schannel.c

/***************************************************************************
 *                                  _   _ ____  _
 *  Project                     ___| | | |  _ \| |
 *                             / __| | | | |_) | |
 *                            | (__| |_| |  _ <| |___
 *                             \___|\___/|_| \_\_____|
 *
 * Copyright (C) 2012 - 2016, Marc Hoersken, <info@marc-hoersken.de>
 * Copyright (C) 2012, Mark Salisbury, <mark.salisbury@hp.com>
 * Copyright (C) 2012 - 2017, 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.haxx.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.
 *
 ***************************************************************************/

/*
 * Source file for all SChannel-specific code for the TLS/SSL layer. No code
 * but vtls.c should ever call or use these functions.
 *
 */

/*
 * Based upon the PolarSSL implementation in polarssl.c and polarssl.h:
 *   Copyright (C) 2010, 2011, Hoi-Ho Chan, <hoiho.chan@gmail.com>
 *
 * Based upon the CyaSSL implementation in cyassl.c and cyassl.h:
 *   Copyright (C) 1998 - 2012, Daniel Stenberg, <daniel@haxx.se>, et al.
 *
 * Thanks for code and inspiration!
 */

#include "curl_setup.h"

#ifdef USE_SCHANNEL

#ifndef USE_WINDOWS_SSPI
#  error "Can't compile SCHANNEL support without SSPI."
#endif

#include "curl_sspi.h"
#include "schannel.h"
#include "vtls.h"
#include "sendf.h"
#include "connect.h" /* for the connect timeout */
#include "strerror.h"
#include "select.h" /* for the socket readyness */
#include "inet_pton.h" /* for IP addr SNI check */
#include "curl_multibyte.h"
#include "warnless.h"
#include "x509asn1.h"
#include "curl_printf.h"
#include "system_win32.h"
#include "hostcheck.h"

 /* The last #include file should be: */
#include "curl_memory.h"
#include "memdebug.h"

/* ALPN requires version 8.1 of the Windows SDK, which was
   shipped with Visual Studio 2013, aka _MSC_VER 1800:

   https://technet.microsoft.com/en-us/library/hh831771%28v=ws.11%29.aspx
*/
#if defined(_MSC_VER) && (_MSC_VER >= 1800) && !defined(_USING_V110_SDK71_)
#  define HAS_ALPN 1
#endif

/* Uncomment to force verbose output
 * #define infof(x, y, ...) printf(y, __VA_ARGS__)
 * #define failf(x, y, ...) printf(y, __VA_ARGS__)
 */

static Curl_recv schannel_recv;
static Curl_send schannel_send;

#ifdef _WIN32_WCE
static CURLcode verify_certificate(struct connectdata *conn, int sockindex);
#endif

static void InitSecBuffer(SecBuffer *buffer, unsigned long BufType,
                          void *BufDataPtr, unsigned long BufByteSize)
{
  buffer->cbBuffer = BufByteSize;
  buffer->BufferType = BufType;
  buffer->pvBuffer = BufDataPtr;
}

static void InitSecBufferDesc(SecBufferDesc *desc, SecBuffer *BufArr,
                              unsigned long NumArrElem)
{
  desc->ulVersion = SECBUFFER_VERSION;
  desc->pBuffers = BufArr;
  desc->cBuffers = NumArrElem;
}

static CURLcode
set_ssl_version_min_max(SCHANNEL_CRED *schannel_cred, struct connectdata *conn)
{
  struct Curl_easy *data = conn->data;
  long ssl_version = SSL_CONN_CONFIG(version);
  long ssl_version_max = SSL_CONN_CONFIG(version_max);
  long i = ssl_version;

  switch(ssl_version_max) {
    case CURL_SSLVERSION_MAX_NONE:
      ssl_version_max = ssl_version << 16;
      break;
    case CURL_SSLVERSION_MAX_DEFAULT:
      ssl_version_max = CURL_SSLVERSION_MAX_TLSv1_2;
      break;
  }
  for(; i <= (ssl_version_max >> 16); ++i) {
    switch(i) {
      case CURL_SSLVERSION_TLSv1_0:
        schannel_cred->grbitEnabledProtocols |= SP_PROT_TLS1_0_CLIENT;
        break;
      case CURL_SSLVERSION_TLSv1_1:
        schannel_cred->grbitEnabledProtocols |= SP_PROT_TLS1_1_CLIENT;
        break;
      case CURL_SSLVERSION_TLSv1_2:
        schannel_cred->grbitEnabledProtocols |= SP_PROT_TLS1_2_CLIENT;
        break;
      case CURL_SSLVERSION_TLSv1_3:
        failf(data, "Schannel: TLS 1.3 is not yet supported");
        return CURLE_SSL_CONNECT_ERROR;
    }
  }
  return CURLE_OK;
}

