path: root/src/lib/x509/x509cert.cpp

/*
* X.509 Certificates
* (C) 1999-2010,2015,2017 Jack Lloyd
* (C) 2016 René Korthaus, Rohde & Schwarz Cybersecurity
*
* Botan is released under the Simplified BSD License (see license.txt)
*/

#include <botan/x509cert.h>
#include <botan/datastor.h>
#include <botan/pk_keys.h>
#include <botan/x509_ext.h>
#include <botan/ber_dec.h>
#include <botan/parsing.h>
#include <botan/bigint.h>
#include <botan/oids.h>
#include <botan/hash.h>
#include <botan/hex.h>
#include <algorithm>
#include <sstream>

namespace Botan {

struct X509_Certificate_Data
   {
   size_t m_version = 0;
   std::vector<uint8_t> m_serial;
   AlgorithmIdentifier m_sig_algo_inner;
   X509_DN m_issuer_dn;
   X509_DN m_subject_dn;
   std::vector<uint8_t> m_issuer_dn_bits;
   std::vector<uint8_t> m_subject_dn_bits;
   X509_Time m_not_before;
   X509_Time m_not_after;
   std::vector<uint8_t> m_subject_public_key_bits;
   AlgorithmIdentifier m_subject_public_key_algid;
   std::vector<uint8_t> m_subject_public_key_bitstring;
   std::vector<uint8_t> m_subject_public_key_bitstring_sha1;

   std::vector<uint8_t> m_v2_issuer_key_id;
   std::vector<uint8_t> m_v2_subject_key_id;
   Extensions m_v3_extensions;

   Key_Constraints m_key_constraints;
   std::vector<OID> m_extended_key_usage;
   std::vector<uint8_t> m_authority_key_id;
   std::vector<uint8_t> m_subject_key_id;

   std::vector<std::string> m_crl_distribution_points;
   std::string m_ocsp_responder;

   size_t m_path_len_constraint = 0;
   bool m_self_signed = false;
   bool m_is_ca_certificate = false;

   AlternativeName m_subject_alt_name;
   AlternativeName m_issuer_alt_name;

   Data_Store m_subject_ds;
   Data_Store m_issuer_ds;
   };

std::string X509_Certificate::PEM_label() const
   {
   return "CERTIFICATE";
   }

std::vector<std::string> X509_Certificate::alternate_PEM_labels() const
   {
   return { "X509 CERTIFICATE" };
   }

X509_Certificate::X509_Certificate(DataSource& src)
   {
   load_data(src);
   }

X509_Certificate::X509_Certificate(const std::vector<uint8_t>& vec)
   {
   DataSource_Memory src(vec.data(), vec.size());
   load_data(src);
   }

#if defined(BOTAN_TARGET_OS_HAS_FILESYSTEM)
X509_Certificate::X509_Certificate(const std::string& fsname)
   {
   DataSource_Stream src(fsname, true);
   load_data(src);
   }
#endif

namespace {

std::unique_ptr<X509_Certificate_Data> parse_x509_cert_body(const X509_Object& obj)
   {
   std::unique_ptr<X509_Certificate_Data> data(new X509_Certificate_Data);

   BER_Decoder tbs_cert(obj.signed_body());
   BigInt serial_bn;

   tbs_cert.decode_optional(data->m_version, ASN1_Tag(0),
                            ASN1_Tag(CONSTRUCTED | CONTEXT_SPECIFIC))
      .decode(serial_bn)
      .decode(data->m_sig_algo_inner)
      .decode(data->m_issuer_dn)
      .start_cons(SEQUENCE)
         .decode(data->m_not_before)
         .decode(data->m_not_after)
      .end_cons()
      .decode(data->m_subject_dn);

   if(data->m_version > 2)
      throw Decoding_Error("Unknown X.509 cert version " + std::to_string(data->m_version));
   if(obj.signature_algorithm() != data->m_sig_algo_inner)
      throw Decoding_Error("X.509 Certificate had differing algorithm identifers in inner and outer ID fields");

   // for general sanity convert wire version (0 based) to standards version (v1 .. v3)
   data->m_version += 1;

   data->m_serial = BigInt::encode(serial_bn);
   data->m_subject_dn_bits = ASN1::put_in_sequence(data->m_subject_dn.get_bits());
   data->m_issuer_dn_bits = ASN1::put_in_sequence(data->m_issuer_dn.get_bits());

