curl/lib/vtls/mbedtls.c

/***************************************************************************
 *                                  _   _ ____  _
 *  Project                     ___| | | |  _ \| |
 *                             / __| | | | |_) | |
 *                            | (__| |_| |  _ <| |___
 *                             \___|\___/|_| \_\_____|
 *
 * Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
 * Copyright (C) Hoi-Ho Chan, <hoiho.chan@gmail.com>
 *
 * 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 mbedTLS-specific code for the TLS/SSL layer. No code
 * but vtls.c should ever call or use these functions.
 *
 */

#include "curl_setup.h"

#ifdef USE_MBEDTLS

/* Define this to enable lots of debugging for mbedTLS */
/* #define MBEDTLS_DEBUG */

#include <mbedtls/version.h>
#if MBEDTLS_VERSION_NUMBER >= 0x02040000
#include <mbedtls/net_sockets.h>
#else
#include <mbedtls/net.h>
#endif
#include <mbedtls/ssl.h>
#include <mbedtls/x509.h>

#include <mbedtls/error.h>
#include <mbedtls/entropy.h>
#include <mbedtls/ctr_drbg.h>
#include <mbedtls/sha256.h>

#if MBEDTLS_VERSION_MAJOR >= 2
#  ifdef MBEDTLS_DEBUG
#    include <mbedtls/debug.h>
#  endif
#endif

#include "urldata.h"
#include "sendf.h"
#include "inet_pton.h"
#include "mbedtls.h"
#include "vtls.h"
#include "vtls_int.h"
#include "parsedate.h"
#include "connect.h" /* for the connect timeout */
#include "select.h"
#include "multiif.h"
#include "mbedtls_threadlock.h"

/* The last 3 #include files should be in this order */
#include "curl_printf.h"
#include "curl_memory.h"
#include "memdebug.h"

/* ALPN for http2 */
#ifdef USE_HTTP2
#  undef HAS_ALPN
#  ifdef MBEDTLS_SSL_ALPN
#    define HAS_ALPN
#  endif
#endif

struct ssl_backend_data {
  mbedtls_ctr_drbg_context ctr_drbg;
  mbedtls_entropy_context entropy;
  mbedtls_ssl_context ssl;
  mbedtls_x509_crt cacert;
  mbedtls_x509_crt clicert;
#ifdef MBEDTLS_X509_CRL_PARSE_C
  mbedtls_x509_crl crl;
#endif
  mbedtls_pk_context pk;
  mbedtls_ssl_config config;
#ifdef HAS_ALPN
  const char *protocols[3];
#endif
};

/* apply threading? */
#if defined(USE_THREADS_POSIX) || defined(USE_THREADS_WIN32)
#define THREADING_SUPPORT
#endif

#ifndef MBEDTLS_ERROR_C
#define mbedtls_strerror(a,b,c) b[0] = 0
#endif

#if defined(THREADING_SUPPORT)
static mbedtls_entropy_context ts_entropy;

static int entropy_init_initialized = 0;

/* start of entropy_init_mutex() */
static void entropy_init_mutex(mbedtls_entropy_context *ctx)
{
  /* lock 0 = entropy_init_mutex() */
  Curl_mbedtlsthreadlock_lock_function(0);
  if(entropy_init_initialized == 0) {
    mbedtls_entropy_init(ctx);
    entropy_init_initialized = 1;
  }
  Curl_mbedtlsthreadlock_unlock_function(0);
}
/* end of entropy_init_mutex() */

/* start of entropy_func_mutex() */
static int entropy_func_mutex(void *data, unsigned char *output, size_t len)
{
  int ret;
  /* lock 1 = entropy_func_mutex() */
  Curl_mbedtlsthreadlock_lock_function(1);
  ret = mbedtls_entropy_func(data, output, len);
  Curl_mbedtlsthreadlock_unlock_function(1);

  return ret;
}
/* end of entropy_func_mutex() */

#endif /* THREADING_SUPPORT */

#ifdef MBEDTLS_DEBUG
static void mbed_debug(void *context, int level, const char *f_name,
                       int line_nb, const char *line)
{
  struct Curl_easy *data = NULL;

  if(!context)
    return;

  data = (struct Curl_easy *)context;

  infof(data, "%s", line);
  (void) level;
}
#else
#endif

static int bio_cf_write(void *bio, const unsigned char *buf, size_t blen)
{
  struct Curl_cfilter *cf = bio;
  struct Curl_easy *data = CF_DATA_CURRENT(cf);
  ssize_t nwritten;
  CURLcode result;

  DEBUGASSERT(data);
  nwritten = Curl_conn_cf_send(cf->next, data, (char *)buf, blen, &result);
  DEBUGF(LOG_CF(data, cf, "bio_cf_out_write(len=%zu) -> %zd, err=%d",
                blen, nwritten, result));
  if(nwritten < 0 && CURLE_AGAIN == result) {
    nwritten = MBEDTLS_ERR_SSL_WANT_WRITE;
  }
  return (int)nwritten;
}

static int bio_cf_read(void *bio, unsigned char *buf, size_t blen)
{
  struct Curl_cfilter *cf = bio;
  struct Curl_easy *data = CF_DATA_CURRENT(cf);
  ssize_t nread;
  CURLcode result;

  DEBUGASSERT(data);
  /* OpenSSL catches this case, so should we. */
  if(!buf)
    return 0;

  nread = Curl_conn_cf_recv(cf->next, data, (char *)buf, blen, &result);
  DEBUGF(LOG_CF(data, cf, "bio_cf_in_read(len=%zu) -> %zd, err=%d",
                blen, nread, result));
  if(nread < 0 && CURLE_AGAIN == result) {
    nread = MBEDTLS_ERR_SSL_WANT_READ;
  }
  return (int)nread;
}

