Adds a VerifyAuditProof method to CTLogVerifier

net/cert/merkle_audit_proof.h

 // 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.
 
 #ifndef NET_CERT_MERKLE_AUDIT_PROOF_H_
 #define NET_CERT_MERKLE_AUDIT_PROOF_H_
 
 #include <stdint.h>
 
 #include <string>
 #include <vector>
 
 #include "base/macros.h"
 #include "net/base/net_export.h"
 
 namespace net {
 namespace ct {
 
 // Returns the length of the audit path for a leaf at |leaf_index| in a Merkle
 // tree containing |tree_size| leaves.
 // The |leaf_index| must be less than the |tree_size|.
 NET_EXPORT uint64_t CalculateAuditPathLength(uint64_t leaf_index,
                                              uint64_t tree_size);
 
 // Audit proof for a Merkle tree leaf, as defined in section 2.1.1. of RFC6962.
 struct NET_EXPORT MerkleAuditProof {
   MerkleAuditProof();
   MerkleAuditProof(uint64_t leaf_index,
+                   uint64_t tree_size,
                    const std::vector<std::string>& audit_path);
   ~MerkleAuditProof();
 
   // Index of the tree leaf in the log.
+  // Must be provided when fetching the proof from the log.
   uint64_t leaf_index = 0;
 
+  // The proof works only in conjunction with an STH for this tree size.
+  // Must be provided when fetching the proof from the log.
+  uint64_t tree_size = 0;
+
   // Audit path nodes.
+  // Using the leaf hash and these nodes, the STH hash can be reconstructed to
+  // prove that leaf was included in the log's tree.
   std::vector<std::string> nodes;
 };
 
 }  // namespace ct
 }  // namespace net
 
 #endif  // NET_CERT_MERKLE_AUDIT_PROOF_H_