Adds function for generating MerkleTreeLeaf hash.

net/cert/merkle_tree_leaf.cc

Index: net/cert/merkle_tree_leaf.cc
diff --git a/net/cert/merkle_tree_leaf.cc b/net/cert/merkle_tree_leaf.cc
index 6d67255e36f49d630f55ed352d7d33a9e93d2ad4..5a87df2e087c443ec03b74d92fdc770e29b0db8a 100644
--- a/net/cert/merkle_tree_leaf.cc
+++ b/net/cert/merkle_tree_leaf.cc
@@ -4,7 +4,9 @@
 
 #include "net/cert/merkle_tree_leaf.h"
 
+#include "crypto/sha2.h"
 #include "net/cert/ct_objects_extractor.h"
+#include "net/cert/ct_serialization.h"
 #include "net/cert/x509_certificate.h"
 
 namespace net {
@@ -15,6 +17,16 @@ MerkleTreeLeaf::MerkleTreeLeaf() {}
 
 MerkleTreeLeaf::~MerkleTreeLeaf() {}
 
+bool Hash(const MerkleTreeLeaf& tree_leaf, std::string* out) {
+  std::string leaf_in_tls_format;
+  if (!EncodeTreeLeaf(tree_leaf, &leaf_in_tls_format))
+    return false;
+
+  // Prepend 0 byte as per RFC 6962, section-2.1
+  *out = crypto::SHA256HashString("\x00" + leaf_in_tls_format);

[eroman, 2016/05/06 23:51:22]

This does not behave as described -- no 0 byte is prepended.

"\x00" is interpreted as a 0-length c-string, since its first character is NUL.

Would be simpler to call string::insert() with '\0' to avoid this.

[Rob Percival, 2016/05/09 12:36:05]

Done.

+  return true;
+}
+
 bool GetMerkleTreeLeaf(const X509Certificate* cert,
                        const SignedCertificateTimestamp* sct,
                        MerkleTreeLeaf* merkle_tree_leaf) {