Split the functionality of ConnectionWriterImpl into layers.

components/net/sf/briar/transport/OutgoingAuthenticationLayerImpl.java

@@ -0,0 +1,52 @@
+package net.sf.briar.transport;
+
+import static net.sf.briar.api.transport.TransportConstants.MAC_LENGTH;
+
+import java.io.IOException;
+import java.security.InvalidKeyException;
+
+import javax.crypto.Mac;
+import javax.crypto.ShortBufferException;
+
+import net.sf.briar.api.crypto.ErasableKey;
+
+class OutgoingAuthenticationLayerImpl implements OutgoingAuthenticationLayer {
+
+	private final OutgoingErrorCorrectionLayer out;
+	private final Mac mac;
+
+	OutgoingAuthenticationLayerImpl(OutgoingErrorCorrectionLayer out, Mac mac,
+			ErasableKey macKey) {
+		this.out = out;
+		this.mac = mac;
+		// Initialise the MAC
+		try {
+			mac.init(macKey);
+		} catch(InvalidKeyException badKey) {
+			throw new IllegalArgumentException(badKey);
+		}
+		macKey.erase();
+		if(mac.getMacLength() != MAC_LENGTH)
+			throw new IllegalArgumentException();
+	}
+
+	public void writeFrame(Frame f) throws IOException {
+		byte[] buf = f.getBuffer();
+		int length = f.getLength() - MAC_LENGTH;
+		mac.update(buf, 0, length);
+		try {
+			mac.doFinal(buf, length);
+		} catch(ShortBufferException badMac) {
+			throw new RuntimeException(badMac);
+		}
+		out.writeFrame(f);
+	}
+
+	public void flush() throws IOException {
+		out.flush();
+	}
+
+	public long getRemainingCapacity() {
+		return out.getRemainingCapacity();
+	}
+}