nmap/nse_ssl_cert.cc

/***************************************************************************
 * nse_ssl_cert.cc -- NSE userdatum representing an SSL certificate.       *
 *                                                                         *
 ***********************IMPORTANT NMAP LICENSE TERMS************************
 *                                                                         *
 * The Nmap Security Scanner is (C) 1996-2018 Insecure.Com LLC ("The Nmap  *
 * Project"). Nmap is also a registered trademark of the Nmap Project.     *
 * This program is free software; you may redistribute and/or modify it    *
 * under the terms of the GNU General Public License as published by the   *
 * Free Software Foundation; Version 2 ("GPL"), BUT ONLY WITH ALL OF THE   *
 * CLARIFICATIONS AND EXCEPTIONS DESCRIBED HEREIN.  This guarantees your   *
 * right to use, modify, and redistribute this software under certain      *
 * conditions.  If you wish to embed Nmap technology into proprietary      *
 * software, we sell alternative licenses (contact sales@nmap.com).        *
 * Dozens of software vendors already license Nmap technology such as      *
 * host discovery, port scanning, OS detection, version detection, and     *
 * the Nmap Scripting Engine.                                              *
 *                                                                         *
 * Note that the GPL places important restrictions on "derivative works",  *
 * yet it does not provide a detailed definition of that term.  To avoid   *
 * misunderstandings, we interpret that term as broadly as copyright law   *
 * allows.  For example, we consider an application to constitute a        *
 * derivative work for the purpose of this license if it does any of the   *
 * following with any software or content covered by this license          *
 * ("Covered Software"):                                                   *
 *                                                                         *
 * o Integrates source code from Covered Software.                         *
 *                                                                         *
 * o Reads or includes copyrighted data files, such as Nmap's nmap-os-db   *
 * or nmap-service-probes.                                                 *
 *                                                                         *
 * o Is designed specifically to execute Covered Software and parse the    *
 * results (as opposed to typical shell or execution-menu apps, which will *
 * execute anything you tell them to).                                     *
 *                                                                         *
 * o Includes Covered Software in a proprietary executable installer.  The *
 * installers produced by InstallShield are an example of this.  Including *
 * Nmap with other software in compressed or archival form does not        *
 * trigger this provision, provided appropriate open source decompression  *
 * or de-archiving software is widely available for no charge.  For the    *
 * purposes of this license, an installer is considered to include Covered *
 * Software even if it actually retrieves a copy of Covered Software from  *
 * another source during runtime (such as by downloading it from the       *
 * Internet).                                                              *
 *                                                                         *
 * o Links (statically or dynamically) to a library which does any of the  *
 * above.                                                                  *
 *                                                                         *
 * o Executes a helper program, module, or script to do any of the above.  *
 *                                                                         *
 * This list is not exclusive, but is meant to clarify our interpretation  *
 * of derived works with some common examples.  Other people may interpret *
 * the plain GPL differently, so we consider this a special exception to   *
 * the GPL that we apply to Covered Software.  Works which meet any of     *
 * these conditions must conform to all of the terms of this license,      *
 * particularly including the GPL Section 3 requirements of providing      *
 * source code and allowing free redistribution of the work as a whole.    *
 *                                                                         *
 * As another special exception to the GPL terms, the Nmap Project grants  *
 * permission to link the code of this program with any version of the     *
 * OpenSSL library which is distributed under a license identical to that  *
 * listed in the included docs/licenses/OpenSSL.txt file, and distribute   *
 * linked combinations including the two.                                  *