static CURLcode
schannel_connect_step1(struct connectdata *conn, int sockindex)
{
  ssize_t written = -1;
  struct Curl_easy *data = conn->data;
  struct ssl_connect_data *connssl = &conn->ssl[sockindex];
  SecBuffer outbuf;
  SecBufferDesc outbuf_desc;
  SecBuffer inbuf;
  SecBufferDesc inbuf_desc;
#ifdef HAS_ALPN
  unsigned char alpn_buffer[128];
#endif
  SCHANNEL_CRED schannel_cred;
  SECURITY_STATUS sspi_status = SEC_E_OK;
  struct curl_schannel_cred *old_cred = NULL;
  struct in_addr addr;
#ifdef ENABLE_IPV6
  struct in6_addr addr6;
#endif
  TCHAR *host_name;
  CURLcode result;
  const char * const hostname = SSL_IS_PROXY() ? conn->http_proxy.host.name :
    conn->host.name;

  infof(data, "schannel: SSL/TLS connection with %s port %hu (step 1/3)\n",
        hostname, conn->remote_port);

  if(Curl_verify_windows_version(5, 1, PLATFORM_WINNT,
                                 VERSION_LESS_THAN_EQUAL)) {
     /* SChannel in Windows XP (OS version 5.1) uses legacy handshakes and
        algorithms that may not be supported by all servers. */
     infof(data, "schannel: WinSSL version is old and may not be able to "
           "connect to some servers due to lack of SNI, algorithms, etc.\n");
  }

#ifdef HAS_ALPN
  /* ALPN is only supported on Windows 8.1 / Server 2012 R2 and above.
     Also it doesn't seem to be supported for Wine, see curl bug #983. */
  connssl->use_alpn = conn->bits.tls_enable_alpn &&
                      !GetProcAddress(GetModuleHandleA("ntdll"),
                                      "wine_get_version") &&
                      Curl_verify_windows_version(6, 3, PLATFORM_WINNT,
                                                  VERSION_GREATER_THAN_EQUAL);
#else
  connssl->use_alpn = false;
#endif

  connssl->cred = NULL;

  /* check for an existing re-usable credential handle */
  if(data->set.general_ssl.sessionid) {
    Curl_ssl_sessionid_lock(conn);
    if(!Curl_ssl_getsessionid(conn, (void **)&old_cred, NULL, sockindex)) {
      connssl->cred = old_cred;
      infof(data, "schannel: re-using existing credential handle\n");

      /* increment the reference counter of the credential/session handle */
      connssl->cred->refcount++;
      infof(data, "schannel: incremented credential handle refcount = %d\n",
            connssl->cred->refcount);
    }
    Curl_ssl_sessionid_unlock(conn);
  }

  if(!connssl->cred) {
    /* setup Schannel API options */
    memset(&schannel_cred, 0, sizeof(schannel_cred));
    schannel_cred.dwVersion = SCHANNEL_CRED_VERSION;

    if(conn->ssl_config.verifypeer) {
#ifdef _WIN32_WCE
      /* certificate validation on CE doesn't seem to work right; we'll
         do it following a more manual process. */
      schannel_cred.dwFlags = SCH_CRED_MANUAL_CRED_VALIDATION |
        SCH_CRED_IGNORE_NO_REVOCATION_CHECK |
        SCH_CRED_IGNORE_REVOCATION_OFFLINE;
#else
      schannel_cred.dwFlags = SCH_CRED_AUTO_CRED_VALIDATION;
      /* TODO s/data->set.ssl.no_revoke/SSL_SET_OPTION(no_revoke)/g */
      if(data->set.ssl.no_revoke)
        schannel_cred.dwFlags |= SCH_CRED_IGNORE_NO_REVOCATION_CHECK |
                                 SCH_CRED_IGNORE_REVOCATION_OFFLINE;
      else
        schannel_cred.dwFlags |= SCH_CRED_REVOCATION_CHECK_CHAIN;
#endif
      if(data->set.ssl.no_revoke)
        infof(data, "schannel: disabled server certificate revocation "
                    "checks\n");
      else
        infof(data, "schannel: checking server certificate revocation\n");
    }
    else {
      schannel_cred.dwFlags = SCH_CRED_MANUAL_CRED_VALIDATION |
        SCH_CRED_IGNORE_NO_REVOCATION_CHECK |
        SCH_CRED_IGNORE_REVOCATION_OFFLINE;
      infof(data, "schannel: disabled server certificate revocation checks\n");
    }

    if(!conn->ssl_config.verifyhost) {
      schannel_cred.dwFlags |= SCH_CRED_NO_SERVERNAME_CHECK;
      infof(data, "schannel: verifyhost setting prevents Schannel from "
            "comparing the supplied target name with the subject "
            "names in server certificates.\n");
    }

