/***************************************************************************
 *                                  _   _ ____  _
 *  Project                     ___| | | |  _ \| |
 *                             / __| | | | |_) | |
 *                            | (__| |_| |  _ <| |___
 *                             \___|\___/|_| \_\_____|
 *
 * Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
 * Copyright (C) Marc Hoersken, <info@marc-hoersken.de>
 * Copyright (C) Mark Salisbury, <mark.salisbury@hp.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 Schannel-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_SCHANNEL

#ifndef USE_WINDOWS_SSPI
#error "cannot compile Schannel support without SSPI."
#endif

#include "vtls/schannel.h"
#include "vtls/schannel_int.h"
#include "vtls/vtls.h"
#include "vtls/vtls_int.h"
#include "vtls/vtls_scache.h"
#include "curl_trc.h"
#include "connect.h" /* for the connect timeout */
#include "curlx/strdup.h"
#include "strerror.h"
#include "select.h" /* for the socket readiness */
#include "curlx/fopen.h"
#include "curlx/multibyte.h"
#include "vtls/x509asn1.h"
#include "system_win32.h"
#include "curlx/version_win32.h"
#include "rand.h"
#include "curlx/strparse.h"
#include "progress.h"
#include "curl_sha256.h"

/* Some verbose debug messages are wrapped by SCH_DEV() instead of DEBUGF()
 * and only shown if CURL_SCHANNEL_DEV_DEBUG was defined at build time. These
 * messages are extra verbose and intended for curl developers debugging
 * Schannel recv decryption and renegotiation.
 */
#ifdef CURL_SCHANNEL_DEV_DEBUG
#define SCH_DEV(x) x
#define SCH_DEV_SHOWBOOL(x)                                   \
  infof(data, "schannel: " #x " %s", (x) ? "TRUE" : "FALSE");
#else
#define SCH_DEV(x) do {} while(0)
#define SCH_DEV_SHOWBOOL(x) do {} while(0)
#endif

/* Offered by mingw-w64 v8+. MS SDK 7.0A+. */
#ifndef SP_PROT_TLS1_0_CLIENT
#define SP_PROT_TLS1_0_CLIENT           SP_PROT_TLS1_CLIENT
#endif
#ifndef SP_PROT_TLS1_1_CLIENT
#define SP_PROT_TLS1_1_CLIENT           0x00000200
#endif
#ifndef SP_PROT_TLS1_2_CLIENT
#define SP_PROT_TLS1_2_CLIENT           0x00000800
#endif

/* Offered by mingw-w64 v8+. MS SDK ~10+/~VS2017+. */
#ifndef SP_PROT_TLS1_3_CLIENT
#define SP_PROT_TLS1_3_CLIENT           0x00002000
#endif
#ifndef SCH_USE_STRONG_CRYPTO
#define SCH_USE_STRONG_CRYPTO           0x00400000
#endif

/* Offered by mingw-w64 v10+. MS SDK 7.0A+. */
#ifndef SECBUFFER_ALERT
#define SECBUFFER_ALERT                 17
#endif

/* Both schannel buffer sizes must be > 0 */
#define CURL_SCHANNEL_BUFFER_INIT_SIZE   4096
#define CURL_SCHANNEL_BUFFER_FREE_SIZE   1024

#define CERT_THUMBPRINT_STR_LEN 40
#define CERT_THUMBPRINT_DATA_LEN 20

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

/* Offered by mingw-w64 v4+. MS SDK 6.0A+. */
#ifndef PKCS12_NO_PERSIST_KEY
#define PKCS12_NO_PERSIST_KEY 0x00008000
#endif

/* Offered by mingw-w64 v4+. MS SDK ~10+/~VS2017+. */
#ifndef CERT_FIND_HAS_PRIVATE_KEY
#define CERT_FIND_HAS_PRIVATE_KEY (21 << CERT_COMPARE_SHIFT)
#endif

/* key to use at `multi->proto_hash` */
#define MPROTO_SCHANNEL_CERT_SHARE_KEY   "tls:schannel:cert:share"

/* ALPN requires version 8.1 of the Windows SDK, which was
   shipped with Visual Studio 2013, aka _MSC_VER 1800:
     https://learn.microsoft.com/previous-versions/windows/it-pro/windows-server-2012-R2-and-2012/hh831771
   Or mingw-w64 9.0 or upper.
*/
#if (defined(__MINGW64_VERSION_MAJOR) && __MINGW64_VERSION_MAJOR >= 9) || \
  (defined(_MSC_VER) && (_MSC_VER >= 1800) && !defined(_USING_V110_SDK71_))
#define HAS_ALPN_SCHANNEL
static bool s_win_has_alpn;
#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 schannel_set_ssl_version_min_max(DWORD *enabled_protocols,
                                                 struct Curl_cfilter *cf,
                                                 struct Curl_easy *data)
{
  struct ssl_primary_config *conn_config = Curl_ssl_cf_get_primary_config(cf);
  long ssl_version = conn_config->version;
  long ssl_version_max = (long)conn_config->version_max;
  long i = ssl_version;

  switch(ssl_version_max) {
  case CURL_SSLVERSION_MAX_NONE:
  case CURL_SSLVERSION_MAX_DEFAULT:

    /* Windows Server 2022 and newer (including Windows 11) support TLS 1.3
       built-in. Previous builds of Windows 10 had broken TLS 1.3
       implementations that could be enabled via registry.
    */
    if(curlx_verify_windows_version(10, 0, 20348, PLATFORM_WINNT,
                                    VERSION_GREATER_THAN_EQUAL)) {
      ssl_version_max = CURL_SSLVERSION_MAX_TLSv1_3;
    }
    else /* Windows 10 and older */
      ssl_version_max = CURL_SSLVERSION_MAX_TLSv1_2;

    break;
  }

  for(; i <= (ssl_version_max >> 16); ++i) {
    switch(i) {
    case CURL_SSLVERSION_TLSv1_0:
      (*enabled_protocols) |= SP_PROT_TLS1_0_CLIENT;
      break;
    case CURL_SSLVERSION_TLSv1_1:
      (*enabled_protocols) |= SP_PROT_TLS1_1_CLIENT;
      break;
    case CURL_SSLVERSION_TLSv1_2:
      (*enabled_protocols) |= SP_PROT_TLS1_2_CLIENT;
      break;
    case CURL_SSLVERSION_TLSv1_3:

      /* Windows Server 2022 and newer */
      if(curlx_verify_windows_version(10, 0, 20348, PLATFORM_WINNT,
                                      VERSION_GREATER_THAN_EQUAL)) {
        (*enabled_protocols) |= SP_PROT_TLS1_3_CLIENT;
        break;
      }
      else { /* Windows 10 and older */
        failf(data, "schannel: TLS 1.3 not supported on Windows prior to 11");
        return CURLE_SSL_CONNECT_ERROR;
      }
    }
  }
  return CURLE_OK;
}

#define CIPHEROPTION(x) { #x, x }

struct algo {
  const char *name;
  int id;
};

static const struct algo algs[] = {
  CIPHEROPTION(CALG_MD2),
  CIPHEROPTION(CALG_MD4),
  CIPHEROPTION(CALG_MD5),
  CIPHEROPTION(CALG_SHA),
  CIPHEROPTION(CALG_SHA1),
  CIPHEROPTION(CALG_MAC),
  CIPHEROPTION(CALG_RSA_SIGN),
  CIPHEROPTION(CALG_DSS_SIGN),
  CIPHEROPTION(CALG_NO_SIGN),
  CIPHEROPTION(CALG_RSA_KEYX),
  CIPHEROPTION(CALG_DES),
  CIPHEROPTION(CALG_3DES_112),
  CIPHEROPTION(CALG_3DES),
  CIPHEROPTION(CALG_DESX),
  CIPHEROPTION(CALG_RC2),
  CIPHEROPTION(CALG_RC4),
  CIPHEROPTION(CALG_SEAL),
  CIPHEROPTION(CALG_DH_SF),
  CIPHEROPTION(CALG_DH_EPHEM),
  CIPHEROPTION(CALG_AGREEDKEY_ANY),
  CIPHEROPTION(CALG_HUGHES_MD5),
  CIPHEROPTION(CALG_SKIPJACK),
  CIPHEROPTION(CALG_TEK),
  CIPHEROPTION(CALG_CYLINK_MEK), /* spellchecker:disable-line */
  CIPHEROPTION(CALG_SSL3_SHAMD5),
  CIPHEROPTION(CALG_SSL3_MASTER),
  CIPHEROPTION(CALG_SCHANNEL_MASTER_HASH),
  CIPHEROPTION(CALG_SCHANNEL_MAC_KEY),
  CIPHEROPTION(CALG_SCHANNEL_ENC_KEY),
  CIPHEROPTION(CALG_PCT1_MASTER),
  CIPHEROPTION(CALG_SSL2_MASTER),
  CIPHEROPTION(CALG_TLS1_MASTER),
  CIPHEROPTION(CALG_RC5),
  CIPHEROPTION(CALG_HMAC),
  CIPHEROPTION(CALG_TLS1PRF),
  CIPHEROPTION(CALG_HASH_REPLACE_OWF),
  CIPHEROPTION(CALG_AES_128),
  CIPHEROPTION(CALG_AES_192),
  CIPHEROPTION(CALG_AES_256),
  CIPHEROPTION(CALG_AES),
  CIPHEROPTION(CALG_SHA_256),
  CIPHEROPTION(CALG_SHA_384),
  CIPHEROPTION(CALG_SHA_512),
  CIPHEROPTION(CALG_ECDH),
/* Offered by mingw-w64 v4+. MS SDK 6.0A+. */
#ifdef CALG_ECMQV
  CIPHEROPTION(CALG_ECMQV),
#endif
  CIPHEROPTION(CALG_ECDSA),
/* Offered by mingw-w64 v7+. MS SDK 7.0A+. */
#ifdef CALG_ECDH_EPHEM
  CIPHEROPTION(CALG_ECDH_EPHEM),
#endif
  { NULL, 0 },
};

static int get_alg_id_by_name(const char *name)
{
  const char *nameEnd = strchr(name, ':');
  size_t n = nameEnd ? (size_t)(nameEnd - name) : strlen(name);
  int i;

  for(i = 0; algs[i].name; i++) {
    if((n == strlen(algs[i].name) && !strncmp(algs[i].name, name, n)))
      return algs[i].id;
  }
  return 0; /* not found */
}

