111 lines
3.5 KiB
Java
111 lines
3.5 KiB
Java
/**
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* Copyright (C) 2011 Whisper Systems
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*
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* This program is free software: you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program. If not, see <http://www.gnu.org/licenses/>.
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*/
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package org.thoughtcrime.securesms.protocol;
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import org.whispersystems.textsecure.util.Base64;
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import java.security.MessageDigest;
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import java.security.NoSuchAlgorithmException;
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import java.security.SecureRandom;
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/**
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* Calculates prefixes that identify a message as
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* being part of an encrypted session. The idea was to
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* make calculating and identifying these prefixes somewhat
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* expensive, so that filtering them en-mass would come at a cost.
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*
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* @author Moxie Marlinspike
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*/
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public abstract class WirePrefix {
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private static final int HASH_ITERATIONS = 1000;
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private static final int PREFIX_BYTES = 3;
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public static final int PREFIX_SIZE = 4;
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public abstract String calculatePrefix(String message);
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public static boolean isKeyExchange(String message) {
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return verifyPrefix("?TSK", message);
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}
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public static boolean isEncryptedMessage(String message) {
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return verifyPrefix("?TSM", message);
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}
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public static String calculateKeyExchangePrefix(String message) {
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return calculatePrefix(("?TSK" + message).getBytes(), PREFIX_BYTES);
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}
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public static String calculateEncryptedMesagePrefix(String message) {
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return calculatePrefix(("?TSM" + message).getBytes(), PREFIX_BYTES);
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}
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private static boolean verifyPrefix(String prefixType, String message) {
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if (message.length() <= PREFIX_SIZE)
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return false;
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String prefix = message.substring(0, PREFIX_SIZE);
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message = message.substring(PREFIX_SIZE);
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String calculatedPrefix = calculatePrefix((prefixType + message).getBytes(), PREFIX_BYTES);
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assert(calculatedPrefix.length() == PREFIX_SIZE);
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return prefix.equals(calculatedPrefix);
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}
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private static String calculatePrefix(byte[] message, int byteCount) {
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try {
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MessageDigest md = MessageDigest.getInstance("SHA1");
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byte[] runningDigest = message;
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for (int i=0;i<HASH_ITERATIONS;i++) {
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runningDigest = md.digest(runningDigest);
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}
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return Base64.encodeBytes(runningDigest, 0, byteCount);
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} catch (NoSuchAlgorithmException e) {
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throw new AssertionError(e);
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}
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}
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public static String calculateEncryptedMmsSubject() {
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try {
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byte[] postfix = new byte[6];
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SecureRandom.getInstance("SHA1PRNG").nextBytes(postfix);
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String postfixString = Base64.encodeBytes(postfix);
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String prefix = calculatePrefix(postfixString.getBytes(), 6);
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return prefix + postfixString;
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} catch (NoSuchAlgorithmException e) {
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throw new AssertionError(e);
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}
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}
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public static boolean isEncryptedMmsSubject(String subject) {
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if (subject.length() < 9)
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return false;
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String prefix = subject.substring(0, 8);
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String postfix = subject.substring(8);
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String calculatedPrefix = calculatePrefix(postfix.getBytes(), 6);
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return calculatedPrefix.equals(prefix);
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}
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}
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