Add secure(OnlineAttackSafe|Unique|OfflineAttackSafe)RandomString()

and replace usages of java.util.UUID in Smack with
secureUniqueRandomString() because it uses a thread-local secure random
number generator.

smack-core/src/main/java/org/jivesoftware/smack/util/StringUtils.java

@@ -17,6 +17,8 @@
 
 package org.jivesoftware.smack.util;
 
+import java.io.IOException;
+import java.nio.CharBuffer;
 import java.nio.charset.StandardCharsets;
 import java.util.Collection;
 import java.util.Iterator;
@@ -264,13 +266,9 @@ public class StringUtils {
     }
 
     /**
-     * Array of numbers and letters of mixed case. Numbers appear in the list
+     * 24 upper case characters from the latin alphabet and numbers without '0' and 'O'.
-     * twice so that there is a more equal chance that a number will be picked.
-     * We can use the array to get a random number or letter by picking a random
-     * array index.
      */
-    private static final char[] numbersAndLetters = ("0123456789abcdefghijklmnopqrstuvwxyz" +
+    private static final char[] UNAMBIGUOUS_NUMBERS_AND_LETTER = "123456789ABCDEFGHIJKLMNPQRSTUVWXYZ".toCharArray();
-                    "ABCDEFGHIJKLMNOPQRSTUVWXYZ").toCharArray();
 
     /**
      * Returns a random String of numbers and letters (lower and upper case)
@@ -289,6 +287,74 @@ public class StringUtils {
         return randomString(length, RandomUtil.RANDOM.get());
     }
 
+    public static String secureOnlineAttackSafeRandomString() {
+        // 34^10 = 2.06e15 possible combinations. Which is enough to protect against online brute force attacks.
+        // See also https://www.grc.com/haystack.htm
+        final int REQUIRED_LENGTH = 10;
+
+        return randomString(RandomUtil.SECURE_RANDOM.get(), UNAMBIGUOUS_NUMBERS_AND_LETTER, REQUIRED_LENGTH);
+    }
+
+    public static String secureUniqueRandomString() {
+        // 34^13 = 8.11e19 possible combinations, which is > 2^64.
+        final int REQUIRED_LENGTH = 13;
+
+        return randomString(RandomUtil.SECURE_RANDOM.get(), UNAMBIGUOUS_NUMBERS_AND_LETTER, REQUIRED_LENGTH);
+    }
+
+    /**
+     * Generate a secure random string with is human readable. The resulting string consists of 24 upper case characters
+     * from the Latin alphabet and numbers without '0' and 'O', grouped into 4-characters chunks, e.g.
+     * "TWNK-KD5Y-MT3T-E1GS-DRDB-KVTW". The characters are randomly selected by a cryptographically secure pseudorandom
+     * number generator (CSPRNG).
+     * <p>
+     * The string can be used a backup "code" for secrets, and is in fact the same as the one backup code specified in
+     * XEP-0373 and the one used by the <a href="https://github.com/open-keychain/open-keychain/wiki/Backups">Backup
+     * Format v2 of OpenKeychain</a>.
+     * </p>
+     *
+     * @see <a href="https://xmpp.org/extensions/xep-0373.html#backup-encryption"> XEP-0373 §5.4 Encrypting the Secret
+     *      Key Backup</a>
+     * @return a human readable secure random string.
+     */
+    public static String secureOfflineAttackSafeRandomString() {
+        // 34^24 = 2^122.10 possible combinations. Which is enough to protect against offline brute force attacks.
+        // See also https://www.grc.com/haystack.htm
+        final int REQUIRED_LENGTH = 24;
+
+        return randomString(RandomUtil.SECURE_RANDOM.get(), UNAMBIGUOUS_NUMBERS_AND_LETTER, REQUIRED_LENGTH);
+    }
+
+    private static final int RANDOM_STRING_CHUNK_SIZE = 4;
+
+    private static String randomString(Random random, char[] alphabet, int numRandomChars) {
+        // The buffer most hold the size of the requested number of random chars and the chunk separators ('-').
+        int bufferSize = numRandomChars + ((numRandomChars - 1) / RANDOM_STRING_CHUNK_SIZE);
+        CharBuffer charBuffer = CharBuffer.allocate(bufferSize);
+
+        try {
+            randomString(charBuffer, random, alphabet, numRandomChars);
+        } catch (IOException e) {
+            // This should never happen if we calcuate the buffer size correctly.
+            throw new AssertionError(e);
+        }
+
+        return charBuffer.flip().toString();
+    }
+
+    private static void randomString(Appendable appendable, Random random, char[] alphabet, int numRandomChars)
+                    throws IOException {
+        for (int randomCharNum = 1; randomCharNum <= numRandomChars; randomCharNum++) {
+            int randomIndex = random.nextInt(alphabet.length);
+            char randomChar = alphabet[randomIndex];
+            appendable.append(randomChar);
+
+            if (randomCharNum % RANDOM_STRING_CHUNK_SIZE == 0 && randomCharNum < numRandomChars) {
+                appendable.append('-');
+            }
+        }
+    }
+
     public static String randomString(final int length) {
         return randomString(length, RandomUtil.SECURE_RANDOM.get());
     }
@@ -298,24 +364,14 @@ public class StringUtils {
             return "";
         }
 