#define NUM_CIPHERS 47 /* There are 47 options listed above */

static CURLcode set_ssl_ciphers(SCHANNEL_CRED *schannel_cred, char *ciphers,
                                ALG_ID *algIds)
{
  const char *startCur = ciphers;
  int algCount = 0;
  while(startCur && *startCur && (algCount < NUM_CIPHERS)) {
    curl_off_t alg;
    if(curlx_str_number(&startCur, &alg, INT_MAX) || !alg)
      alg = get_alg_id_by_name(startCur);

    if(alg)
      algIds[algCount++] = (ALG_ID)alg;
    else if(!strncmp(startCur, "USE_STRONG_CRYPTO",
                     sizeof("USE_STRONG_CRYPTO") - 1) ||
            !strncmp(startCur, "SCH_USE_STRONG_CRYPTO",
                     sizeof("SCH_USE_STRONG_CRYPTO") - 1))
      schannel_cred->dwFlags |= SCH_USE_STRONG_CRYPTO;
    else
      return CURLE_SSL_CIPHER;
    startCur = strchr(startCur, ':');
    if(startCur)
      startCur++;
  }
  schannel_cred->palgSupportedAlgs = algIds;
  schannel_cred->cSupportedAlgs = (DWORD)algCount;
  return CURLE_OK;
}

/* Function allocates memory for store_path only if CURLE_OK is returned */
static CURLcode get_cert_location(TCHAR *path, DWORD *store_name,
                                  TCHAR **store_path, TCHAR **thumbprint)
{
  TCHAR *sep;
  TCHAR *store_path_start;
  size_t store_name_len;

  sep = _tcschr(path, TEXT('\\'));
  if(!sep)
    return CURLE_SSL_CERTPROBLEM;

  store_name_len = sep - path;

  if(_tcsncmp(path, TEXT("CurrentUser"), store_name_len) == 0)
    *store_name = CERT_SYSTEM_STORE_CURRENT_USER;
  else if(_tcsncmp(path, TEXT("LocalMachine"), store_name_len) == 0)
    *store_name = CERT_SYSTEM_STORE_LOCAL_MACHINE;
  else if(_tcsncmp(path, TEXT("CurrentService"), store_name_len) == 0)
    *store_name = CERT_SYSTEM_STORE_CURRENT_SERVICE;
  else if(_tcsncmp(path, TEXT("Services"), store_name_len) == 0)
    *store_name = CERT_SYSTEM_STORE_SERVICES;
  else if(_tcsncmp(path, TEXT("Users"), store_name_len) == 0)
    *store_name = CERT_SYSTEM_STORE_USERS;
  else if(_tcsncmp(path, TEXT("CurrentUserGroupPolicy"), store_name_len) == 0)
    *store_name = CERT_SYSTEM_STORE_CURRENT_USER_GROUP_POLICY;
  else if(_tcsncmp(path, TEXT("LocalMachineGroupPolicy"), store_name_len) == 0)
    *store_name = CERT_SYSTEM_STORE_LOCAL_MACHINE_GROUP_POLICY;
  else if(_tcsncmp(path, TEXT("LocalMachineEnterprise"), store_name_len) == 0)
    *store_name = CERT_SYSTEM_STORE_LOCAL_MACHINE_ENTERPRISE;
  else
    return CURLE_SSL_CERTPROBLEM;

  store_path_start = sep + 1;

  sep = _tcschr(store_path_start, TEXT('\\'));
  if(!sep)
    return CURLE_SSL_CERTPROBLEM;

  *thumbprint = sep + 1;
  if(_tcslen(*thumbprint) != CERT_THUMBPRINT_STR_LEN)
    return CURLE_SSL_CERTPROBLEM;

  *sep = TEXT('\0');
  *store_path = curlx_tcsdup(store_path_start);
  *sep = TEXT('\\');
  if(!*store_path)
    return CURLE_OUT_OF_MEMORY;

  return CURLE_OK;
}

static CURLcode get_client_cert(struct Curl_easy *data,
                                HCERTSTORE *out_cert_store,
                                PCCERT_CONTEXT *out_cert_context)
{
  PCCERT_CONTEXT client_cert = NULL;
  HCERTSTORE client_cert_store = NULL;
  CURLcode result = CURLE_OK;

  if(data->set.ssl.primary.clientcert || data->set.ssl.primary.cert_blob) {
    DWORD cert_store_name = 0;
    TCHAR *cert_store_path = NULL;
    TCHAR *cert_thumbprint_str = NULL;
    TCHAR cert_thumbprint_buf[CERT_THUMBPRINT_STR_LEN + 1];
    CRYPT_HASH_BLOB cert_thumbprint;
    BYTE cert_thumbprint_data[CERT_THUMBPRINT_DATA_LEN];
    HCERTSTORE cert_store = NULL;
    FILE *fInCert = NULL;
    void *certdata = NULL;
    size_t certsize = 0;
    bool blob = data->set.ssl.primary.cert_blob != NULL;

    if(blob) {
      certdata = data->set.ssl.primary.cert_blob->data;
      certsize = data->set.ssl.primary.cert_blob->len;
    }
    else {
      TCHAR *cert_path =
        curlx_convert_UTF8_to_tchar(data->set.ssl.primary.clientcert);
      if(!cert_path)
        return CURLE_OUT_OF_MEMORY;

      result = get_cert_location(cert_path, &cert_store_name,
                                 &cert_store_path, &cert_thumbprint_str);

      /* 'cert_thumbprint_str' points in to the allocated 'cert_path', copy
         the data. The string is verified to be CERT_THUMBPRINT_STR_LEN bytes
         long within the get_cert_location() function. */
      if(!result && cert_thumbprint_str) {
        memcpy(cert_thumbprint_buf, cert_thumbprint_str,
               sizeof(cert_thumbprint_buf));
        cert_thumbprint_str = cert_thumbprint_buf;
      }

      curlx_free(cert_path);
      if(result && (data->set.ssl.primary.clientcert[0] != '\0'))
        fInCert = curlx_fopen(data->set.ssl.primary.clientcert, "rb");

      if(result && !fInCert) {
        failf(data, "schannel: Failed to get certificate location"
              " or file for %s",
              data->set.ssl.primary.clientcert);
        return result;
      }
    }

    if((fInCert || blob) && data->set.ssl.cert_type &&
       !curl_strequal(data->set.ssl.cert_type, "P12")) {
      failf(data, "schannel: certificate format compatibility error "
            "for %s",
            blob ? "(memory blob)" : data->set.ssl.primary.clientcert);
      curlx_free(cert_store_path);
      if(fInCert)
        curlx_fclose(fInCert);
      return CURLE_SSL_CERTPROBLEM;
    }

    if(fInCert || blob) {
      /* Reading a .p12 or .pfx file, like the example at bottom of
         https://learn.microsoft.com/archive/msdn-technet-forums/3e7bc95f-b21a-4bcd-bd2c-7f996718cae5
      */
      CRYPT_DATA_BLOB datablob;
      WCHAR *pszPassword;
      size_t pwd_len = 0;
      int cert_find_flags;
      const char *cert_showfilename_error = blob ?
        "(memory blob)" : data->set.ssl.primary.clientcert;
      curlx_free(cert_store_path);
      if(fInCert) {
        long cert_tell = 0;
        bool continue_reading = fseek(fInCert, 0, SEEK_END) == 0;
        if(continue_reading)
          cert_tell = ftell(fInCert);
        if(cert_tell < 0)
          continue_reading = FALSE;
        else
          certsize = (size_t)cert_tell;
        if(continue_reading)
          continue_reading = fseek(fInCert, 0, SEEK_SET) == 0;
        if(continue_reading && (certsize < CURL_MAX_INPUT_LENGTH))
          certdata = curlx_malloc(certsize + 1);
        if((!certdata) ||
           ((int) fread(certdata, certsize, 1, fInCert) != 1))
          continue_reading = FALSE;
        curlx_fclose(fInCert);
        if(!continue_reading) {
          failf(data, "schannel: Failed to read cert file %s",
                data->set.ssl.primary.clientcert);
          curlx_free(certdata);
          return CURLE_SSL_CERTPROBLEM;
        }
      }

      /* Convert key-pair data to the in-memory certificate store */
      datablob.pbData = (BYTE *)certdata;
      datablob.cbData = (DWORD)certsize;

      if(data->set.ssl.key_passwd)
        pwd_len = strlen(data->set.ssl.key_passwd);
      pszPassword = (WCHAR *)curlx_malloc(sizeof(WCHAR) * (pwd_len + 1));
      if(pszPassword) {
        int str_w_len = 0;
        if(pwd_len > 0)
          str_w_len = MultiByteToWideChar(CP_UTF8,
                                          MB_ERR_INVALID_CHARS,
                                          data->set.ssl.key_passwd,
                                          (int)pwd_len,
                                          pszPassword, (int)(pwd_len + 1));

        if((str_w_len >= 0) && (str_w_len <= (int)pwd_len))
          pszPassword[str_w_len] = 0;
        else
          pszPassword[0] = 0;

        cert_store = PFXImportCertStore(&datablob, pszPassword,
                                        PKCS12_NO_PERSIST_KEY);
        curlx_free(pszPassword);
      }
      if(!blob)
        curlx_free(certdata);
      if(!cert_store) {
        DWORD errorcode = GetLastError();
        if(errorcode == ERROR_INVALID_PASSWORD)
          failf(data, "schannel: Failed to import cert file %s, "
                "password is bad",
                cert_showfilename_error);
        else
          failf(data, "schannel: Failed to import cert file %s, "
                "last error is 0x%08lx",
                cert_showfilename_error, errorcode);
        return CURLE_SSL_CERTPROBLEM;
      }

