Delete XML whitespace before feeding the Base64 decoder

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

@@ -543,9 +543,15 @@ public class StringUtils {
         return cs.toString();
     }
 
-    private static final Pattern XMLWHITESPACE = Pattern.compile("[\t\n\r ]");
+    /**
+     * Defined by XML 1.0 § 2.3 as:
+     *  S      ::=      (#x20 | #x9 | #xD | #xA)+
+     *
+     * @see <a href="https://www.w3.org/TR/xml/#sec-white-space">XML 1.0 § 2.3</a>
+     */
+    private static final Pattern XML_WHITESPACE = Pattern.compile("[\t\n\r ]");
 
     public static String deleteXmlWhitespace(String string) {
-        return XMLWHITESPACE.matcher(string).replaceAll("");
+        return XML_WHITESPACE.matcher(string).replaceAll("");
     }
 }

smack-core/src/main/java/org/jivesoftware/smack/util/stringencoder/Base64.java

@@ -19,6 +19,7 @@ package org.jivesoftware.smack.util.stringencoder;
 import java.nio.charset.StandardCharsets;
 
 import org.jivesoftware.smack.util.Objects;
+import org.jivesoftware.smack.util.StringUtils;
 
 public class Base64 {
 
@@ -57,6 +58,9 @@ public class Base64 {
     // TODO: We really should not mask the IllegalArgumentException. But some unit test depend on this behavior, like
     // ibb.packet.DataPacketExtension.shouldReturnNullIfDataIsInvalid().
     public static final byte[] decode(String string) {
+        // xs:base64Binary may include XML whitespace which we need to delete before feeding the string into the Base64
+        // decoder. See also XML Schema Part 2: Datatypes Second Edition § 3.2.16.
+        string = StringUtils.deleteXmlWhitespace(string);
         try {
             return base64encoder.decode(string);
         } catch (IllegalArgumentException e) {