Adds a MerkleAuditProof struct to net/cert.

net/cert/merkle_audit_proof.cc

Index: net/cert/merkle_audit_proof.cc
diff --git a/net/cert/merkle_audit_proof.cc b/net/cert/merkle_audit_proof.cc
new file mode 100644
index 0000000000000000000000000000000000000000..81a8fac5d62dbab3594a14ffc56b2b9a72326c9a
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+++ b/net/cert/merkle_audit_proof.cc
@@ -0,0 +1,41 @@
+// 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 "net/cert/merkle_audit_proof.h"
+
+#include "base/logging.h"
+
+namespace net {
+namespace ct {
+
+uint64_t CalculateAuditPathLength(uint64_t leaf_index, uint64_t tree_size) {
+  // Algorithm taken from
+  // https://github.com/google/certificate-transparency-rfcs/blob/c8844de6bd0b5d3d16bac79865e6edef533d760b/dns/draft-ct-over-dns.md#retrieve-merkle-audit-proof-from-log-by-leaf-hash.

[Eran Messeri, 2016/05/25 09:59:55]

Nit: The ct-over-dns is what you may call a living document.
I'd swap this comment line with the next one (15) as the RFC is the
authoritative source.

[Rob Percival, 2016/05/25 10:14:07]

Done.

+  // RFC6962, section 2.1.1, describes audit paths.
+  DCHECK_LT(leaf_index, tree_size);
+  uint64_t length = 0;
+  uint64_t index = leaf_index;
+  uint64_t last_node = tree_size - 1;
+
+  while (last_node != 0) {
+    if ((index % 2 != 0) || index != last_node)
+      ++length;
+    index /= 2;
+    last_node /= 2;
+  }
+
+  return length;
+}
+
+MerkleAuditProof::MerkleAuditProof() {}
+
+MerkleAuditProof::MerkleAuditProof(const std::string& log_id,
+                                   uint64_t leaf_index,
+                                   const std::vector<std::string>& audit_path)
+    : log_id(log_id), leaf_index(leaf_index), nodes(audit_path) {}
+
+MerkleAuditProof::~MerkleAuditProof() {}
+
+}  // namespace ct
+}  // namespace net