/*
 *  profile
 */
static const mbedtls_x509_crt_profile mbedtls_x509_crt_profile_fr =
{
  /* Hashes from SHA-1 and above */
  MBEDTLS_X509_ID_FLAG(MBEDTLS_MD_SHA1) |
  MBEDTLS_X509_ID_FLAG(MBEDTLS_MD_RIPEMD160) |
  MBEDTLS_X509_ID_FLAG(MBEDTLS_MD_SHA224) |
  MBEDTLS_X509_ID_FLAG(MBEDTLS_MD_SHA256) |
  MBEDTLS_X509_ID_FLAG(MBEDTLS_MD_SHA384) |
  MBEDTLS_X509_ID_FLAG(MBEDTLS_MD_SHA512),
  0xFFFFFFF, /* Any PK alg    */
  0xFFFFFFF, /* Any curve     */
  1024,      /* RSA min key len */
};

/* See https://tls.mbed.org/discussions/generic/
   howto-determine-exact-buffer-len-for-mbedtls_pk_write_pubkey_der
*/
#define RSA_PUB_DER_MAX_BYTES   (38 + 2 * MBEDTLS_MPI_MAX_SIZE)
#define ECP_PUB_DER_MAX_BYTES   (30 + 2 * MBEDTLS_ECP_MAX_BYTES)

#define PUB_DER_MAX_BYTES   (RSA_PUB_DER_MAX_BYTES > ECP_PUB_DER_MAX_BYTES ? \
                             RSA_PUB_DER_MAX_BYTES : ECP_PUB_DER_MAX_BYTES)

static CURLcode mbedtls_version_from_curl(int *mbedver, long version)
{
#if MBEDTLS_VERSION_NUMBER >= 0x03000000
  switch(version) {
    case CURL_SSLVERSION_TLSv1_0:
    case CURL_SSLVERSION_TLSv1_1:
    case CURL_SSLVERSION_TLSv1_2:
      *mbedver = MBEDTLS_SSL_MINOR_VERSION_3;
      return CURLE_OK;
    case CURL_SSLVERSION_TLSv1_3:
      break;
  }
#else
  switch(version) {
    case CURL_SSLVERSION_TLSv1_0:
      *mbedver = MBEDTLS_SSL_MINOR_VERSION_1;
      return CURLE_OK;
    case CURL_SSLVERSION_TLSv1_1:
      *mbedver = MBEDTLS_SSL_MINOR_VERSION_2;
      return CURLE_OK;
    case CURL_SSLVERSION_TLSv1_2:
      *mbedver = MBEDTLS_SSL_MINOR_VERSION_3;
      return CURLE_OK;
    case CURL_SSLVERSION_TLSv1_3:
      break;
  }
#endif

  return CURLE_SSL_CONNECT_ERROR;
}

static CURLcode
set_ssl_version_min_max(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);
#if MBEDTLS_VERSION_NUMBER >= 0x03000000
  int mbedtls_ver_min = MBEDTLS_SSL_MINOR_VERSION_3;
  int mbedtls_ver_max = MBEDTLS_SSL_MINOR_VERSION_3;
#else
  int mbedtls_ver_min = MBEDTLS_SSL_MINOR_VERSION_1;
  int mbedtls_ver_max = MBEDTLS_SSL_MINOR_VERSION_1;
#endif
  long ssl_version = conn_config->version;
  long ssl_version_max = conn_config->version_max;
  CURLcode result = CURLE_OK;

  DEBUGASSERT(backend);

  switch(ssl_version) {
    case CURL_SSLVERSION_DEFAULT:
    case CURL_SSLVERSION_TLSv1:
      ssl_version = CURL_SSLVERSION_TLSv1_0;
      break;
  }

  switch(ssl_version_max) {
    case CURL_SSLVERSION_MAX_NONE:
    case CURL_SSLVERSION_MAX_DEFAULT:
      ssl_version_max = CURL_SSLVERSION_MAX_TLSv1_2;
      break;
  }

  result = mbedtls_version_from_curl(&mbedtls_ver_min, ssl_version);
  if(result) {
    failf(data, "unsupported min version passed via CURLOPT_SSLVERSION");
    return result;
  }
  result = mbedtls_version_from_curl(&mbedtls_ver_max, ssl_version_max >> 16);
  if(result) {
    failf(data, "unsupported max version passed via CURLOPT_SSLVERSION");
    return result;
  }

  mbedtls_ssl_conf_min_version(&backend->config, MBEDTLS_SSL_MAJOR_VERSION_3,
                               mbedtls_ver_min);
  mbedtls_ssl_conf_max_version(&backend->config, MBEDTLS_SSL_MAJOR_VERSION_3,
                               mbedtls_ver_max);

  return result;
}

static CURLcode
mbed_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);
  const struct curl_blob *ca_info_blob = conn_config->ca_info_blob;
  struct ssl_config_data *ssl_config = Curl_ssl_cf_get_config(cf, data);
  const char * const ssl_cafile =
    /* CURLOPT_CAINFO_BLOB overrides CURLOPT_CAINFO */
    (ca_info_blob ? NULL : conn_config->CAfile);
  const bool verifypeer = conn_config->verifypeer;
  const char * const ssl_capath = conn_config->CApath;
  char * const ssl_cert = ssl_config->primary.clientcert;
  const struct curl_blob *ssl_cert_blob = ssl_config->primary.cert_blob;
  const char * const ssl_crlfile = ssl_config->primary.CRLfile;
  const char *hostname = connssl->hostname;
  int ret = -1;
  char errorbuf[128];