-        byte[] randomBytes = new byte[length];
-        random.nextBytes(randomBytes);
         char[] randomChars = new char[length];
         for (int i = 0; i < length; i++) {
-            randomChars[i] = getPrintableChar(randomBytes[i]);
+            int index = random.nextInt(UNAMBIGUOUS_NUMBERS_AND_LETTER.length);
+            randomChars[i] = UNAMBIGUOUS_NUMBERS_AND_LETTER[index];
         }
         return new String(randomChars);
     }
 
-    private static char getPrintableChar(byte indexByte) {
-        assert (numbersAndLetters.length < Byte.MAX_VALUE * 2);
-
-        // Convert indexByte as it where an unsigned byte by promoting it to int
-        // and masking it with 0xff. Yields results from 0 - 254.
-        int index = indexByte & 0xff;
-        return numbersAndLetters[index % numbersAndLetters.length];
-    }
-
     /**
      * Returns true if CharSequence is not null and is not empty, false otherwise.
      * Examples:

smack-experimental/src/main/java/org/jivesoftware/smackx/mam/MamManager.java

@@ -1,6 +1,6 @@
 /**
  *
- * Copyright © 2017-2018 Florian Schmaus, 2016-2017 Fernando Ramirez
+ * Copyright © 2017-2019 Florian Schmaus, 2016-2017 Fernando Ramirez
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -23,7 +23,6 @@ import java.util.Date;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
-import java.util.UUID;
 import java.util.WeakHashMap;
 
 import org.jivesoftware.smack.ConnectionCreationListener;
@@ -456,7 +455,7 @@ public final class MamManager extends Manager {
 
     public MamQuery queryArchive(MamQueryArgs mamQueryArgs) throws NoResponseException, XMPPErrorException,
                     NotConnectedException, NotLoggedInException, InterruptedException {
-        String queryId = UUID.randomUUID().toString();
+        String queryId = StringUtils.secureUniqueRandomString();
         String node = mamQueryArgs.node;
         DataForm dataForm = mamQueryArgs.getDataForm();
 
@@ -515,7 +514,7 @@ public final class MamManager extends Manager {
     public List<FormField> retrieveFormFields(String node)
                     throws NoResponseException, XMPPErrorException, NotConnectedException,
             InterruptedException, NotLoggedInException {
-        String queryId = UUID.randomUUID().toString();
+        String queryId = StringUtils.secureUniqueRandomString();
         MamQueryIQ mamQueryIq = new MamQueryIQ(queryId, node, null);
         mamQueryIq.setTo(archiveAddress);
 
