Extract Certificate Transparency SCTs from stapled OCSP responses

net/cert/ct_objects_extractor_nss.cc

Index: net/cert/ct_objects_extractor_nss.cc
diff --git a/net/cert/ct_objects_extractor_nss.cc b/net/cert/ct_objects_extractor_nss.cc
index 0f353489e3d33a585dca01befc6006ff00ef28bd..f80ad2f3ab221521ba9214fbe11a20c31cff3b29 100644
--- a/net/cert/ct_objects_extractor_nss.cc
+++ b/net/cert/ct_objects_extractor_nss.cc
@@ -10,6 +10,7 @@
 #include <secoid.h>
 
 #include "base/lazy_instance.h"
+#include "base/sha1.h"
 #include "crypto/scoped_nss_types.h"
 #include "crypto/sha2.h"
 #include "net/cert/asn1_util.h"
@@ -22,6 +23,12 @@ namespace ct {
 
 namespace {
 
+// NSS black magic to get the address of externally defined template at runtime.
+SEC_ASN1_MKSUB(SEC_IntegerTemplate)
+SEC_ASN1_MKSUB(SECOID_AlgorithmIDTemplate)
+SEC_ASN1_MKSUB(SEC_GeneralizedTimeTemplate)
+SEC_ASN1_MKSUB(CERT_SequenceOfCertExtensionTemplate)
+
 // Wrapper class to convert a X509Certificate::OSCertHandle directly
 // into a CERTCertificate* usable with other NSS functions. This is used for
 // platforms where X509Certificate::OSCertHandle refers to a different type
@@ -62,6 +69,27 @@ const char kEmbeddedSCTDescription[] =
     "X.509v3 Certificate Transparency Embedded Signed Certificate Timestamp "
     "List";
 
+// The wire form of the OID 1.3.6.1.4.1.11129.2.4.5 - OCSP SingleExtension for
+// X.509v3 Certificate Transparency Signed Certificate Timestamp List, see
+// Section 3.3 of RFC6962.
+const unsigned char kOCSPExtensionOid[] = {0x2B, 0x06, 0x01, 0x04, 0x01,
+                                           0xD6, 0x79, 0x02, 0x04, 0x05};
+
+const SECItem kOCSPExtensionOidItem = {
+  siBuffer, const_cast<unsigned char*>(kOCSPExtensionOid),
+  sizeof(kOCSPExtensionOid)
+};
+
+// id-ad-ocsp: 1.3.6.1.5.5.7.48.1.1
+const unsigned char kBasicOCSPResponseOid[] = {0x2B, 0x06, 0x01, 0x05, 0x05,
+                                               0x07, 0x30, 0x01, 0x01};
+
+const SECItem kBasicOCSPResponseOidItem = {
+  siBuffer, const_cast<unsigned char*>(kBasicOCSPResponseOid),
+  sizeof(kBasicOCSPResponseOid)
+};
+
+
 // Initializes the necessary NSS internals for use with Certificate
 // Transparency.
 class CTInitSingleton {
@@ -108,12 +136,12 @@ base::LazyInstance<CTInitSingleton>::Leaky g_ct_singleton =
     LAZY_INSTANCE_INITIALIZER;
 
 // Obtains the data for an X.509v3 certificate extension identified by |oid|
-// and encoded as an OCTET STRING. Returns true if the extension was found,
-// updating |ext_data| to be the extension data after removing the DER
-// encoding of OCTET STRING.
-bool GetOctetStringExtension(CERTCertificate* cert,
-                             SECOidTag oid,
-                             std::string* extension_data) {
+// and encoded as an OCTET STRING. Returns true if the extension was found in
+// the certificate, updating |ext_data| to be the extension data after removing
+// the DER encoding of OCTET STRING.
+bool GetCertOctetStringExtension(CERTCertificate* cert,
+                                 SECOidTag oid,
+                                 std::string* extension_data) {
   SECItem extension;
   SECStatus rv = CERT_FindCertExtension(cert, oid, &extension);
   if (rv != SECSuccess)
@@ -133,6 +161,48 @@ bool GetOctetStringExtension(CERTCertificate* cert,
   return rv == SECSuccess;
 }
 
