JzlibInputOutputStream.java

@@ -58,7 +58,9 @@ public class JzlibInputOutputStream extends XMPPInputOutputStream {
     @Override
     public OutputStream getOutputStream(OutputStream outputStream) throws IOException {
         final DeflaterOutputStream os = new DeflaterOutputStream(outputStream);
+        if (flushMethod == FlushMethod.SYNC_FLUSH) {
             os.setSyncFlush(true);
+        }
 
         return os;
     }

smack-core/src/main/java/org/jivesoftware/smack/compression/Java7ZlibInputOutputStream.java

@@ -48,6 +48,9 @@ public class Java7ZlibInputOutputStream extends XMPPInputOutputStream {
     private final static boolean supported;
     private final static int compressionLevel = Deflater.DEFAULT_COMPRESSION;
 
+    private static final int SYNC_FLUSH_INT = 2;
+    private static final int FULL_FLUSH_INT = 3;
+
     static {
         Method m = null;
         try {
@@ -100,25 +103,23 @@ public class Java7ZlibInputOutputStream extends XMPPInputOutputStream {
 
     @Override
     public OutputStream getOutputStream(OutputStream outputStream) {
+        final int flushMethodInt;
+        if (flushMethod == FlushMethod.SYNC_FLUSH) {
+            flushMethodInt = SYNC_FLUSH_INT;
+        } else {
+            flushMethodInt = FULL_FLUSH_INT;
+        }
         return new DeflaterOutputStream(outputStream, new Deflater(compressionLevel)) {
             public void flush() throws IOException {
                 if (!supported) {
                     super.flush();
                     return;
                 }
-                int count = 0;
-                if (!def.needsInput()) {
-                    do {
-                        count = def.deflate(buf, 0, buf.length);
-                        out.write(buf, 0, count);
-                    } while (count > 0);
-                    out.flush();
-                }
                 try {
-                    do {
+                    int count;
-                        count = (Integer) method.invoke(def, buf, 0, buf.length, 2);
+                    while ((count = (Integer) method.invoke(def, buf, 0, buf.length, flushMethodInt)) != 0) {
                         out.write(buf, 0, count);
-                    } while (count > 0);
+                    }
                 } catch (IllegalArgumentException e) {
                     throw new IOException("Can't flush");
                 } catch (IllegalAccessException e) {

smack-core/src/main/java/org/jivesoftware/smack/compression/XMPPInputOutputStream.java

@@ -20,6 +20,21 @@ import java.io.InputStream;
 import java.io.OutputStream;
 
 public abstract class XMPPInputOutputStream {
+
+    protected static FlushMethod flushMethod;
+
+    /**
+     * Set the used flushed method when compressing data. The default is full flush which may not
+     * achieve the best compression ratio, but provides better security against certain attacks.
+     * Only use sync flush if you fully understand the implications.
+     * 
+     * @see <a href="https://blog.thijsalkema.de/blog/2014/08/07/https-attacks-and-xmpp-2-crime-and-breach/">Attacks against XMPP when using compression</a>
+     * @param flushMethod
+     */
+    public static void setFlushMethod(FlushMethod flushMethod) {
+        XMPPInputOutputStream.flushMethod = flushMethod;
+    }
+
     protected String compressionMethod;
 
     public String getCompressionMethod() {
@@ -31,4 +46,9 @@ public abstract class XMPPInputOutputStream {
     public abstract InputStream getInputStream(InputStream inputStream) throws Exception;
 
     public abstract OutputStream getOutputStream(OutputStream outputStream) throws Exception;
+
+    public enum FlushMethod {
+        FULL_FLUSH,
+        SYNC_FLUSH,
+    }
 }