[Proximity auth] Disallow '+' and '/' in incoming base64-encoded strings.

components/proximity_auth/cryptauth/base64url.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/proximity_auth/cryptauth/base64url.h"
 
 #include "base/base64.h"
 #include "base/strings/string_util.h"
 
 namespace proximity_auth {
 
 void Base64UrlEncode(const std::string& decoded_input,
                      std::string* encoded_output) {
   base::Base64Encode(decoded_input, encoded_output);
   base::ReplaceChars(*encoded_output, "+", "-", encoded_output);
   base::ReplaceChars(*encoded_output, "/", "_", encoded_output);
 }
 
 bool Base64UrlDecode(const std::string& encoded_input,
                      std::string* decoded_output) {
+  // Bail on malformed strings, which already contain a '+' or a '/'. All valid
+  // strings should escape these special characters as '-' and '_',
+  // respectively.
+  if (encoded_input.find_first_of("+/") != std::string::npos)
+    return false;
+
   std::string adjusted_encoded_input = encoded_input;
   base::ReplaceChars(adjusted_encoded_input, "-", "+", &adjusted_encoded_input);
   base::ReplaceChars(adjusted_encoded_input, "_", "/", &adjusted_encoded_input);
 
   return base::Base64Decode(adjusted_encoded_input, decoded_output);
 }
 
 }  // namespace proximity_auth