    switch(conn->ssl_config.version) {
    case CURL_SSLVERSION_DEFAULT:
    case CURL_SSLVERSION_TLSv1:
      schannel_cred.grbitEnabledProtocols = SP_PROT_TLS1_0_CLIENT |
        SP_PROT_TLS1_1_CLIENT |
        SP_PROT_TLS1_2_CLIENT;
      break;
    case CURL_SSLVERSION_TLSv1_0:
    case CURL_SSLVERSION_TLSv1_1:
    case CURL_SSLVERSION_TLSv1_2:
    case CURL_SSLVERSION_TLSv1_3:
      {
        result = set_ssl_version_min_max(&schannel_cred, conn);
        if(result != CURLE_OK)
          return result;
        break;
      }
    case CURL_SSLVERSION_SSLv3:
      schannel_cred.grbitEnabledProtocols = SP_PROT_SSL3_CLIENT;
      break;
    case CURL_SSLVERSION_SSLv2:
      schannel_cred.grbitEnabledProtocols = SP_PROT_SSL2_CLIENT;
      break;
    default:
      failf(data, "Unrecognized parameter passed via CURLOPT_SSLVERSION");
      return CURLE_SSL_CONNECT_ERROR;
    }

    /* allocate memory for the re-usable credential handle */
    connssl->cred = (struct curl_schannel_cred *)
      malloc(sizeof(struct curl_schannel_cred));
    if(!connssl->cred) {
      failf(data, "schannel: unable to allocate memory");
      return CURLE_OUT_OF_MEMORY;
    }
    memset(connssl->cred, 0, sizeof(struct curl_schannel_cred));
    connssl->cred->refcount = 1;

    /* https://msdn.microsoft.com/en-us/library/windows/desktop/aa374716.aspx
       */
    sspi_status =
      s_pSecFn->AcquireCredentialsHandle(NULL, (TCHAR *)UNISP_NAME,
                                         SECPKG_CRED_OUTBOUND, NULL,
                                         &schannel_cred, NULL, NULL,
                                         &connssl->cred->cred_handle,
                                         &connssl->cred->time_stamp);

    if(sspi_status != SEC_E_OK) {
      if(sspi_status == SEC_E_WRONG_PRINCIPAL)
        failf(data, "schannel: SNI or certificate check failed: %s",
              Curl_sspi_strerror(conn, sspi_status));
      else
        failf(data, "schannel: AcquireCredentialsHandle failed: %s",
              Curl_sspi_strerror(conn, sspi_status));
      Curl_safefree(connssl->cred);
      return CURLE_SSL_CONNECT_ERROR;
    }
  }

  /* Warn if SNI is disabled due to use of an IP address */
  if(Curl_inet_pton(AF_INET, hostname, &addr)
#ifdef ENABLE_IPV6
     || Curl_inet_pton(AF_INET6, hostname, &addr6)
#endif
    ) {
    infof(data, "schannel: using IP address, SNI is not supported by OS.\n");
  }

#ifdef HAS_ALPN
  if(connssl->use_alpn) {
    int cur = 0;
    int list_start_index = 0;
    unsigned int *extension_len = NULL;
    unsigned short* list_len = NULL;

    /* The first four bytes will be an unsigned int indicating number
       of bytes of data in the rest of the the buffer. */
    extension_len = (unsigned int *)(&alpn_buffer[cur]);
    cur += sizeof(unsigned int);

    /* The next four bytes are an indicator that this buffer will contain
       ALPN data, as opposed to NPN, for example. */
    *(unsigned int *)&alpn_buffer[cur] =
      SecApplicationProtocolNegotiationExt_ALPN;
    cur += sizeof(unsigned int);

    /* The next two bytes will be an unsigned short indicating the number
       of bytes used to list the preferred protocols. */
    list_len = (unsigned short*)(&alpn_buffer[cur]);
    cur += sizeof(unsigned short);

    list_start_index = cur;

#ifdef USE_NGHTTP2
    if(data->set.httpversion >= CURL_HTTP_VERSION_2) {
      memcpy(&alpn_buffer[cur], NGHTTP2_PROTO_ALPN, NGHTTP2_PROTO_ALPN_LEN);
      cur += NGHTTP2_PROTO_ALPN_LEN;
      infof(data, "schannel: ALPN, offering %s\n", NGHTTP2_PROTO_VERSION_ID);
    }
#endif

    alpn_buffer[cur++] = ALPN_HTTP_1_1_LENGTH;
    memcpy(&alpn_buffer[cur], ALPN_HTTP_1_1, ALPN_HTTP_1_1_LENGTH);
    cur += ALPN_HTTP_1_1_LENGTH;
    infof(data, "schannel: ALPN, offering %s\n", ALPN_HTTP_1_1);

    *list_len = curlx_uitous(cur - list_start_index);
    *extension_len = *list_len + sizeof(unsigned int) + sizeof(unsigned short);