   BER_Object public_key = tbs_cert.get_next_object();
   if(public_key.type_tag != SEQUENCE || public_key.class_tag != CONSTRUCTED)
      throw BER_Bad_Tag("X509_Certificate: Unexpected tag for public key",
                        public_key.type_tag, public_key.class_tag);

   // validate_public_key_params(public_key.value);
   AlgorithmIdentifier public_key_alg_id;
   BER_Decoder(public_key.value).decode(public_key_alg_id).discard_remaining();

   std::vector<std::string> public_key_info =
      split_on(OIDS::oid2str(public_key_alg_id.get_oid()), '/');

   if(!public_key_info.empty() && public_key_info[0] == "RSA")
      {
      // RFC4055: If PublicKeyAlgo = PSS or OAEP: limit the use of the public key exclusively to either RSASSA - PSS or RSAES - OAEP
      if(public_key_info.size() >= 2)
         {
         if(public_key_info[1] == "EMSA4")
            {
            /*
            When the RSA private key owner wishes to limit the use of the public
            key exclusively to RSASSA-PSS, then the id-RSASSA-PSS object
            identifier MUST be used in the algorithm field within the subject
            public key information, and, if present, the parameters field MUST
            contain RSASSA-PSS-params.

            All parameters in the signature structure algorithm identifier MUST
            match the parameters in the key structure algorithm identifier
            except the saltLength field. The saltLength field in the signature parameters
            MUST be greater or equal to that in the key parameters field.

            ToDo: Allow salt length to be greater
            */
            if(public_key_alg_id != obj.signature_algorithm())
               {
               throw Decoding_Error("Algorithm identifier mismatch");
               }
            }
         if(public_key_info[1] == "OAEP")
            {
            throw Decoding_Error("Decoding subject public keys of type RSAES-OAEP is currently not supported");
            }
         }
      else
         {
         // oid = rsaEncryption -> parameters field MUST contain NULL
         if(public_key_alg_id != AlgorithmIdentifier(public_key_alg_id.get_oid(), AlgorithmIdentifier::USE_NULL_PARAM))
            {
            throw Decoding_Error("Parameters field MUST contain NULL");
            }
         }
      }

   data->m_subject_public_key_bits = unlock(public_key.value);

   BER_Decoder(data->m_subject_public_key_bits)
      .decode(data->m_subject_public_key_algid)
      .decode(data->m_subject_public_key_bitstring, BIT_STRING);

   tbs_cert.decode_optional_string(data->m_v2_issuer_key_id, BIT_STRING, 1);
   tbs_cert.decode_optional_string(data->m_v2_subject_key_id, BIT_STRING, 2);

   BER_Object v3_exts_data = tbs_cert.get_next_object();
   if(v3_exts_data.type_tag == 3 &&
      v3_exts_data.class_tag == ASN1_Tag(CONSTRUCTED | CONTEXT_SPECIFIC))
      {
      BER_Decoder(v3_exts_data.value).decode(data->m_v3_extensions).verify_end();
      }
   else if(v3_exts_data.type_tag != NO_OBJECT)
      throw BER_Bad_Tag("Unknown tag in X.509 cert",
                        v3_exts_data.type_tag, v3_exts_data.class_tag);

   if(tbs_cert.more_items())
      throw Decoding_Error("TBSCertificate has extra data after extensions block");

   // Now cache some fields from the extensions
   if(auto ext = data->m_v3_extensions.get_extension_object_as<Cert_Extension::Key_Usage>())
      {
      data->m_key_constraints = ext->get_constraints();
      }
   else
      {
      data->m_key_constraints = NO_CONSTRAINTS;
      }

   if(auto ext = data->m_v3_extensions.get_extension_object_as<Cert_Extension::Subject_Key_ID>())
      {
      data->m_subject_key_id = ext->get_key_id();
      }

   if(auto ext = data->m_v3_extensions.get_extension_object_as<Cert_Extension::Authority_Key_ID>())
      {
      data->m_authority_key_id = ext->get_key_id();
      }

   if(auto ext = data->m_v3_extensions.get_extension_object_as<Cert_Extension::Basic_Constraints>())
      {
      if(ext->get_is_ca() == true)
         {
         if(data->m_key_constraints == NO_CONSTRAINTS ||
            (data->m_key_constraints & KEY_CERT_SIGN))
            {
            data->m_is_ca_certificate = true;
            data->m_path_len_constraint = ext->get_path_limit();
            }
         }
      }