 *                                                                         *
 * The Nmap Project has permission to redistribute Npcap, a packet         *
 * capturing driver and library for the Microsoft Windows platform.        *
 * Npcap is a separate work with it's own license rather than this Nmap    *
 * license.  Since the Npcap license does not permit redistribution        *
 * without special permission, our Nmap Windows binary packages which      *
 * contain Npcap may not be redistributed without special permission.      *
 *                                                                         *
 * Any redistribution of Covered Software, including any derived works,    *
 * must obey and carry forward all of the terms of this license, including *
 * obeying all GPL rules and restrictions.  For example, source code of    *
 * the whole work must be provided and free redistribution must be         *
 * allowed.  All GPL references to "this License", are to be treated as    *
 * including the terms and conditions of this license text as well.        *
 *                                                                         *
 * Because this license imposes special exceptions to the GPL, Covered     *
 * Work may not be combined (even as part of a larger work) with plain GPL *
 * software.  The terms, conditions, and exceptions of this license must   *
 * be included as well.  This license is incompatible with some other open *
 * source licenses as well.  In some cases we can relicense portions of    *
 * Nmap or grant special permissions to use it in other open source        *
 * software.  Please contact fyodor@nmap.org with any such requests.       *
 * Similarly, we don't incorporate incompatible open source software into  *
 * Covered Software without special permission from the copyright holders. *
 *                                                                         *
 * If you have any questions about the licensing restrictions on using     *
 * Nmap in other works, we are happy to help.  As mentioned above, we also *
 * offer an alternative license to integrate Nmap into proprietary         *
 * applications and appliances.  These contracts have been sold to dozens  *
 * of software vendors, and generally include a perpetual license as well  *
 * as providing support and updates.  They also fund the continued         *
 * development of Nmap.  Please email sales@nmap.com for further           *
 * information.                                                            *
 *                                                                         *
 * If you have received a written license agreement or contract for        *
 * Covered Software stating terms other than these, you may choose to use  *
 * and redistribute Covered Software under those terms instead of these.   *
 *                                                                         *
 * Source is provided to this software because we believe users have a     *
 * right to know exactly what a program is going to do before they run it. *
 * This also allows you to audit the software for security holes.          *
 *                                                                         *
 * Source code also allows you to port Nmap to new platforms, fix bugs,    *
 * and add new features.  You are highly encouraged to send your changes   *
 * to the dev@nmap.org mailing list for possible incorporation into the    *
 * main distribution.  By sending these changes to Fyodor or one of the    *
 * Insecure.Org development mailing lists, or checking them into the Nmap  *
 * source code repository, it is understood (unless you specify            *
 * otherwise) that you are offering the Nmap Project the unlimited,        *
 * non-exclusive right to reuse, modify, and relicense the code.  Nmap     *
 * will always be available Open Source, but this is important because     *
 * the inability to relicense code has caused devastating problems for     *
 * other Free Software projects (such as KDE and NASM).  We also           *
 * occasionally relicense the code to third parties as discussed above.    *
 * If you wish to specify special license conditions of your               *
 * contributions, just say so when you send them.                          *
 *                                                                         *
 * This program is distributed in the hope that it will be useful, but     *
 * WITHOUT ANY WARRANTY; without even the implied warranty of              *
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the Nmap      *
 * license file for more details (it's in a COPYING file included with     *
 * Nmap, and also available from https://svn.nmap.org/nmap/COPYING)        *
 *                                                                         *
 ***************************************************************************/