+// NSS offers CERT_FindCertExtension for certificates, but that only accepts
+// CERTCertificate* inputs, so the method below extracts the SCT extension
+// directly from the CERTCertExtension** of an OCSP response.
+//
+// Obtains the data for an OCSP extension identified by kOCSPExtensionOidItem
+// and encoded as an OCTET STRING. Returns true if the extension was found in
+// |extensions|, updating |extension_data| to be the extension data after
+// removing the DER encoding of OCTET STRING.
+bool GetSCTListFromOCSPExtension(PLArenaPool* arena,
+                                 const CERTCertExtension* const* extensions,
+                                 std::string* extension_data) {
+  if (!extensions)
+    return false;
+
+  const CERTCertExtension* match = NULL;
+
+  for (const CERTCertExtension* const* exts = extensions; *exts; ++exts) {
+    const CERTCertExtension* ext = *exts;
+    if (SECITEM_ItemsAreEqual(&kOCSPExtensionOidItem, &ext->id)) {
+      match = ext;
+      break;
+    }
+  }
+
+  if (!match)
+    return false;
+
+  SECItem contents;
+  // SEC_QuickDERDecodeItem sets |contents| to point to |match|, so it is not
+  // necessary to free the contents of |contents|.
+  SECStatus rv = SEC_QuickDERDecodeItem(arena, &contents,
+                                        SEC_ASN1_GET(SEC_OctetStringTemplate),
+                                        &match->value);
+  if (rv != SECSuccess)
+    return false;
+
+  base::StringPiece parsed_data(reinterpret_cast<char*>(contents.data),
+                                contents.len);
+  parsed_data.CopyToString(extension_data);
+  return true;
+}
+
 // Given a |cert|, extract the TBSCertificate from this certificate, also
 // removing the X.509 extension with OID 1.3.6.1.4.1.11129.2.4.2 (that is,
 // the embedded SCT)
@@ -180,6 +250,179 @@ bool ExtractTBSCertWithoutSCTs(CERTCertificate* cert,
   return true;
 }
 