   if(auto ext = data->m_v3_extensions.get_extension_object_as<Cert_Extension::Issuer_Alternative_Name>())
      {
      data->m_issuer_alt_name = ext->get_alt_name();
      }

   if(auto ext = data->m_v3_extensions.get_extension_object_as<Cert_Extension::Subject_Alternative_Name>())
      {
      data->m_subject_alt_name = ext->get_alt_name();
      }

   if(auto ext = data->m_v3_extensions.get_extension_object_as<Cert_Extension::Extended_Key_Usage>())
      {
      data->m_extended_key_usage = ext->get_oids();
      }

   if(auto ext = data->m_v3_extensions.get_extension_object_as<Cert_Extension::Authority_Information_Access>())
      {
      data->m_ocsp_responder = ext->ocsp_responder();
      }

   if(auto ext = data->m_v3_extensions.get_extension_object_as<Cert_Extension::CRL_Distribution_Points>())
      {
      data->m_crl_distribution_points = ext->crl_distribution_urls();
      }

   // Check for self-signed vs self-issued certificates
   if(data->m_subject_dn == data->m_issuer_dn)
      {
      try
         {
         std::unique_ptr<Public_Key> pub_key(
            X509::load_key(ASN1::put_in_sequence(data->m_subject_public_key_bits)));
         data->m_self_signed = obj.check_signature(*pub_key);
         }
      catch(Decoding_Error&)
         {
         // ignore errors here to allow parsing to continue
         }
      }

   std::unique_ptr<HashFunction> sha1(HashFunction::create("SHA-1"));
   if(sha1)
      {
      sha1->update(data->m_subject_public_key_bitstring);
      data->m_subject_public_key_bitstring_sha1 = sha1->final_stdvec();
      // otherwise left as empty, and we will throw if subject_public_key_bitstring_sha1 is called
      }

   data->m_subject_ds.add(data->m_subject_dn.contents());
   data->m_issuer_ds.add(data->m_issuer_dn.contents());
   data->m_v3_extensions.contents_to(data->m_subject_ds, data->m_issuer_ds);

   return data;
   }

}

/*
* Decode the TBSCertificate data
*/
void X509_Certificate::force_decode()
   {
   m_data.reset();

   std::unique_ptr<X509_Certificate_Data> data = parse_x509_cert_body(*this);

   m_data.reset(data.release());
   }

const X509_Certificate_Data& X509_Certificate::data() const
   {
   if(m_data == nullptr)
      {
      throw Invalid_State("X509_Certificate uninitialized");
      }
   return *m_data.get();
   }

uint32_t X509_Certificate::x509_version() const
   {
   return data().m_version;
   }

bool X509_Certificate::is_self_signed() const
   {
   return data().m_self_signed;
   }

const X509_Time& X509_Certificate::not_before() const
   {
   return data().m_not_before;
   }

const X509_Time& X509_Certificate::not_after() const
   {
   return data().m_not_after;
   }

const AlgorithmIdentifier& X509_Certificate::subject_public_key_algo() const
   {
   return data().m_subject_public_key_algid;
   }

const std::vector<uint8_t>& X509_Certificate::v2_issuer_key_id() const
   {
   return data().m_v2_issuer_key_id;
   }

const std::vector<uint8_t>& X509_Certificate::v2_subject_key_id() const
   {
   return data().m_v2_subject_key_id;
   }

const std::vector<uint8_t>& X509_Certificate::subject_public_key_bits() const
   {
   return data().m_subject_public_key_bits;
   }

const std::vector<uint8_t>& X509_Certificate::subject_public_key_bitstring() const
   {
   return data().m_subject_public_key_bitstring;
   }

std::vector<uint8_t> X509_Certificate::subject_public_key_bitstring_sha1() const
   {
   if(data().m_subject_public_key_bitstring_sha1.empty())
      throw Encoding_Error("X509_Certificate::subject_public_key_bitstring_sha1 called but SHA-1 disabled in build");

   return data().m_subject_public_key_bitstring_sha1;
   }

const std::vector<uint8_t>& X509_Certificate::authority_key_id() const
   {
   return data().m_authority_key_id;
   }

const std::vector<uint8_t>& X509_Certificate::subject_key_id() const
   {
   return data().m_subject_key_id;
   }

const std::vector<uint8_t>& X509_Certificate::serial_number() const
   {
   return data().m_serial;
   }

const X509_DN& X509_Certificate::issuer_dn() const
   {
   return data().m_issuer_dn;
   }

const X509_DN& X509_Certificate::subject_dn() const
   {
   return data().m_subject_dn;
   }

const std::vector<uint8_t>& X509_Certificate::raw_issuer_dn() const
   {
   return data().m_issuer_dn_bits;
   }

const std::vector<uint8_t>& X509_Certificate::raw_subject_dn() const
   {
   return data().m_subject_dn_bits;
   }

bool X509_Certificate::is_CA_cert() const
   {
   return data().m_is_ca_certificate;
   }

uint32_t X509_Certificate::path_limit() const
   {
   return data().m_path_len_constraint;
   }