/* $Id:$ */

#include "nbase.h"

#ifdef HAVE_CONFIG_H
#include "nmap_config.h"
#endif

#include <assert.h>
#include <ctype.h>
#include <errno.h>
#include <string.h>
#include <openssl/bn.h>
#include <openssl/bio.h>
#include <openssl/pem.h>
#include <openssl/rsa.h>
#include <openssl/ssl.h>
#include <openssl/x509.h>
#include <openssl/x509v3.h>
#include <openssl/evp.h>
#include <openssl/err.h>

#if (OPENSSL_VERSION_NUMBER >= 0x10100000L) && !defined LIBRESSL_VERSION_NUMBER
/* Technically some of these things were added in 0x10100006
 * but that was pre-release. */
#define HAVE_OPAQUE_STRUCTS 1
#else
#define X509_get0_notBefore X509_get_notBefore
#define X509_get0_notAfter X509_get_notAfter
#endif


#include "nse_lua.h"

#include "nse_nsock.h"

struct cert_userdata {
  X509 *cert;
  int attributes_table;
};

/* from nse_openssl.cc */
typedef struct bignum_data {
  BIGNUM * bn;
} bignum_data_t;


SSL *nse_nsock_get_ssl(lua_State *L);

/* This is a reference to a table that will be used as the metatable for
   certificate attribute tables. It has an __index entry that points to the
   global table of certificate functions like digest. */
static int ssl_cert_methods_index_ref = LUA_NOREF;

/* Calculate the digest of the certificate using the given algorithm. */
static int ssl_cert_digest(lua_State *L)
{
  struct cert_userdata *udata;
  const char *algorithm;
  unsigned char buf[256];
  unsigned int n;
  const EVP_MD *md;

  udata = (struct cert_userdata *) luaL_checkudata(L, 1, "SSL_CERT");
  algorithm = luaL_checkstring(L, 2);

  md = EVP_get_digestbyname(algorithm);
  if (md == NULL)
      return 0;

  n = sizeof(buf);
  if (X509_digest(udata->cert, md, buf, &n) != 1)
      return 0;
  lua_pushlstring(L, (char *) buf, n);

  return 1;
}

/* These are the contents of the table that is pointed to by the table that has
   ssl_cert_methods_index_ref as a reference. */
static struct luaL_Reg ssl_cert_methods[] = {
  { "digest", ssl_cert_digest },
  { NULL, NULL },
};

/* This is a helper function for x509_name_to_table. It takes the ASN1_OBJECT
   passed as an argument, turns it into a table key, and pushes it on the stack.
   The key is a string (like "commonName") if the object has an NID known by
   OBJ_obj2nid; otherwise it is an array containing the OID components as
   strings: { "2", "5", "4", "3" }. */
static void obj_to_key(lua_State *L, const ASN1_OBJECT *obj)
{
  int nid;

  nid = OBJ_obj2nid(obj);
  if (nid == NID_undef) {
    size_t size = 1;
    char *buf = (char *) lua_newuserdata(L, size);
    const char *p, *q;
    int i, n;

    while ((n = OBJ_obj2txt(buf, size, obj, 1)) < 0 || (unsigned) n >= size) {
      size = size * 2;
      buf = (char *) lua_newuserdata(L, size);
      memcpy(lua_touserdata(L, -1), lua_touserdata(L, -2), lua_rawlen(L, -2));
      lua_replace(L, -2);
    }

    lua_newtable(L);

    i = 1;
    p = buf;
    q = p;
    while (*q != '\0') {
      q = strchr(p, '.');
      if (q == NULL)
        q = strchr(p, '\0');
      lua_pushlstring(L, p, q - p);
      lua_rawseti(L, -2, i++);
      p = q + 1;
    }
    lua_replace(L, -2); /* replace userdata with table */
  } else {
    lua_pushstring(L, OBJ_nid2ln(nid));
  }
}

/* This is a helper function for l_get_ssl_certificate. It builds a table from
   the given X509_NAME, using keys returned from obj_to_key as keys. The result
   is pushed on the stack. */
static void x509_name_to_table(lua_State *L, X509_NAME *name)
{
  int i;

  lua_createtable(L, 0, X509_NAME_entry_count(name));

  for (i = 0; i < X509_NAME_entry_count(name); i++) {
    X509_NAME_ENTRY *entry;
    const ASN1_OBJECT *obj;
    const ASN1_STRING *value;

    entry = X509_NAME_get_entry(name, i);
    obj = X509_NAME_ENTRY_get_object(entry);
    value = X509_NAME_ENTRY_get_data(entry);

    obj_to_key(L, obj);
    lua_pushlstring(L, (const char *) value->data, value->length);

    lua_settable(L, -3);
  }
}

static bool x509_extensions_to_table(lua_State *L, const STACK_OF(X509_EXTENSION) *exts)
{
  if (sk_X509_EXTENSION_num(exts) <= 0)
    return false;

  lua_createtable(L, sk_X509_EXTENSION_num(exts), 0);

  for (int i = 0; i < sk_X509_EXTENSION_num(exts); i++) {
    ASN1_OBJECT *obj;
    X509_EXTENSION *ext;
    char *value = NULL;
    BIO *out;

    ext = sk_X509_EXTENSION_value(exts, i);
    obj = X509_EXTENSION_get_object(ext);