    InitSecBuffer(&inbuf, SECBUFFER_APPLICATION_PROTOCOLS, alpn_buffer, cur);
    InitSecBufferDesc(&inbuf_desc, &inbuf, 1);
  }
  else
  {
    InitSecBuffer(&inbuf, SECBUFFER_EMPTY, NULL, 0);
    InitSecBufferDesc(&inbuf_desc, &inbuf, 1);
  }
#else /* HAS_ALPN */
  InitSecBuffer(&inbuf, SECBUFFER_EMPTY, NULL, 0);
  InitSecBufferDesc(&inbuf_desc, &inbuf, 1);
#endif

  /* setup output buffer */
  InitSecBuffer(&outbuf, SECBUFFER_EMPTY, NULL, 0);
  InitSecBufferDesc(&outbuf_desc, &outbuf, 1);

  /* setup request flags */
  connssl->req_flags = ISC_REQ_SEQUENCE_DETECT | ISC_REQ_REPLAY_DETECT |
    ISC_REQ_CONFIDENTIALITY | ISC_REQ_ALLOCATE_MEMORY |
    ISC_REQ_STREAM;

  /* allocate memory for the security context handle */
  connssl->ctxt = (struct curl_schannel_ctxt *)
    malloc(sizeof(struct curl_schannel_ctxt));
  if(!connssl->ctxt) {
    failf(data, "schannel: unable to allocate memory");
    return CURLE_OUT_OF_MEMORY;
  }
  memset(connssl->ctxt, 0, sizeof(struct curl_schannel_ctxt));

  host_name = Curl_convert_UTF8_to_tchar(hostname);
  if(!host_name)
    return CURLE_OUT_OF_MEMORY;

  /* Schannel InitializeSecurityContext:
     https://msdn.microsoft.com/en-us/library/windows/desktop/aa375924.aspx

     At the moment we don't pass inbuf unless we're using ALPN since we only
     use it for that, and Wine (for which we currently disable ALPN) is giving
     us problems with inbuf regardless. https://github.com/curl/curl/issues/983
  */
  sspi_status = s_pSecFn->InitializeSecurityContext(
    &connssl->cred->cred_handle, NULL, host_name, connssl->req_flags, 0, 0,
    (connssl->use_alpn ? &inbuf_desc : NULL),
    0, &connssl->ctxt->ctxt_handle,
    &outbuf_desc, &connssl->ret_flags, &connssl->ctxt->time_stamp);

  Curl_unicodefree(host_name);

  if(sspi_status != SEC_I_CONTINUE_NEEDED) {
    if(sspi_status == SEC_E_WRONG_PRINCIPAL)
      failf(data, "schannel: SNI or certificate check failed: %s",
            Curl_sspi_strerror(conn, sspi_status));
    else
      failf(data, "schannel: initial InitializeSecurityContext failed: %s",
            Curl_sspi_strerror(conn, sspi_status));
    Curl_safefree(connssl->ctxt);
    return CURLE_SSL_CONNECT_ERROR;
  }

  infof(data, "schannel: sending initial handshake data: "
        "sending %lu bytes...\n", outbuf.cbBuffer);

  /* send initial handshake data which is now stored in output buffer */
  result = Curl_write_plain(conn, conn->sock[sockindex], outbuf.pvBuffer,
                            outbuf.cbBuffer, &written);
  s_pSecFn->FreeContextBuffer(outbuf.pvBuffer);
  if((result != CURLE_OK) || (outbuf.cbBuffer != (size_t) written)) {
    failf(data, "schannel: failed to send initial handshake data: "
          "sent %zd of %lu bytes", written, outbuf.cbBuffer);
    return CURLE_SSL_CONNECT_ERROR;
  }

  infof(data, "schannel: sent initial handshake data: "
        "sent %zd bytes\n", written);

  connssl->recv_unrecoverable_err = CURLE_OK;
  connssl->recv_sspi_close_notify = false;
  connssl->recv_connection_closed = false;

  /* continue to second handshake step */
  connssl->connecting_state = ssl_connect_2;

  return CURLE_OK;
}

static CURLcode
schannel_connect_step2(struct connectdata *conn, int sockindex)
{
  int i;
  ssize_t nread = -1, written = -1;
  struct Curl_easy *data = conn->data;
  struct ssl_connect_data *connssl = &conn->ssl[sockindex];
  unsigned char *reallocated_buffer;
  size_t reallocated_length;
  SecBuffer outbuf[3];
  SecBufferDesc outbuf_desc;
  SecBuffer inbuf[2];
  SecBufferDesc inbuf_desc;
  SECURITY_STATUS sspi_status = SEC_E_OK;
  TCHAR *host_name;
  CURLcode result;
  bool doread;
  const char * const hostname = SSL_IS_PROXY() ? conn->http_proxy.host.name :
    conn->host.name;

  doread = (connssl->connecting_state != ssl_connect_2_writing) ? TRUE : FALSE;

  infof(data, "schannel: SSL/TLS connection with %s port %hu (step 2/3)\n",
        hostname, conn->remote_port);

  if(!connssl->cred || !connssl->ctxt)
    return CURLE_SSL_CONNECT_ERROR;