Key_Constraints X509_Certificate::constraints() const
   {
   return data().m_key_constraints;
   }

const std::vector<OID>& X509_Certificate::extended_key_usage() const
   {
   return data().m_extended_key_usage;
   }

std::vector<OID> X509_Certificate::certificate_policy_oids() const
   {
   if(auto ext = v3_extensions().get_extension_object_as<Cert_Extension::Certificate_Policies>())
      {
      return ext->get_policy_oids();
      }
   return std::vector<OID>();
   }

/*
* Return the name constraints
*/
NameConstraints X509_Certificate::name_constraints() const
   {
   if(auto ext = v3_extensions().get_extension_object_as<Cert_Extension::Name_Constraints>())
      {
      return ext->get_name_constraints();
      }
   return NameConstraints(); // no constraints
   }

const Extensions& X509_Certificate::v3_extensions() const
   {
   return data().m_v3_extensions;
   }

bool X509_Certificate::allowed_usage(Key_Constraints usage) const
   {
   if(constraints() == NO_CONSTRAINTS)
      return true;
   return ((constraints() & usage) == usage);
   }

bool X509_Certificate::allowed_extended_usage(const std::string& usage) const
   {
   return allowed_extended_usage(OIDS::str2oid(usage));
   }

bool X509_Certificate::allowed_extended_usage(const OID& usage) const
   {
   const std::vector<OID>& ex = extended_key_usage();
   if(ex.empty())
      return true;

   if(std::find(ex.begin(), ex.end(), usage) != ex.end())
      return true;

   return false;
   }

bool X509_Certificate::allowed_usage(Usage_Type usage) const
   {
   // These follow suggestions in RFC 5280 4.2.1.12

   switch(usage)
      {
      case Usage_Type::UNSPECIFIED:
         return true;

      case Usage_Type::TLS_SERVER_AUTH:
         return (allowed_usage(KEY_AGREEMENT) || allowed_usage(KEY_ENCIPHERMENT) || allowed_usage(DIGITAL_SIGNATURE)) && allowed_extended_usage("PKIX.ServerAuth");

      case Usage_Type::TLS_CLIENT_AUTH:
         return (allowed_usage(DIGITAL_SIGNATURE) || allowed_usage(KEY_AGREEMENT)) && allowed_extended_usage("PKIX.ClientAuth");

      case Usage_Type::OCSP_RESPONDER:
         return (allowed_usage(DIGITAL_SIGNATURE) || allowed_usage(NON_REPUDIATION)) && allowed_extended_usage("PKIX.OCSPSigning");

      case Usage_Type::CERTIFICATE_AUTHORITY:
         return is_CA_cert();
      }

   return false;
   }

bool X509_Certificate::has_constraints(Key_Constraints constraints) const
   {
   if(this->constraints() == NO_CONSTRAINTS)
      {
      return false;
      }

   return ((this->constraints() & constraints) != 0);
   }

bool X509_Certificate::has_ex_constraint(const std::string& ex_constraint) const
   {
   return has_ex_constraint(OIDS::str2oid(ex_constraint));
   }

bool X509_Certificate::has_ex_constraint(const OID& usage) const
   {
   const std::vector<OID>& ex = extended_key_usage();
   return (std::find(ex.begin(), ex.end(), usage) != ex.end());
   }

/*
* Return if a certificate extension is marked critical
*/
bool X509_Certificate::is_critical(const std::string& ex_name) const
   {
   return v3_extensions().critical_extension_set(OIDS::str2oid(ex_name));
   }

std::string X509_Certificate::ocsp_responder() const
   {
   return data().m_ocsp_responder;
   }

std::string X509_Certificate::crl_distribution_point() const
   {
   // just returns the first (arbitrarily)
   if(data().m_crl_distribution_points.size() > 0)
      return data().m_crl_distribution_points[0];
   return "";
   }

const AlternativeName& X509_Certificate::subject_alt_name() const
   {
   return data().m_subject_alt_name;
   }

const AlternativeName& X509_Certificate::issuer_alt_name() const
   {
   return data().m_issuer_alt_name;
   }