  DEBUGASSERT(backend);

  if((conn_config->version == CURL_SSLVERSION_SSLv2) ||
     (conn_config->version == CURL_SSLVERSION_SSLv3)) {
    failf(data, "Not supported SSL version");
    return CURLE_NOT_BUILT_IN;
  }

#ifdef THREADING_SUPPORT
  entropy_init_mutex(&ts_entropy);
  mbedtls_ctr_drbg_init(&backend->ctr_drbg);

  ret = mbedtls_ctr_drbg_seed(&backend->ctr_drbg, entropy_func_mutex,
                              &ts_entropy, NULL, 0);
  if(ret) {
    mbedtls_strerror(ret, errorbuf, sizeof(errorbuf));
    failf(data, "mbedtls_ctr_drbg_seed returned (-0x%04X) %s",
          -ret, errorbuf);
    return CURLE_FAILED_INIT;
  }
#else
  mbedtls_entropy_init(&backend->entropy);
  mbedtls_ctr_drbg_init(&backend->ctr_drbg);

  ret = mbedtls_ctr_drbg_seed(&backend->ctr_drbg, mbedtls_entropy_func,
                              &backend->entropy, NULL, 0);
  if(ret) {
    mbedtls_strerror(ret, errorbuf, sizeof(errorbuf));
    failf(data, "mbedtls_ctr_drbg_seed returned (-0x%04X) %s",
          -ret, errorbuf);
    return CURLE_FAILED_INIT;
  }
#endif /* THREADING_SUPPORT */

  /* Load the trusted CA */
  mbedtls_x509_crt_init(&backend->cacert);

  if(ca_info_blob && verifypeer) {
    /* Unfortunately, mbedtls_x509_crt_parse() requires the data to be null
       terminated even when provided the exact length, forcing us to waste
       extra memory here. */
    unsigned char *newblob = malloc(ca_info_blob->len + 1);
    if(!newblob)
      return CURLE_OUT_OF_MEMORY;
    memcpy(newblob, ca_info_blob->data, ca_info_blob->len);
    newblob[ca_info_blob->len] = 0; /* null terminate */
    ret = mbedtls_x509_crt_parse(&backend->cacert, newblob,
                                 ca_info_blob->len + 1);
    free(newblob);
    if(ret<0) {
      mbedtls_strerror(ret, errorbuf, sizeof(errorbuf));
      failf(data, "Error importing ca cert blob - mbedTLS: (-0x%04X) %s",
            -ret, errorbuf);
      return CURLE_SSL_CERTPROBLEM;
    }
  }

  if(ssl_cafile && verifypeer) {
#ifdef MBEDTLS_FS_IO
    ret = mbedtls_x509_crt_parse_file(&backend->cacert, ssl_cafile);

    if(ret<0) {
      mbedtls_strerror(ret, errorbuf, sizeof(errorbuf));
      failf(data, "Error reading ca cert file %s - mbedTLS: (-0x%04X) %s",
            ssl_cafile, -ret, errorbuf);
      return CURLE_SSL_CACERT_BADFILE;
    }
#else
    failf(data, "mbedtls: functions that use the filesystem not built in");
    return CURLE_NOT_BUILT_IN;
#endif
  }

  if(ssl_capath) {
#ifdef MBEDTLS_FS_IO
    ret = mbedtls_x509_crt_parse_path(&backend->cacert, ssl_capath);

    if(ret<0) {
      mbedtls_strerror(ret, errorbuf, sizeof(errorbuf));
      failf(data, "Error reading ca cert path %s - mbedTLS: (-0x%04X) %s",
            ssl_capath, -ret, errorbuf);

      if(verifypeer)
        return CURLE_SSL_CACERT_BADFILE;
    }
#else
    failf(data, "mbedtls: functions that use the filesystem not built in");
    return CURLE_NOT_BUILT_IN;
#endif
  }

  /* Load the client certificate */
  mbedtls_x509_crt_init(&backend->clicert);

  if(ssl_cert) {
#ifdef MBEDTLS_FS_IO
    ret = mbedtls_x509_crt_parse_file(&backend->clicert, ssl_cert);

    if(ret) {
      mbedtls_strerror(ret, errorbuf, sizeof(errorbuf));
      failf(data, "Error reading client cert file %s - mbedTLS: (-0x%04X) %s",
            ssl_cert, -ret, errorbuf);

      return CURLE_SSL_CERTPROBLEM;
    }
#else
    failf(data, "mbedtls: functions that use the filesystem not built in");
    return CURLE_NOT_BUILT_IN;
#endif
  }

  if(ssl_cert_blob) {
    /* Unfortunately, mbedtls_x509_crt_parse() requires the data to be null
       terminated even when provided the exact length, forcing us to waste
       extra memory here. */
    unsigned char *newblob = malloc(ssl_cert_blob->len + 1);
    if(!newblob)
      return CURLE_OUT_OF_MEMORY;
    memcpy(newblob, ssl_cert_blob->data, ssl_cert_blob->len);
    newblob[ssl_cert_blob->len] = 0; /* null terminate */
    ret = mbedtls_x509_crt_parse(&backend->clicert, newblob,
                                 ssl_cert_blob->len + 1);
    free(newblob);

    if(ret) {
      mbedtls_strerror(ret, errorbuf, sizeof(errorbuf));
      failf(data, "Error reading private key %s - mbedTLS: (-0x%04X) %s",
            ssl_config->key, -ret, errorbuf);
      return CURLE_SSL_CERTPROBLEM;
    }
  }