#define NSE_NUM_X509_EXTENSION_FIELDS 3
    lua_createtable(L, 0, NSE_NUM_X509_EXTENSION_FIELDS);
    char objname[256];
    long len = 0;
    len = OBJ_obj2txt(objname, 256, obj, 0);
    lua_pushlstring(L, objname, MIN(len, 256));
    lua_setfield(L, -2, "name");


    if (X509_EXTENSION_get_critical(ext)) {
      lua_pushboolean(L, true);
      lua_setfield(L, -2, "critical");
    }

    out = BIO_new(BIO_s_mem());
    if (!X509V3_EXT_print(out, ext, 0, 0)) {
      lua_pushboolean(L, true);
      lua_setfield(L, -2, "error");
    }
    else {
      len = BIO_get_mem_data(out, &value);
      lua_pushlstring(L, value, len);
      lua_setfield(L, -2, "value");
    }
    BIO_free_all(out);

    lua_rawseti(L, -2, i+1);
  }

  return true;

}

/* Parse as a decimal integer the len characters starting at s. This function
   can only process positive numbers; if the return value is negative then a
   parsing error occurred. */
static int parse_int(const unsigned char *s, size_t len)
{
  char buf[32];
  char *tail;
  long v;

  if (len == 0)
    return -1;
  if (!isdigit((int) (unsigned char) s[0]))
    return -1;
  if (len > sizeof(buf) - 1)
    return -1;
  memcpy(buf, s, len);
  buf[len] = '\0';

  errno = 0;
  v = strtol(buf, &tail, 10);
  if (errno != 0 || *tail != '\0')
    return -1;
  if ((int) v != v || v < 0)
    return -1;

  return (int) v;
}

/* This is a helper function for asn1_time_to_obj. It parses a textual ASN1_TIME
   value and stores the time in the given struct tm. It returns 0 on success and
   -1 on a parse error. */
static int time_to_tm(const ASN1_TIME *t, struct tm *result)
{
  const unsigned char *p;

  p = t->data;
  if (t->length == 13 && t->data[t->length - 1] == 'Z') {
    /* yymmddhhmmssZ */
    int year;

    year = parse_int(t->data, 2);
    if (year < 0)
      return -1;
    /* "In coming up with the worlds least efficient machine-readable time
       encoding format, the ISO nevertheless decided to forgo the encoding of
       centuries, a problem which has been kludged around by redefining the time
       as UTCTime if the date is 2049 or earlier, and GeneralizedTime if the date
       is 2050 or later."
       http://www.cs.auckland.ac.nz/~pgut001/pubs/x509guide.txt */
    if (year < 50)
      result->tm_year = 2000 + year;
    else
      result->tm_year = 1900 + year;
    p = t->data + 2;
  } else if (t->length == 15 && t->data[t->length - 1] == 'Z') {
    /* yyyymmddhhmmssZ */
    result->tm_year = parse_int(t->data, 4);
    if (result->tm_year < 0)
      return -1;
    p = t->data + 4;
  } else {
    return -1;
  }

  result->tm_mon = parse_int(p, 2);
  /* struct tm uses zero-indexed months. */
  if (result->tm_mon == 0)
    return -1;
  result->tm_mon--;
  result->tm_mday = parse_int(p + 2, 2);
  result->tm_hour = parse_int(p + 4, 2);
  result->tm_min = parse_int(p + 6, 2);
  result->tm_sec = parse_int(p + 8, 2);

  if (result->tm_mon < 0 || result->tm_mday < 0 || result->tm_hour < 0
      || result->tm_min < 0 || result->tm_sec < 0) {
    return -1;
  }