      /* CERT_FIND_HAS_PRIVATE_KEY is only available in Windows 8 / Server
         2012, (NT v6.2). For earlier versions we use CURL_FIND_ANY. */
      if(curlx_verify_windows_version(6, 2, 0, PLATFORM_WINNT,
                                      VERSION_GREATER_THAN_EQUAL))
        cert_find_flags = CERT_FIND_HAS_PRIVATE_KEY;
      else
        cert_find_flags = CERT_FIND_ANY;

      client_cert =
        CertFindCertificateInStore(cert_store,
                                   X509_ASN_ENCODING | PKCS_7_ASN_ENCODING, 0,
                                   cert_find_flags, NULL, NULL);

      if(!client_cert) {
        failf(data, "schannel: Failed to get certificate from file %s"
              ", last error is 0x%08lx",
              cert_showfilename_error, GetLastError());
        CertCloseStore(cert_store, 0);
        return CURLE_SSL_CERTPROBLEM;
      }
    }
    else {
      cert_store =
        CertOpenStore(
#ifdef UNICODE
                      CERT_STORE_PROV_SYSTEM_W,
#else
                      CERT_STORE_PROV_SYSTEM_A,
#endif
                      0,
                      (HCRYPTPROV)NULL,
                      CERT_STORE_OPEN_EXISTING_FLAG | cert_store_name,
                      cert_store_path);
      if(!cert_store) {
        char *path_utf8 =
          curlx_convert_tchar_to_UTF8(cert_store_path);
        failf(data, "schannel: Failed to open cert store %lx %s, "
              "last error is 0x%08lx",
              cert_store_name, (path_utf8 ? path_utf8 : "(unknown)"),
              GetLastError());
        curlx_free(cert_store_path);
        curlx_free(path_utf8);
        return CURLE_SSL_CERTPROBLEM;
      }
      curlx_free(cert_store_path);

      cert_thumbprint.pbData = cert_thumbprint_data;
      cert_thumbprint.cbData = CERT_THUMBPRINT_DATA_LEN;

      if(!CryptStringToBinary(cert_thumbprint_str,
                              CERT_THUMBPRINT_STR_LEN,
                              CRYPT_STRING_HEX,
                              cert_thumbprint_data,
                              &cert_thumbprint.cbData,
                              NULL, NULL)) {
        CertCloseStore(cert_store, 0);
        return CURLE_SSL_CERTPROBLEM;
      }

      client_cert =
        CertFindCertificateInStore(cert_store,
                                   X509_ASN_ENCODING | PKCS_7_ASN_ENCODING, 0,
                                   CERT_FIND_HASH, &cert_thumbprint, NULL);
      if(!client_cert) {
        /* CRYPT_E_NOT_FOUND / E_INVALIDARG */
        CertCloseStore(cert_store, 0);
        failf(data, "schannel: client cert not found in cert store");
        return CURLE_SSL_CERTPROBLEM;
      }
    }
    client_cert_store = cert_store;
  }

  *out_cert_store = client_cert_store;
  *out_cert_context = client_cert;

  return CURLE_OK;
}

static CURLcode acquire_sspi_handle(struct Curl_cfilter *cf,
                                    struct Curl_easy *data,
                                    struct schannel_ssl_backend_data *backend,
                                    PCCERT_CONTEXT client_cert,
                                    DWORD flags,
                                    DWORD enabled_protocols)
{
  struct ssl_primary_config *conn_config = Curl_ssl_cf_get_primary_config(cf);
  SECURITY_STATUS sspi_status = SEC_E_OK;
  CURLcode result;

  /* We support TLS 1.3 starting in Windows 10 version 1809 (OS build 17763) as
     long as the user did not set a legacy algorithm list
     (CURLOPT_SSL_CIPHER_LIST). */
  if(!conn_config->cipher_list &&
     curlx_verify_windows_version(10, 0, 17763, PLATFORM_WINNT,
                                  VERSION_GREATER_THAN_EQUAL)) {

    SCH_CREDENTIALS credentials = { 0 };
    TLS_PARAMETERS tls_parameters = { 0 };
    CRYPTO_SETTINGS crypto_settings[1];
    PCCERT_CONTEXT client_certs[1];

    if(client_cert)
      client_certs[0] = client_cert;
    else
      client_certs[0] = NULL;

    memset(crypto_settings, 0, sizeof(crypto_settings));

    tls_parameters.pDisabledCrypto = crypto_settings;

    /* The number of blocked suites */
    tls_parameters.cDisabledCrypto = (DWORD)0;
    credentials.pTlsParameters = &tls_parameters;
    credentials.cTlsParameters = 1;

    credentials.dwVersion = SCH_CREDENTIALS_VERSION;
    credentials.dwFlags = flags | SCH_USE_STRONG_CRYPTO;

    credentials.pTlsParameters->grbitDisabledProtocols = ~enabled_protocols;

    if(client_certs[0]) {
      credentials.cCreds = 1;
      credentials.paCred = client_certs;
    }

    sspi_status =
      Curl_pSecFn->AcquireCredentialsHandle(NULL,
                                            (TCHAR *)CURL_UNCONST(UNISP_NAME),
                                            SECPKG_CRED_OUTBOUND, NULL,
                                            &credentials, NULL, NULL,
                                            &backend->cred->cred_handle, NULL);
  }
  else {
    /* Pre-Windows 10 1809 or the user set a legacy algorithm list.
       Schannel does not negotiate TLS 1.3 when SCHANNEL_CRED is used. */
    ALG_ID algIds[NUM_CIPHERS];
    char *ciphers = conn_config->cipher_list;
    SCHANNEL_CRED schannel_cred = { 0 };
    PCCERT_CONTEXT client_certs[1];

    if(client_cert)
      client_certs[0] = client_cert;
    else
      client_certs[0] = NULL;

    schannel_cred.dwVersion = SCHANNEL_CRED_VERSION;
    schannel_cred.dwFlags = flags;
    schannel_cred.grbitEnabledProtocols = enabled_protocols;

    if(ciphers) {
      if((enabled_protocols & SP_PROT_TLS1_3_CLIENT)) {
        infof(data, "schannel: WARNING: This version of Schannel "
              "negotiates a less-secure TLS version than TLS 1.3 because the "
              "user set an algorithm cipher list.");
      }
      result = set_ssl_ciphers(&schannel_cred, ciphers, algIds);
      if(result) {
        failf(data, "schannel: Failed setting algorithm cipher list");
        return result;
      }
    }
    else {
      schannel_cred.dwFlags = flags | SCH_USE_STRONG_CRYPTO;
    }

    if(client_certs[0]) {
      schannel_cred.cCreds = 1;
      schannel_cred.paCred = client_certs;
    }

    sspi_status =
      Curl_pSecFn->AcquireCredentialsHandle(NULL,
                                            (TCHAR *)CURL_UNCONST(UNISP_NAME),
                                            SECPKG_CRED_OUTBOUND, NULL,
                                            &schannel_cred, NULL, NULL,
                                            &backend->cred->cred_handle, NULL);
  }

  if(sspi_status != SEC_E_OK) {
    char buffer[STRERROR_LEN];
    failf(data, "schannel: AcquireCredentialsHandle failed: %s",
          Curl_sspi_strerror(sspi_status, buffer, sizeof(buffer)));
    switch(sspi_status) {
    case SEC_E_INSUFFICIENT_MEMORY:
      return CURLE_OUT_OF_MEMORY;
    case SEC_E_NO_CREDENTIALS:
    case SEC_E_SECPKG_NOT_FOUND:
    case SEC_E_NOT_OWNER:
    case SEC_E_UNKNOWN_CREDENTIALS:
    case SEC_E_INTERNAL_ERROR:
    default:
      return CURLE_SSL_CONNECT_ERROR;
    }
  }

  return CURLE_OK;
}

static CURLcode schannel_acquire_credential_handle(struct Curl_cfilter *cf,
                                                   struct Curl_easy *data)
{
  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);

  PCCERT_CONTEXT client_cert = NULL;
  HCERTSTORE client_cert_store = NULL;
  CURLcode result;

  /* setup Schannel API options */
  DWORD flags = 0;
  DWORD enabled_protocols = 0;

  struct schannel_ssl_backend_data *backend =
    (struct schannel_ssl_backend_data *)(connssl->backend);

  DEBUGASSERT(backend);

  if(conn_config->verifypeer) {
    if(backend->use_manual_cred_validation)
      flags = SCH_CRED_MANUAL_CRED_VALIDATION;
    else
      flags = SCH_CRED_AUTO_CRED_VALIDATION;

    if(ssl_config->no_revoke) {
      flags |= SCH_CRED_IGNORE_NO_REVOCATION_CHECK |
               SCH_CRED_IGNORE_REVOCATION_OFFLINE;

      DEBUGF(infof(data, "schannel: disabled server certificate revocation "
                         "checks"));
    }
    else if(ssl_config->revoke_best_effort) {
      flags |= SCH_CRED_IGNORE_NO_REVOCATION_CHECK |
               SCH_CRED_IGNORE_REVOCATION_OFFLINE |
               SCH_CRED_REVOCATION_CHECK_CHAIN;

      DEBUGF(infof(data, "schannel: ignore revocation offline errors"));
    }
    else {
      flags |= SCH_CRED_REVOCATION_CHECK_CHAIN;

      DEBUGF(infof(data, "schannel: checking server certificate revocation"));
    }
  }
  else {
    flags = SCH_CRED_MANUAL_CRED_VALIDATION |
            SCH_CRED_IGNORE_NO_REVOCATION_CHECK |
            SCH_CRED_IGNORE_REVOCATION_OFFLINE;
    DEBUGF(infof(data, "schannel: disabled server cert revocation checks"));
  }

  if(!conn_config->verifyhost) {
    flags |= SCH_CRED_NO_SERVERNAME_CHECK;
    DEBUGF(infof(data, "schannel: verifyhost setting prevents Schannel from "
                       "comparing the supplied target name with the subject "
                       "names in server certificates."));
  }

  if(!ssl_config->auto_client_cert) {
    flags &= ~(DWORD)SCH_CRED_USE_DEFAULT_CREDS;
    flags |= SCH_CRED_NO_DEFAULT_CREDS;
    infof(data, "schannel: disabled automatic use of client certificate");
  }
  else
    infof(data, "schannel: enabled automatic use of client certificate");

