curl/lib/vtls/gtls.c

/***************************************************************************
 *                                  _   _ ____  _
 *  Project                     ___| | | |  _ \| |
 *                             / __| | | | |_) | |
 *                            | (__| |_| |  _ <| |___
 *                             \___|\___/|_| \_\_____|
 *
 * Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
 *
 * This software is licensed as described in the file COPYING, which
 * you should have received as part of this distribution. The terms
 * are also available at https://curl.se/docs/copyright.html.
 *
 * You may opt to use, copy, modify, merge, publish, distribute and/or sell
 * copies of the Software, and permit persons to whom the Software is
 * furnished to do so, under the terms of the COPYING file.
 *
 * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
 * KIND, either express or implied.
 *
 * SPDX-License-Identifier: curl
 *
 ***************************************************************************/

/*
 * Source file for all GnuTLS-specific code for the TLS/SSL layer. No code
 * but vtls.c should ever call or use these functions.
 *
 * Note: don't use the GnuTLS' *_t variable type names in this source code,
 * since they were not present in 1.0.X.
 */

#include "curl_setup.h"

#ifdef USE_GNUTLS

#include <gnutls/abstract.h>
#include <gnutls/gnutls.h>
#include <gnutls/x509.h>
#include <gnutls/crypto.h>
#include <nettle/sha2.h>

#include "urldata.h"
#include "sendf.h"
#include "inet_pton.h"
#include "gtls.h"
#include "vtls.h"
#include "vtls_int.h"
#include "vauth/vauth.h"
#include "parsedate.h"
#include "connect.h" /* for the connect timeout */
#include "select.h"
#include "strcase.h"
#include "warnless.h"
#include "x509asn1.h"
#include "multiif.h"
#include "curl_printf.h"
#include "curl_memory.h"
/* The last #include file should be: */
#include "memdebug.h"

/* Enable GnuTLS debugging by defining GTLSDEBUG */
/*#define GTLSDEBUG */

#ifdef GTLSDEBUG
static void tls_log_func(int level, const char *str)
{
    fprintf(stderr, "|<%d>| %s", level, str);
}
#endif
static bool gtls_inited = FALSE;

#if !defined(GNUTLS_VERSION_NUMBER) || (GNUTLS_VERSION_NUMBER < 0x03010a)
#error "too old GnuTLS version"
#endif

# include <gnutls/ocsp.h>

struct ssl_backend_data {
  struct gtls_instance gtls;
};

static ssize_t gtls_push(void *s, const void *buf, size_t blen)
{
  struct Curl_cfilter *cf = s;
  struct ssl_connect_data *connssl = cf->ctx;
  struct Curl_easy *data = CF_DATA_CURRENT(cf);
  ssize_t nwritten;
  CURLcode result;

  DEBUGASSERT(data);
  nwritten = Curl_conn_cf_send(cf->next, data, buf, blen, &result);
  if(nwritten < 0) {
    gnutls_transport_set_errno(connssl->backend->gtls.session,
                               (CURLE_AGAIN == result)? EAGAIN : EINVAL);
    nwritten = -1;
  }
  return nwritten;
}

static ssize_t gtls_pull(void *s, void *buf, size_t blen)
{
  struct Curl_cfilter *cf = s;
  struct ssl_connect_data *connssl = cf->ctx;
  struct Curl_easy *data = CF_DATA_CURRENT(cf);
  ssize_t nread;
  CURLcode result;

  DEBUGASSERT(data);
  nread = Curl_conn_cf_recv(cf->next, data, buf, blen, &result);
  if(nread < 0) {
    gnutls_transport_set_errno(connssl->backend->gtls.session,
                               (CURLE_AGAIN == result)? EAGAIN : EINVAL);
    nread = -1;
  }
  return nread;
}

/* gtls_init()
 *
 * Global GnuTLS init, called from Curl_ssl_init(). This calls functions that
 * are not thread-safe and thus this function itself is not thread-safe and
 * must only be called from within curl_global_init() to keep the thread
 * situation under control!
 */
static int gtls_init(void)
{
  int ret = 1;
  if(!gtls_inited) {
    ret = gnutls_global_init()?0:1;
#ifdef GTLSDEBUG
    gnutls_global_set_log_function(tls_log_func);
    gnutls_global_set_log_level(2);
#endif
    gtls_inited = TRUE;
  }
  return ret;
}

static void gtls_cleanup(void)
{
  if(gtls_inited) {
    gnutls_global_deinit();
    gtls_inited = FALSE;
  }
}

#ifndef CURL_DISABLE_VERBOSE_STRINGS
static void showtime(struct Curl_easy *data,
                     const char *text,
                     time_t stamp)
{
  struct tm buffer;
  const struct tm *tm = &buffer;
  char str[96];
  CURLcode result = Curl_gmtime(stamp, &buffer);
  if(result)
    return;

  msnprintf(str,
            sizeof(str),
            "  %s: %s, %02d %s %4d %02d:%02d:%02d GMT",
            text,
            Curl_wkday[tm->tm_wday?tm->tm_wday-1:6],
            tm->tm_mday,
            Curl_month[tm->tm_mon],
            tm->tm_year + 1900,
            tm->tm_hour,
            tm->tm_min,
            tm->tm_sec);
  infof(data, "%s", str);
}
#endif

static gnutls_datum_t load_file(const char *file)
{
  FILE *f;
  gnutls_datum_t loaded_file = { NULL, 0 };
  long filelen;
  void *ptr;

  f = fopen(file, "rb");
  if(!f)
    return loaded_file;
  if(fseek(f, 0, SEEK_END) != 0
     || (filelen = ftell(f)) < 0
     || fseek(f, 0, SEEK_SET) != 0
     || !(ptr = malloc((size_t)filelen)))
    goto out;
  if(fread(ptr, 1, (size_t)filelen, f) < (size_t)filelen) {
    free(ptr);
    goto out;
  }

  loaded_file.data = ptr;
  loaded_file.size = (unsigned int)filelen;
out:
  fclose(f);
  return loaded_file;
}

static void unload_file(gnutls_datum_t data)
{
  free(data.data);
}


/* this function does a SSL/TLS (re-)handshake */
static CURLcode handshake(struct Curl_cfilter *cf,
                          struct Curl_easy *data,
                          bool duringconnect,
                          bool nonblocking)
{
  struct ssl_connect_data *connssl = cf->ctx;
  struct ssl_backend_data *backend = connssl->backend;
  gnutls_session_t session;
  curl_socket_t sockfd = Curl_conn_cf_get_socket(cf, data);

  DEBUGASSERT(backend);
  session = backend->gtls.session;

  for(;;) {
    timediff_t timeout_ms;
    int rc;

    /* check allowed time left */
    timeout_ms = Curl_timeleft(data, NULL, duringconnect);

    if(timeout_ms < 0) {
      /* no need to continue if time already is up */
      failf(data, "SSL connection timeout");
      return CURLE_OPERATION_TIMEDOUT;
    }