  /* buffer to store previously received and decrypted data */
  if(connssl->decdata_buffer == NULL) {
    connssl->decdata_offset = 0;
    connssl->decdata_length = CURL_SCHANNEL_BUFFER_INIT_SIZE;
    connssl->decdata_buffer = malloc(connssl->decdata_length);
    if(connssl->decdata_buffer == NULL) {
      failf(data, "schannel: unable to allocate memory");
      return CURLE_OUT_OF_MEMORY;
    }
  }

  /* buffer to store previously received and encrypted data */
  if(connssl->encdata_buffer == NULL) {
    connssl->encdata_offset = 0;
    connssl->encdata_length = CURL_SCHANNEL_BUFFER_INIT_SIZE;
    connssl->encdata_buffer = malloc(connssl->encdata_length);
    if(connssl->encdata_buffer == NULL) {
      failf(data, "schannel: unable to allocate memory");
      return CURLE_OUT_OF_MEMORY;
    }
  }

  /* if we need a bigger buffer to read a full message, increase buffer now */
  if(connssl->encdata_length - connssl->encdata_offset <
     CURL_SCHANNEL_BUFFER_FREE_SIZE) {
    /* increase internal encrypted data buffer */
    reallocated_length = connssl->encdata_offset +
      CURL_SCHANNEL_BUFFER_FREE_SIZE;
    reallocated_buffer = realloc(connssl->encdata_buffer,
                                 reallocated_length);

    if(reallocated_buffer == NULL) {
      failf(data, "schannel: unable to re-allocate memory");
      return CURLE_OUT_OF_MEMORY;
    }
    else {
      connssl->encdata_buffer = reallocated_buffer;
      connssl->encdata_length = reallocated_length;
    }
  }

  for(;;) {
    if(doread) {
      /* read encrypted handshake data from socket */
      result = Curl_read_plain(conn->sock[sockindex],
                               (char *) (connssl->encdata_buffer +
                                         connssl->encdata_offset),
                               connssl->encdata_length -
                               connssl->encdata_offset,
                               &nread);
      if(result == CURLE_AGAIN) {
        if(connssl->connecting_state != ssl_connect_2_writing)
          connssl->connecting_state = ssl_connect_2_reading;
        infof(data, "schannel: failed to receive handshake, "
              "need more data\n");
        return CURLE_OK;
      }
      else if((result != CURLE_OK) || (nread == 0)) {
        failf(data, "schannel: failed to receive handshake, "
              "SSL/TLS connection failed");
        return CURLE_SSL_CONNECT_ERROR;
      }

      /* increase encrypted data buffer offset */
      connssl->encdata_offset += nread;
    }

    infof(data, "schannel: encrypted data buffer: offset %zu length %zu\n",
          connssl->encdata_offset, connssl->encdata_length);

    /* setup input buffers */
    InitSecBuffer(&inbuf[0], SECBUFFER_TOKEN, malloc(connssl->encdata_offset),
                  curlx_uztoul(connssl->encdata_offset));
    InitSecBuffer(&inbuf[1], SECBUFFER_EMPTY, NULL, 0);
    InitSecBufferDesc(&inbuf_desc, inbuf, 2);

    /* setup output buffers */
    InitSecBuffer(&outbuf[0], SECBUFFER_TOKEN, NULL, 0);
    InitSecBuffer(&outbuf[1], SECBUFFER_ALERT, NULL, 0);
    InitSecBuffer(&outbuf[2], SECBUFFER_EMPTY, NULL, 0);
    InitSecBufferDesc(&outbuf_desc, outbuf, 3);

    if(inbuf[0].pvBuffer == NULL) {
      failf(data, "schannel: unable to allocate memory");
      return CURLE_OUT_OF_MEMORY;
    }

    /* copy received handshake data into input buffer */
    memcpy(inbuf[0].pvBuffer, connssl->encdata_buffer,
           connssl->encdata_offset);

    host_name = Curl_convert_UTF8_to_tchar(hostname);
    if(!host_name)
      return CURLE_OUT_OF_MEMORY;

    /* https://msdn.microsoft.com/en-us/library/windows/desktop/aa375924.aspx
       */
    sspi_status = s_pSecFn->InitializeSecurityContext(
      &connssl->cred->cred_handle, &connssl->ctxt->ctxt_handle,
      host_name, connssl->req_flags, 0, 0, &inbuf_desc, 0, NULL,
      &outbuf_desc, &connssl->ret_flags, &connssl->ctxt->time_stamp);

    Curl_unicodefree(host_name);

    /* free buffer for received handshake data */
    Curl_safefree(inbuf[0].pvBuffer);