  return 0;
}

/* This is a helper function for asn1_time_to_obj. It converts a struct tm into
   a date table as returned by the Lua date os.date("!*t"), with the exception
   that the wday and yday fields are not present. */
static void tm_to_table(lua_State *L, const struct tm *tm)
{
#define NSE_NUM_TM_FIELDS 6
  lua_createtable(L, 0, NSE_NUM_TM_FIELDS);

  lua_pushnumber(L, tm->tm_year);
  lua_setfield(L, -2, "year");
  /* Lua uses one-indexed months. */
  lua_pushnumber(L, tm->tm_mon + 1);
  lua_setfield(L, -2, "month");
  lua_pushnumber(L, tm->tm_mday);
  lua_setfield(L, -2, "day");
  lua_pushnumber(L, tm->tm_hour);
  lua_setfield(L, -2, "hour");
  lua_pushnumber(L, tm->tm_min);
  lua_setfield(L, -2, "min");
  lua_pushnumber(L, tm->tm_sec);
  lua_setfield(L, -2, "sec");
  /* Omit tm_wday and tm_yday. */
}

/* This is a helper function for x509_validity_to_table. It takes the given
   ASN1_TIME and converts it to a value on the stack, which is one of
     nil, if the time is NULL;
     a date table, if the date can be parsed; and
     a string of the raw bytes, if the date cannot be parsed. */
static void asn1_time_to_obj(lua_State *L, const ASN1_TIME *s)
{
  struct tm tm;

  if (s == NULL) {
      lua_pushnil(L);
  } else if (time_to_tm(s, &tm) == 0) {
      tm_to_table(L, &tm);
  } else {
      lua_pushlstring(L, (const char *) s->data, s->length);
  }
}

/* This is a helper function for x509_validity_to_table. It builds a table with
   the two members "notBefore" and "notAfter", whose values are what is returned
   from asn1_time_to_obj. */
static void x509_validity_to_table(lua_State *L, X509 *cert)
{
#define NSE_NUM_VALIDITY_FIELDS 2
  lua_createtable(L, 0, NSE_NUM_VALIDITY_FIELDS);

  asn1_time_to_obj(L, X509_get0_notBefore(cert));
  lua_setfield(L, -2, "notBefore");
  asn1_time_to_obj(L, X509_get0_notAfter(cert));
  lua_setfield(L, -2, "notAfter");
}

/* This is a helper function for l_get_ssl_certificate. It converts the
   certificate into a PEM-encoded string on the stack. */
static void cert_pem_to_string(lua_State *L, X509 *cert)
{
  BIO *bio;
  char *buf;
  long size;

  bio = BIO_new(BIO_s_mem());
  assert(bio != NULL);

  assert(PEM_write_bio_X509(bio, cert));

  size = BIO_get_mem_data(bio, &buf);
  lua_pushlstring(L, buf, size);

  BIO_vfree(bio);
}

/* This is a helper function for l_get_ssl_certificate. It converts the
   public-key type to a string. */
static const char *pkey_type_to_string(int type)
{
  switch (type) {
  case EVP_PKEY_RSA:
    return "rsa";
  case EVP_PKEY_DSA:
    return "dsa";
  case EVP_PKEY_DH:
    return "dh";
#ifdef HAVE_OPENSSL_EC
  case EVP_PKEY_EC:
    return "ec";
#endif
  default:
    return "unknown";
  }
}