+// The following code is adapted from the NSS OCSP module, in order to expose
+// the internal structure of an OCSP response.
+
+// ResponseBytes ::=       SEQUENCE {
+//     responseType   OBJECT IDENTIFIER,
+//     response       OCTET STRING }
+struct ResponseBytes {
+  SECItem response_type;
+  SECItem der_response;
+};
+
+const SEC_ASN1Template kResponseBytesTemplate[] = {
+  { SEC_ASN1_SEQUENCE, 0, NULL, sizeof(ResponseBytes) },
+  { SEC_ASN1_OBJECT_ID, offsetof(ResponseBytes, response_type) },
+  { SEC_ASN1_OCTET_STRING, offsetof(ResponseBytes, der_response) },
+  { 0 }
+};
+
+// OCSPResponse ::=     SEQUENCE {
+//      responseStatus          OCSPResponseStatus,
+//      responseBytes           [0] EXPLICIT ResponseBytes OPTIONAL }
+struct OCSPResponse {
+  SECItem response_status;
+  // This indirection is needed because |response_bytes| is an optional
+  // component and we need a way to determine if it is missing.
+  ResponseBytes* response_bytes;
+};
+
+const SEC_ASN1Template kPointerToResponseBytesTemplate[] = {
+  { SEC_ASN1_POINTER, 0, kResponseBytesTemplate }
+};
+
+const SEC_ASN1Template kOCSPResponseTemplate[] = {
+  { SEC_ASN1_SEQUENCE, 0, NULL, sizeof(OCSPResponse) },
+  { SEC_ASN1_ENUMERATED, offsetof(OCSPResponse, response_status) },
+  { SEC_ASN1_OPTIONAL | SEC_ASN1_EXPLICIT | SEC_ASN1_CONSTRUCTED |
+    SEC_ASN1_CONTEXT_SPECIFIC | 0, offsetof(OCSPResponse, response_bytes),
+    kPointerToResponseBytesTemplate },
+  { 0 }
+};
+
+// CertID          ::=     SEQUENCE {
+//   hashAlgorithm       AlgorithmIdentifier,
+//   issuerNameHash      OCTET STRING, -- Hash of Issuer's DN
+//   issuerKeyHash       OCTET STRING, -- Hash of Issuers public key
+//   serialNumber        CertificateSerialNumber }
+struct CertID {
+  SECAlgorithmID hash_algorithm;
+  SECItem issuer_name_hash;
+  SECItem issuer_key_hash;
+  SECItem serial_number;
+};
+
+const SEC_ASN1Template kCertIDTemplate[] = {
+  { SEC_ASN1_SEQUENCE, 0, NULL, sizeof(CertID) },
+  { SEC_ASN1_INLINE | SEC_ASN1_XTRN, offsetof(CertID, hash_algorithm),
+    SEC_ASN1_SUB(SECOID_AlgorithmIDTemplate) },
+  { SEC_ASN1_OCTET_STRING, offsetof(CertID, issuer_name_hash) },
+  { SEC_ASN1_OCTET_STRING, offsetof(CertID, issuer_key_hash) },
+  { SEC_ASN1_INTEGER, offsetof(CertID, serial_number) },
+  { 0 }
+};
+
+// SingleResponse ::= SEQUENCE {
+//   certID                       CertID,
+//   certStatus                   CertStatus,
+//   thisUpdate                   GeneralizedTime,
+//   nextUpdate           [0]     EXPLICIT GeneralizedTime OPTIONAL,
+//   singleExtensions     [1]     EXPLICIT Extensions OPTIONAL }
+struct SingleResponse {
+  CertID cert_id;
+  // The following three fields are not used.
+  SECItem der_cert_status;
+  SECItem this_update;
+  SECItem next_update;
+  CERTCertExtension** single_extensions;
+};
+
+const SEC_ASN1Template kSingleResponseTemplate[] = {
+  { SEC_ASN1_SEQUENCE, 0, NULL, sizeof(SingleResponse) },
+  { SEC_ASN1_INLINE, offsetof(SingleResponse, cert_id), kCertIDTemplate },
+  // Really a CHOICE but we make it an ANY because  we don't care about the
+  // contents of this field.
+  // TODO(ekasper): use SEC_ASN1_CHOICE.
+  { SEC_ASN1_ANY, offsetof(SingleResponse, der_cert_status) },
+  { SEC_ASN1_GENERALIZED_TIME, offsetof(SingleResponse, this_update) },
+  { SEC_ASN1_OPTIONAL | SEC_ASN1_EXPLICIT |
+    SEC_ASN1_CONSTRUCTED | SEC_ASN1_CONTEXT_SPECIFIC | SEC_ASN1_XTRN | 0,
+    offsetof(SingleResponse, next_update),
+    SEC_ASN1_SUB(SEC_GeneralizedTimeTemplate) },
+  { SEC_ASN1_OPTIONAL | SEC_ASN1_EXPLICIT | SEC_ASN1_CONSTRUCTED |
+    SEC_ASN1_CONTEXT_SPECIFIC | SEC_ASN1_XTRN | 1,
+    offsetof(SingleResponse, single_extensions),
+    SEC_ASN1_SUB(CERT_SequenceOfCertExtensionTemplate) },
+  { 0 }
+};
+
+// ResponseData ::= SEQUENCE {
+//   version              [0] EXPLICIT Version DEFAULT v1,
+//   responderID              ResponderID,
+//   producedAt               GeneralizedTime,
+//   responses                SEQUENCE OF SingleResponse,
+//   responseExtensions   [1] EXPLICIT Extensions OPTIONAL }
+struct ResponseData {
+  // The first three fields are not used.
+  SECItem version;
+  SECItem der_responder_id;
+  SECItem produced_at;
+  SingleResponse** single_responses;
+  // Skip extensions.
+};
+
+const SEC_ASN1Template kResponseDataTemplate[] = {
+  { SEC_ASN1_SEQUENCE, 0, NULL, sizeof(ResponseData) },
+  { SEC_ASN1_OPTIONAL | SEC_ASN1_EXPLICIT | SEC_ASN1_CONSTRUCTED |
+    SEC_ASN1_CONTEXT_SPECIFIC | SEC_ASN1_XTRN | 0,
+    offsetof(ResponseData, version), SEC_ASN1_SUB(SEC_IntegerTemplate) },
+  // Really a CHOICE but we make it an ANY because  we don't care about the
+  // contents of this field.
+  // TODO(ekasper): use SEC_ASN1_CHOICE.
+  { SEC_ASN1_ANY, offsetof(ResponseData, der_responder_id) },
+  { SEC_ASN1_GENERALIZED_TIME, offsetof(ResponseData, produced_at) },
+  { SEC_ASN1_SEQUENCE_OF, offsetof(ResponseData, single_responses),
+    kSingleResponseTemplate },
+  { SEC_ASN1_SKIP_REST },
+  { 0 }
+};
+
+// BasicOCSPResponse       ::= SEQUENCE {
+//   tbsResponseData      ResponseData,
+//   signatureAlgorithm   AlgorithmIdentifier,
+//   signature            BIT STRING,
+//   certs                [0] EXPLICIT SEQUENCE OF Certificate OPTIONAL }
+struct BasicOCSPResponse {
+  ResponseData tbs_response_data;
+  // We do not care about the rest.
+};
+
+const SEC_ASN1Template kBasicOCSPResponseTemplate[] = {
+  { SEC_ASN1_SEQUENCE, 0, NULL, sizeof(BasicOCSPResponse) },
+  { SEC_ASN1_INLINE, offsetof(BasicOCSPResponse, tbs_response_data),
+    kResponseDataTemplate },
+  { SEC_ASN1_SKIP_REST },
+  { 0 }
+};
+
+bool StringEqualToSECItem(const std::string& value1, const SECItem& value2) {
+  if (value1.size() != value2.len)
+    return false;
+  return memcmp(value1.data(), value2.data, value2.len) == 0;
+}
+
+// TODO(ekasper): also use the issuer name hash in matching.
+bool CertIDMatches(const CertID& cert_id,
+                   const std::string& serial_number,
+                   const std::string& issuer_key_sha1_hash,
+                   const std::string& issuer_key_sha256_hash) {
+  if (!StringEqualToSECItem(serial_number, cert_id.serial_number))
+    return false;
+
+  SECOidTag hash_alg = SECOID_FindOIDTag(&cert_id.hash_algorithm.algorithm);
+  switch (hash_alg) {
+    case SEC_OID_SHA1:
+      return StringEqualToSECItem(issuer_key_sha1_hash,
+                                  cert_id.issuer_key_hash);
+    case SEC_OID_SHA256:
+      return StringEqualToSECItem(issuer_key_sha256_hash,
+                                  cert_id.issuer_key_hash);
+    default:
+      return false;
+  }
+}
+
 }  // namespace
 
