Implementation with tests. Resolves #285

api/src/main/java/io/jsonwebtoken/io/Base64.java

@@ -27,6 +27,7 @@ final class Base64 { //final and package-protected on purpose
     private static final char[] BASE64URL_ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_".toCharArray();
     private static final int[] BASE64_IALPHABET = new int[256];
     private static final int[] BASE64URL_IALPHABET = new int[256];
+    private static final int IALPHABET_MAX_INDEX = BASE64_IALPHABET.length - 1;
 
     static {
         Arrays.fill(BASE64_IALPHABET, -1);
@@ -56,6 +57,10 @@ final class Base64 { //final and package-protected on purpose
     // *  char[] version
     // ****************************************************************************************
 
+    private String getName() {
+        return urlsafe ? "base64url" : "base64"; // RFC 4648 codec names are all lowercase
+    }
+
     /**
      * Encodes a raw byte array into a BASE64 <code>char[]</code> representation in accordance with RFC 2045.
      *
@@ -194,6 +199,15 @@ final class Base64 { //final and package-protected on purpose
     }
     */
 
+    private int ctoi(char c) {
+        int i = c > IALPHABET_MAX_INDEX ? -1 : IALPHABET[c];
+        if (i < 0) {
+            String msg = "Illegal " + getName() + " character: '" + c + "'";
+            throw new DecodingException(msg);
+        }
+        return i;
+    }
+
     /**
      * Decodes a BASE64 encoded char array that is known to be reasonably well formatted. The preconditions are:<br>
      * + The array must have a line length of 76 chars OR no line separators at all (one line).<br>
@@ -237,7 +251,7 @@ final class Base64 { //final and package-protected on purpose
         for (int cc = 0, eLen = (len / 3) * 3; d < eLen; ) {
 
             // Assemble three bytes into an int from four "valid" characters.
-            int i = IALPHABET[sArr[sIx++]] << 18 | IALPHABET[sArr[sIx++]] << 12 | IALPHABET[sArr[sIx++]] << 6 | IALPHABET[sArr[sIx++]];
+            int i = ctoi(sArr[sIx++]) << 18 | ctoi(sArr[sIx++]) << 12 | ctoi(sArr[sIx++]) << 6 | ctoi(sArr[sIx++]);
 
             // Add the bytes
             dArr[d++] = (byte) (i >> 16);
@@ -255,7 +269,7 @@ final class Base64 { //final and package-protected on purpose
             // Decode last 1-3 bytes (incl '=') into 1-3 bytes
             int i = 0;
             for (int j = 0; sIx <= eIx - pad; j++) {
-                i |= IALPHABET[sArr[sIx++]] << (18 - j * 6);
+                i |= ctoi(sArr[sIx++]) << (18 - j * 6);
             }
 
             for (int r = 16; d < len; r -= 8) {

api/src/main/java/io/jsonwebtoken/io/CodecException.java

@@ -5,6 +5,10 @@ package io.jsonwebtoken.io;
  */
 public class CodecException extends IOException {
 
+    public CodecException(String message) {
+        super(message);
+    }
+
     public CodecException(String message, Throwable cause) {
         super(message, cause);
     }

api/src/main/java/io/jsonwebtoken/io/DecodingException.java

@@ -5,6 +5,10 @@ package io.jsonwebtoken.io;
  */
 public class DecodingException extends CodecException {
 
+    public DecodingException(String message) {
+        super(message);
+    }
+
     public DecodingException(String message, Throwable cause) {
         super(message, cause);
     }

api/src/test/groovy/io/jsonwebtoken/io/Base64Test.groovy

@@ -23,6 +23,16 @@ class Base64Test {
                姚奊べ 椥䤥グ曣わ 栨キョ奎婨榞 ちょ䰯 Ủ䧞以ケ妣 誧姨のドゥろ よ苯礊 く涥, りゅぽ槞 馣ぢゃ尦䦎ぎ
                大た䏩䰥ぐ 郎きや楺橯 䧎キェ, 難ゞ滧 栧择 谯䧟簨訧ぎょ 椥䤥グ曣わ'''
 
+    @Test
+    void testBase64Name() {
+        assertEquals 'base64', Base64.DEFAULT.getName() // RFC 4648 codec name is all lowercase
+    }
+
+    @Test
+    void testBase64UrlName() {
+        assertEquals 'base64url', Base64.URL_SAFE.getName() // RFC 4648 codec name is all lowercase
+    }
+
     @Test
     void testEncodeToStringWithNullArgument() {
         String s = Base64.DEFAULT.encodeToString(null, false)
@@ -70,6 +80,39 @@ class Base64Test {
         assertEquals expected, result
     }
 
+    @Test
+    void testDecodeFastWithIntermediateIllegalInboundCharacters() {
+        def encoded = 'SGVsbG8g*5LiW55WM'
+        try {
+            Base64.DEFAULT.decodeFast(encoded.toCharArray())
+            fail()
+        } catch (DecodingException de) {
+            assertEquals 'Illegal base64 character: \'*\'', de.getMessage()
+        }
+    }
+
+    @Test
+    void testDecodeFastWithIntermediateIllegalOutOfBoundCharacters() {
+        def encoded = 'SGVsbG8g世5LiW55WM'
+        try {
+            Base64.DEFAULT.decodeFast(encoded.toCharArray())
+            fail()
+        } catch (DecodingException de) {
+            assertEquals 'Illegal base64 character: \'世\'', de.getMessage()
+        }
+    }
+
+    @Test
+    void testDecodeFastWithIntermediateIllegalSpaceCharacters() {
+        def encoded = 'SGVsbG8g 5LiW55WM'
+        try {
+            Base64.DEFAULT.decodeFast(encoded.toCharArray())
+            fail()
+        } catch (DecodingException de) {
+            assertEquals 'Illegal base64 character: \' \'', de.getMessage()
+        }
+    }
+
     @Test
     void testDecodeFastWithLineSeparators() {