@@ -601,7 +600,8 @@ public final class MamManager extends Manager {
 
         private List<Message> page(RSMSet requestRsmSet) throws NoResponseException, XMPPErrorException,
                         NotConnectedException, NotLoggedInException, InterruptedException {
-            MamQueryIQ mamQueryIQ = new MamQueryIQ(UUID.randomUUID().toString(), node, form);
+            String queryId = StringUtils.secureUniqueRandomString();
+            MamQueryIQ mamQueryIQ = new MamQueryIQ(queryId, node, form);
             mamQueryIQ.setType(IQ.Type.set);
             mamQueryIQ.setTo(archiveAddress);
             mamQueryIQ.addExtension(requestRsmSet);

smack-experimental/src/main/java/org/jivesoftware/smackx/sid/element/StableAndUniqueIdElement.java

@@ -16,8 +16,6 @@
  */
 package org.jivesoftware.smackx.sid.element;
 
-import java.util.UUID;
-
 import org.jivesoftware.smack.packet.ExtensionElement;
 import org.jivesoftware.smack.util.StringUtils;
 
@@ -28,7 +26,7 @@ public abstract class StableAndUniqueIdElement implements ExtensionElement {
     private final String id;
 
     public StableAndUniqueIdElement() {
-        this.id = UUID.randomUUID().toString();
+        this.id = StringUtils.secureUniqueRandomString();
     }
 
     public String getId() {

smack-experimental/src/test/java/org/jivesoftware/smackx/sid/StableUniqueStanzaIdTest.java

@@ -65,7 +65,7 @@ public class StableUniqueStanzaIdTest extends SmackTestSuite {
         OriginIdElement element = new OriginIdElement();
         assertNotNull(element);
         assertEquals(StableUniqueStanzaIdManager.NAMESPACE, element.getNamespace());
-        assertEquals(36, element.getId().length());
+        assertEquals(16, element.getId().length());
     }
 
     @Test

smack-extensions/src/main/java/org/jivesoftware/smackx/bytestreams/ibb/InBandBytestreamManager.java

@@ -20,7 +20,6 @@ import java.util.Collections;
 import java.util.LinkedList;
 import java.util.List;
 import java.util.Map;
-import java.util.Random;
 import java.util.WeakHashMap;
 import java.util.concurrent.ConcurrentHashMap;
 
@@ -36,6 +35,7 @@ import org.jivesoftware.smack.XMPPException;
 import org.jivesoftware.smack.XMPPException.XMPPErrorException;
 import org.jivesoftware.smack.packet.IQ;
 import org.jivesoftware.smack.packet.StanzaError;
+import org.jivesoftware.smack.util.StringUtils;
 
 import org.jivesoftware.smackx.bytestreams.BytestreamListener;
 import org.jivesoftware.smackx.bytestreams.BytestreamManager;
@@ -138,9 +138,6 @@ public final class InBandBytestreamManager extends Manager implements Bytestream
     /* prefix used to generate session IDs */
     private static final String SESSION_ID_PREFIX = "jibb_";
 
-    /* random generator to create session IDs */
-    private static final Random randomGenerator = new Random();
-
     /* stores one InBandBytestreamManager for each XMPP connection */
     private static final Map<XMPPConnection, InBandBytestreamManager> managers = new WeakHashMap<>();
 
@@ -490,7 +487,7 @@ public final class InBandBytestreamManager extends Manager implements Bytestream
     private static String getNextSessionID() {
         StringBuilder buffer = new StringBuilder();
         buffer.append(SESSION_ID_PREFIX);
-        buffer.append(randomGenerator.nextInt(Integer.MAX_VALUE) + randomGenerator.nextInt(Integer.MAX_VALUE));
+        buffer.append(StringUtils.secureOnlineAttackSafeRandomString());
         return buffer.toString();
     }
 

smack-extensions/src/main/java/org/jivesoftware/smackx/bytestreams/socks5/Socks5BytestreamManager.java