    /* check if the handshake was incomplete */
    if(sspi_status == SEC_E_INCOMPLETE_MESSAGE) {
      connssl->connecting_state = ssl_connect_2_reading;
      infof(data, "schannel: received incomplete message, need more data\n");
      return CURLE_OK;
    }

    /* If the server has requested a client certificate, attempt to continue
       the handshake without one. This will allow connections to servers which
       request a client certificate but do not require it. */
    if(sspi_status == SEC_I_INCOMPLETE_CREDENTIALS &&
       !(connssl->req_flags & ISC_REQ_USE_SUPPLIED_CREDS)) {
      connssl->req_flags |= ISC_REQ_USE_SUPPLIED_CREDS;
      connssl->connecting_state = ssl_connect_2_writing;
      infof(data, "schannel: a client certificate has been requested\n");
      return CURLE_OK;
    }

    /* check if the handshake needs to be continued */
    if(sspi_status == SEC_I_CONTINUE_NEEDED || sspi_status == SEC_E_OK) {
      for(i = 0; i < 3; i++) {
        /* search for handshake tokens that need to be send */
        if(outbuf[i].BufferType == SECBUFFER_TOKEN && outbuf[i].cbBuffer > 0) {
          infof(data, "schannel: sending next handshake data: "
                "sending %lu bytes...\n", outbuf[i].cbBuffer);

          /* send handshake token to server */
          result = Curl_write_plain(conn, conn->sock[sockindex],
                                    outbuf[i].pvBuffer, outbuf[i].cbBuffer,
                                    &written);
          if((result != CURLE_OK) ||
             (outbuf[i].cbBuffer != (size_t) written)) {
            failf(data, "schannel: failed to send next handshake data: "
                  "sent %zd of %lu bytes", written, outbuf[i].cbBuffer);
            return CURLE_SSL_CONNECT_ERROR;
          }
        }

        /* free obsolete buffer */
        if(outbuf[i].pvBuffer != NULL) {
          s_pSecFn->FreeContextBuffer(outbuf[i].pvBuffer);
        }
      }
    }
    else {
      if(sspi_status == SEC_E_WRONG_PRINCIPAL)
        failf(data, "schannel: SNI or certificate check failed: %s",
              Curl_sspi_strerror(conn, sspi_status));
      else
        failf(data, "schannel: next InitializeSecurityContext failed: %s",
              Curl_sspi_strerror(conn, sspi_status));
      return CURLE_SSL_CONNECT_ERROR;
    }

    /* check if there was additional remaining encrypted data */
    if(inbuf[1].BufferType == SECBUFFER_EXTRA && inbuf[1].cbBuffer > 0) {
      infof(data, "schannel: encrypted data length: %lu\n", inbuf[1].cbBuffer);
      /*
        There are two cases where we could be getting extra data here:
        1) If we're renegotiating a connection and the handshake is already
        complete (from the server perspective), it can encrypted app data
        (not handshake data) in an extra buffer at this point.
        2) (sspi_status == SEC_I_CONTINUE_NEEDED) We are negotiating a
        connection and this extra data is part of the handshake.
        We should process the data immediately; waiting for the socket to
        be ready may fail since the server is done sending handshake data.
      */
      /* check if the remaining data is less than the total amount
         and therefore begins after the already processed data */
      if(connssl->encdata_offset > inbuf[1].cbBuffer) {
        memmove(connssl->encdata_buffer,
                (connssl->encdata_buffer + connssl->encdata_offset) -
                inbuf[1].cbBuffer, inbuf[1].cbBuffer);
        connssl->encdata_offset = inbuf[1].cbBuffer;
        if(sspi_status == SEC_I_CONTINUE_NEEDED) {
          doread = FALSE;
          continue;
        }
      }
    }
    else {
      connssl->encdata_offset = 0;
    }
    break;
  }

  /* check if the handshake needs to be continued */
  if(sspi_status == SEC_I_CONTINUE_NEEDED) {
    connssl->connecting_state = ssl_connect_2_reading;
    return CURLE_OK;
  }

  /* check if the handshake is complete */
  if(sspi_status == SEC_E_OK) {
    connssl->connecting_state = ssl_connect_3;
    infof(data, "schannel: SSL/TLS handshake complete\n");
  }

#ifdef _WIN32_WCE
  /* Windows CE doesn't do any server certificate validation.
     We have to do it manually. */
  if(conn->ssl_config.verifypeer)
    return verify_certificate(conn, sockindex);
#endif

  return CURLE_OK;
}

static CURLcode
schannel_connect_step3(struct connectdata *conn, int sockindex)
{
  CURLcode result = CURLE_OK;
  struct Curl_easy *data = conn->data;
  struct ssl_connect_data *connssl = &conn->ssl[sockindex];
  SECURITY_STATUS sspi_status = SEC_E_OK;
  CERT_CONTEXT *ccert_context = NULL;
  const char * const hostname = SSL_IS_PROXY() ? conn->http_proxy.host.name :
    conn->host.name;
#ifdef HAS_ALPN
  SecPkgContext_ApplicationProtocol alpn_result;
#endif