  DEBUGASSERT(conn_config->version != CURL_SSLVERSION_DEFAULT);
  switch(conn_config->version) {
  case CURL_SSLVERSION_TLSv1:
  case CURL_SSLVERSION_TLSv1_0:
  case CURL_SSLVERSION_TLSv1_1:
  case CURL_SSLVERSION_TLSv1_2:
  case CURL_SSLVERSION_TLSv1_3: {
    result = schannel_set_ssl_version_min_max(&enabled_protocols, cf, data);
    if(result)
      return result;
    break;
  }
  case CURL_SSLVERSION_SSLv3:
  case CURL_SSLVERSION_SSLv2:
    failf(data, "SSL versions not supported");
    return CURLE_NOT_BUILT_IN;
  default:
    failf(data, "Unrecognized parameter passed via CURLOPT_SSLVERSION");
    return CURLE_SSL_CONNECT_ERROR;
  }

  result = get_client_cert(data, &client_cert_store, &client_cert);
  if(result)
    return result;

  /* allocate memory for the reusable credential handle */
  backend->cred = (struct Curl_schannel_cred *)
    curlx_calloc(1, sizeof(struct Curl_schannel_cred));
  if(!backend->cred) {
    failf(data, "schannel: unable to allocate memory");

    if(client_cert)
      CertFreeCertificateContext(client_cert);
    if(client_cert_store)
      CertCloseStore(client_cert_store, 0);

    return CURLE_OUT_OF_MEMORY;
  }
  backend->cred->refcount = 1;

  /* Since we did not persist the key, we need to extend the store's
   * lifetime until the end of the connection
   */
  backend->cred->client_cert_store = client_cert_store;

  result = acquire_sspi_handle(cf, data, backend, client_cert,
                               flags, enabled_protocols);

  if(client_cert)
    CertFreeCertificateContext(client_cert);

  return result;
}

static CURLcode schannel_connect_step1(struct Curl_cfilter *cf,
                                       struct Curl_easy *data)
{
  size_t written = 0;
  struct ssl_connect_data *connssl = cf->ctx;
  struct schannel_ssl_backend_data *backend =
    (struct schannel_ssl_backend_data *)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);
  SecBuffer outbuf;
  SecBufferDesc outbuf_desc;
  SecBuffer inbuf;
  SecBufferDesc inbuf_desc;
#ifdef HAS_ALPN_SCHANNEL
  unsigned char alpn_buffer[128];
#endif
  SECURITY_STATUS sspi_status = SEC_E_OK;
  CURLcode result;

  DEBUGASSERT(backend);
  DEBUGF(infof(data, "schannel: SSL/TLS connection with %s port %d (step 1/3)",
               connssl->peer.hostname, connssl->peer.port));

#ifdef HAS_ALPN_SCHANNEL
  backend->use_alpn = connssl->alpn && s_win_has_alpn;
#else
  backend->use_alpn = FALSE;
#endif

  if(conn_config->CAfile || conn_config->ca_info_blob) {
    if(curlx_verify_windows_version(6, 1, 0, PLATFORM_WINNT,
                                    VERSION_GREATER_THAN_EQUAL)) {
      backend->use_manual_cred_validation = TRUE;
    }
    else {
      failf(data, "schannel: this version of Windows is too old to support "
                  "certificate verification via CA bundle file.");
      return CURLE_SSL_CACERT_BADFILE;
    }
  }
  else
    backend->use_manual_cred_validation = FALSE;

  backend->cred = NULL;

  /* check for an existing reusable credential handle */
  if(Curl_ssl_scache_use(cf, data)) {
    struct Curl_schannel_cred *old_cred;
    Curl_ssl_scache_lock(data);
    old_cred = Curl_ssl_scache_get_obj(cf, data, connssl->peer.scache_key);
    if(old_cred) {
      backend->cred = old_cred;
      DEBUGF(infof(data, "schannel: reusing existing credential handle"));

      /* increment the reference counter of the credential/session handle */
      backend->cred->refcount++;
      DEBUGF(infof(data,
                   "schannel: incremented credential handle refcount = %d",
                   backend->cred->refcount));
    }
    Curl_ssl_scache_unlock(data);
  }

  if(!backend->cred) {
    char *snihost;
    result = schannel_acquire_credential_handle(cf, data);
    if(result || !backend->cred)
      return result;
    /* schannel_acquire_credential_handle() sets backend->cred accordingly or
       it returns error otherwise. */

    /* A hostname associated with the credential is needed by
       InitializeSecurityContext for SNI and other reasons. */
    snihost = connssl->peer.sni ? connssl->peer.sni : connssl->peer.hostname;
    backend->cred->sni_hostname = curlx_convert_UTF8_to_tchar(snihost);
    if(!backend->cred->sni_hostname)
      return CURLE_OUT_OF_MEMORY;
  }

  /* Warn if SNI is disabled due to use of an IP address */
  if(connssl->peer.type != CURL_SSL_PEER_DNS) {
    infof(data, "schannel: using IP address, SNI is not supported by OS.");
  }

#ifdef HAS_ALPN_SCHANNEL
  if(backend->use_alpn) {
    int cur = 0;
    int list_start_index = 0;
    unsigned int *extension_len = NULL;
    unsigned short *list_len = NULL;
    struct alpn_proto_buf proto;

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

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

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

    list_start_index = cur;

    result = Curl_alpn_to_proto_buf(&proto, connssl->alpn);
    if(result) {
      failf(data, "Error setting ALPN");
      return CURLE_SSL_CONNECT_ERROR;
    }
    memcpy(&alpn_buffer[cur], proto.data, proto.len);
    cur += proto.len;

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

    InitSecBuffer(&inbuf, SECBUFFER_APPLICATION_PROTOCOLS, alpn_buffer, cur);
    InitSecBufferDesc(&inbuf_desc, &inbuf, 1);

    Curl_alpn_to_proto_str(&proto, connssl->alpn);
    infof(data, VTLS_INFOF_ALPN_OFFER_1STR, proto.data);
  }
  else {
    InitSecBuffer(&inbuf, SECBUFFER_EMPTY, NULL, 0);
    InitSecBufferDesc(&inbuf_desc, &inbuf, 1);
  }
#else /* HAS_ALPN_SCHANNEL */
  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);

  /* security request flags */
  backend->req_flags =
    ISC_REQ_SEQUENCE_DETECT | ISC_REQ_REPLAY_DETECT |
    ISC_REQ_CONFIDENTIALITY | ISC_REQ_ALLOCATE_MEMORY |
    ISC_REQ_STREAM |
    (!ssl_config->auto_client_cert ? ISC_REQ_USE_SUPPLIED_CREDS : 0);

  /* allocate memory for the security context handle */
  backend->ctxt = (struct Curl_schannel_ctxt *)
    curlx_calloc(1, sizeof(struct Curl_schannel_ctxt));
  if(!backend->ctxt) {
    failf(data, "schannel: unable to allocate memory");
    return CURLE_OUT_OF_MEMORY;
  }

  /* Schannel InitializeSecurityContext:
     https://learn.microsoft.com/windows/win32/api/rrascfg/nn-rrascfg-ieapproviderconfig

     At the moment we do not pass inbuf unless we are 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 = Curl_pSecFn->InitializeSecurityContext(
    &backend->cred->cred_handle, NULL, backend->cred->sni_hostname,
    backend->req_flags, 0, 0,
    (backend->use_alpn ? &inbuf_desc : NULL),
    0, &backend->ctxt->ctxt_handle,
    &outbuf_desc, &backend->ret_flags, NULL);

  if(sspi_status != SEC_I_CONTINUE_NEEDED) {
    char buffer[STRERROR_LEN];
    Curl_safefree(backend->ctxt);
    switch(sspi_status) {
    case SEC_E_INSUFFICIENT_MEMORY:
      failf(data, "schannel: initial InitializeSecurityContext failed: %s",
            Curl_sspi_strerror(sspi_status, buffer, sizeof(buffer)));
      return CURLE_OUT_OF_MEMORY;
    case SEC_E_WRONG_PRINCIPAL:
      failf(data, "schannel: SNI or certificate check failed: %s",
            Curl_sspi_strerror(sspi_status, buffer, sizeof(buffer)));
      return CURLE_PEER_FAILED_VERIFICATION;
#if 0
    case SEC_E_INVALID_HANDLE:
    case SEC_E_INVALID_TOKEN:
    case SEC_E_LOGON_DENIED:
    case SEC_E_TARGET_UNKNOWN:
    case SEC_E_NO_AUTHENTICATING_AUTHORITY:
    case SEC_E_INTERNAL_ERROR:
    case SEC_E_NO_CREDENTIALS:
    case SEC_E_UNSUPPORTED_FUNCTION:
    case SEC_E_APPLICATION_PROTOCOL_MISMATCH:
#endif
    default:
      failf(data, "schannel: initial InitializeSecurityContext failed: %s",
            Curl_sspi_strerror(sspi_status, buffer, sizeof(buffer)));
      return CURLE_SSL_CONNECT_ERROR;
    }
  }

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

  /* send initial handshake data which is now stored in output buffer */
  result = Curl_conn_cf_send(cf->next, data,
                             (const uint8_t *)outbuf.pvBuffer,
                             outbuf.cbBuffer, FALSE,
                             &written);
  Curl_pSecFn->FreeContextBuffer(outbuf.pvBuffer);
  if(result || (outbuf.cbBuffer != written)) {
    failf(data, "schannel: failed to send initial handshake data: "
          "sent %zu of %lu bytes", written, outbuf.cbBuffer);
    return CURLE_SSL_CONNECT_ERROR;
  }

  DEBUGF(infof(data, "schannel: sent initial handshake data: "
               "sent %zu bytes", written));

  backend->recv_unrecoverable_err = CURLE_OK;
  backend->recv_sspi_close_notify = FALSE;
  backend->recv_connection_closed = FALSE;
  backend->recv_renegotiating = FALSE;
  backend->renegotiate_state.started = FALSE;
  backend->encdata_is_incomplete = FALSE;