@@ -24,7 +24,6 @@ import java.util.HashSet;
 import java.util.LinkedList;
 import java.util.List;
 import java.util.Map;
-import java.util.Random;
 import java.util.Set;
 import java.util.WeakHashMap;
 import java.util.concurrent.ConcurrentHashMap;
@@ -44,6 +43,7 @@ import org.jivesoftware.smack.XMPPException.XMPPErrorException;
 import org.jivesoftware.smack.packet.IQ;
 import org.jivesoftware.smack.packet.Stanza;
 import org.jivesoftware.smack.packet.StanzaError;
+import org.jivesoftware.smack.util.StringUtils;
 
 import org.jivesoftware.smackx.bytestreams.BytestreamListener;
 import org.jivesoftware.smackx.bytestreams.BytestreamManager;
@@ -114,9 +114,6 @@ public final class Socks5BytestreamManager extends Manager implements Bytestream
     /* prefix used to generate session IDs */
     private static final String SESSION_ID_PREFIX = "js5_";
 
-    /* random generator to create session IDs */
-    private static final Random randomGenerator = new Random();
-
     /* stores one Socks5BytestreamManager for each XMPP connection */
     private static final Map<XMPPConnection, Socks5BytestreamManager> managers = new WeakHashMap<>();
 
@@ -759,7 +756,7 @@ public final class Socks5BytestreamManager extends Manager implements Bytestream
     private static String getNextSessionID() {
         StringBuilder buffer = new StringBuilder();
         buffer.append(SESSION_ID_PREFIX);
-        buffer.append(randomGenerator.nextInt(Integer.MAX_VALUE) + randomGenerator.nextInt(Integer.MAX_VALUE));
+        buffer.append(StringUtils.secureOnlineAttackSafeRandomString());
         return buffer.toString();
     }
 

smack-im/src/main/java/org/jivesoftware/smack/chat/ChatManager.java

@@ -20,7 +20,6 @@ package org.jivesoftware.smack.chat;
 import java.util.Collections;
 import java.util.Map;
 import java.util.Set;
-import java.util.UUID;
 import java.util.WeakHashMap;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.CopyOnWriteArraySet;
@@ -42,6 +41,7 @@ import org.jivesoftware.smack.filter.ThreadFilter;
 import org.jivesoftware.smack.packet.Message;
 import org.jivesoftware.smack.packet.Message.Type;
 import org.jivesoftware.smack.packet.Stanza;
+import org.jivesoftware.smack.util.StringUtils;
 
 import org.jxmpp.jid.EntityBareJid;
 import org.jxmpp.jid.EntityJid;
@@ -407,7 +407,7 @@ public final class ChatManager extends Manager{
      * @return the next id.
      */
     private static String nextID() {
-        return UUID.randomUUID().toString();
+        return StringUtils.secureUniqueRandomString();
     }
 
     public static void setDefaultMatchMode(MatchMode mode) {

smack-openpgp/src/main/java/org/jivesoftware/smackx/ox/util/SecretKeyBackupHelper.java

@@ -18,9 +18,9 @@ package org.jivesoftware.smackx.ox.util;
 
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
-import java.security.SecureRandom;
 import java.util.Set;
 
+import org.jivesoftware.smack.util.StringUtils;
 import org.jivesoftware.smack.util.stringencoder.Base64;
 
 import org.jivesoftware.smackx.ox.crypto.OpenPgpProvider;
@@ -52,26 +52,7 @@ public class SecretKeyBackupHelper {
      * @return backup code
      */
     public static String generateBackupPassword() {
-        final String alphabet = "123456789ABCDEFGHIJKLMNPQRSTUVWXYZ";
+        return StringUtils.secureOfflineAttackSafeRandomString();
-        final int len = alphabet.length();
-        SecureRandom random = new SecureRandom();
-        StringBuilder code = new StringBuilder(29);
-
-        // 6 blocks
-        for (int i = 0; i < 6; i++) {
-
-            // of 4 chars
-            for (int j = 0; j < 4; j++) {
-                char c = alphabet.charAt(random.nextInt(len));
-                code.append(c);
-            }
-
-            // dash after every block except the last one
-            if (i != 5) {
-                code.append('-');
-            }
-        }
-        return code.toString();
     }
 
     /**