  /* Load the client private key */
  mbedtls_pk_init(&backend->pk);

  if(ssl_config->key || ssl_config->key_blob) {
    if(ssl_config->key) {
#ifdef MBEDTLS_FS_IO
#if MBEDTLS_VERSION_NUMBER >= 0x03000000
      ret = mbedtls_pk_parse_keyfile(&backend->pk, ssl_config->key,
                                     ssl_config->key_passwd,
                                     mbedtls_ctr_drbg_random,
                                     &backend->ctr_drbg);
#else
      ret = mbedtls_pk_parse_keyfile(&backend->pk, ssl_config->key,
                                     ssl_config->key_passwd);
#endif

      if(ret) {
        mbedtls_strerror(ret, errorbuf, sizeof(errorbuf));
        failf(data, "Error reading private key %s - mbedTLS: (-0x%04X) %s",
              ssl_config->key, -ret, errorbuf);
        return CURLE_SSL_CERTPROBLEM;
      }
#else
      failf(data, "mbedtls: functions that use the filesystem not built in");
      return CURLE_NOT_BUILT_IN;
#endif
    }
    else {
      const struct curl_blob *ssl_key_blob = ssl_config->key_blob;
      const unsigned char *key_data =
        (const unsigned char *)ssl_key_blob->data;
      const char *passwd = ssl_config->key_passwd;
#if MBEDTLS_VERSION_NUMBER >= 0x03000000
      ret = mbedtls_pk_parse_key(&backend->pk, key_data, ssl_key_blob->len,
                                 (const unsigned char *)passwd,
                                 passwd ? strlen(passwd) : 0,
                                 mbedtls_ctr_drbg_random,
                                 &backend->ctr_drbg);
#else
      ret = mbedtls_pk_parse_key(&backend->pk, key_data, ssl_key_blob->len,
                                 (const unsigned char *)passwd,
                                 passwd ? strlen(passwd) : 0);
#endif

      if(ret) {
        mbedtls_strerror(ret, errorbuf, sizeof(errorbuf));
        failf(data, "Error parsing private key - mbedTLS: (-0x%04X) %s",
              -ret, errorbuf);
        return CURLE_SSL_CERTPROBLEM;
      }
    }

    if(ret == 0 && !(mbedtls_pk_can_do(&backend->pk, MBEDTLS_PK_RSA) ||
                     mbedtls_pk_can_do(&backend->pk, MBEDTLS_PK_ECKEY)))
      ret = MBEDTLS_ERR_PK_TYPE_MISMATCH;
  }

  /* Load the CRL */
#ifdef MBEDTLS_X509_CRL_PARSE_C
  mbedtls_x509_crl_init(&backend->crl);

  if(ssl_crlfile) {
#ifdef MBEDTLS_FS_IO
    ret = mbedtls_x509_crl_parse_file(&backend->crl, ssl_crlfile);

    if(ret) {
      mbedtls_strerror(ret, errorbuf, sizeof(errorbuf));
      failf(data, "Error reading CRL file %s - mbedTLS: (-0x%04X) %s",
            ssl_crlfile, -ret, errorbuf);

      return CURLE_SSL_CRL_BADFILE;
    }
#else
    failf(data, "mbedtls: functions that use the filesystem not built in");
    return CURLE_NOT_BUILT_IN;
#endif
  }
#else
  if(ssl_crlfile) {
    failf(data, "mbedtls: crl support not built in");
    return CURLE_NOT_BUILT_IN;
  }
#endif

  infof(data, "mbedTLS: Connecting to %s:%d", hostname, connssl->port);

  mbedtls_ssl_config_init(&backend->config);
  ret = mbedtls_ssl_config_defaults(&backend->config,
                                    MBEDTLS_SSL_IS_CLIENT,
                                    MBEDTLS_SSL_TRANSPORT_STREAM,
                                    MBEDTLS_SSL_PRESET_DEFAULT);
  if(ret) {
    failf(data, "mbedTLS: ssl_config failed");
    return CURLE_SSL_CONNECT_ERROR;
  }

  mbedtls_ssl_init(&backend->ssl);
  if(mbedtls_ssl_setup(&backend->ssl, &backend->config)) {
    failf(data, "mbedTLS: ssl_init failed");
    return CURLE_SSL_CONNECT_ERROR;
  }

  /* new profile with RSA min key len = 1024 ... */
  mbedtls_ssl_conf_cert_profile(&backend->config,
                                &mbedtls_x509_crt_profile_fr);

  switch(conn_config->version) {
  case CURL_SSLVERSION_DEFAULT:
  case CURL_SSLVERSION_TLSv1:
#if MBEDTLS_VERSION_NUMBER < 0x03000000
    mbedtls_ssl_conf_min_version(&backend->config, MBEDTLS_SSL_MAJOR_VERSION_3,
                                 MBEDTLS_SSL_MINOR_VERSION_1);
    infof(data, "mbedTLS: Set min SSL version to TLS 1.0");
    break;
#endif
  case CURL_SSLVERSION_TLSv1_0:
  case CURL_SSLVERSION_TLSv1_1:
  case CURL_SSLVERSION_TLSv1_2:
  case CURL_SSLVERSION_TLSv1_3:
    {
      CURLcode result = set_ssl_version_min_max(cf, data);
      if(result != CURLE_OK)
        return result;
      break;
    }
  default:
    failf(data, "Unrecognized parameter passed via CURLOPT_SSLVERSION");
    return CURLE_SSL_CONNECT_ERROR;
  }

  mbedtls_ssl_conf_authmode(&backend->config, MBEDTLS_SSL_VERIFY_OPTIONAL);

  mbedtls_ssl_conf_rng(&backend->config, mbedtls_ctr_drbg_random,
                       &backend->ctr_drbg);
  mbedtls_ssl_set_bio(&backend->ssl, cf, bio_cf_write, bio_cf_read,
                      NULL /*  rev_timeout() */);

  mbedtls_ssl_conf_ciphersuites(&backend->config,
                                mbedtls_ssl_list_ciphersuites());

