Use the substr API correctly in FullHashToHashPrefix.

components/safe_browsing_db/v4_protocol_manager_util.cc

 // Copyright 2016 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/safe_browsing_db/v4_protocol_manager_util.h"
 
 #include "base/base64.h"
 #include "base/hash.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/rand_util.h"
@@ skipping 287 matching lines @@
   }
 }
 
 // static
 bool V4ProtocolManagerUtil::FullHashToHashPrefix(const FullHash& full_hash,
                                                  PrefixSize prefix_size,
                                                  HashPrefix* hash_prefix) {
   if (full_hash.size() < prefix_size) {
     return false;
   }
-  *hash_prefix = full_hash.substr(prefix_size);
+  *hash_prefix = full_hash.substr(0, prefix_size);
   return true;
 }
 
 // static
 bool V4ProtocolManagerUtil::FullHashToSmallestHashPrefix(
     const FullHash& full_hash,
     HashPrefix* hash_prefix) {
   return FullHashToHashPrefix(full_hash, kMinHashPrefixLength, hash_prefix);
 }
 
@@ skipping 254 matching lines @@
 
   const std::string hash = base::SHA1HashString(packed_ip);
   DCHECK_EQ(20u, hash.size());
   hashed_encoded_ip->resize(hash.size() + 1);
   hashed_encoded_ip->replace(0, hash.size(), hash);
   (*hashed_encoded_ip)[hash.size()] = static_cast<unsigned char>(128);
   return true;
 }
 
 }  // namespace safe_browsing