    /* if ssl is expecting something, check if it's available. */
    if(connssl->connecting_state == ssl_connect_2_reading
       || connssl->connecting_state == ssl_connect_2_writing) {
      int what;
      curl_socket_t writefd = ssl_connect_2_writing ==
        connssl->connecting_state?sockfd:CURL_SOCKET_BAD;
      curl_socket_t readfd = ssl_connect_2_reading ==
        connssl->connecting_state?sockfd:CURL_SOCKET_BAD;

      what = Curl_socket_check(readfd, CURL_SOCKET_BAD, writefd,
                               nonblocking?0:
                               timeout_ms?timeout_ms:1000);
      if(what < 0) {
        /* fatal error */
        failf(data, "select/poll on SSL socket, errno: %d", SOCKERRNO);
        return CURLE_SSL_CONNECT_ERROR;
      }
      else if(0 == what) {
        if(nonblocking)
          return CURLE_OK;
        else if(timeout_ms) {
          /* timeout */
          failf(data, "SSL connection timeout at %ld", (long)timeout_ms);
          return CURLE_OPERATION_TIMEDOUT;
        }
      }
      /* socket is readable or writable */
    }

    rc = gnutls_handshake(session);

    if((rc == GNUTLS_E_AGAIN) || (rc == GNUTLS_E_INTERRUPTED)) {
      connssl->connecting_state =
        gnutls_record_get_direction(session)?
        ssl_connect_2_writing:ssl_connect_2_reading;
      continue;
    }
    else if((rc < 0) && !gnutls_error_is_fatal(rc)) {
      const char *strerr = NULL;

      if(rc == GNUTLS_E_WARNING_ALERT_RECEIVED) {
        int alert = gnutls_alert_get(session);
        strerr = gnutls_alert_get_name(alert);
      }

      if(!strerr)
        strerr = gnutls_strerror(rc);

      infof(data, "gnutls_handshake() warning: %s", strerr);
      continue;
    }
    else if(rc < 0) {
      const char *strerr = NULL;

      if(rc == GNUTLS_E_FATAL_ALERT_RECEIVED) {
        int alert = gnutls_alert_get(session);
        strerr = gnutls_alert_get_name(alert);
      }

      if(!strerr)
        strerr = gnutls_strerror(rc);

      failf(data, "gnutls_handshake() failed: %s", strerr);
      return CURLE_SSL_CONNECT_ERROR;
    }

    /* Reset our connect state machine */
    connssl->connecting_state = ssl_connect_1;
    return CURLE_OK;
  }
}

static gnutls_x509_crt_fmt_t do_file_type(const char *type)
{
  if(!type || !type[0])
    return GNUTLS_X509_FMT_PEM;
  if(strcasecompare(type, "PEM"))
    return GNUTLS_X509_FMT_PEM;
  if(strcasecompare(type, "DER"))
    return GNUTLS_X509_FMT_DER;
  return GNUTLS_X509_FMT_PEM; /* default to PEM */
}

#define GNUTLS_CIPHERS "NORMAL:-ARCFOUR-128:-CTYPE-ALL:+CTYPE-X509"
/* If GnuTLS was compiled without support for SRP it will error out if SRP is
   requested in the priority string, so treat it specially
 */
#define GNUTLS_SRP "+SRP"

static CURLcode
set_ssl_version_min_max(struct Curl_easy *data,
                        struct ssl_primary_config *conn_config,
                        const char **prioritylist,
                        const char *tls13support)
{
  long ssl_version = conn_config->version;
  long ssl_version_max = conn_config->version_max;

  if((ssl_version == CURL_SSLVERSION_DEFAULT) ||
     (ssl_version == CURL_SSLVERSION_TLSv1))
    ssl_version = CURL_SSLVERSION_TLSv1_0;
  if(ssl_version_max == CURL_SSLVERSION_MAX_NONE)
    ssl_version_max = CURL_SSLVERSION_MAX_DEFAULT;
  if(!tls13support) {
    /* If the running GnuTLS doesn't support TLS 1.3, we must not specify a
       prioritylist involving that since it will make GnuTLS return an en
       error back at us */
    if((ssl_version_max == CURL_SSLVERSION_MAX_TLSv1_3) ||
       (ssl_version_max == CURL_SSLVERSION_MAX_DEFAULT)) {
      ssl_version_max = CURL_SSLVERSION_MAX_TLSv1_2;
    }
  }
  else if(ssl_version_max == CURL_SSLVERSION_MAX_DEFAULT) {
    ssl_version_max = CURL_SSLVERSION_MAX_TLSv1_3;
  }

  switch(ssl_version | ssl_version_max) {
  case CURL_SSLVERSION_TLSv1_0 | CURL_SSLVERSION_MAX_TLSv1_0:
    *prioritylist = GNUTLS_CIPHERS ":-VERS-SSL3.0:-VERS-TLS-ALL:"
      "+VERS-TLS1.0";
    return CURLE_OK;
  case CURL_SSLVERSION_TLSv1_0 | CURL_SSLVERSION_MAX_TLSv1_1:
    *prioritylist = GNUTLS_CIPHERS ":-VERS-SSL3.0:-VERS-TLS-ALL:"
      "+VERS-TLS1.1:+VERS-TLS1.0";
    return CURLE_OK;
  case CURL_SSLVERSION_TLSv1_0 | CURL_SSLVERSION_MAX_TLSv1_2:
    *prioritylist = GNUTLS_CIPHERS ":-VERS-SSL3.0:-VERS-TLS-ALL:"
      "+VERS-TLS1.2:+VERS-TLS1.1:+VERS-TLS1.0";
    return CURLE_OK;
  case CURL_SSLVERSION_TLSv1_1 | CURL_SSLVERSION_MAX_TLSv1_1:
    *prioritylist = GNUTLS_CIPHERS ":-VERS-SSL3.0:-VERS-TLS-ALL:"
      "+VERS-TLS1.1";
    return CURLE_OK;
  case CURL_SSLVERSION_TLSv1_1 | CURL_SSLVERSION_MAX_TLSv1_2:
    *prioritylist = GNUTLS_CIPHERS ":-VERS-SSL3.0:-VERS-TLS-ALL:"
      "+VERS-TLS1.2:+VERS-TLS1.1";
    return CURLE_OK;
  case CURL_SSLVERSION_TLSv1_2 | CURL_SSLVERSION_MAX_TLSv1_2:
    *prioritylist = GNUTLS_CIPHERS ":-VERS-SSL3.0:-VERS-TLS-ALL:"
      "+VERS-TLS1.2";
    return CURLE_OK;
  case CURL_SSLVERSION_TLSv1_3 | CURL_SSLVERSION_MAX_TLSv1_3:
    *prioritylist = GNUTLS_CIPHERS ":-VERS-SSL3.0:-VERS-TLS-ALL:"
      "+VERS-TLS1.3";
    return CURLE_OK;
  case CURL_SSLVERSION_TLSv1_0 | CURL_SSLVERSION_MAX_TLSv1_3:
    *prioritylist = GNUTLS_CIPHERS ":-VERS-SSL3.0";
    return CURLE_OK;
  case CURL_SSLVERSION_TLSv1_1 | CURL_SSLVERSION_MAX_TLSv1_3:
    *prioritylist = GNUTLS_CIPHERS ":-VERS-SSL3.0:-VERS-TLS-ALL:"
      "+VERS-TLS1.3:+VERS-TLS1.2:+VERS-TLS1.1";
    return CURLE_OK;
  case CURL_SSLVERSION_TLSv1_2 | CURL_SSLVERSION_MAX_TLSv1_3:
    *prioritylist = GNUTLS_CIPHERS ":-VERS-SSL3.0:-VERS-TLS-ALL:"
      "+VERS-TLS1.3:+VERS-TLS1.2";
    return CURLE_OK;
  }

  failf(data, "GnuTLS: cannot set ssl protocol");
  return CURLE_SSL_CONNECT_ERROR;
}