#if defined(MBEDTLS_SSL_RENEGOTIATION)
  mbedtls_ssl_conf_renegotiation(&backend->config,
                                 MBEDTLS_SSL_RENEGOTIATION_ENABLED);
#endif

#if defined(MBEDTLS_SSL_SESSION_TICKETS)
  mbedtls_ssl_conf_session_tickets(&backend->config,
                                   MBEDTLS_SSL_SESSION_TICKETS_DISABLED);
#endif

  /* Check if there's a cached ID we can/should use here! */
  if(ssl_config->primary.sessionid) {
    void *old_session = NULL;

    Curl_ssl_sessionid_lock(data);
    if(!Curl_ssl_getsessionid(cf, data, &old_session, NULL)) {
      ret = mbedtls_ssl_set_session(&backend->ssl, old_session);
      if(ret) {
        Curl_ssl_sessionid_unlock(data);
        failf(data, "mbedtls_ssl_set_session returned -0x%x", -ret);
        return CURLE_SSL_CONNECT_ERROR;
      }
      infof(data, "mbedTLS re-using session");
    }
    Curl_ssl_sessionid_unlock(data);
  }

  mbedtls_ssl_conf_ca_chain(&backend->config,
                            &backend->cacert,
#ifdef MBEDTLS_X509_CRL_PARSE_C
                            &backend->crl);
#else
                            NULL);
#endif

  if(ssl_config->key || ssl_config->key_blob) {
    mbedtls_ssl_conf_own_cert(&backend->config,
                              &backend->clicert, &backend->pk);
  }
  {
    char *snihost = Curl_ssl_snihost(data, hostname, NULL);
    if(!snihost || mbedtls_ssl_set_hostname(&backend->ssl, snihost)) {
      /* mbedtls_ssl_set_hostname() sets the name to use in CN/SAN checks and
         the name to set in the SNI extension. So even if curl connects to a
         host specified as an IP address, this function must be used. */
      failf(data, "Failed to set SNI");
      return CURLE_SSL_CONNECT_ERROR;
    }
  }

#ifdef HAS_ALPN
  if(connssl->alpn) {
    struct alpn_proto_buf proto;
    size_t i;

    for(i = 0; i < connssl->alpn->count; ++i) {
      backend->protocols[i] = connssl->alpn->entries[i];
    }
    /* this function doesn't clone the protocols array, which is why we need
       to keep it around */
    if(mbedtls_ssl_conf_alpn_protocols(&backend->config,
                                       &backend->protocols[0])) {
      failf(data, "Failed setting ALPN protocols");
      return CURLE_SSL_CONNECT_ERROR;
    }
    Curl_alpn_to_proto_str(&proto, connssl->alpn);
    infof(data, VTLS_INFOF_ALPN_OFFER_1STR, proto.data);
  }
#endif

#ifdef MBEDTLS_DEBUG
  /* In order to make that work in mbedtls MBEDTLS_DEBUG_C must be defined. */
  mbedtls_ssl_conf_dbg(&backend->config, mbed_debug, data);
  /* - 0 No debug
   * - 1 Error
   * - 2 State change
   * - 3 Informational
   * - 4 Verbose
   */
  mbedtls_debug_set_threshold(4);
#endif

  /* give application a chance to interfere with mbedTLS set up. */
  if(data->set.ssl.fsslctx) {
    ret = (*data->set.ssl.fsslctx)(data, &backend->config,
                                   data->set.ssl.fsslctxp);
    if(ret) {
      failf(data, "error signaled by ssl ctx callback");
      return ret;
    }
  }

  connssl->connecting_state = ssl_connect_2;

  return CURLE_OK;
}

static CURLcode
mbed_connect_step2(struct Curl_cfilter *cf, struct Curl_easy *data)
{
  int ret;
  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);
  const mbedtls_x509_crt *peercert;
  const char * const pinnedpubkey = Curl_ssl_cf_is_proxy(cf)?
    data->set.str[STRING_SSL_PINNEDPUBLICKEY_PROXY]:
    data->set.str[STRING_SSL_PINNEDPUBLICKEY];

  DEBUGASSERT(backend);

  ret = mbedtls_ssl_handshake(&backend->ssl);

  if(ret == MBEDTLS_ERR_SSL_WANT_READ) {
    connssl->connecting_state = ssl_connect_2_reading;
    return CURLE_OK;
  }
  else if(ret == MBEDTLS_ERR_SSL_WANT_WRITE) {
    connssl->connecting_state = ssl_connect_2_writing;
    return CURLE_OK;
  }
  else if(ret) {
    char errorbuf[128];
    mbedtls_strerror(ret, errorbuf, sizeof(errorbuf));
    failf(data, "ssl_handshake returned - mbedTLS: (-0x%04X) %s",
          -ret, errorbuf);
    return CURLE_SSL_CONNECT_ERROR;
  }

  infof(data, "mbedTLS: Handshake complete, cipher is %s",
        mbedtls_ssl_get_ciphersuite(&backend->ssl));

  ret = mbedtls_ssl_get_verify_result(&backend->ssl);

  if(!conn_config->verifyhost)
    /* Ignore hostname errors if verifyhost is disabled */
    ret &= ~MBEDTLS_X509_BADCERT_CN_MISMATCH;

  if(ret && conn_config->verifypeer) {
    if(ret & MBEDTLS_X509_BADCERT_EXPIRED)
      failf(data, "Cert verify failed: BADCERT_EXPIRED");

    else if(ret & MBEDTLS_X509_BADCERT_REVOKED)
      failf(data, "Cert verify failed: BADCERT_REVOKED");

