Certificate Transparency: Code for unpacking EV cert hashes whitelist

net/cert/x509_certificate_win.cc

Index: net/cert/x509_certificate_win.cc
diff --git a/net/cert/x509_certificate_win.cc b/net/cert/x509_certificate_win.cc
index c679107371a8594e8202b725bf4078d46f7b8074..6b7a6d2bf1e1c75d4fdba3753b19becc9a0257eb 100644
--- a/net/cert/x509_certificate_win.cc
+++ b/net/cert/x509_certificate_win.cc
@@ -14,6 +14,7 @@
 #include "base/strings/utf_string_conversions.h"
 #include "crypto/capi_util.h"
 #include "crypto/scoped_capi_types.h"
+#include "crypto/sha2.h"
 #include "net/base/net_errors.h"
 
 #pragma comment(lib, "crypt32.lib")
@@ -313,6 +314,25 @@ SHA1HashValue X509Certificate::CalculateFingerprint(
   return sha1;
 }
 
+// static
+SHA256HashValue X509Certificate::CalculateFingerprint256(OSCertHandle cert) {
+  DCHECK(NULL != cert->pbCertEncoded);
+  DCHECK_NE(static_cast<DWORD>(0), cert->cbCertEncoded);
+
+  SHA256HashValue sha256;
+  DWORD sha256_size = sizeof(sha256.data);
+
+  // The reason crypto::SHA256HashString is used here rather than
+  // CryptHashCertificate (which is used for SHA1) is that:
+  // * On Windows XP there is no CSP that supports SHA-256.
+  // * On Windows Vista it is necessary to acquire a non-default CSP
+  //   to get SHA-256 capabilities, which introduces significant overhead.

[Ryan Sleevi, 2014/09/11 23:44:39]

So, I should have been clearer on the comment. The reason why
CryptHashCertificate is used above is that it's just a shim for
CertGetCertificateContextProperty(), but in a foward-compatible way.

That is, Windows maintains a SHA-1 hash for all certificates, always,
internally, but they reserve the right to change that (but largely, wont)

I'd simply update this comment
// Use crypto::SHA256HashString for two reasons:
// * < Windows Vista does not have universal SHA-256 support
// * It's more efficient for > Windows Vista (less overhead)

[Eran Messeri, 2014/10/01 16:08:36]

Done, thanks for the explanation.

+  base::StringPiece der_cert(reinterpret_cast<const char*>(cert->pbCertEncoded),
+                             cert->cbCertEncoded);
+  crypto::SHA256HashString(der_cert, sha256.data, sha256_size);
+  return sha256;
+}
+
 // TODO(wtc): This function is implemented with NSS low-level hash
 // functions to ensure it is fast.  Reimplement this function with
 // CryptoAPI.  May need to cache the HCRYPTPROV to reduce the overhead.