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

  return CURLE_OK;
}

static CURLcode schannel_error(struct Curl_easy *data,
                               SECURITY_STATUS sspi_status)
{
  char buffer[STRERROR_LEN];
  switch(sspi_status) {
  case SEC_E_INSUFFICIENT_MEMORY:
    failf(data, "schannel: next InitializeSecurityContext failed: %s",
          Curl_sspi_strerror(sspi_status, buffer, sizeof(buffer)));
    return CURLE_OUT_OF_MEMORY;
  case SEC_E_WRONG_PRINCIPAL:
    failf(data, "schannel: SNI or certificate check failed: %s",
          Curl_sspi_strerror(sspi_status, buffer, sizeof(buffer)));
    return CURLE_PEER_FAILED_VERIFICATION;
  case SEC_E_UNTRUSTED_ROOT:
    failf(data, "schannel: %s",
          Curl_sspi_strerror(sspi_status, buffer, sizeof(buffer)));
    return CURLE_PEER_FAILED_VERIFICATION;
#if 0
  case SEC_E_INVALID_HANDLE:
  case SEC_E_INVALID_TOKEN:
  case SEC_E_LOGON_DENIED:
  case SEC_E_TARGET_UNKNOWN:
  case SEC_E_NO_AUTHENTICATING_AUTHORITY:
  case SEC_E_INTERNAL_ERROR:
  case SEC_E_NO_CREDENTIALS:
  case SEC_E_UNSUPPORTED_FUNCTION:
  case SEC_E_APPLICATION_PROTOCOL_MISMATCH:
#endif
  default:
    failf(data, "schannel: next InitializeSecurityContext failed: %s",
          Curl_sspi_strerror(sspi_status, buffer, sizeof(buffer)));
    return CURLE_SSL_CONNECT_ERROR;
  }
}

static CURLcode schannel_pkp_pin_peer_pubkey(struct Curl_cfilter *cf,
                                             struct Curl_easy *data,
                                             const char *pinnedpubkey)
{
  struct ssl_connect_data *connssl = cf->ctx;
  struct schannel_ssl_backend_data *backend =
    (struct schannel_ssl_backend_data *)connssl->backend;
  CERT_CONTEXT *pCertContextServer = NULL;

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

  DEBUGASSERT(backend);

  /* if a path was not specified, do not pin */
  if(!pinnedpubkey)
    return CURLE_OK;

  do {
    SECURITY_STATUS sspi_status;
    const char *x509_der;
    DWORD x509_der_len;
    struct Curl_X509certificate x509_parsed;
    struct Curl_asn1Element *pubkey;

    sspi_status =
      Curl_pSecFn->QueryContextAttributes(&backend->ctxt->ctxt_handle,
                                          SECPKG_ATTR_REMOTE_CERT_CONTEXT,
                                          &pCertContextServer);

    if((sspi_status != SEC_E_OK) || !pCertContextServer) {
      char buffer[STRERROR_LEN];
      failf(data, "schannel: Failed to read remote certificate context: %s",
            Curl_sspi_strerror(sspi_status, buffer, sizeof(buffer)));
      break; /* failed */
    }

    if(!(((pCertContextServer->dwCertEncodingType & X509_ASN_ENCODING) != 0) &&
         (pCertContextServer->cbCertEncoded > 0)))
      break;

    x509_der = (const char *)pCertContextServer->pbCertEncoded;
    x509_der_len = pCertContextServer->cbCertEncoded;
    memset(&x509_parsed, 0, sizeof(x509_parsed));
    if(Curl_parseX509(&x509_parsed, x509_der, x509_der + x509_der_len))
      break;

    pubkey = &x509_parsed.subjectPublicKeyInfo;
    if(!pubkey->header || pubkey->end <= pubkey->header) {
      failf(data, "SSL: failed retrieving public key from server certificate");
      break;
    }

    result = Curl_pin_peer_pubkey(data,
                                  pinnedpubkey,
                                  (const unsigned char *)pubkey->header,
                                  (size_t)(pubkey->end - pubkey->header));
    if(result) {
      failf(data, "SSL: public key does not match pinned public key");
    }
  } while(0);

  if(pCertContextServer)
    CertFreeCertificateContext(pCertContextServer);

  return result;
}

static CURLcode ensure_encoding_size(struct Curl_easy *data,
                                     struct sbuffer *encdata,
                                     size_t min_length)
{
  size_t size;
  DEBUGASSERT(encdata->length >= encdata->offset);
  if(encdata->length < encdata->offset)
    return CURLE_FAILED_INIT;
  size = encdata->length - encdata->offset;
  if(size < CURL_SCHANNEL_BUFFER_FREE_SIZE || encdata->length < min_length) {
    unsigned char *buffer;
    size_t length = encdata->offset + CURL_SCHANNEL_BUFFER_FREE_SIZE;
    if(length < min_length)
      length = min_length;
    buffer = curlx_realloc(encdata->buffer, length);
    if(!buffer) {
      failf(data, "schannel: unable to re-allocate memory");
      return CURLE_OUT_OF_MEMORY;
    }

    encdata->buffer = buffer;
    encdata->length = length;
    SCH_DEV(infof(data, "schannel: encdata.buffer resized %zu",
                  encdata->length));
  }
  return CURLE_OK;
}

static CURLcode ensure_decoding_size(struct Curl_easy *data,
                                     struct sbuffer *decdata,
                                     size_t nowsize,
                                     size_t len)
{
  size_t size = nowsize > CURL_SCHANNEL_BUFFER_FREE_SIZE ?
    nowsize : CURL_SCHANNEL_BUFFER_FREE_SIZE;
  DEBUGASSERT(decdata->length >= decdata->offset);
  if(decdata->length < decdata->offset)
    return CURLE_FAILED_INIT;
  else if(decdata->length - decdata->offset < size ||
          decdata->length < len) {
    /* increase internal decrypted data buffer */
    size_t length = decdata->offset + size;
    unsigned char *buffer;
    /* make sure that the requested amount of data fits */
    if(length < len)
      length = len;

    buffer = curlx_realloc(decdata->buffer, length);
    if(!buffer) {
      failf(data, "schannel: unable to re-allocate memory");
      return CURLE_OUT_OF_MEMORY;
    }
    decdata->buffer = buffer;
    decdata->length = length;
  }
  return CURLE_OK;
}

static CURLcode schannel_connect_step2(struct Curl_cfilter *cf,
                                       struct Curl_easy *data)
{
  struct ssl_connect_data *connssl = cf->ctx;
  struct schannel_ssl_backend_data *backend =
    (struct schannel_ssl_backend_data *)connssl->backend;
  struct ssl_primary_config *conn_config = Curl_ssl_cf_get_primary_config(cf);
  int i;
  size_t nread = 0;
  SecBuffer outbuf[3];
  SecBufferDesc outbuf_desc;
  SecBuffer inbuf[2];
  SecBufferDesc inbuf_desc;
  SECURITY_STATUS sspi_status = SEC_E_OK;
  CURLcode result;
  bool doread;
  const char *pubkey_ptr;

  DEBUGASSERT(backend);

  doread = (connssl->io_need & CURL_SSL_IO_NEED_SEND) ? FALSE : TRUE;
  connssl->io_need = CURL_SSL_IO_NEED_NONE;

  DEBUGF(infof(data, "schannel: SSL/TLS connection with %s port %d (step 2/3)",
               connssl->peer.hostname, connssl->peer.port));

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

  /* buffer to store previously received and decrypted data */
  if(!backend->decdata.buffer) {
    backend->decdata.offset = 0;
    backend->decdata.length = CURL_SCHANNEL_BUFFER_INIT_SIZE;
    backend->decdata.buffer = curlx_malloc(backend->decdata.length);
    if(!backend->decdata.buffer) {
      failf(data, "schannel: unable to allocate memory");
      return CURLE_OUT_OF_MEMORY;
    }
  }

  /* buffer to store previously received and encrypted data */
  if(!backend->encdata.buffer) {
    backend->encdata_is_incomplete = FALSE;
    backend->encdata.offset = 0;
    backend->encdata.length = CURL_SCHANNEL_BUFFER_INIT_SIZE;
    backend->encdata.buffer = curlx_malloc(backend->encdata.length);
    if(!backend->encdata.buffer) {
      failf(data, "schannel: unable to allocate memory");
      return CURLE_OUT_OF_MEMORY;
    }
  }

  result = ensure_encoding_size(data, &backend->encdata, 0);
  if(result)
    return result;

  for(;;) {
    if(doread) {
      /* read encrypted handshake data from socket */
      result = Curl_conn_cf_recv(cf->next, data,
                                 (char *)(backend->encdata.buffer +
                                          backend->encdata.offset),
                                 backend->encdata.length -
                                 backend->encdata.offset,
                                 &nread);
      if(result == CURLE_AGAIN) {
        if(!backend->encdata.offset || backend->encdata_is_incomplete) {
          connssl->io_need = CURL_SSL_IO_NEED_RECV;
          DEBUGF(infof(data, "schannel: failed to receive handshake, "
                       "need more data"));
          return CURLE_OK;
        }
        else {
          DEBUGF(infof(data, "schannel: no new handshake data received, "
                       "continuing to process existing handshake data"));
        }
      }
      else if(result || (nread == 0)) {
        failf(data, "schannel: failed to receive handshake, "
              "SSL/TLS connection failed");
        return CURLE_SSL_CONNECT_ERROR;
      }
      else {
        /* increase encrypted data buffer offset */
        backend->encdata.offset += nread;
        backend->encdata_is_incomplete = FALSE;
        SCH_DEV(infof(data, "schannel: encrypted data got %zu", nread));
      }
    }

    SCH_DEV(infof(data,
                  "schannel: encrypted data buffer: offset %zu length %zu",
                  backend->encdata.offset, backend->encdata.length));

