Add a function for parsing RFC 5280's "Certificate".

net/cert/internal/parse_certificate.cc

+// Copyright 2015 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/internal/parse_certificate.h"
+
+#include "net/der/input.h"
+#include "net/der/parse_values.h"
+#include "net/der/parser.h"
+
+namespace net {
+
+namespace {
+
+// Returns true if |input| is a SEQUENCE and nothing else.
+WARN_UNUSED_RESULT bool IsSequenceTLV(const der::Input& input) {

[davidben, 2015/08/14 17:14:50]

[Oh. You'd convinced me in the other CL that it's fine to leave that to the
caller. I have no complaints about actually checking either though. Your call.]

[eroman, 2015/08/14 17:41:00]

Since the tests rely on this now, I will go ahead and keep it :)

+  der::Parser parser(input);
+  der::Parser unused_sequence_parser;
+  if (!parser.ReadSequence(&unused_sequence_parser))
+    return false;
+  // Should by a single SEQUENCE by definition of the function.
+  return !parser.HasMore();
+}
+
+// Reads a SEQUENCE from |parser| and writes the full tag-length-value into
+// |out|. On failure |parser| may or may not have been advanced.
+WARN_UNUSED_RESULT bool ReadSequenceTLV(der::Parser* parser, der::Input* out) {
+  return parser->ReadRawTLV(out) && IsSequenceTLV(*out);
+}
+
+}  // namespace
+
+bool ParseCertificate(const der::Input& certificate_tlv,
+                      ParsedCertificate* out) {
+  der::Parser parser(certificate_tlv);
+
+  //   Certificate  ::=  SEQUENCE  {
+  der::Parser certificate_parser;
+  if (!parser.ReadSequence(&certificate_parser))
+    return false;
+
+  //        tbsCertificate       TBSCertificate,
+  if (!ReadSequenceTLV(&certificate_parser, &out->tbs_certificate_tlv))
+    return false;
+
+  //        signatureAlgorithm   AlgorithmIdentifier,
+  if (!ReadSequenceTLV(&certificate_parser, &out->signature_algorithm_tlv))
+    return false;
+
+  //        signatureValue       BIT STRING  }
+  if (!certificate_parser.ReadBitString(&out->signature_value))
+    return false;
+
+  // There isn't an extension point at the end of Certificate.
+  if (certificate_parser.HasMore())
+    return false;
+
+  // By definition the input was a single Certificate, so there shouldn't be
+  // unconsumed data.
+  if (parser.HasMore())
+    return false;
+
+  return true;
+}
+
+}  // namespace net