 bool ExtractEmbeddedSCTList(X509Certificate::OSCertHandle cert,
@@ -190,8 +433,9 @@ bool ExtractEmbeddedSCTList(X509Certificate::OSCertHandle cert,
   if (!leaf_cert.cert)
     return false;
 
-  return GetOctetStringExtension(
-      leaf_cert.cert.get(), g_ct_singleton.Get().embedded_oid(), sct_list);
+  return GetCertOctetStringExtension(leaf_cert.cert.get(),
+                                     g_ct_singleton.Get().embedded_oid(),
+                                     sct_list);
 }
 
 bool GetPrecertLogEntry(X509Certificate::OSCertHandle leaf,
@@ -269,6 +513,97 @@ bool GetX509LogEntry(X509Certificate::OSCertHandle leaf, LogEntry* result) {
   return true;
 }
 
+bool ExtractSCTListFromOCSPResponse(X509Certificate::OSCertHandle issuer,
+                                    const std::string& cert_serial_number,
+                                    const std::string& ocsp_response,
+                                    std::string* sct_list) {
+  DCHECK(issuer);
+  crypto::ScopedPLArenaPool arena(PORT_NewArena(DER_DEFAULT_CHUNKSIZE));
+
+  OCSPResponse response;
+  memset(&response, 0, sizeof(response));
+
+  SECItem src = { siBuffer,
+                  reinterpret_cast<unsigned char*>(const_cast<char*>(
+                      ocsp_response.data())), ocsp_response.size() };

