Issue jetty#4128 - Add missing padding and use URL decoder

Signed-off-by: Travis Spencer <travis@curity.io>

jetty-openid/src/main/java/org/eclipse/jetty/security/openid/OpenIdCredentials.java

@@ -158,9 +158,9 @@ protected Map<String, Object> decodeJWT(String jwt) throws IOException
         if (sections.length != 3)
             throw new IllegalArgumentException("JWT does not contain 3 sections");
 
-        Base64.Decoder decoder = Base64.getDecoder();
-        String jwtHeaderString = new String(decoder.decode(sections[0]), StandardCharsets.UTF_8);
-        String jwtClaimString = new String(decoder.decode(sections[1]), StandardCharsets.UTF_8);
+        Base64.Decoder decoder = Base64.getUrlDecoder();
+        String jwtHeaderString = new String(decoder.decode(padJWTSection(sections[0])), StandardCharsets.UTF_8);
+        String jwtClaimString = new String(decoder.decode(padJWTSection(sections[1])), StandardCharsets.UTF_8);
         String jwtSignature = sections[2];
 
         Map<String, Object> jwtHeader = (Map)JSON.parse(jwtHeaderString);
@@ -175,6 +175,29 @@ and the Token Endpoint (which it is in this flow), the TLS server validation
         return (Map)JSON.parse(jwtClaimString);
     }
 
+    private static byte[] padJWTSection(String unpaddedEncodedJwtSection)
+    {
+        int length = unpaddedEncodedJwtSection.length();
+        int remainder = length % 4;
+        byte[] bytes;
+
+        if (remainder > 0)
+        {
+            int paddingNeeded = 4 - remainder;
+            byte[] padding = { '=', '=', '=' };
+            bytes = new byte[length + paddingNeeded];
+
+            System.arraycopy(unpaddedEncodedJwtSection.getBytes(), 0, bytes, 0, length);
+            System.arraycopy(padding, 0, bytes, length, paddingNeeded);
+        }
+        else
+        {
+            bytes = unpaddedEncodedJwtSection.getBytes();
+        }
+
+        return bytes;
+    }
+
     private Map<String, Object> claimAuthCode(String authCode) throws IOException
     {
         if (LOG.isDebugEnabled())