    /* setup input buffers */
    InitSecBuffer(&inbuf[0], SECBUFFER_TOKEN,
                  curlx_malloc(backend->encdata.offset),
                  curlx_uztoul(backend->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) {
      failf(data, "schannel: unable to allocate memory");
      return CURLE_OUT_OF_MEMORY;
    }

    /* copy received handshake data into input buffer */
    memcpy(inbuf[0].pvBuffer, backend->encdata.buffer,
           backend->encdata.offset);

    /* The socket must be writeable (or a poll error occurred) before we call
       InitializeSecurityContext to continue processing the received TLS
       records. This is because that function is not idempotent and we do not
       support partial save/resume sending replies of handshake tokens. */
    if(!SOCKET_WRITABLE(Curl_conn_cf_get_socket(cf, data), 0)) {
      SCH_DEV(infof(data, "schannel: handshake waiting for writeable socket"));
      connssl->io_need = CURL_SSL_IO_NEED_SEND;
      curlx_free(inbuf[0].pvBuffer);
      return CURLE_OK;
    }

    sspi_status = Curl_pSecFn->InitializeSecurityContext(
      &backend->cred->cred_handle, &backend->ctxt->ctxt_handle,
      backend->cred->sni_hostname, backend->req_flags,
      0, 0, &inbuf_desc, 0, NULL,
      &outbuf_desc, &backend->ret_flags, NULL);

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

    /* check if the handshake was incomplete */
    switch(sspi_status) {
    case SEC_E_INCOMPLETE_MESSAGE:
      backend->encdata_is_incomplete = TRUE;
      connssl->io_need = CURL_SSL_IO_NEED_RECV;
      DEBUGF(infof(data,
                   "schannel: received incomplete message, need more data"));
      return CURLE_OK;

    case SEC_I_CONTINUE_NEEDED:
    case SEC_E_OK:
      /* check if the handshake needs to be continued */
      result = CURLE_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) {
          size_t written = 0;
          DEBUGF(infof(data, "schannel: sending next handshake data: "
                       "sending %lu bytes.", outbuf[i].cbBuffer));

          /* send handshake token to server */
          result = Curl_conn_cf_send(cf->next, data,
                                     (const uint8_t *)outbuf[i].pvBuffer,
                                     outbuf[i].cbBuffer,
                                     FALSE, &written);
          if(result || (outbuf[i].cbBuffer != written)) {
            failf(data, "schannel: failed to send next handshake data: "
                  "sent %zu of %lu bytes", written, outbuf[i].cbBuffer);
            result = CURLE_SSL_CONNECT_ERROR;
          }
        }
      }
      for(i = 0; i < 3; i++) {
        /* free obsolete buffer */
        if(outbuf[i].pvBuffer)
          Curl_pSecFn->FreeContextBuffer(outbuf[i].pvBuffer);
      }
      if(result)
        return result;
      break;

    case SEC_I_INCOMPLETE_CREDENTIALS:
      if(!(backend->req_flags & ISC_REQ_USE_SUPPLIED_CREDS)) {
        /* If the server has requested a client certificate, attempt to
           continue the handshake without one. This allows connections to
           servers which request a client certificate but do not require
           it. */
        backend->req_flags |= ISC_REQ_USE_SUPPLIED_CREDS;
        connssl->io_need = CURL_SSL_IO_NEED_SEND;
        DEBUGF(infof(data,
                     "schannel: a client certificate has been requested"));
        return CURLE_OK;
      }
      FALLTHROUGH();

    default:
      return schannel_error(data, sspi_status);
    }

    /* check if there was additional remaining encrypted data */
    if(inbuf[1].BufferType == SECBUFFER_EXTRA && inbuf[1].cbBuffer > 0) {
      SCH_DEV(infof(data, "schannel: encrypted data length: %lu",
                    inbuf[1].cbBuffer));
      /*
        There are two cases where we could be getting extra data here:
        1. If we are 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(backend->encdata.offset > inbuf[1].cbBuffer) {
        memmove(backend->encdata.buffer,
                (backend->encdata.buffer + backend->encdata.offset) -
                inbuf[1].cbBuffer, inbuf[1].cbBuffer);
        backend->encdata.offset = inbuf[1].cbBuffer;
        if(sspi_status == SEC_I_CONTINUE_NEEDED) {
          doread = FALSE;
          continue;
        }
      }
    }
    else {
      backend->encdata.offset = 0;
    }
    break;
  }

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

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

#ifndef CURL_DISABLE_PROXY
  pubkey_ptr = Curl_ssl_cf_is_proxy(cf) ?
    data->set.str[STRING_SSL_PINNEDPUBLICKEY_PROXY] :
    data->set.str[STRING_SSL_PINNEDPUBLICKEY];
#else
  pubkey_ptr = data->set.str[STRING_SSL_PINNEDPUBLICKEY];
#endif
  if(pubkey_ptr) {
    result = schannel_pkp_pin_peer_pubkey(cf, data, pubkey_ptr);
    if(result) {
      failf(data, "SSL: public key does not match pinned public key");
      return result;
    }
  }

  if(conn_config->verifypeer && backend->use_manual_cred_validation) {
    /* Certificate verification also verifies the hostname if verifyhost */
    return Curl_verify_certificate(cf, data);
  }

  /* Verify the hostname manually when certificate verification is disabled,
     because in that case Schannel does not verify it. */
  if(!conn_config->verifypeer && conn_config->verifyhost)
    return Curl_verify_host(cf, data);

  return CURLE_OK;
}

static bool valid_cert_encoding(const CERT_CONTEXT *cert_context)
{
  return (cert_context != NULL) &&
    ((cert_context->dwCertEncodingType & X509_ASN_ENCODING) != 0) &&
    (cert_context->pbCertEncoded != NULL) &&
    (cert_context->cbCertEncoded > 0);
}

typedef bool (*Read_crt_func)(const CERT_CONTEXT *ccert_context,
                              bool reverse_order, void *arg);

static void traverse_cert_store(const CERT_CONTEXT *context,
                                Read_crt_func func, void *arg)
{
  const CERT_CONTEXT *current_context = NULL;
  bool should_continue = TRUE;
  bool first = TRUE;
  bool reverse_order = FALSE;
  while(should_continue &&
        (current_context = CertEnumCertificatesInStore(
          context->hCertStore,
          current_context)) != NULL) {
    /* Windows 11 22H2 OS Build 22621.674 or higher enumerates certificates in
       leaf-to-root order while all previous versions of Windows enumerate
       certificates in root-to-leaf order. Determine the order of enumeration
       by comparing SECPKG_ATTR_REMOTE_CERT_CONTEXT's pbCertContext with the
       first certificate's pbCertContext. */
    if(first && context->pbCertEncoded != current_context->pbCertEncoded)
      reverse_order = TRUE;
    should_continue = func(current_context, reverse_order, arg);
    first = FALSE;
  }

  if(current_context)
    CertFreeCertificateContext(current_context);
}

static bool cert_counter_callback(const CERT_CONTEXT *ccert_context,
                                  bool reverse_order, void *certs_count)
{
  (void)reverse_order;
  if(valid_cert_encoding(ccert_context))
    (*(int *)certs_count)++;
  if(*(int *)certs_count > MAX_ALLOWED_CERT_AMOUNT)
    return FALSE;
  return TRUE;
}

struct Adder_args {
  struct Curl_easy *data;
  CURLcode result;
  int idx;
  int certs_count;
};

static bool add_cert_to_certinfo(const CERT_CONTEXT *ccert_context,
                                 bool reverse_order, void *raw_arg)
{
  struct Adder_args *args = (struct Adder_args *)raw_arg;
  args->result = CURLE_OK;
  if(valid_cert_encoding(ccert_context)) {
    const char *beg = (const char *)ccert_context->pbCertEncoded;
    const char *end = beg + ccert_context->cbCertEncoded;
    int insert_index = reverse_order ? (args->certs_count - 1) - args->idx :
                       args->idx;
    args->result = Curl_extract_certinfo(args->data, insert_index,
                                         beg, end);
    args->idx++;
  }
  return args->result == CURLE_OK;
}

static void schannel_session_free(void *sessionid)
{
  /* this is expected to be called under sessionid lock */
  struct Curl_schannel_cred *cred = sessionid;

  if(cred) {
    cred->refcount--;
    if(cred->refcount == 0) {
      Curl_pSecFn->FreeCredentialsHandle(&cred->cred_handle);
      curlx_free(cred->sni_hostname);
      if(cred->client_cert_store) {
        CertCloseStore(cred->client_cert_store, 0);
        cred->client_cert_store = NULL;
      }
      Curl_safefree(cred);
    }
  }
}

static CURLcode schannel_connect_step3(struct Curl_cfilter *cf,
                                       struct Curl_easy *data)
{
  struct ssl_connect_data *connssl = cf->ctx;
  struct schannel_ssl_backend_data *backend =
    (struct schannel_ssl_backend_data *)connssl->backend;
  CURLcode result = CURLE_OK;
  SECURITY_STATUS sspi_status = SEC_E_OK;
  CERT_CONTEXT *ccert_context = NULL;
#ifdef HAS_ALPN_SCHANNEL
  SecPkgContext_ApplicationProtocol alpn_result;
#endif

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

  DEBUGF(infof(data, "schannel: SSL/TLS connection with %s port %d (step 3/3)",
               connssl->peer.hostname, connssl->peer.port));

  if(!backend->cred)
    return CURLE_SSL_CONNECT_ERROR;

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

#ifdef HAS_ALPN_SCHANNEL
  if(backend->use_alpn) {
    sspi_status =
      Curl_pSecFn->QueryContextAttributes(&backend->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) {
      if(backend->recv_renegotiating &&
         connssl->negotiated.alpn &&
         strncmp(connssl->negotiated.alpn,
                 (const char *)alpn_result.ProtocolId,
                 alpn_result.ProtocolIdSize)) {
        /* Renegotiation selected a different protocol now, we cannot
         * deal with this */
        failf(data, "schannel: server selected an ALPN protocol too late");
        return CURLE_SSL_CONNECT_ERROR;
      }
      Curl_alpn_set_negotiated(cf, data, connssl, alpn_result.ProtocolId,
                               alpn_result.ProtocolIdSize);
    }
    else {
      if(!backend->recv_renegotiating)
        Curl_alpn_set_negotiated(cf, data, connssl, NULL, 0);
    }
  }
#endif

