Check that table evenly divides 256 when generating random string.

webrtc/base/helpers.cc

 /*
  *  Copyright 2004 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 201 matching lines @@
   }
   return true;
 }
 
 std::string CreateRandomString(size_t len) {
   std::string str;
   CreateRandomString(len, &str);
   return str;
 }
 
-bool CreateRandomString(size_t len,
+static bool CreateRandomString(size_t len,
                         const char* table, int table_size,
                         std::string* str) {
   str->clear();
+  // Avoid biased modulo division below.
+  if (256 % table_size) {
+    LOG(LS_ERROR) << "Table size must divide 256 evenly!";
+    return false;
+  }
   std::unique_ptr<uint8_t[]> bytes(new uint8_t[len]);
   if (!Rng().Generate(bytes.get(), len)) {
     LOG(LS_ERROR) << "Failed to generate random string!";
     return false;
   }
   str->reserve(len);
   for (size_t i = 0; i < len; ++i) {
     str->push_back(table[bytes[i] % table_size]);
   }
   return true;
@@ skipping 70 matching lines @@
   } while (id == 0);
   return id;
 }
 
 double CreateRandomDouble() {
   return CreateRandomId() / (std::numeric_limits<uint32_t>::max() +
                              std::numeric_limits<double>::epsilon());
 }
 
 }  // namespace rtc