session-android/src/org/thoughtcrime/securesms/sms/SmsTransportDetails.java

/** 
 * Copyright (C) 2011 Whisper Systems
 * 
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * 
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
package org.thoughtcrime.securesms.sms;

import android.util.Log;

import org.thoughtcrime.securesms.crypto.SessionCipher;
import org.thoughtcrime.securesms.crypto.TransportDetails;
import org.thoughtcrime.securesms.protocol.WirePrefix;
import org.thoughtcrime.securesms.util.Base64;

import java.io.IOException;

public class SmsTransportDetails implements TransportDetails {
	
  public static final int SMS_SIZE           = 160;
  public static final int MULTIPART_SMS_SIZE = 153;

  public static final int BASE_MAX_BYTES                = Base64.getEncodedBytesForTarget(SMS_SIZE - WirePrefix.PREFIX_SIZE);
  public static final int SINGLE_MESSAGE_MAX_BYTES      = BASE_MAX_BYTES - MultipartSmsTransportMessage.SINGLE_MESSAGE_MULTIPART_OVERHEAD;
  public static final int MULTI_MESSAGE_MAX_BYTES       = BASE_MAX_BYTES - MultipartSmsTransportMessage.MULTI_MESSAGE_MULTIPART_OVERHEAD;
  public static final int FIRST_MULTI_MESSAGE_MAX_BYTES = BASE_MAX_BYTES - MultipartSmsTransportMessage.FIRST_MULTI_MESSAGE_MULTIPART_OVERHEAD;

  public static final int ENCRYPTED_SINGLE_MESSAGE_BODY_MAX_SIZE = SINGLE_MESSAGE_MAX_BYTES - SessionCipher.ENCRYPTED_MESSAGE_OVERHEAD;

  @Override
  public byte[] encodeMessage(byte[] messageWithMac) {
    String encodedMessage = Base64.encodeBytesWithoutPadding(messageWithMac);
    Log.w("SmsTransportDetails", "Encoded Message Length: " + encodedMessage.length());
    return encodedMessage.getBytes();
  }

  @Override
  public byte[] decodeMessage(byte[] encodedMessageBytes) throws IOException {
    String encodedMessage = new String(encodedMessageBytes);
    return Base64.decodeWithoutPadding(encodedMessage);
  }

  @Override
  public byte[] stripPaddedMessage(byte[] messageWithPadding) {
    int paddingBeginsIndex = 0;

    for (int i=1;i<messageWithPadding.length;i++) {
      if (messageWithPadding[i] == (byte)0x00) {
        paddingBeginsIndex = i;
        break;
      }
    }

    if (paddingBeginsIndex == 0)
      return messageWithPadding;

    byte[] message = new byte[paddingBeginsIndex];
    System.arraycopy(messageWithPadding, 0, message, 0, message.length);

    return message;
  }

  @Override
  public byte[] getPaddedMessageBody(byte[] messageBody) {
    int paddedBodySize = getMaxBodySizeForBytes(messageBody.length);
    Log.w("SessionCipher", "Padding message body out to: " + paddedBodySize);

    byte[] paddedBody = new byte[paddedBodySize];
    System.arraycopy(messageBody, 0, paddedBody, 0, messageBody.length);

    return paddedBody;
  }

  private int getMaxBodySizeForBytes(int bodyLength) {
    int encryptedBodyLength   = bodyLength + SessionCipher.ENCRYPTED_MESSAGE_OVERHEAD;
    int messageRecordsForBody = getMessageCountForBytes(encryptedBodyLength);

    if (messageRecordsForBody == 1) {
      return ENCRYPTED_SINGLE_MESSAGE_BODY_MAX_SIZE;
    } else {
      return
          FIRST_MULTI_MESSAGE_MAX_BYTES +
          (MULTI_MESSAGE_MAX_BYTES * (messageRecordsForBody-1)) -
              SessionCipher.ENCRYPTED_MESSAGE_OVERHEAD;
    }
  }

  public int getMessageCountForBytes(int bytes) {
    if (bytes <= SINGLE_MESSAGE_MAX_BYTES)
      return 1;

    bytes = Math.max(bytes - FIRST_MULTI_MESSAGE_MAX_BYTES, 0);

    int messageCount = 1 + (bytes / MULTI_MESSAGE_MAX_BYTES);
    int remainder    = bytes % MULTI_MESSAGE_MAX_BYTES;

    if (remainder > 0)
      messageCount++;

    return messageCount;
  }
}