int lua_push_ecdhparams(lua_State *L, EVP_PKEY *pubkey) {
#ifdef HAVE_OPENSSL_EC
  EC_KEY *ec_key = EVP_PKEY_get1_EC_KEY(pubkey);
  const EC_GROUP *group = EC_KEY_get0_group(ec_key);
  int nid;
  /* This structure (ecdhparams.curve_params) comes from tls.lua */
  lua_createtable(L, 0, 1); /* ecdhparams */
  lua_createtable(L, 0, 2); /* curve_params */
  if ((nid = EC_GROUP_get_curve_name(group)) != 0) {
    lua_pushstring(L, OBJ_nid2sn(nid));
    lua_setfield(L, -2, "curve");
    lua_pushstring(L, "namedcurve");
    lua_setfield(L, -2, "ec_curve_type");
  }
  else {
    /* According to RFC 5480 section 2.1.1, explicit curves must not be used with
       X.509. This may change in the future, but for now it doesn't seem worth it
       to add in code to extract the extra parameters. */
    nid = EC_METHOD_get_field_type(EC_GROUP_method_of(group));
    if (nid == NID_X9_62_prime_field) {
      lua_pushstring(L, "explicit_prime");
    }
    else if (nid == NID_X9_62_characteristic_two_field) {
      lua_pushstring(L, "explicit_char2");
    }
    else {
      /* Something weird happened. */
      lua_pushstring(L, "UNKNOWN");
    }
    lua_setfield(L, -2, "ec_curve_type");
  }
  lua_setfield(L, -2, "curve_params");
  EC_KEY_free(ec_key);
  return 1;
#else
  return 0;
#endif
}

static int parse_ssl_cert(lua_State *L, X509 *cert);

int l_parse_ssl_certificate(lua_State *L)
{
  X509 *cert;
  size_t l;
  const char *der;

  der = luaL_checklstring(L, 1, &l);
  if (der == NULL) {
    lua_pushnil(L);
    return 1;
  }

  cert = d2i_X509(NULL, (const unsigned char **) &der, l);
  if (cert == NULL) {
    lua_pushnil(L);
    return 1;
  }
  return parse_ssl_cert(L, cert);
}

int l_get_ssl_certificate(lua_State *L)
{
  SSL *ssl;
  X509 *cert;

  ssl = nse_nsock_get_ssl(L);
  cert = SSL_get_peer_certificate(ssl);
  if (cert == NULL) {
    lua_pushnil(L);
    return 1;
  }
  return parse_ssl_cert(L, cert);
}

static int parse_ssl_cert(lua_State *L, X509 *cert)
{
  struct cert_userdata *udata;
  X509_NAME *subject, *issuer;
  EVP_PKEY *pubkey;
  int pkey_type;

  udata = (struct cert_userdata *) lua_newuserdata(L, sizeof(*udata));
  udata->cert = cert;

#define NSE_NUM_CERT_FIELDS 7
  lua_createtable(L, 0, NSE_NUM_CERT_FIELDS);

  subject = X509_get_subject_name(cert);
  if (subject != NULL) {
    x509_name_to_table(L, subject);
    lua_setfield(L, -2, "subject");
  }

#if HAVE_OPAQUE_STRUCTS
  const char *sig_algo = OBJ_nid2ln(X509_get_signature_nid(cert));
#else
  const char *sig_algo = OBJ_nid2ln(OBJ_obj2nid(cert->sig_alg->algorithm));
#endif
  lua_pushstring(L, sig_algo);
  lua_setfield(L, -2, "sig_algorithm");

  issuer = X509_get_issuer_name(cert);
  if (issuer != NULL) {
    x509_name_to_table(L, issuer);
    lua_setfield(L, -2, "issuer");
  }

  x509_validity_to_table(L, cert);
  lua_setfield(L, -2, "validity");

  cert_pem_to_string(L, cert);
  lua_setfield(L, -2, "pem");

#if HAVE_OPAQUE_STRUCTS
  if (x509_extensions_to_table(L, X509_get0_extensions(cert))) {
#else
  if (x509_extensions_to_table(L, cert->cert_info->extensions)) {
#endif
    lua_setfield(L, -2, "extensions");
  }