[wtc, 2013/12/15 05:31:14]

Emilia: you can add a static_cast<unsigned int> to ocsp_response.size(). If we
want to be extra careful, we can add a check also:

  // A stapled OCSP response cannot overflow the 3-byte length. 
  if (ocsp_response.size() > 0xffffff)  // we can use a smaller max size.
    return false;

Note also that Chromium's base/safe_numerics.h has base::checked_numeric_cast<>.

[ekasper, 2013/12/16 12:35:42]

We know that we're safe given the source of those responses is NSS - but it
doesn't cost anything to be correct so I've done it.
Ah, good to know. But this CHECKs so doesn't seem right for untrusted inputs.

+
+  // |response| will point directly into |src|, so it's not necessary to
+  // free the |response| contents, but they may only be used while |src|
+  // is valid (i.e., in this method).
+  SECStatus rv = SEC_QuickDERDecodeItem(arena.get(), &response,
+                                        kOCSPResponseTemplate, &src);
+  if (rv != SECSuccess)
+    return false;
+
+  if (!response.response_bytes)
+    return false;
+
+  if (!SECITEM_ItemsAreEqual(&kBasicOCSPResponseOidItem,
+                             &response.response_bytes->response_type)) {
+    return false;
+  }
+
+  BasicOCSPResponse basic_response;
+  memset(&basic_response, 0, sizeof(basic_response));
+
+  rv = SEC_QuickDERDecodeItem(arena.get(), &basic_response,
+                              kBasicOCSPResponseTemplate,
+                              &response.response_bytes->der_response);
+  if (rv != SECSuccess)
+    return false;
+
+  SingleResponse** responses =
+      basic_response.tbs_response_data.single_responses;
+  if (!responses)
+    return false;
+
+  std::string issuer_der;
+  if (!X509Certificate::GetDEREncoded(issuer, &issuer_der))
+    return false;
+
+  base::StringPiece issuer_spki;
+  if (!asn1::ExtractSPKIFromDERCert(issuer_der, &issuer_spki))
+    return false;
+
+  // In OCSP, only the key itself is under hash.
+  base::StringPiece issuer_spk;
+  if (!asn1::ExtractSubjectPublicKeyFromSPKI(issuer_spki, &issuer_spk))
+    return false;
+
+  // ExtractSubjectPublicKey... does not remove the initial octet encoding the
+  // number of unused bits in the ASN.1 BIT STRING so we do it here. For public
+  // keys, the bitstring is in practice always byte-aligned.
+  if (issuer_spk.empty() || issuer_spk[0] != 0)
+    return false;
+  issuer_spk.remove_prefix(1);
+
+  // NSS OCSP lib recognizes SHA1, MD5 and MD2; MD5 and MD2 are dead but
+  // https://bugzilla.mozilla.org/show_bug.cgi?id=663315 will add SHA-256
+  // and SHA-384.
+  // TODO(ekasper): add SHA-384 to crypto/sha2.h and here if it proves
+  // necessary.
+  // TODO(ekasper): only compute the hashes on demand.
+  std::string issuer_key_sha256_hash = crypto::SHA256HashString(issuer_spk);
+  std::string issuer_key_sha1_hash = base::SHA1HashString(
+      issuer_spk.as_string());
+
+  const SingleResponse* match = NULL;
+  for (const SingleResponse* const* resps = responses; *resps; ++resps) {
+    const SingleResponse* resp = *resps;
+    if (CertIDMatches(resp->cert_id, cert_serial_number,
+                      issuer_key_sha1_hash, issuer_key_sha256_hash)) {
+      match = resp;
+      break;
+    }
+  }
+
+  if (!match)
+    return false;
+
+  return GetSCTListFromOCSPExtension(arena.get(), match->single_extensions,
+                                     sct_list);
+}
+
 }  // namespace ct
 
 }  // namespace net