    else if(ret & MBEDTLS_X509_BADCERT_CN_MISMATCH)
      failf(data, "Cert verify failed: BADCERT_CN_MISMATCH");

    else if(ret & MBEDTLS_X509_BADCERT_NOT_TRUSTED)
      failf(data, "Cert verify failed: BADCERT_NOT_TRUSTED");

    else if(ret & MBEDTLS_X509_BADCERT_FUTURE)
      failf(data, "Cert verify failed: BADCERT_FUTURE");

    return CURLE_PEER_FAILED_VERIFICATION;
  }

  peercert = mbedtls_ssl_get_peer_cert(&backend->ssl);

  if(peercert && data->set.verbose) {
    const size_t bufsize = 16384;
    char *buffer = malloc(bufsize);

    if(!buffer)
      return CURLE_OUT_OF_MEMORY;

    if(mbedtls_x509_crt_info(buffer, bufsize, "* ", peercert) > 0)
      infof(data, "Dumping cert info: %s", buffer);
    else
      infof(data, "Unable to dump certificate information");

    free(buffer);
  }

  if(pinnedpubkey) {
    int size;
    CURLcode result;
    mbedtls_x509_crt *p = NULL;
    unsigned char *pubkey = NULL;

#if MBEDTLS_VERSION_NUMBER == 0x03000000
    if(!peercert || !peercert->MBEDTLS_PRIVATE(raw).MBEDTLS_PRIVATE(p) ||
       !peercert->MBEDTLS_PRIVATE(raw).MBEDTLS_PRIVATE(len)) {
#else
    if(!peercert || !peercert->raw.p || !peercert->raw.len) {
#endif
      failf(data, "Failed due to missing peer certificate");
      return CURLE_SSL_PINNEDPUBKEYNOTMATCH;
    }

    p = calloc(1, sizeof(*p));

    if(!p)
      return CURLE_OUT_OF_MEMORY;

    pubkey = malloc(PUB_DER_MAX_BYTES);

    if(!pubkey) {
      result = CURLE_OUT_OF_MEMORY;
      goto pinnedpubkey_error;
    }

    mbedtls_x509_crt_init(p);

    /* Make a copy of our const peercert because mbedtls_pk_write_pubkey_der
       needs a non-const key, for now.
       https://github.com/ARMmbed/mbedtls/issues/396 */
#if MBEDTLS_VERSION_NUMBER == 0x03000000
    if(mbedtls_x509_crt_parse_der(p,
                        peercert->MBEDTLS_PRIVATE(raw).MBEDTLS_PRIVATE(p),
                        peercert->MBEDTLS_PRIVATE(raw).MBEDTLS_PRIVATE(len))) {
#else
    if(mbedtls_x509_crt_parse_der(p, peercert->raw.p, peercert->raw.len)) {
#endif
      failf(data, "Failed copying peer certificate");
      result = CURLE_SSL_PINNEDPUBKEYNOTMATCH;
      goto pinnedpubkey_error;
    }

#if MBEDTLS_VERSION_NUMBER == 0x03000000
    size = mbedtls_pk_write_pubkey_der(&p->MBEDTLS_PRIVATE(pk), pubkey,
                                       PUB_DER_MAX_BYTES);
#else
    size = mbedtls_pk_write_pubkey_der(&p->pk, pubkey, PUB_DER_MAX_BYTES);
#endif

    if(size <= 0) {
      failf(data, "Failed copying public key from peer certificate");
      result = CURLE_SSL_PINNEDPUBKEYNOTMATCH;
      goto pinnedpubkey_error;
    }

    /* mbedtls_pk_write_pubkey_der writes data at the end of the buffer. */
    result = Curl_pin_peer_pubkey(data,
                                  pinnedpubkey,
                                  &pubkey[PUB_DER_MAX_BYTES - size], size);
    pinnedpubkey_error:
    mbedtls_x509_crt_free(p);
    free(p);
    free(pubkey);
    if(result) {
      return result;
    }
  }

#ifdef HAS_ALPN
  if(connssl->alpn) {
    const char *proto = mbedtls_ssl_get_alpn_protocol(&backend->ssl);

    Curl_alpn_set_negotiated(cf, data, (const unsigned char *)proto,
                             proto? strlen(proto) : 0);
  }
#endif

  connssl->connecting_state = ssl_connect_3;
  infof(data, "SSL connected");

  return CURLE_OK;
}

static CURLcode
mbed_connect_step3(struct Curl_cfilter *cf, struct Curl_easy *data)
{
  CURLcode retcode = CURLE_OK;
  struct ssl_connect_data *connssl = cf->ctx;
  struct ssl_backend_data *backend = connssl->backend;
  struct ssl_config_data *ssl_config = Curl_ssl_cf_get_config(cf, data);

  DEBUGASSERT(ssl_connect_3 == connssl->connecting_state);
  DEBUGASSERT(backend);

  if(ssl_config->primary.sessionid) {
    int ret;
    mbedtls_ssl_session *our_ssl_sessionid;
    void *old_ssl_sessionid = NULL;
    bool added = FALSE;

    our_ssl_sessionid = malloc(sizeof(mbedtls_ssl_session));
    if(!our_ssl_sessionid)
      return CURLE_OUT_OF_MEMORY;

    mbedtls_ssl_session_init(our_ssl_sessionid);

    ret = mbedtls_ssl_get_session(&backend->ssl, our_ssl_sessionid);
    if(ret) {
      if(ret != MBEDTLS_ERR_SSL_ALLOC_FAILED)
        mbedtls_ssl_session_free(our_ssl_sessionid);
      free(our_ssl_sessionid);
      failf(data, "mbedtls_ssl_get_session returned -0x%x", -ret);
      return CURLE_SSL_CONNECT_ERROR;
    }