CURLcode gtls_client_init(struct Curl_easy *data,
                          struct ssl_primary_config *config,
                          struct ssl_config_data *ssl_config,
                          const char *hostname,
                          struct gtls_instance *gtls,
                          long *pverifyresult)
{
  unsigned int init_flags;
  int rc;
  bool sni = TRUE; /* default is SNI enabled */
#ifdef ENABLE_IPV6
  struct in6_addr addr;
#else
  struct in_addr addr;
#endif
  const char *prioritylist;
  const char *err = NULL;
  const char *tls13support;
  CURLcode result;

  if(!gtls_inited)
    gtls_init();

  *pverifyresult = 0;

  if(config->version == CURL_SSLVERSION_SSLv2) {
    failf(data, "GnuTLS does not support SSLv2");
    return CURLE_SSL_CONNECT_ERROR;
  }
  else if(config->version == CURL_SSLVERSION_SSLv3)
    sni = FALSE; /* SSLv3 has no SNI */

  /* allocate a cred struct */
  rc = gnutls_certificate_allocate_credentials(&gtls->cred);
  if(rc != GNUTLS_E_SUCCESS) {
    failf(data, "gnutls_cert_all_cred() failed: %s", gnutls_strerror(rc));
    return CURLE_SSL_CONNECT_ERROR;
  }

#ifdef USE_GNUTLS_SRP
  if(config->username && Curl_auth_allowed_to_host(data)) {
    infof(data, "Using TLS-SRP username: %s", config->username);

    rc = gnutls_srp_allocate_client_credentials(&gtls->srp_client_cred);
    if(rc != GNUTLS_E_SUCCESS) {
      failf(data, "gnutls_srp_allocate_client_cred() failed: %s",
            gnutls_strerror(rc));
      return CURLE_OUT_OF_MEMORY;
    }

    rc = gnutls_srp_set_client_credentials(gtls->srp_client_cred,
                                           config->username,
                                           config->password);
    if(rc != GNUTLS_E_SUCCESS) {
      failf(data, "gnutls_srp_set_client_cred() failed: %s",
            gnutls_strerror(rc));
      return CURLE_BAD_FUNCTION_ARGUMENT;
    }
  }
#endif

  if(config->CAfile) {
    /* set the trusted CA cert bundle file */
    gnutls_certificate_set_verify_flags(gtls->cred,
                                        GNUTLS_VERIFY_ALLOW_X509_V1_CA_CRT);

    rc = gnutls_certificate_set_x509_trust_file(gtls->cred,
                                                config->CAfile,
                                                GNUTLS_X509_FMT_PEM);
    if(rc < 0) {
      infof(data, "error reading ca cert file %s (%s)",
            config->CAfile, gnutls_strerror(rc));
      if(config->verifypeer) {
        *pverifyresult = rc;
        return CURLE_SSL_CACERT_BADFILE;
      }
    }
    else
      infof(data, "found %d certificates in %s", rc, config->CAfile);
  }

  if(config->CApath) {
    /* set the trusted CA cert directory */
    rc = gnutls_certificate_set_x509_trust_dir(gtls->cred,
                                               config->CApath,
                                               GNUTLS_X509_FMT_PEM);
    if(rc < 0) {
      infof(data, "error reading ca cert file %s (%s)",
            config->CApath, gnutls_strerror(rc));
      if(config->verifypeer) {
        *pverifyresult = rc;
        return CURLE_SSL_CACERT_BADFILE;
      }
    }
    else
      infof(data, "found %d certificates in %s", rc, config->CApath);
  }

#ifdef CURL_CA_FALLBACK
  /* use system ca certificate store as fallback */
  if(config->verifypeer && !(config->CAfile || config->CApath)) {
    /* this ignores errors on purpose */
    gnutls_certificate_set_x509_system_trust(gtls->cred);
  }
#endif

  if(config->CRLfile) {
    /* set the CRL list file */
    rc = gnutls_certificate_set_x509_crl_file(gtls->cred,
                                              config->CRLfile,
                                              GNUTLS_X509_FMT_PEM);
    if(rc < 0) {
      failf(data, "error reading crl file %s (%s)",
            config->CRLfile, gnutls_strerror(rc));
      return CURLE_SSL_CRL_BADFILE;
    }
    else
      infof(data, "found %d CRL in %s", rc, config->CRLfile);
  }

  /* Initialize TLS session as a client */
  init_flags = GNUTLS_CLIENT;

#if defined(GNUTLS_FORCE_CLIENT_CERT)
  init_flags |= GNUTLS_FORCE_CLIENT_CERT;
#endif

#if defined(GNUTLS_NO_TICKETS)
  /* Disable TLS session tickets */
  init_flags |= GNUTLS_NO_TICKETS;
#endif

  rc = gnutls_init(&gtls->session, init_flags);
  if(rc != GNUTLS_E_SUCCESS) {
    failf(data, "gnutls_init() failed: %d", rc);
    return CURLE_SSL_CONNECT_ERROR;
  }

  if((0 == Curl_inet_pton(AF_INET, hostname, &addr)) &&
#ifdef ENABLE_IPV6
     (0 == Curl_inet_pton(AF_INET6, hostname, &addr)) &&
#endif
     sni) {
    size_t snilen;
    char *snihost = Curl_ssl_snihost(data, hostname, &snilen);
    if(!snihost || gnutls_server_name_set(gtls->session, GNUTLS_NAME_DNS,
                                          snihost, snilen) < 0) {
      failf(data, "Failed to set SNI");
      return CURLE_SSL_CONNECT_ERROR;
    }
  }

  /* Use default priorities */
  rc = gnutls_set_default_priority(gtls->session);
  if(rc != GNUTLS_E_SUCCESS)
    return CURLE_SSL_CONNECT_ERROR;

  /* "In GnuTLS 3.6.5, TLS 1.3 is enabled by default" */
  tls13support = gnutls_check_version("3.6.5");

  /* Ensure +SRP comes at the *end* of all relevant strings so that it can be
   * removed if a run-time error indicates that SRP is not supported by this
   * GnuTLS version */

  if(config->version == CURL_SSLVERSION_SSLv2 ||
     config->version == CURL_SSLVERSION_SSLv3) {
    failf(data, "GnuTLS does not support SSLv2 or SSLv3");
    return CURLE_SSL_CONNECT_ERROR;
  }

  if(config->version == CURL_SSLVERSION_TLSv1_3) {
    if(!tls13support) {
      failf(data, "This GnuTLS installation does not support TLS 1.3");
      return CURLE_SSL_CONNECT_ERROR;
    }
  }

  /* At this point we know we have a supported TLS version, so set it */
  result = set_ssl_version_min_max(data, config, &prioritylist, tls13support);
  if(result)
    return result;