/*
* Return information about the subject
*/
std::vector<std::string>
X509_Certificate::subject_info(const std::string& req) const
   {
   if(subject_dn().has_field(req))
      return subject_dn().get_attribute(req);

   if(subject_alt_name().has_field(req))
      return subject_alt_name().get_attribute(req);

   // These will be removed later:
   if(req == "X509.Certificate.v2.key_id")
      return {hex_encode(this->v2_subject_key_id())};
   if(req == "X509v3.SubjectKeyIdentifier")
      return {hex_encode(this->subject_key_id())};
   if(req == "X509.Certificate.dn_bits")
      return {hex_encode(this->raw_subject_dn())};
   if(req == "X509.Certificate.start")
      return {not_before().to_string()};
   if(req == "X509.Certificate.end")
      return {not_after().to_string()};

   if(req == "X509.Certificate.version")
      return {std::to_string(x509_version())};
   if(req == "X509.Certificate.serial")
      return {hex_encode(serial_number())};

   return data().m_subject_ds.get(req);
   }

/*
* Return information about the issuer
*/
std::vector<std::string>
X509_Certificate::issuer_info(const std::string& req) const
   {
   if(issuer_dn().has_field(req))
      return issuer_dn().get_attribute(req);

   if(issuer_alt_name().has_field(req))
      return issuer_alt_name().get_attribute(req);

   // These will be removed later:
   if(req == "X509.Certificate.v2.key_id")
      return {hex_encode(this->v2_issuer_key_id())};
   if(req == "X509v3.AuthorityKeyIdentifier")
      return {hex_encode(this->authority_key_id())};
   if(req == "X509.Certificate.dn_bits")
      return {hex_encode(this->raw_issuer_dn())};

   return data().m_issuer_ds.get(req);
   }

/*
* Return the public key in this certificate
*/
std::unique_ptr<Public_Key> X509_Certificate::load_subject_public_key() const
   {
   try
      {
      return std::unique_ptr<Public_Key>(X509::load_key(ASN1::put_in_sequence(this->subject_public_key_bits())));
      }
   catch(std::exception& e)
      {
      throw Decoding_Error("X509_Certificate::load_subject_public_key", e.what());
      }
   }

std::vector<uint8_t> X509_Certificate::raw_issuer_dn_sha256() const
   {
   std::unique_ptr<HashFunction> hash(HashFunction::create_or_throw("SHA-256"));
   hash->update(raw_issuer_dn());
   return hash->final_stdvec();
   }

std::vector<uint8_t> X509_Certificate::raw_subject_dn_sha256() const
   {
   std::unique_ptr<HashFunction> hash(HashFunction::create("SHA-256"));
   hash->update(raw_subject_dn());
   return hash->final_stdvec();
   }

namespace {

/*
* Lookup each OID in the vector
*/
std::vector<std::string> lookup_oids(const std::vector<OID>& oids)
   {
   std::vector<std::string> out;

   for(const OID& oid : oids)
      {
      out.push_back(OIDS::oid2str(oid));
      }
   return out;
   }

}

/*
* Return the list of extended key usage OIDs
*/
std::vector<std::string> X509_Certificate::ex_constraints() const
   {
   return lookup_oids(extended_key_usage());
   }

/*
* Return the list of certificate policies
*/
std::vector<std::string> X509_Certificate::policies() const
   {
   return lookup_oids(certificate_policy_oids());
   }

std::string X509_Certificate::fingerprint(const std::string& hash_name) const
   {
   return create_hex_fingerprint(this->BER_encode(), hash_name);
   }

bool X509_Certificate::matches_dns_name(const std::string& name) const
   {
   if(name.empty())
      return false;

   std::vector<std::string> issued_names = subject_info("DNS");