  /* save the current session data for possible reuse */
  if(Curl_ssl_scache_use(cf, data)) {
    Curl_ssl_scache_lock(data);
    /* Up ref count since call takes ownership */
    backend->cred->refcount++;
    result = Curl_ssl_scache_add_obj(cf, data, connssl->peer.scache_key,
                                     backend->cred, schannel_session_free);
    Curl_ssl_scache_unlock(data);
    if(result)
      return result;
  }

  if(data->set.ssl.certinfo) {
    int certs_count = 0;
    sspi_status =
      Curl_pSecFn->QueryContextAttributes(&backend->ctxt->ctxt_handle,
                                          SECPKG_ATTR_REMOTE_CERT_CONTEXT,
                                          &ccert_context);

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

    traverse_cert_store(ccert_context, cert_counter_callback, &certs_count);
    if(certs_count > MAX_ALLOWED_CERT_AMOUNT) {
      failf(data, "%d certificates is more than allowed (%u)",
            certs_count, MAX_ALLOWED_CERT_AMOUNT);
      CertFreeCertificateContext(ccert_context);
      return CURLE_SSL_CONNECT_ERROR;
    }

    result = Curl_ssl_init_certinfo(data, certs_count);
    if(!result) {
      struct Adder_args args;
      args.data = data;
      args.idx = 0;
      args.certs_count = certs_count;
      args.result = CURLE_OK;
      traverse_cert_store(ccert_context, add_cert_to_certinfo, &args);
      result = args.result;
    }
    CertFreeCertificateContext(ccert_context);
    if(result)
      return result;
  }

  connssl->connecting_state = ssl_connect_done;

  return CURLE_OK;
}

static CURLcode schannel_connect(struct Curl_cfilter *cf,
                                 struct Curl_easy *data,
                                 bool *done)
{
  struct ssl_connect_data *connssl = cf->ctx;
  CURLcode result;

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

  *done = FALSE;

  if(ssl_connect_1 == connssl->connecting_state) {
    result = schannel_connect_step1(cf, data);
    if(result)
      return result;
  }

  if(ssl_connect_2 == connssl->connecting_state) {
    result = schannel_connect_step2(cf, data);
    if(result)
      return result;
  }

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

  if(ssl_connect_done == connssl->connecting_state) {
    connssl->state = ssl_connection_complete;

#ifdef SECPKG_ATTR_ENDPOINT_BINDINGS  /* mingw-w64 v9+. MS SDK 7.0A+. */
    /* When SSPI is used in combination with Schannel
     * we need the Schannel context to create the Schannel
     * binding to pass the IIS extended protection checks.
     * Available on Windows 7 or later.
     */
    {
      struct schannel_ssl_backend_data *backend =
        (struct schannel_ssl_backend_data *)connssl->backend;
      DEBUGASSERT(backend);
      cf->conn->sslContext = &backend->ctxt->ctxt_handle;
    }
#endif

    *done = TRUE;
  }

  return CURLE_OK;
}

enum schannel_renegotiate_caller_t {
  SCH_RENEG_CALLER_IS_RECV,
  SCH_RENEG_CALLER_IS_SEND
};

#define MAX_RENEG_BLOCK_TIME (7 * 1000) /* 7 seconds in milliseconds */

/* This function renegotiates the connection due to a server request received
   by schannel_recv. This function returns CURLE_AGAIN if the renegotiation is
   incomplete. In that case, we remain in the renegotiation (connecting) stage
   and future calls to schannel_recv and schannel_send must call this function
   first to complete the renegotiation. */
static CURLcode schannel_recv_renegotiate(
  struct Curl_cfilter *cf, struct Curl_easy *data,
  enum schannel_renegotiate_caller_t caller)
{
  CURLcode result;
  curl_socket_t sockfd;
  struct ssl_connect_data *connssl = cf->ctx;
  struct schannel_ssl_backend_data *backend =
    (struct schannel_ssl_backend_data *)connssl->backend;
  struct schannel_renegotiate_state *rs = &backend->renegotiate_state;

  if(!backend || !backend->recv_renegotiating) {
    failf(data, "schannel: unexpected call to schannel_recv_renegotiate");
    return CURLE_SSL_CONNECT_ERROR;
  }
  DEBUGASSERT(caller <= SCH_RENEG_CALLER_IS_SEND);
  if(caller == SCH_RENEG_CALLER_IS_RECV)
    SCH_DEV(infof(data, "schannel: renegotiation caller is schannel_recv"));
  else
    SCH_DEV(infof(data, "schannel: renegotiation caller is schannel_send"));

  sockfd = Curl_conn_cf_get_socket(cf, data);

  if(sockfd == CURL_SOCKET_BAD) {
    failf(data, "schannel: renegotiation missing socket");
    return CURLE_SSL_CONNECT_ERROR;
  }

  if(!rs->started) { /* new renegotiation */
    infof(data, "schannel: renegotiating SSL/TLS connection");
    DEBUGASSERT(connssl->state == ssl_connection_complete);
    DEBUGASSERT(connssl->connecting_state == ssl_connect_done);
    connssl->state = ssl_connection_negotiating;
    connssl->connecting_state = ssl_connect_2;
    memset(rs, 0, sizeof(*rs));
    rs->io_need = CURL_SSL_IO_NEED_SEND;
    rs->start_time = *Curl_pgrs_now(data);
    rs->started = TRUE;
  }

  for(;;) {
    bool block_read, block_write, blocking, done;
    curl_socket_t readfd, writefd;
    timediff_t elapsed;

    elapsed = curlx_ptimediff_ms(Curl_pgrs_now(data), &rs->start_time);
    if(elapsed >= MAX_RENEG_BLOCK_TIME) {
      failf(data, "schannel: renegotiation timeout");
      result = CURLE_SSL_CONNECT_ERROR;
      break;
    }

    /* the current io_need state may have been overwritten since the last time
       this function was called. restore the io_need state needed to continue
       the renegotiation. */

    connssl->io_need = rs->io_need;

    result = schannel_connect(cf, data, &done);

    rs->io_need = connssl->io_need;

    if(!result && !done)
      result = CURLE_AGAIN;

    if(result != CURLE_AGAIN)
      break;

    readfd = (rs->io_need & CURL_SSL_IO_NEED_RECV) ? sockfd : CURL_SOCKET_BAD;
    writefd = (rs->io_need & CURL_SSL_IO_NEED_SEND) ? sockfd : CURL_SOCKET_BAD;

    if(readfd == CURL_SOCKET_BAD && writefd == CURL_SOCKET_BAD)
      continue;

    /* connect should not have requested io read and write together */
    DEBUGASSERT(readfd == CURL_SOCKET_BAD || writefd == CURL_SOCKET_BAD);

    /* This function is partially blocking to avoid a stoppage that would
     * occur if the user is waiting on the socket only in one direction.
     *
     * For example, if the user has called recv then they may not be waiting
     * for a writeable socket and vice versa, so we block to avoid that.
     *
     * In practice a wait is unlikely to occur. For caller recv if handshake
     * data needs to be sent then we block for a writeable socket that should
     * be writeable immediately except for OS resource constraints. For caller
     * send if handshake data needs to be received then we block for a readable
     * socket, which could take some time, but it is more likely the user has
     * called recv since they had called it prior (only recv can start
     * renegotiation and probably the user is going to call it again to get
     * more of their data before calling send).
     */

    block_read = (caller == SCH_RENEG_CALLER_IS_SEND) ? TRUE : FALSE;
    block_write = (caller == SCH_RENEG_CALLER_IS_RECV) ? TRUE : FALSE;

    blocking = (block_read && (readfd != CURL_SOCKET_BAD)) ||
               (block_write && (writefd != CURL_SOCKET_BAD));

    SCH_DEV_SHOWBOOL(block_read);
    SCH_DEV_SHOWBOOL(block_write);
    SCH_DEV_SHOWBOOL(blocking);

    for(;;) {
      int what;
      timediff_t timeout_ms, remaining;

      result = Curl_pgrsUpdate(data);
      if(result)
        break;

      elapsed = curlx_ptimediff_ms(Curl_pgrs_now(data), &rs->start_time);
      if(elapsed >= MAX_RENEG_BLOCK_TIME) {
        failf(data, "schannel: renegotiation timeout");
        result = CURLE_SSL_CONNECT_ERROR;
        break;
      }
      remaining = MAX_RENEG_BLOCK_TIME - elapsed;

      if(blocking) {
        timeout_ms = Curl_timeleft_ms(data);

        if(timeout_ms < 0) {
          result = CURLE_OPERATION_TIMEDOUT;
          break;
        }

        /* the blocking is in intervals so that the progress function can be
           called every second */
        if(!timeout_ms || timeout_ms > 1000)
          timeout_ms = 1000;

        if(timeout_ms > remaining)
          timeout_ms = remaining;
      }
      else
        timeout_ms = 0;

      SCH_DEV(infof(data, "schannel: renegotiation wait until socket is"
                    "%s%s for up to %" FMT_TIMEDIFF_T " ms",
                    ((readfd != CURL_SOCKET_BAD) ? " readable" : ""),
                    ((writefd != CURL_SOCKET_BAD) ? " writeable" : ""),
                    timeout_ms));

      what = Curl_socket_check(readfd, CURL_SOCKET_BAD, writefd, timeout_ms);

      if(what > 0 && (what & (CURL_CSELECT_IN | CURL_CSELECT_OUT))) {
        SCH_DEV(infof(data, "schannel: renegotiation socket %s%s",
                      ((what & CURL_CSELECT_IN) ? "CURL_CSELECT_IN " : ""),
                      ((what & CURL_CSELECT_OUT) ? "CURL_CSELECT_OUT " : "")));
        result = CURLE_AGAIN;
        break;
      }
      else if(!what) {
        SCH_DEV(infof(data, "schannel: renegotiation socket timeout"));
        if(blocking)
          continue;
        else
          return CURLE_AGAIN;
      }

      failf(data, "schannel: socket error during renegotiation");
      result = CURLE_SSL_CONNECT_ERROR;
      break;
    }
    if(result != CURLE_AGAIN)
      break;
  }