  DEBUGASSERT(ssl_connect_3 == connssl->connecting_state);

  infof(data, "schannel: SSL/TLS connection with %s port %hu (step 3/3)\n",
        hostname, conn->remote_port);

  if(!connssl->cred)
    return CURLE_SSL_CONNECT_ERROR;

  /* check if the required context attributes are met */
  if(connssl->ret_flags != connssl->req_flags) {
    if(!(connssl->ret_flags & ISC_RET_SEQUENCE_DETECT))
      failf(data, "schannel: failed to setup sequence detection");
    if(!(connssl->ret_flags & ISC_RET_REPLAY_DETECT))
      failf(data, "schannel: failed to setup replay detection");
    if(!(connssl->ret_flags & ISC_RET_CONFIDENTIALITY))
      failf(data, "schannel: failed to setup confidentiality");
    if(!(connssl->ret_flags & ISC_RET_ALLOCATED_MEMORY))
      failf(data, "schannel: failed to setup memory allocation");
    if(!(connssl->ret_flags & ISC_RET_STREAM))
      failf(data, "schannel: failed to setup stream orientation");
    return CURLE_SSL_CONNECT_ERROR;
  }

#ifdef HAS_ALPN
  if(connssl->use_alpn) {
    sspi_status = s_pSecFn->QueryContextAttributes(&connssl->ctxt->ctxt_handle,
      SECPKG_ATTR_APPLICATION_PROTOCOL, &alpn_result);

    if(sspi_status != SEC_E_OK) {
      failf(data, "schannel: failed to retrieve ALPN result");
      return CURLE_SSL_CONNECT_ERROR;
    }

    if(alpn_result.ProtoNegoStatus ==
       SecApplicationProtocolNegotiationStatus_Success) {

      infof(data, "schannel: ALPN, server accepted to use %.*s\n",
        alpn_result.ProtocolIdSize, alpn_result.ProtocolId);

#ifdef USE_NGHTTP2
      if(alpn_result.ProtocolIdSize == NGHTTP2_PROTO_VERSION_ID_LEN &&
         !memcmp(NGHTTP2_PROTO_VERSION_ID, alpn_result.ProtocolId,
          NGHTTP2_PROTO_VERSION_ID_LEN)) {
        conn->negnpn = CURL_HTTP_VERSION_2;
      }
      else
#endif
      if(alpn_result.ProtocolIdSize == ALPN_HTTP_1_1_LENGTH &&
         !memcmp(ALPN_HTTP_1_1, alpn_result.ProtocolId,
           ALPN_HTTP_1_1_LENGTH)) {
        conn->negnpn = CURL_HTTP_VERSION_1_1;
      }
    }
    else
      infof(data, "ALPN, server did not agree to a protocol\n");
  }
#endif

  /* save the current session data for possible re-use */
  if(data->set.general_ssl.sessionid) {
    bool incache;
    struct curl_schannel_cred *old_cred = NULL;

    Curl_ssl_sessionid_lock(conn);
    incache = !(Curl_ssl_getsessionid(conn, (void **)&old_cred, NULL,
                                      sockindex));
    if(incache) {
      if(old_cred != connssl->cred) {
        infof(data, "schannel: old credential handle is stale, removing\n");
        /* we're not taking old_cred ownership here, no refcount++ is needed */
        Curl_ssl_delsessionid(conn, (void *)old_cred);
        incache = FALSE;
      }
    }
    if(!incache) {
      result = Curl_ssl_addsessionid(conn, (void *)connssl->cred,
                                     sizeof(struct curl_schannel_cred),
                                     sockindex);
      if(result) {
        Curl_ssl_sessionid_unlock(conn);
        failf(data, "schannel: failed to store credential handle");
        return result;
      }
      else {
        /* this cred session is now also referenced by sessionid cache */
        connssl->cred->refcount++;
        infof(data, "schannel: stored credential handle in session cache\n");
      }
    }
    Curl_ssl_sessionid_unlock(conn);
  }

  if(data->set.ssl.certinfo) {
    sspi_status = s_pSecFn->QueryContextAttributes(&connssl->ctxt->ctxt_handle,
      SECPKG_ATTR_REMOTE_CERT_CONTEXT, &ccert_context);

    if((sspi_status != SEC_E_OK) || (ccert_context == NULL)) {
      failf(data, "schannel: failed to retrieve remote cert context");
      return CURLE_SSL_CONNECT_ERROR;
    }

    result = Curl_ssl_init_certinfo(data, 1);
    if(!result) {
      if(((ccert_context->dwCertEncodingType & X509_ASN_ENCODING) != 0) &&
         (ccert_context->cbCertEncoded > 0)) {