   // Fall back to CN only if no DNS names are set (RFC 6125 sec 6.4.4)
   if(issued_names.empty())
      issued_names = subject_info("Name");

   for(size_t i = 0; i != issued_names.size(); ++i)
      {
      if(host_wildcard_match(issued_names[i], name))
         return true;
      }

   return false;
   }

/*
* Compare two certificates for equality
*/
bool X509_Certificate::operator==(const X509_Certificate& other) const
   {
   return (this->signature() == other.signature() &&
           this->signature_algorithm() == other.signature_algorithm() &&
           this->signed_body() == other.signed_body());
   }

bool X509_Certificate::operator<(const X509_Certificate& other) const
   {
   /* If signature values are not equal, sort by lexicographic ordering of that */
   if(this->signature() != other.signature())
      {
      return (this->signature() < other.signature());
      }

   // Then compare the signed contents
   return this->signed_body() < other.signed_body();
   }

/*
* X.509 Certificate Comparison
*/
bool operator!=(const X509_Certificate& cert1, const X509_Certificate& cert2)
   {
   return !(cert1 == cert2);
   }

std::string X509_Certificate::to_string() const
   {
   std::ostringstream out;

   out << "Version: " << this->x509_version() << "\n";
   out << "Subject: " << subject_dn() << "\n";
   out << "Issuer: " << issuer_dn() << "\n";
   out << "Issued: " << this->not_before().readable_string() << "\n";
   out << "Expires: " << this->not_after().readable_string() << "\n";

   out << "Constraints:\n";
   Key_Constraints constraints = this->constraints();
   if(constraints == NO_CONSTRAINTS)
      out << " None\n";
   else
      {
      if(constraints & DIGITAL_SIGNATURE)
         out << "   Digital Signature\n";
      if(constraints & NON_REPUDIATION)
         out << "   Non-Repudiation\n";
      if(constraints & KEY_ENCIPHERMENT)
         out << "   Key Encipherment\n";
      if(constraints & DATA_ENCIPHERMENT)
         out << "   Data Encipherment\n";
      if(constraints & KEY_AGREEMENT)
         out << "   Key Agreement\n";
      if(constraints & KEY_CERT_SIGN)
         out << "   Cert Sign\n";
      if(constraints & CRL_SIGN)
         out << "   CRL Sign\n";
      if(constraints & ENCIPHER_ONLY)
         out << "   Encipher Only\n";
      if(constraints & DECIPHER_ONLY)
         out << "   Decipher Only\n";
      }

   const std::vector<OID> policies = this->certificate_policy_oids();
   if(!policies.empty())
      {
      out << "Policies: " << "\n";
      for(auto oid : policies)
         out << "   " << oid.as_string() << "\n";
      }

   std::vector<OID> ex_constraints = this->extended_key_usage();
   if(!ex_constraints.empty())
      {
      out << "Extended Constraints:\n";
      for(size_t i = 0; i != ex_constraints.size(); i++)
         out << "   " << OIDS::oid2str(ex_constraints[i]) << "\n";
      }

   NameConstraints name_constraints = this->name_constraints();

   if(!name_constraints.permitted().empty() ||
         !name_constraints.excluded().empty())
      {
      out << "Name Constraints:\n";

      if(!name_constraints.permitted().empty())
         {
         out << "   Permit";
         for(auto st: name_constraints.permitted())
            {
            out << " " << st.base();
            }
         out << "\n";
         }

      if(!name_constraints.excluded().empty())
         {
         out << "   Exclude";
         for(auto st: name_constraints.excluded())
            {
            out << " " << st.base();
            }
         out << "\n";
         }
      }

   if(!ocsp_responder().empty())
      out << "OCSP responder " << ocsp_responder() << "\n";
   if(!crl_distribution_point().empty())
      out << "CRL " << crl_distribution_point() << "\n";

   out << "Signature algorithm: " <<
      OIDS::oid2str(this->signature_algorithm().get_oid()) << "\n";

   out << "Serial number: " << hex_encode(this->serial_number()) << "\n";

   if(this->authority_key_id().size())
     out << "Authority keyid: " << hex_encode(this->authority_key_id()) << "\n";

   if(this->subject_key_id().size())
     out << "Subject keyid: " << hex_encode(this->subject_key_id()) << "\n";

   try
      {
      std::unique_ptr<Public_Key> pubkey(this->subject_public_key());
      out << "Public Key [" << pubkey->algo_name() << "-" << pubkey->key_length() << "]\n\n";
      out << X509::PEM_encode(*pubkey);
      }
   catch(Decoding_Error&)
      {
      const AlgorithmIdentifier& alg_id = this->subject_public_key_algo();
      out << "Failed to decode key with oid " << alg_id.get_oid().as_string() << "\n";
      }

   return out.str();
   }

}