Adds a MerkleAuditProof struct to net/cert.

net/cert/merkle_audit_proof.h

Index: net/cert/merkle_audit_proof.h
diff --git a/net/cert/merkle_audit_proof.h b/net/cert/merkle_audit_proof.h
new file mode 100644
index 0000000000000000000000000000000000000000..2f6fdb93b9fd83036e7e3d48270e0df84106d053
--- /dev/null
+++ b/net/cert/merkle_audit_proof.h
@@ -0,0 +1,46 @@
+// 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(const std::string& log_id,
+                   uint64_t leaf_index,
+                   const std::vector<std::string>& audit_path);
+  ~MerkleAuditProof();
+
+  // The origin of this proof.
+  std::string log_id;
+
+  // Index of the tree leaf in the log.
+  uint64_t leaf_index = 0;
+
+  // Audit path nodes.
+  std::vector<std::string> nodes;
+};
+
+}  // namespace ct
+}  // namespace net
+
+#endif  // NET_CERT_MERKLE_AUDIT_PROOF_H_