        const char *beg = (const char *) ccert_context->pbCertEncoded;
        const char *end = beg + ccert_context->cbCertEncoded;
        result = Curl_extract_certinfo(conn, 0, beg, end);
      }
    }
    CertFreeCertificateContext(ccert_context);
    if(result)
      return result;
  }

  connssl->connecting_state = ssl_connect_done;

  return CURLE_OK;
}

static CURLcode
schannel_connect_common(struct connectdata *conn, int sockindex,
                        bool nonblocking, bool *done)
{
  CURLcode result;
  struct Curl_easy *data = conn->data;
  struct ssl_connect_data *connssl = &conn->ssl[sockindex];
  curl_socket_t sockfd = conn->sock[sockindex];
  time_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) {
    /* check 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/TLS connection timeout");
      return CURLE_OPERATION_TIMEDOUT;
    }

    result = schannel_connect_step1(conn, sockindex);
    if(result)
      return result;
  }

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

    /* check 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/TLS 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/TLS 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/TLS 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.
     */
    result = schannel_connect_step2(conn, sockindex);
    if(result || (nonblocking &&
                  (ssl_connect_2 == connssl->connecting_state ||
                   ssl_connect_2_reading == connssl->connecting_state ||
                   ssl_connect_2_writing == connssl->connecting_state)))
      return result;

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

  if(ssl_connect_3 == connssl->connecting_state) {
    result = schannel_connect_step3(conn, sockindex);
    if(result)
      return result;
  }

  if(ssl_connect_done == connssl->connecting_state) {
    connssl->state = ssl_connection_complete;
    conn->recv[sockindex] = schannel_recv;
    conn->send[sockindex] = schannel_send;
    *done = TRUE;
  }
  else
    *done = FALSE;

  /* reset our connection state machine */
  connssl->connecting_state = ssl_connect_1;

  return CURLE_OK;
}

static ssize_t
schannel_send(struct connectdata *conn, int sockindex,
              const void *buf, size_t len, CURLcode *err)
{
  ssize_t written = -1;
  size_t data_len = 0;
  unsigned char *data = NULL;
  struct ssl_connect_data *connssl = &conn->ssl[sockindex];
  SecBuffer outbuf[4];
  SecBufferDesc outbuf_desc;
  SECURITY_STATUS sspi_status = SEC_E_OK;
  CURLcode result;

  /* check if the maximum stream sizes were queried */
  if(connssl->stream_sizes.cbMaximumMessage == 0) {
    sspi_status = s_pSecFn->QueryContextAttributes(
      &connssl->ctxt->ctxt_handle,
      SECPKG_ATTR_STREAM_SIZES,
      &connssl->stream_sizes);
    if(sspi_status != SEC_E_OK) {
      *err = CURLE_SEND_ERROR;
      return -1;
    }
  }

  /* check if the buffer is longer than the maximum message length */
  if(len > connssl->stream_sizes.cbMaximumMessage) {
    *err = CURLE_SEND_ERROR;
    return -1;
  }

  /* calculate the complete message length and allocate a buffer for it */
  data_len = connssl->stream_sizes.cbHeader + len +
    connssl->stream_sizes.cbTrailer;
  data = (unsigned char *) malloc(data_len);
  if(data == NULL) {
    *err = CURLE_OUT_OF_MEMORY;
    return -1;
  }

  /* setup output buffers (header, data, trailer, empty) */
  InitSecBuffer(&outbuf[0], SECBUFFER_STREAM_HEADER,
                data, connssl->stream_sizes.cbHeader);
  InitSecBuffer(&outbuf[1], SECBUFFER_DATA,
                data + connssl->stream_sizes.cbHeader, curlx_uztoul(len));
  InitSecBuffer(&outbuf[2], SECBUFFER_STREAM_TRAILER,
                data + connssl->stream_sizes.cbHeader + len,
                connssl->stream_sizes.cbTrailer);
  InitSecBuffer(&outbuf[3], SECBUFFER_EMPTY, NULL, 0);
  InitSecBufferDesc(&outbuf_desc, outbuf, 4);

  /* copy data into output buffer */
  memcpy(outbuf[1].pvBuffer, buf, len);

  /* https://msdn.microsoft.com/en-us/library/windows/desktop/aa375390.aspx */
  sspi_status = s_pSecFn->EncryptMessage(&connssl->ctxt->ctxt_handle, 0,
                                         &outbuf_desc, 0);

  /* check if the message was encrypted */
  if(sspi_status == SEC_E_OK) {
    written = 0;

    /* send the encrypted message including header, data and trailer */
    len = outbuf[0].cbBuffer + outbuf[1].cbBuffer + outbuf[2].cbBuffer;

    /*
      It's important to send the full message which includes the header,
      encrypted payload, and trailer.  Until the client receives all the
      data a coherent message has not been delivered and the client