#ifdef USE_GNUTLS_SRP
  /* Only add SRP to the cipher list if SRP is requested. Otherwise
   * GnuTLS will disable TLS 1.3 support. */
  if(config->username) {
    size_t len = strlen(prioritylist);

    char *prioritysrp = malloc(len + sizeof(GNUTLS_SRP) + 1);
    if(!prioritysrp)
      return CURLE_OUT_OF_MEMORY;
    strcpy(prioritysrp, prioritylist);
    strcpy(prioritysrp + len, ":" GNUTLS_SRP);
    rc = gnutls_priority_set_direct(gtls->session, prioritysrp, &err);
    free(prioritysrp);

    if((rc == GNUTLS_E_INVALID_REQUEST) && err) {
      infof(data, "This GnuTLS does not support SRP");
    }
  }
  else {
#endif
    infof(data, "GnuTLS ciphers: %s", prioritylist);
    rc = gnutls_priority_set_direct(gtls->session, prioritylist, &err);
#ifdef USE_GNUTLS_SRP
  }
#endif

  if(rc != GNUTLS_E_SUCCESS) {
    failf(data, "Error %d setting GnuTLS cipher list starting with %s",
          rc, err);
    return CURLE_SSL_CONNECT_ERROR;
  }

  if(config->clientcert) {
    if(ssl_config->key_passwd) {
      const unsigned int supported_key_encryption_algorithms =
        GNUTLS_PKCS_USE_PKCS12_3DES | GNUTLS_PKCS_USE_PKCS12_ARCFOUR |
        GNUTLS_PKCS_USE_PKCS12_RC2_40 | GNUTLS_PKCS_USE_PBES2_3DES |
        GNUTLS_PKCS_USE_PBES2_AES_128 | GNUTLS_PKCS_USE_PBES2_AES_192 |
        GNUTLS_PKCS_USE_PBES2_AES_256;
      rc = gnutls_certificate_set_x509_key_file2(
           gtls->cred,
           config->clientcert,
           ssl_config->key ? ssl_config->key : config->clientcert,
           do_file_type(ssl_config->cert_type),
           ssl_config->key_passwd,
           supported_key_encryption_algorithms);
      if(rc != GNUTLS_E_SUCCESS) {
        failf(data,
              "error reading X.509 potentially-encrypted key file: %s",
              gnutls_strerror(rc));
        return CURLE_SSL_CONNECT_ERROR;
      }
    }
    else {
      if(gnutls_certificate_set_x509_key_file(
           gtls->cred,
           config->clientcert,
           ssl_config->key ? ssl_config->key : config->clientcert,
           do_file_type(ssl_config->cert_type) ) !=
         GNUTLS_E_SUCCESS) {
        failf(data, "error reading X.509 key or certificate file");
        return CURLE_SSL_CONNECT_ERROR;
      }
    }
  }

#ifdef USE_GNUTLS_SRP
  /* put the credentials to the current session */
  if(config->username) {
    rc = gnutls_credentials_set(gtls->session, GNUTLS_CRD_SRP,
                                gtls->srp_client_cred);
    if(rc != GNUTLS_E_SUCCESS) {
      failf(data, "gnutls_credentials_set() failed: %s", gnutls_strerror(rc));
      return CURLE_SSL_CONNECT_ERROR;
    }
  }
  else
#endif
  {
    rc = gnutls_credentials_set(gtls->session, GNUTLS_CRD_CERTIFICATE,
                                gtls->cred);
    if(rc != GNUTLS_E_SUCCESS) {
      failf(data, "gnutls_credentials_set() failed: %s", gnutls_strerror(rc));
      return CURLE_SSL_CONNECT_ERROR;
    }
  }

  if(config->verifystatus) {
    rc = gnutls_ocsp_status_request_enable_client(gtls->session,
                                                  NULL, 0, NULL);
    if(rc != GNUTLS_E_SUCCESS) {
      failf(data, "gnutls_ocsp_status_request_enable_client() failed: %d", rc);
      return CURLE_SSL_CONNECT_ERROR;
    }
  }

  return CURLE_OK;
}

static CURLcode
gtls_connect_step1(struct Curl_cfilter *cf, struct Curl_easy *data)
{
  struct ssl_connect_data *connssl = cf->ctx;
  struct ssl_backend_data *backend = connssl->backend;
  struct ssl_primary_config *conn_config = Curl_ssl_cf_get_primary_config(cf);
  struct ssl_config_data *ssl_config = Curl_ssl_cf_get_config(cf, data);
  long * const pverifyresult = &ssl_config->certverifyresult;
  CURLcode result;

  DEBUGASSERT(backend);

  if(connssl->state == ssl_connection_complete)
    /* to make us tolerant against being called more than once for the
       same connection */
    return CURLE_OK;

  result = gtls_client_init(data, conn_config, ssl_config,
                            connssl->hostname,
                            &backend->gtls, pverifyresult);
  if(result)
    return result;

  if(connssl->alpn) {
    struct alpn_proto_buf proto;
    gnutls_datum_t alpn[ALPN_ENTRIES_MAX];
    size_t i;

    for(i = 0; i < connssl->alpn->count; ++i) {
      alpn[i].data = (unsigned char *)connssl->alpn->entries[i];
      alpn[i].size = (unsigned)strlen(connssl->alpn->entries[i]);
    }
    if(gnutls_alpn_set_protocols(backend->gtls.session, alpn,
                                 (unsigned)connssl->alpn->count, 0)) {
      failf(data, "failed setting ALPN");
      return CURLE_SSL_CONNECT_ERROR;
    }
    Curl_alpn_to_proto_str(&proto, connssl->alpn);
    infof(data, VTLS_INFOF_ALPN_OFFER_1STR, proto.data);
  }

  /* This might be a reconnect, so we check for a session ID in the cache
     to speed up things */
  if(conn_config->sessionid) {
    void *ssl_sessionid;
    size_t ssl_idsize;

    Curl_ssl_sessionid_lock(data);
    if(!Curl_ssl_getsessionid(cf, data, &ssl_sessionid, &ssl_idsize)) {
      /* we got a session id, use it! */
      gnutls_session_set_data(backend->gtls.session,
                              ssl_sessionid, ssl_idsize);

      /* Informational message */
      infof(data, "SSL re-using session ID");
    }
    Curl_ssl_sessionid_unlock(data);
  }

  /* register callback functions and handle to send and receive data. */
  gnutls_transport_set_ptr(backend->gtls.session, cf);
  gnutls_transport_set_push_function(backend->gtls.session, gtls_push);
  gnutls_transport_set_pull_function(backend->gtls.session, gtls_pull);

  return CURLE_OK;
}

static CURLcode pkp_pin_peer_pubkey(struct Curl_easy *data,
                                    gnutls_x509_crt_t cert,
                                    const char *pinnedpubkey)
{
  /* Scratch */
  size_t len1 = 0, len2 = 0;
  unsigned char *buff1 = NULL;

  gnutls_pubkey_t key = NULL;

  /* Result is returned to caller */
  CURLcode result = CURLE_SSL_PINNEDPUBKEYNOTMATCH;

  /* if a path wasn't specified, don't pin */
  if(!pinnedpubkey)
    return CURLE_OK;

  if(!cert)
    return result;

  do {
    int ret;

    /* Begin Gyrations to get the public key     */
    gnutls_pubkey_init(&key);

    ret = gnutls_pubkey_import_x509(key, cert, 0);
    if(ret < 0)
      break; /* failed */

    ret = gnutls_pubkey_export(key, GNUTLS_X509_FMT_DER, NULL, &len1);
    if(ret != GNUTLS_E_SHORT_MEMORY_BUFFER || len1 == 0)
      break; /* failed */

    buff1 = malloc(len1);
    if(!buff1)
      break; /* failed */

    len2 = len1;

    ret = gnutls_pubkey_export(key, GNUTLS_X509_FMT_DER, buff1, &len2);
    if(ret < 0 || len1 != len2)
      break; /* failed */