  pubkey = X509_get_pubkey(cert);
  if (pubkey == NULL) {
    lua_pushnil(L);
    lua_pushfstring(L, "Error parsing cert: %s", ERR_error_string(ERR_get_error(), NULL));
    return 2;
  }
#define NSE_NUM_PKEY_FIELDS 4
  lua_createtable(L, 0, NSE_NUM_PKEY_FIELDS);
#if HAVE_OPAQUE_STRUCTS
  pkey_type = EVP_PKEY_base_id(pubkey);
#else
  pkey_type = EVP_PKEY_type(pubkey->type);
#endif
#ifdef HAVE_OPENSSL_EC
  if (pkey_type == EVP_PKEY_EC) {
    lua_push_ecdhparams(L, pubkey);
    lua_setfield(L, -2, "ecdhparams");
  }
  else
#endif
  if (pkey_type == EVP_PKEY_RSA) {
    RSA *rsa = EVP_PKEY_get1_RSA(pubkey);
    /* exponent */
    bignum_data_t * data = (bignum_data_t *) lua_newuserdata( L, sizeof(bignum_data_t));
    luaL_getmetatable( L, "BIGNUM" );
    lua_setmetatable( L, -2 );
  #if HAVE_OPAQUE_STRUCTS
    const BIGNUM *n, *e;
    RSA_get0_key(rsa, &n, &e, NULL);
    data->bn = (BIGNUM*) e;
  #else
    data->bn = rsa->e;
  #endif
    lua_setfield(L, -2, "exponent");
    /* modulus */
    data = (bignum_data_t *) lua_newuserdata( L, sizeof(bignum_data_t));
    luaL_getmetatable( L, "BIGNUM" );
    lua_setmetatable( L, -2 );
  #if HAVE_OPAQUE_STRUCTS
    data->bn = (BIGNUM*) n;
  #else
    data->bn = rsa->n;
  #endif
    lua_setfield(L, -2, "modulus");
  }
  lua_pushstring(L, pkey_type_to_string(pkey_type));
  lua_setfield(L, -2, "type");
  lua_pushinteger(L, EVP_PKEY_bits(pubkey));
  lua_setfield(L, -2, "bits");
  lua_setfield(L, -2, "pubkey");
  EVP_PKEY_free(pubkey);

  /* At this point the certificate-specific table of attributes is at the top of
     the stack. We give it a metatable with an __index entry that points into
     the global shared table of certificate functions. */
  lua_rawgeti(L, LUA_REGISTRYINDEX, ssl_cert_methods_index_ref);
  lua_setmetatable(L, -2);

  udata->attributes_table = luaL_ref(L, LUA_REGISTRYINDEX);

  luaL_getmetatable(L, "SSL_CERT");
  lua_setmetatable(L, -2);

  return 1;
}

static int l_ssl_cert_index(lua_State *L)
{
  struct cert_userdata *udata;

  udata = (struct cert_userdata *) luaL_checkudata(L, 1, "SSL_CERT");
  lua_rawgeti(L, LUA_REGISTRYINDEX, udata->attributes_table);
  /* The key. */
  lua_pushvalue(L, 2);
  /* Look it up in the table of attributes. */
  lua_gettable(L, -2);

  return 1;
}

static int l_ssl_cert_gc(lua_State *L)
{
  struct cert_userdata *udata;

  udata = (struct cert_userdata *) luaL_checkudata(L, 1, "SSL_CERT");
  X509_free(udata->cert);
  luaL_unref(L, LUA_REGISTRYINDEX, udata->attributes_table);

  return 0;
}

void nse_nsock_init_ssl_cert(lua_State *L)
{
  luaL_newmetatable(L, "SSL_CERT");
  lua_pushcclosure(L, l_ssl_cert_index, 0);
  lua_setfield(L, -2, "__index");
  lua_pushcclosure(L, l_ssl_cert_gc, 0);
  lua_setfield(L, -2, "__gc");

  /* Create a table with an __index entry that will be used as a metatable for
     per-certificate attribute tables. This gives the tables access to the
     global shared table of certificate functions. */
  lua_newtable(L);
  lua_newtable(L);
  luaL_setfuncs(L, ssl_cert_methods, 0);
  lua_setfield(L, -2, "__index");
  ssl_cert_methods_index_ref = luaL_ref(L, LUA_REGISTRYINDEX);
}