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
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index 0000000000000000000000000000000000000000..e206a46c79c987d2047342d77a75e588776c9a23
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+++ b/net/cert/merkle_audit_proof.cc
@@ -0,0 +1,36 @@
+// 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"
+
+namespace net {
+namespace ct {
+
+uint64_t CalculateAuditPathLength(uint64_t leaf_index, uint64_t tree_size) {
+  // As described in RFC6962, section 4.5

[Eran Messeri, 2016/05/23 13:10:51]

The length calculation is not described in this section...

[Rob Percival, 2016/05/24 17:10:04]

Fixed to point to the CT over DNS draft RFC.

+  uint64_t length = 0;
+  uint64_t ln = tree_size - 1;

[Eran Messeri, 2016/05/23 13:10:51]

Can you think of more informative names to describe the values held in these
variables?

[Rob Percival, 2016/05/24 17:10:04]

Done.

+  uint64_t li = leaf_index;
+
+  while (ln != 0) {
+    if ((li & 1) || li < ln)

[Eran Messeri, 2016/05/23 13:10:51]

(1) Better documentation (copy from the DNS I-D if possible).
(2) li > ln should never happen (should be checked as a precondition probably),
so I'd suggest replacing the li < ln check with !(li == ln) for clarity.

[Rob Percival, 2016/05/24 17:10:04]

1. The DNS I-D doesn't really document this, it just describes the steps the
algorithm takes in pseudo-code (but doesn't explain the reasoning behind it). To
usefully document it here, I'd probably have to explain Merkle trees and audit
proofs, so is just linking to the appropriate section of RFC6962 ok?
2. Done. By the way, our Python code in GitHub also uses < rather than !=
(https://github.com/google/certificate-transparency/blob/af98904302724c29aa665...).

+      ++length;
+    li >>= 1;
+    ln >>= 1;
+  }
+
+  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