    /* End Gyrations */

    /* The one good exit point */
    result = Curl_pin_peer_pubkey(data, pinnedpubkey, buff1, len1);
  } while(0);

  if(key)
    gnutls_pubkey_deinit(key);

  Curl_safefree(buff1);

  return result;
}

CURLcode
Curl_gtls_verifyserver(struct Curl_easy *data,
                       gnutls_session_t session,
                       struct ssl_primary_config *config,
                       struct ssl_config_data *ssl_config,
                       const char *hostname,
                       const char *dispname,
                       const char *pinned_key)
{
  unsigned int cert_list_size;
  const gnutls_datum_t *chainp;
  unsigned int verify_status = 0;
  gnutls_x509_crt_t x509_cert, x509_issuer;
  gnutls_datum_t issuerp;
  gnutls_datum_t certfields;
  char certname[65] = ""; /* limited to 64 chars by ASN.1 */
  size_t size;
  time_t certclock;
  const char *ptr;
  int rc;
  CURLcode result = CURLE_OK;
#ifndef CURL_DISABLE_VERBOSE_STRINGS
  unsigned int algo;
  unsigned int bits;
  gnutls_protocol_t version = gnutls_protocol_get_version(session);
#endif
  long * const certverifyresult = &ssl_config->certverifyresult;

  /* the name of the cipher suite used, e.g. ECDHE_RSA_AES_256_GCM_SHA384. */
  ptr = gnutls_cipher_suite_get_name(gnutls_kx_get(session),
                                     gnutls_cipher_get(session),
                                     gnutls_mac_get(session));

  infof(data, "SSL connection using %s / %s",
        gnutls_protocol_get_name(version), ptr);

  /* This function will return the peer's raw certificate (chain) as sent by
     the peer. These certificates are in raw format (DER encoded for
     X.509). In case of a X.509 then a certificate list may be present. The
     first certificate in the list is the peer's certificate, following the
     issuer's certificate, then the issuer's issuer etc. */

  chainp = gnutls_certificate_get_peers(session, &cert_list_size);
  if(!chainp) {
    if(config->verifypeer ||
       config->verifyhost ||
       config->issuercert) {
#ifdef USE_GNUTLS_SRP
      if(ssl_config->primary.username && !config->verifypeer &&
         gnutls_cipher_get(session)) {
        /* no peer cert, but auth is ok if we have SRP user and cipher and no
           peer verify */
      }
      else {
#endif
        failf(data, "failed to get server cert");
        *certverifyresult = GNUTLS_E_NO_CERTIFICATE_FOUND;
        return CURLE_PEER_FAILED_VERIFICATION;
#ifdef USE_GNUTLS_SRP
      }
#endif
    }
    infof(data, " common name: WARNING couldn't obtain");
  }

  if(data->set.ssl.certinfo && chainp) {
    unsigned int i;

    result = Curl_ssl_init_certinfo(data, cert_list_size);
    if(result)
      return result;

    for(i = 0; i < cert_list_size; i++) {
      const char *beg = (const char *) chainp[i].data;
      const char *end = beg + chainp[i].size;

      result = Curl_extract_certinfo(data, i, beg, end);
      if(result)
        return result;
    }
  }

  if(config->verifypeer) {
    /* This function will try to verify the peer's certificate and return its
       status (trusted, invalid etc.). The value of status should be one or
       more of the gnutls_certificate_status_t enumerated elements bitwise
       or'd. To avoid denial of service attacks some default upper limits
       regarding the certificate key size and chain size are set. To override
       them use gnutls_certificate_set_verify_limits(). */

    rc = gnutls_certificate_verify_peers2(session, &verify_status);
    if(rc < 0) {
      failf(data, "server cert verify failed: %d", rc);
      *certverifyresult = rc;
      return CURLE_SSL_CONNECT_ERROR;
    }

    *certverifyresult = verify_status;

    /* verify_status is a bitmask of gnutls_certificate_status bits */
    if(verify_status & GNUTLS_CERT_INVALID) {
      if(config->verifypeer) {
        failf(data, "server certificate verification failed. CAfile: %s "
              "CRLfile: %s", config->CAfile ? config->CAfile:
              "none",
              ssl_config->primary.CRLfile ?
              ssl_config->primary.CRLfile : "none");
        return CURLE_PEER_FAILED_VERIFICATION;
      }
      else
        infof(data, "  server certificate verification FAILED");
    }
    else
      infof(data, "  server certificate verification OK");
  }
  else
    infof(data, "  server certificate verification SKIPPED");

  if(config->verifystatus) {
    if(gnutls_ocsp_status_request_is_checked(session, 0) == 0) {
      gnutls_datum_t status_request;
      gnutls_ocsp_resp_t ocsp_resp;

      gnutls_ocsp_cert_status_t status;
      gnutls_x509_crl_reason_t reason;

      rc = gnutls_ocsp_status_request_get(session, &status_request);

      infof(data, " server certificate status verification FAILED");

      if(rc == GNUTLS_E_REQUESTED_DATA_NOT_AVAILABLE) {
        failf(data, "No OCSP response received");
        return CURLE_SSL_INVALIDCERTSTATUS;
      }

      if(rc < 0) {
        failf(data, "Invalid OCSP response received");
        return CURLE_SSL_INVALIDCERTSTATUS;
      }

      gnutls_ocsp_resp_init(&ocsp_resp);

      rc = gnutls_ocsp_resp_import(ocsp_resp, &status_request);
      if(rc < 0) {
        failf(data, "Invalid OCSP response received");
        return CURLE_SSL_INVALIDCERTSTATUS;
      }

      (void)gnutls_ocsp_resp_get_single(ocsp_resp, 0, NULL, NULL, NULL, NULL,
                                        &status, NULL, NULL, NULL, &reason);

      switch(status) {
      case GNUTLS_OCSP_CERT_GOOD:
        break;

      case GNUTLS_OCSP_CERT_REVOKED: {
        const char *crl_reason;

        switch(reason) {
          default:
          case GNUTLS_X509_CRLREASON_UNSPECIFIED:
            crl_reason = "unspecified reason";
            break;

          case GNUTLS_X509_CRLREASON_KEYCOMPROMISE:
            crl_reason = "private key compromised";
            break;

          case GNUTLS_X509_CRLREASON_CACOMPROMISE:
            crl_reason = "CA compromised";
            break;

          case GNUTLS_X509_CRLREASON_AFFILIATIONCHANGED:
            crl_reason = "affiliation has changed";
            break;

          case GNUTLS_X509_CRLREASON_SUPERSEDED:
            crl_reason = "certificate superseded";
            break;

          case GNUTLS_X509_CRLREASON_CESSATIONOFOPERATION:
            crl_reason = "operation has ceased";
            break;

          case GNUTLS_X509_CRLREASON_CERTIFICATEHOLD:
            crl_reason = "certificate is on hold";
            break;

          case GNUTLS_X509_CRLREASON_REMOVEFROMCRL:
            crl_reason = "will be removed from delta CRL";
            break;

          case GNUTLS_X509_CRLREASON_PRIVILEGEWITHDRAWN:
            crl_reason = "privilege withdrawn";
            break;

          case GNUTLS_X509_CRLREASON_AACOMPROMISE:
            crl_reason = "AA compromised";
            break;
        }

        failf(data, "Server certificate was revoked: %s", crl_reason);
        break;
      }

      default:
      case GNUTLS_OCSP_CERT_UNKNOWN:
        failf(data, "Server certificate status is unknown");
        break;
      }

      gnutls_ocsp_resp_deinit(ocsp_resp);

      return CURLE_SSL_INVALIDCERTSTATUS;
    }
    else
      infof(data, "  server certificate status verification OK");
  }
  else
    infof(data, "  server certificate status verification SKIPPED");

