[proto] Fixing Bugs in ASN.1 PDU, ASN.1 Universal Types, and X.509 Ce…

…rtificate (google#72)

* Fix ASN.1 encoding issues in X.509 & Univ. Types.

- GeneralizedTime and UTCTime were flipped for encoding.
- Version is EXPLICIT tagged. The encoder was incorrectly
using IMPLICIT tagging.

* Improve Comment for VersionNumber & Add Val0 back.

- Make the coment more clear for encoding the VersionNumber
in X.509 cert to der.
- Add Val0 back in ASN.1 PDU proto to be able to generate
context-specific 0, amongst other things.

proto/asn1-pdu/asn1_universal_types_to_der.cc

@@ -111,7 +111,7 @@ void Encode(const UTCTime& utc_time, std::vector<uint8_t>& der) {
   // after the value is encoded.
   const size_t tag_len_pos = der.size();
 
-  EncodeTimestamp(utc_time.time_stamp(), false, der);
+  EncodeTimestamp(utc_time.time_stamp(), true, der);
 
   // Check if encoding was successful.
   if (der.size() != tag_len_pos) {
@@ -126,7 +126,7 @@ void Encode(const GeneralizedTime& generalized_time,
   // after the value is encoded.
   const size_t tag_len_pos = der.size();
 
-  EncodeTimestamp(generalized_time.time_stamp(), true, der);
+  EncodeTimestamp(generalized_time.time_stamp(), false, der);
 
   // Check if encoding was successful.
   if (der.size() != tag_len_pos) {

proto/asn1-pdu/x509_certificate_to_der.cc

@@ -307,12 +307,19 @@ DECLARE_ENCODE_FUNCTION(ValiditySequence) {
 }
 
 DECLARE_ENCODE_FUNCTION(VersionNumber) {
-  // |version| is Context-specific with tag number 0 (RFC 5280, 4.1 & 4.1.2.1).
-  // Takes on values 0, 1 and 2, so only require length of 1 to
-  // encode it (RFC 5280, 4.1 & 4.1.2.1).
-  std::vector<uint8_t> der_version = {kAsn1ContextSpecific | 0x00, 0x01,
-                                      static_cast<uint8_t>(val)};
-  der.insert(der.end(), der_version.begin(), der_version.end());
+  // RFC 5280, 4.1 & 4.1.2.1:
+  // version         [0]  EXPLICIT Version DEFAULT v1,
+  // Version  ::=  INTEGER  {  v1(0), v2(1), v3(2)  }
+  //
+  // X.690 (2015), 11.5: DEFAULT value in a sequence field is not encoded
+  if (val != 0) {
+    // Use a fixed buffer for the EXPLICIT encoding, since the version is always
+    // a one byte INTEGER.
+    std::vector<uint8_t> der_version = {
+        kAsn1ContextSpecific | kAsn1Constructed | 0x00, 0x03, kAsn1Integer,
+        0x01, static_cast<uint8_t>(val)};
+    der.insert(der.end(), der_version.begin(), der_version.end());
+  }
 }
 
 DECLARE_ENCODE_FUNCTION(TBSCertificateSequence) {
@@ -342,8 +349,8 @@ DECLARE_ENCODE_FUNCTION(TBSCertificateSequence) {
   if (val.has_subject_unique_id()) {
     size_t pos_of_tag = der.size();
     Encode(val.subject_unique_id(), der);
-    // |subject_unqiue_id| is Context-specific with tag number 2 (RFC 5280, 4.1
-    // & 4.1.2.8).
+    // |subject_unqiue_id| is Context-specific with tag number 2 (RFC
+    // 5280, 4.1 & 4.1.2.8).
     ReplaceTag(kAsn1ContextSpecific | 0x02, pos_of_tag, der);
   }
   if (val.has_extensions()) {