    /* If there's already a matching session in the cache, delete it */
    Curl_ssl_sessionid_lock(data);
    if(!Curl_ssl_getsessionid(cf, data, &old_ssl_sessionid, NULL))
      Curl_ssl_delsessionid(data, old_ssl_sessionid);

    retcode = Curl_ssl_addsessionid(cf, data, our_ssl_sessionid,
                                    0, &added);
    Curl_ssl_sessionid_unlock(data);
    if(!added) {
      mbedtls_ssl_session_free(our_ssl_sessionid);
      free(our_ssl_sessionid);
    }
    if(retcode) {
      failf(data, "failed to store ssl session");
      return retcode;
    }
  }

  connssl->connecting_state = ssl_connect_done;

  return CURLE_OK;
}

static ssize_t mbed_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;
  int ret = -1;

  (void)data;
  DEBUGASSERT(backend);
  ret = mbedtls_ssl_write(&backend->ssl, (unsigned char *)mem, len);

  if(ret < 0) {
    *curlcode = (ret == MBEDTLS_ERR_SSL_WANT_WRITE) ?
      CURLE_AGAIN : CURLE_SEND_ERROR;
    ret = -1;
  }

  return ret;
}

static void mbedtls_close_all(struct Curl_easy *data)
{
  (void)data;
}

static void mbedtls_close(struct Curl_cfilter *cf, struct Curl_easy *data)
{
  struct ssl_connect_data *connssl = cf->ctx;
  struct ssl_backend_data *backend = connssl->backend;
  char buf[32];

  (void)data;
  DEBUGASSERT(backend);

  /* Maybe the server has already sent a close notify alert.
     Read it to avoid an RST on the TCP connection. */
  (void)mbedtls_ssl_read(&backend->ssl, (unsigned char *)buf, sizeof(buf));

  mbedtls_pk_free(&backend->pk);
  mbedtls_x509_crt_free(&backend->clicert);
  mbedtls_x509_crt_free(&backend->cacert);
#ifdef MBEDTLS_X509_CRL_PARSE_C
  mbedtls_x509_crl_free(&backend->crl);
#endif
  mbedtls_ssl_config_free(&backend->config);
  mbedtls_ssl_free(&backend->ssl);
  mbedtls_ctr_drbg_free(&backend->ctr_drbg);
#ifndef THREADING_SUPPORT
  mbedtls_entropy_free(&backend->entropy);
#endif /* THREADING_SUPPORT */
}

static ssize_t mbed_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;
  int ret = -1;
  ssize_t len = -1;

  (void)data;
  DEBUGASSERT(backend);

  ret = mbedtls_ssl_read(&backend->ssl, (unsigned char *)buf,
                         buffersize);

  if(ret <= 0) {
    if(ret == MBEDTLS_ERR_SSL_PEER_CLOSE_NOTIFY)
      return 0;

    *curlcode = (ret == MBEDTLS_ERR_SSL_WANT_READ) ?
      CURLE_AGAIN : CURLE_RECV_ERROR;
    return -1;
  }

  len = ret;

  return len;
}

static void mbedtls_session_free(void *ptr)
{
  mbedtls_ssl_session_free(ptr);
  free(ptr);
}

static size_t mbedtls_version(char *buffer, size_t size)
{
#ifdef MBEDTLS_VERSION_C
  /* if mbedtls_version_get_number() is available it is better */
  unsigned int version = mbedtls_version_get_number();
  return msnprintf(buffer, size, "mbedTLS/%u.%u.%u", version>>24,
                   (version>>16)&0xff, (version>>8)&0xff);
#else
  return msnprintf(buffer, size, "mbedTLS/%s", MBEDTLS_VERSION_STRING);
#endif
}

static CURLcode mbedtls_random(struct Curl_easy *data,
                               unsigned char *entropy, size_t length)
{
#if defined(MBEDTLS_CTR_DRBG_C)
  int ret = -1;
  char errorbuf[128];
  mbedtls_entropy_context ctr_entropy;
  mbedtls_ctr_drbg_context ctr_drbg;
  mbedtls_entropy_init(&ctr_entropy);
  mbedtls_ctr_drbg_init(&ctr_drbg);

  ret = mbedtls_ctr_drbg_seed(&ctr_drbg, mbedtls_entropy_func,
                              &ctr_entropy, NULL, 0);

  if(ret) {
    mbedtls_strerror(ret, errorbuf, sizeof(errorbuf));
    failf(data, "mbedtls_ctr_drbg_seed returned (-0x%04X) %s",
          -ret, errorbuf);
  }
  else {
    ret = mbedtls_ctr_drbg_random(&ctr_drbg, entropy, length);

    if(ret) {
      mbedtls_strerror(ret, errorbuf, sizeof(errorbuf));
      failf(data, "mbedtls_ctr_drbg_random returned (-0x%04X) %s",
            -ret, errorbuf);
    }
  }

  mbedtls_ctr_drbg_free(&ctr_drbg);
  mbedtls_entropy_free(&ctr_entropy);

  return ret == 0 ? CURLE_OK : CURLE_FAILED_INIT;
#elif defined(MBEDTLS_HAVEGE_C)
  mbedtls_havege_state hs;
  mbedtls_havege_init(&hs);
  mbedtls_havege_random(&hs, entropy, length);
  mbedtls_havege_free(&hs);
  return CURLE_OK;
#else
  return CURLE_NOT_BUILT_IN;
#endif
}

static CURLcode
mbed_connect_common(struct Curl_cfilter *cf, struct Curl_easy *data,
                    bool nonblocking,
                    bool *done)
{
  CURLcode retcode;
  struct ssl_connect_data *connssl = cf->ctx;
  curl_socket_t sockfd = Curl_conn_cf_get_socket(cf, data);
  timediff_t timeout_ms;
  int what;