  /* initialize an X.509 certificate structure. */
  gnutls_x509_crt_init(&x509_cert);

  if(chainp)
    /* convert the given DER or PEM encoded Certificate to the native
       gnutls_x509_crt_t format */
    gnutls_x509_crt_import(x509_cert, chainp, GNUTLS_X509_FMT_DER);

  if(config->issuercert) {
    gnutls_x509_crt_init(&x509_issuer);
    issuerp = load_file(config->issuercert);
    gnutls_x509_crt_import(x509_issuer, &issuerp, GNUTLS_X509_FMT_PEM);
    rc = gnutls_x509_crt_check_issuer(x509_cert, x509_issuer);
    gnutls_x509_crt_deinit(x509_issuer);
    unload_file(issuerp);
    if(rc <= 0) {
      failf(data, "server certificate issuer check failed (IssuerCert: %s)",
            config->issuercert?config->issuercert:"none");
      gnutls_x509_crt_deinit(x509_cert);
      return CURLE_SSL_ISSUER_ERROR;
    }
    infof(data, "  server certificate issuer check OK (Issuer Cert: %s)",
          config->issuercert?config->issuercert:"none");
  }

  size = sizeof(certname);
  rc = gnutls_x509_crt_get_dn_by_oid(x509_cert, GNUTLS_OID_X520_COMMON_NAME,
                                     0, /* the first and only one */
                                     FALSE,
                                     certname,
                                     &size);
  if(rc) {
    infof(data, "error fetching CN from cert:%s",
          gnutls_strerror(rc));
  }

  /* This function will check if the given certificate's subject matches the
     given hostname. This is a basic implementation of the matching described
     in RFC2818 (HTTPS), which takes into account wildcards, and the subject
     alternative name PKIX extension. Returns non zero on success, and zero on
     failure. */
  rc = gnutls_x509_crt_check_hostname(x509_cert, hostname);
#if GNUTLS_VERSION_NUMBER < 0x030306
  /* Before 3.3.6, gnutls_x509_crt_check_hostname() didn't check IP
     addresses. */
  if(!rc) {
#ifdef ENABLE_IPV6
    #define use_addr in6_addr
#else
    #define use_addr in_addr
#endif
    unsigned char addrbuf[sizeof(struct use_addr)];
    size_t addrlen = 0;

    if(Curl_inet_pton(AF_INET, hostname, addrbuf) > 0)
      addrlen = 4;
#ifdef ENABLE_IPV6
    else if(Curl_inet_pton(AF_INET6, hostname, addrbuf) > 0)
      addrlen = 16;
#endif

    if(addrlen) {
      unsigned char certaddr[sizeof(struct use_addr)];
      int i;

      for(i = 0; ; i++) {
        size_t certaddrlen = sizeof(certaddr);
        int ret = gnutls_x509_crt_get_subject_alt_name(x509_cert, i, certaddr,
                                                       &certaddrlen, NULL);
        /* If this happens, it wasn't an IP address. */
        if(ret == GNUTLS_E_SHORT_MEMORY_BUFFER)
          continue;
        if(ret < 0)
          break;
        if(ret != GNUTLS_SAN_IPADDRESS)
          continue;
        if(certaddrlen == addrlen && !memcmp(addrbuf, certaddr, addrlen)) {
          rc = 1;
          break;
        }
      }
    }
  }
#endif
  if(!rc) {
    if(config->verifyhost) {
      failf(data, "SSL: certificate subject name (%s) does not match "
            "target host name '%s'", certname, dispname);
      gnutls_x509_crt_deinit(x509_cert);
      return CURLE_PEER_FAILED_VERIFICATION;
    }
    else
      infof(data, "  common name: %s (does not match '%s')",
            certname, dispname);
  }
  else
    infof(data, "  common name: %s (matched)", certname);

  /* Check for time-based validity */
  certclock = gnutls_x509_crt_get_expiration_time(x509_cert);

  if(certclock == (time_t)-1) {
    if(config->verifypeer) {
      failf(data, "server cert expiration date verify failed");
      *certverifyresult = GNUTLS_CERT_EXPIRED;
      gnutls_x509_crt_deinit(x509_cert);
      return CURLE_SSL_CONNECT_ERROR;
    }
    else
      infof(data, "  server certificate expiration date verify FAILED");
  }
  else {
    if(certclock < time(NULL)) {
      if(config->verifypeer) {
        failf(data, "server certificate expiration date has passed.");
        *certverifyresult = GNUTLS_CERT_EXPIRED;
        gnutls_x509_crt_deinit(x509_cert);
        return CURLE_PEER_FAILED_VERIFICATION;
      }
      else
        infof(data, "  server certificate expiration date FAILED");
    }
    else
      infof(data, "  server certificate expiration date OK");
  }

  certclock = gnutls_x509_crt_get_activation_time(x509_cert);

  if(certclock == (time_t)-1) {
    if(config->verifypeer) {
      failf(data, "server cert activation date verify failed");
      *certverifyresult = GNUTLS_CERT_NOT_ACTIVATED;
      gnutls_x509_crt_deinit(x509_cert);
      return CURLE_SSL_CONNECT_ERROR;
    }
    else
      infof(data, "  server certificate activation date verify FAILED");
  }
  else {
    if(certclock > time(NULL)) {
      if(config->verifypeer) {
        failf(data, "server certificate not activated yet.");
        *certverifyresult = GNUTLS_CERT_NOT_ACTIVATED;
        gnutls_x509_crt_deinit(x509_cert);
        return CURLE_PEER_FAILED_VERIFICATION;
      }
      else
        infof(data, "  server certificate activation date FAILED");
    }
    else
      infof(data, "  server certificate activation date OK");
  }

  if(pinned_key) {
    result = pkp_pin_peer_pubkey(data, x509_cert, pinned_key);
    if(result != CURLE_OK) {
      failf(data, "SSL: public key does not match pinned public key");
      gnutls_x509_crt_deinit(x509_cert);
      return result;
    }
  }

  /* Show:

  - subject
  - start date
  - expire date
  - common name
  - issuer

  */

#ifndef CURL_DISABLE_VERBOSE_STRINGS
  /* public key algorithm's parameters */
  algo = gnutls_x509_crt_get_pk_algorithm(x509_cert, &bits);
  infof(data, "  certificate public key: %s",
        gnutls_pk_algorithm_get_name(algo));

  /* version of the X.509 certificate. */
  infof(data, "  certificate version: #%d",
        gnutls_x509_crt_get_version(x509_cert));


  rc = gnutls_x509_crt_get_dn2(x509_cert, &certfields);
  if(rc)
    infof(data, "Failed to get certificate name");
  else {
    infof(data, "  subject: %s", certfields.data);

    certclock = gnutls_x509_crt_get_activation_time(x509_cert);
    showtime(data, "start date", certclock);

    certclock = gnutls_x509_crt_get_expiration_time(x509_cert);
    showtime(data, "expire date", certclock);

    gnutls_free(certfields.data);
  }

  rc = gnutls_x509_crt_get_issuer_dn2(x509_cert, &certfields);
  if(rc)
    infof(data, "Failed to get certificate issuer");
  else {
    infof(data, "  issuer: %s", certfields.data);

    gnutls_free(certfields.data);
  }
#endif

  gnutls_x509_crt_deinit(x509_cert);

  return result;
}

static CURLcode gtls_verifyserver(struct Curl_cfilter *cf,
                                  struct Curl_easy *data,
                                  gnutls_session_t session)
{
  struct ssl_connect_data *connssl = cf->ctx;
  struct ssl_primary_config *conn_config = Curl_ssl_cf_get_primary_config(cf);
  struct ssl_config_data *ssl_config = Curl_ssl_cf_get_config(cf, data);
  const char *pinned_key = Curl_ssl_cf_is_proxy(cf)?
    data->set.str[STRING_SSL_PINNEDPUBLICKEY_PROXY]:
    data->set.str[STRING_SSL_PINNEDPUBLICKEY];
  CURLcode result;

  result = Curl_gtls_verifyserver(data, session, conn_config, ssl_config,
                                  connssl->hostname, connssl->dispname,
                                  pinned_key);
  if(result)
    goto out;

  if(cf->conn->bits.tls_enable_alpn) {
    gnutls_datum_t proto;
    int rc;

    rc = gnutls_alpn_get_selected_protocol(session, &proto);
    if(rc == 0)
      Curl_alpn_set_negotiated(cf, data, proto.data, proto.size);
    else
      Curl_alpn_set_negotiated(cf, data, NULL, 0);
  }

  if(ssl_config->primary.sessionid) {
    /* we always unconditionally get the session id here, as even if we
       already got it from the cache and asked to use it in the connection, it
       might've been rejected and then a new one is in use now and we need to
       detect that. */
    void *connect_sessionid;
    size_t connect_idsize = 0;

    /* get the session ID data size */
    gnutls_session_get_data(session, NULL, &connect_idsize);
    connect_sessionid = malloc(connect_idsize); /* get a buffer for it */

    if(connect_sessionid) {
      bool incache;
      bool added = FALSE;
      void *ssl_sessionid;

      /* extract session ID to the allocated buffer */
      gnutls_session_get_data(session, connect_sessionid, &connect_idsize);

      Curl_ssl_sessionid_lock(data);
      incache = !(Curl_ssl_getsessionid(cf, data, &ssl_sessionid, NULL));
      if(incache) {
        /* there was one before in the cache, so instead of risking that the
           previous one was rejected, we just kill that and store the new */
        Curl_ssl_delsessionid(data, ssl_sessionid);
      }

      /* store this session id */
      result = Curl_ssl_addsessionid(cf, data, connect_sessionid,
                                     connect_idsize, &added);
      Curl_ssl_sessionid_unlock(data);
      if(!added)
        free(connect_sessionid);
      if(result) {
        result = CURLE_OUT_OF_MEMORY;
      }
    }
    else
      result = CURLE_OUT_OF_MEMORY;
  }

out:
  return result;
}

/*
 * This function is called after the TCP connect has completed. Setup the TLS
 * layer and do all necessary magic.
 */
/* We use connssl->connecting_state to keep track of the connection status;
   there are three states: 'ssl_connect_1' (not started yet or complete),
   'ssl_connect_2_reading' (waiting for data from server), and
   'ssl_connect_2_writing' (waiting to be able to write).
 */
static CURLcode
gtls_connect_common(struct Curl_cfilter *cf,
                    struct Curl_easy *data,
                    bool nonblocking,
                    bool *done)
{
  struct ssl_connect_data *connssl = cf->ctx;
  int rc;
  CURLcode result = CURLE_OK;

  /* Initiate the connection, if not already done */
  if(ssl_connect_1 == connssl->connecting_state) {
    rc = gtls_connect_step1(cf, data);
    if(rc) {
      result = rc;
      goto out;
    }
  }

  rc = handshake(cf, data, TRUE, nonblocking);
  if(rc) {
    /* handshake() sets its own error message with failf() */
    result = rc;
    goto out;
  }

  /* Finish connecting once the handshake is done */
  if(ssl_connect_1 == connssl->connecting_state) {
    struct ssl_backend_data *backend = connssl->backend;
    gnutls_session_t session;
    DEBUGASSERT(backend);
    session = backend->gtls.session;
    rc = gtls_verifyserver(cf, data, session);
    if(rc) {
      result = rc;
      goto out;
    }
    connssl->state = ssl_connection_complete;
  }

out:
  *done = ssl_connect_1 == connssl->connecting_state;

  return result;
}

static CURLcode gtls_connect_nonblocking(struct Curl_cfilter *cf,
                                         struct Curl_easy *data,
                                         bool *done)
{
  return gtls_connect_common(cf, data, TRUE, done);
}

static CURLcode gtls_connect(struct Curl_cfilter *cf,
                             struct Curl_easy *data)
{
  CURLcode result;
  bool done = FALSE;

  result = gtls_connect_common(cf, data, FALSE, &done);
  if(result)
    return result;

  DEBUGASSERT(done);

  return CURLE_OK;
}

static bool gtls_data_pending(struct Curl_cfilter *cf,
                              const struct Curl_easy *data)
{
  struct ssl_connect_data *ctx = cf->ctx;

  (void)data;
  DEBUGASSERT(ctx && ctx->backend);
  if(ctx->backend->gtls.session &&
     0 != gnutls_record_check_pending(ctx->backend->gtls.session))
    return TRUE;
  return FALSE;
}

static ssize_t gtls_send(struct Curl_cfilter *cf,
                         struct Curl_easy *data,
                         const void *mem,
                         size_t len,
                         CURLcode *curlcode)
{
  struct ssl_connect_data *connssl = cf->ctx;
  struct ssl_backend_data *backend = connssl->backend;
  ssize_t rc;

  (void)data;
  DEBUGASSERT(backend);
  rc = gnutls_record_send(backend->gtls.session, mem, len);

  if(rc < 0) {
    *curlcode = (rc == GNUTLS_E_AGAIN)
      ? CURLE_AGAIN
      : CURLE_SEND_ERROR;

    rc = -1;
  }

  return rc;
}

static void gtls_close(struct Curl_cfilter *cf,
                       struct Curl_easy *data)
{
  struct ssl_connect_data *connssl = cf->ctx;
  struct ssl_backend_data *backend = connssl->backend;

  (void) data;
  DEBUGASSERT(backend);

  if(backend->gtls.session) {
    char buf[32];
    /* Maybe the server has already sent a close notify alert.
       Read it to avoid an RST on the TCP connection. */
    (void)gnutls_record_recv(backend->gtls.session, buf, sizeof(buf));
    gnutls_bye(backend->gtls.session, GNUTLS_SHUT_WR);
    gnutls_deinit(backend->gtls.session);
    backend->gtls.session = NULL;
  }
  if(backend->gtls.cred) {
    gnutls_certificate_free_credentials(backend->gtls.cred);
    backend->gtls.cred = NULL;
  }
#ifdef USE_GNUTLS_SRP
  if(backend->gtls.srp_client_cred) {
    gnutls_srp_free_client_credentials(backend->gtls.srp_client_cred);
    backend->gtls.srp_client_cred = NULL;
  }
#endif
}