  /* check if the connection has already been established */
  if(ssl_connection_complete == connssl->state) {
    *done = TRUE;
    return CURLE_OK;
  }

  if(ssl_connect_1 == connssl->connecting_state) {
    /* Find out how much more time we're allowed */
    timeout_ms = Curl_timeleft(data, NULL, TRUE);

    if(timeout_ms < 0) {
      /* no need to continue if time already is up */
      failf(data, "SSL connection timeout");
      return CURLE_OPERATION_TIMEDOUT;
    }
    retcode = mbed_connect_step1(cf, data);
    if(retcode)
      return retcode;
  }

  while(ssl_connect_2 == connssl->connecting_state ||
        ssl_connect_2_reading == connssl->connecting_state ||
        ssl_connect_2_writing == connssl->connecting_state) {

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

    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) {

      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);
      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) {
          *done = FALSE;
          return CURLE_OK;
        }
        else {
          /* timeout */
          failf(data, "SSL connection timeout");
          return CURLE_OPERATION_TIMEDOUT;
        }
      }
      /* socket is readable or writable */
    }

    /* Run transaction, and return to the caller if it failed or if
     * this connection is part of a multi handle and this loop would
     * execute again. This permits the owner of a multi handle to
     * abort a connection attempt before step2 has completed while
     * ensuring that a client using select() or epoll() will always
     * have a valid fdset to wait on.
     */
    retcode = mbed_connect_step2(cf, data);
    if(retcode || (nonblocking &&
                   (ssl_connect_2 == connssl->connecting_state ||
                    ssl_connect_2_reading == connssl->connecting_state ||
                    ssl_connect_2_writing == connssl->connecting_state)))
      return retcode;

  } /* repeat step2 until all transactions are done. */

  if(ssl_connect_3 == connssl->connecting_state) {
    retcode = mbed_connect_step3(cf, data);
    if(retcode)
      return retcode;
  }

  if(ssl_connect_done == connssl->connecting_state) {
    connssl->state = ssl_connection_complete;
    *done = TRUE;
  }
  else
    *done = FALSE;

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

  return CURLE_OK;
}

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


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

  retcode = mbed_connect_common(cf, data, FALSE, &done);
  if(retcode)
    return retcode;

  DEBUGASSERT(done);

  return CURLE_OK;
}

/*
 * return 0 error initializing SSL
 * return 1 SSL initialized successfully
 */
static int mbedtls_init(void)
{
  return Curl_mbedtlsthreadlock_thread_setup();
}

static void mbedtls_cleanup(void)
{
  (void)Curl_mbedtlsthreadlock_thread_cleanup();
}

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

  (void)data;
  DEBUGASSERT(ctx && ctx->backend);
  return mbedtls_ssl_get_bytes_avail(&ctx->backend->ssl) != 0;
}

static CURLcode mbedtls_sha256sum(const unsigned char *input,
                                  size_t inputlen,
                                  unsigned char *sha256sum,
                                  size_t sha256len UNUSED_PARAM)
{
  /* TODO: explain this for different mbedtls 2.x vs 3 version */
  (void)sha256len;
#if MBEDTLS_VERSION_NUMBER < 0x02070000
  mbedtls_sha256(input, inputlen, sha256sum, 0);
#else
  /* returns 0 on success, otherwise failure */
#if MBEDTLS_VERSION_NUMBER >= 0x03000000
  if(mbedtls_sha256(input, inputlen, sha256sum, 0) != 0)
#else
  if(mbedtls_sha256_ret(input, inputlen, sha256sum, 0) != 0)
#endif
    return CURLE_BAD_FUNCTION_ARGUMENT;
#endif
  return CURLE_OK;
}

static void *mbedtls_get_internals(struct ssl_connect_data *connssl,
                                   CURLINFO info UNUSED_PARAM)
{
  struct ssl_backend_data *backend = connssl->backend;
  (void)info;
  DEBUGASSERT(backend);
  return &backend->ssl;
}

const struct Curl_ssl Curl_ssl_mbedtls = {
  { CURLSSLBACKEND_MBEDTLS, "mbedtls" }, /* info */

  SSLSUPP_CA_PATH |
  SSLSUPP_CAINFO_BLOB |
  SSLSUPP_PINNEDPUBKEY |
  SSLSUPP_SSL_CTX |
  SSLSUPP_HTTPS_PROXY,

  sizeof(struct ssl_backend_data),

  mbedtls_init,                     /* init */
  mbedtls_cleanup,                  /* cleanup */
  mbedtls_version,                  /* version */
  Curl_none_check_cxn,              /* check_cxn */
  Curl_none_shutdown,               /* shutdown */
  mbedtls_data_pending,             /* data_pending */
  mbedtls_random,                   /* random */
  Curl_none_cert_status_request,    /* cert_status_request */
  mbedtls_connect,                  /* connect */
  mbedtls_connect_nonblocking,      /* connect_nonblocking */
  Curl_ssl_get_select_socks,                 /* getsock */
  mbedtls_get_internals,            /* get_internals */
  mbedtls_close,                    /* close_one */
  mbedtls_close_all,                /* close_all */
  mbedtls_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 */
  mbedtls_sha256sum,                /* sha256sum */
  NULL,                             /* associate_connection */
  NULL,                             /* disassociate_connection */
  NULL,                             /* free_multi_ssl_backend_data */
  mbed_recv,                        /* recv decrypted data */
  mbed_send,                        /* send data to encrypt */
};

#endif /* USE_MBEDTLS */