/*
 * This function is called to shut down the SSL layer but keep the
 * socket open (CCC - Clear Command Channel)
 */
static int gtls_shutdown(struct Curl_cfilter *cf,
                         struct Curl_easy *data)
{
  struct ssl_connect_data *connssl = cf->ctx;
  struct ssl_config_data *ssl_config = Curl_ssl_cf_get_config(cf, data);
  struct ssl_backend_data *backend = connssl->backend;
  int retval = 0;

  DEBUGASSERT(backend);

#ifndef CURL_DISABLE_FTP
  /* This has only been tested on the proftpd server, and the mod_tls code
     sends a close notify alert without waiting for a close notify alert in
     response. Thus we wait for a close notify alert from the server, but
     we do not send one. Let's hope other servers do the same... */

  if(data->set.ftp_ccc == CURLFTPSSL_CCC_ACTIVE)
    gnutls_bye(backend->gtls.session, GNUTLS_SHUT_WR);
#endif

  if(backend->gtls.session) {
    ssize_t result;
    bool done = FALSE;
    char buf[120];

    while(!done) {
      int what = SOCKET_READABLE(Curl_conn_cf_get_socket(cf, data),
                                 SSL_SHUTDOWN_TIMEOUT);
      if(what > 0) {
        /* Something to read, let's do it and hope that it is the close
           notify alert from the server */
        result = gnutls_record_recv(backend->gtls.session,
                                    buf, sizeof(buf));
        switch(result) {
        case 0:
          /* This is the expected response. There was no data but only
             the close notify alert */
          done = TRUE;
          break;
        case GNUTLS_E_AGAIN:
        case GNUTLS_E_INTERRUPTED:
          infof(data, "GNUTLS_E_AGAIN || GNUTLS_E_INTERRUPTED");
          break;
        default:
          retval = -1;
          done = TRUE;
          break;
        }
      }
      else if(0 == what) {
        /* timeout */
        failf(data, "SSL shutdown timeout");
        done = TRUE;
      }
      else {
        /* anything that gets here is fatally bad */
        failf(data, "select/poll on SSL socket, errno: %d", SOCKERRNO);
        retval = -1;
        done = TRUE;
      }
    }
    gnutls_deinit(backend->gtls.session);
  }
  gnutls_certificate_free_credentials(backend->gtls.cred);

#ifdef USE_GNUTLS_SRP
  if(ssl_config->primary.username)
    gnutls_srp_free_client_credentials(backend->gtls.srp_client_cred);
#endif

  backend->gtls.cred = NULL;
  backend->gtls.session = NULL;

  return retval;
}

static ssize_t gtls_recv(struct Curl_cfilter *cf,
                         struct Curl_easy *data,
                         char *buf,
                         size_t buffersize,
                         CURLcode *curlcode)
{
  struct ssl_connect_data *connssl = cf->ctx;
  struct ssl_backend_data *backend = connssl->backend;
  ssize_t ret;

  (void)data;
  DEBUGASSERT(backend);

  ret = gnutls_record_recv(backend->gtls.session, buf, buffersize);
  if((ret == GNUTLS_E_AGAIN) || (ret == GNUTLS_E_INTERRUPTED)) {
    *curlcode = CURLE_AGAIN;
    ret = -1;
    goto out;
  }

  if(ret == GNUTLS_E_REHANDSHAKE) {
    /* BLOCKING call, this is bad but a work-around for now. Fixing this "the
       proper way" takes a whole lot of work. */
    CURLcode result = handshake(cf, data, FALSE, FALSE);
    if(result)
      /* handshake() writes error message on its own */
      *curlcode = result;
    else
      *curlcode = CURLE_AGAIN; /* then return as if this was a wouldblock */
    ret = -1;
    goto out;
  }

  if(ret < 0) {
    failf(data, "GnuTLS recv error (%d): %s",

          (int)ret, gnutls_strerror((int)ret));
    *curlcode = CURLE_RECV_ERROR;
    ret = -1;
    goto out;
  }

out:
  return ret;
}

static void gtls_session_free(void *ptr)
{
  free(ptr);
}

static size_t gtls_version(char *buffer, size_t size)
{
  return msnprintf(buffer, size, "GnuTLS/%s", gnutls_check_version(NULL));
}

/* data might be NULL! */
static CURLcode gtls_random(struct Curl_easy *data,
                            unsigned char *entropy, size_t length)
{
  int rc;
  (void)data;
  rc = gnutls_rnd(GNUTLS_RND_RANDOM, entropy, length);
  return rc?CURLE_FAILED_INIT:CURLE_OK;
}

static CURLcode gtls_sha256sum(const unsigned char *tmp, /* input */
                               size_t tmplen,
                               unsigned char *sha256sum, /* output */
                               size_t sha256len)
{
  struct sha256_ctx SHA256pw;
  sha256_init(&SHA256pw);
  sha256_update(&SHA256pw, (unsigned int)tmplen, tmp);
  sha256_digest(&SHA256pw, (unsigned int)sha256len, sha256sum);
  return CURLE_OK;
}

static bool gtls_cert_status_request(void)
{
  return TRUE;
}

static void *gtls_get_internals(struct ssl_connect_data *connssl,
                                CURLINFO info UNUSED_PARAM)
{
  struct ssl_backend_data *backend = connssl->backend;
  (void)info;
  DEBUGASSERT(backend);
  return backend->gtls.session;
}

const struct Curl_ssl Curl_ssl_gnutls = {
  { CURLSSLBACKEND_GNUTLS, "gnutls" }, /* info */

  SSLSUPP_CA_PATH  |
  SSLSUPP_CERTINFO |
  SSLSUPP_PINNEDPUBKEY |
  SSLSUPP_HTTPS_PROXY,

  sizeof(struct ssl_backend_data),

  gtls_init,                     /* init */
  gtls_cleanup,                  /* cleanup */
  gtls_version,                  /* version */
  Curl_none_check_cxn,           /* check_cxn */
  gtls_shutdown,                 /* shutdown */
  gtls_data_pending,             /* data_pending */
  gtls_random,                   /* random */
  gtls_cert_status_request,      /* cert_status_request */
  gtls_connect,                  /* connect */
  gtls_connect_nonblocking,      /* connect_nonblocking */
  Curl_ssl_get_select_socks,              /* getsock */
  gtls_get_internals,            /* get_internals */
  gtls_close,                    /* close_one */
  Curl_none_close_all,           /* close_all */
  gtls_session_free,             /* session_free */
  Curl_none_set_engine,          /* set_engine */
  Curl_none_set_engine_default,  /* set_engine_default */
  Curl_none_engines_list,        /* engines_list */
  Curl_none_false_start,         /* false_start */
  gtls_sha256sum,                /* sha256sum */
  NULL,                          /* associate_connection */
  NULL,                          /* disassociate_connection */
  NULL,                          /* free_multi_ssl_backend_data */
  gtls_recv,                     /* recv decrypted data */
  gtls_send,                     /* send data to encrypt */
};

#endif /* USE_GNUTLS */