  DEBUGASSERT(result != CURLE_AGAIN);

  rs->started = FALSE;
  backend->recv_renegotiating = FALSE;
  connssl->io_need = CURL_SSL_IO_NEED_NONE;

  if(result)
    failf(data, "schannel: renegotiation failed");
  else
    infof(data, "schannel: SSL/TLS connection renegotiated");

  return result;
}

static CURLcode schannel_send(struct Curl_cfilter *cf, struct Curl_easy *data,
                              const void *buf, size_t len, size_t *pnwritten)
{
  size_t data_len = 0;
  unsigned char *ptr = NULL;
  struct ssl_connect_data *connssl = cf->ctx;
  SecBuffer outbuf[4];
  SecBufferDesc outbuf_desc;
  SECURITY_STATUS sspi_status = SEC_E_OK;
  CURLcode result = CURLE_OK;
  struct schannel_ssl_backend_data *backend =
    (struct schannel_ssl_backend_data *)connssl->backend;

  DEBUGASSERT(backend);
  *pnwritten = 0;

  if(backend->recv_renegotiating) {
    result = schannel_recv_renegotiate(cf, data, SCH_RENEG_CALLER_IS_SEND);
    if(result)
      return result;
  }

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

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

  /* calculate the complete message length and allocate a buffer for it */
  data_len = backend->stream_sizes.cbHeader + len +
    backend->stream_sizes.cbTrailer;
  ptr = (unsigned char *)curlx_malloc(data_len);
  if(!ptr) {
    return CURLE_OUT_OF_MEMORY;
  }

  /* setup output buffers (header, data, trailer, empty) */
  InitSecBuffer(&outbuf[0], SECBUFFER_STREAM_HEADER,
                ptr, backend->stream_sizes.cbHeader);
  InitSecBuffer(&outbuf[1], SECBUFFER_DATA,
                ptr + backend->stream_sizes.cbHeader, curlx_uztoul(len));
  InitSecBuffer(&outbuf[2], SECBUFFER_STREAM_TRAILER,
                ptr + backend->stream_sizes.cbHeader + len,
                backend->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://learn.microsoft.com/windows/win32/api/sspi/nf-sspi-encryptmessage */
  sspi_status = Curl_pSecFn->EncryptMessage(&backend->ctxt->ctxt_handle, 0,
                                            &outbuf_desc, 0);

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

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

    /*
      it is 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
      cannot read any of it.

      If we wanted to buffer the unwritten encrypted bytes, we would
      tell the client that all data it has requested to be sent has been
      sent. The unwritten encrypted bytes would be the first bytes to
      send on the next invocation.
      Here's the catch with this - if we tell the client that all the
      bytes have been sent, does the client call this method again to
      send the buffered data?  Looking at who calls this function, it
      seems the answer is NO.
    */

    /* send entire message or fail */
    while(len > *pnwritten) {
      size_t this_write = 0;
      int what;
      timediff_t timeout_ms = Curl_timeleft_ms(data);
      if(timeout_ms < 0) {
        /* we already got the timeout */
        failf(data, "schannel: timed out sending data (bytes sent: %zu)",
              *pnwritten);
        result = CURLE_OPERATION_TIMEDOUT;
        break;
      }
      else if(!timeout_ms)
        timeout_ms = TIMEDIFF_T_MAX;
      what = SOCKET_WRITABLE(Curl_conn_cf_get_socket(cf, data), timeout_ms);
      if(what < 0) {
        /* fatal error */
        failf(data, "select/poll on SSL socket, errno: %d", SOCKERRNO);
        result = CURLE_SEND_ERROR;
        break;
      }
      else if(what == 0) {
        failf(data, "schannel: timed out sending data (bytes sent: %zu)",
              *pnwritten);
        result = CURLE_OPERATION_TIMEDOUT;
        break;
      }
      /* socket is writable */

       result = Curl_conn_cf_send(cf->next, data,
                                  (const uint8_t *)ptr + *pnwritten,
                                  len - *pnwritten,
                                  FALSE, &this_write);
      if(result == CURLE_AGAIN)
        continue;
      else if(result) {
        break;
      }

      *pnwritten += this_write;
    }
  }
  else if(sspi_status == SEC_E_INSUFFICIENT_MEMORY) {
    result = CURLE_OUT_OF_MEMORY;
  }
  else {
    result = CURLE_SEND_ERROR;
  }

  Curl_safefree(ptr);

  if(len == *pnwritten)
    /* Encrypted message including header, data and trailer entirely sent.
       The return value is the number of unencrypted bytes that were sent. */
    *pnwritten = outbuf[1].cbBuffer;

  return result;
}

static CURLcode schannel_recv(struct Curl_cfilter *cf, struct Curl_easy *data,
                              char *buf, size_t len, size_t *pnread)
{
  size_t size = 0;
  size_t nread = 0;
  struct ssl_connect_data *connssl = cf->ctx;
  SecBuffer inbuf[4];
  SecBufferDesc inbuf_desc;
  SECURITY_STATUS sspi_status = SEC_E_OK;
  struct schannel_ssl_backend_data *backend =
    (struct schannel_ssl_backend_data *)connssl->backend;
  CURLcode result = CURLE_OK;

  DEBUGASSERT(backend);
  *pnread = 0;

  if(backend->recv_renegotiating) {
    result = schannel_recv_renegotiate(cf, data, SCH_RENEG_CALLER_IS_RECV);
    if(result)
      return result;
  }

  /****************************************************************************
   * Do not return or set backend->recv_unrecoverable_err unless in the
   * cleanup. The pattern for return error is set *err, optional infof, goto
   * cleanup.
   *
   * Some verbose debug messages are wrapped by SCH_DEV() instead of DEBUGF()
   * and only shown if CURL_SCHANNEL_DEV_DEBUG was defined at build time. These
   * messages are extra verbose and intended for curl developers debugging
   * Schannel recv decryption.
   *
   * Our priority is to always return as much decrypted data to the caller as
   * possible, even if an error occurs. The state of the decrypted buffer must
   * always be valid. Transfer of decrypted data to the caller's buffer is
   * handled in the cleanup.
   */

  SCH_DEV(infof(data, "schannel: client wants to read %zu bytes", len));

  if(len && len <= backend->decdata.offset) {
    SCH_DEV(infof(data,
                  "schannel: enough decrypted data is already available"));
    goto cleanup;
  }
  else if(backend->recv_unrecoverable_err) {
    result = backend->recv_unrecoverable_err;
    infof(data, "schannel: an unrecoverable error occurred in a prior call");
    goto cleanup;
  }
  else if(backend->recv_sspi_close_notify) {
    /* once a server has indicated shutdown there is no more encrypted data */
    infof(data, "schannel: server indicated shutdown in a prior call");
    goto cleanup;
  }
  /* it is debatable what to return when !len. Regardless we cannot return
     immediately because there may be data to decrypt (in the case we want to
     decrypt all encrypted cached data) so handle !len later in cleanup.
  */
  else if(len && !backend->recv_connection_closed) {
    /* the encrypted buffer must be large enough to hold all the bytes
       requested and some TLS record overhead. 'len' is a buffer size, so this
       integer math cannot overflow. */
    const size_t min_encdata_length = len + CURL_SCHANNEL_BUFFER_FREE_SIZE;

    /* make sure encrypt buffer fits the requested amount of data */
    result = ensure_encoding_size(data, &backend->encdata, min_encdata_length);
    if(result)
      goto cleanup;

    SCH_DEV(infof(data,
                  "schannel: encrypted data buffer: offset %zu length %zu",
                  backend->encdata.offset, backend->encdata.length));

    /* read encrypted data from socket */
    result = Curl_conn_cf_recv(cf->next, data,
                               (char *)(backend->encdata.buffer +
                                        backend->encdata.offset),
                               backend->encdata.length -
                               backend->encdata.offset,
                               &nread);
    if(result) {
      if(result == CURLE_AGAIN)
        SCH_DEV(infof(data, "schannel: recv returned CURLE_AGAIN"));
      else {
        infof(data, "schannel: recv returned error %d", result);
        backend->recv_unrecoverable_err = result;
      }
    }
    else if(nread == 0) {
      backend->recv_connection_closed = TRUE;
      DEBUGF(infof(data, "schannel: server closed the connection"));
    }
    else {
      backend->encdata.offset += nread;
      backend->encdata_is_incomplete = FALSE;
      SCH_DEV(infof(data, "schannel: encrypted data got %zu", nread));
    }
  }

  SCH_DEV(infof(data, "schannel: encrypted data buffer: offset %zu length %zu",
                backend->encdata.offset, backend->encdata.length));

  /* decrypt loop */
  while(backend->encdata.offset > 0 && sspi_status == SEC_E_OK &&
        (!len || backend->decdata.offset < len ||
         backend->recv_connection_closed)) {
    /* prepare data buffer for DecryptMessage call */
    InitSecBuffer(&inbuf[0], SECBUFFER_DATA, backend->encdata.buffer,
                  curlx_uztoul(backend->encdata.offset));

    /* we need 3 more empty input buffers for possible output */
    InitSecBuffer(&inbuf[1], SECBUFFER_EMPTY, NULL, 0);
    InitSecBuffer(&inbuf[2], SECBUFFER_EMPTY, NULL, 0);
    InitSecBuffer(&inbuf[3], SECBUFFER_EMPTY, NULL, 0);
    InitSecBufferDesc(&inbuf_desc, inbuf, 4);

    /* https://learn.microsoft.com/windows/win32/api/sspi/nf-sspi-decryptmessage
     */
    sspi_status = Curl_pSecFn->DecryptMessage(&backend->ctxt->ctxt_handle,
                                              &inbuf_desc, 0, NULL);

    /* check if everything went fine (server may want to renegotiate
       or shutdown the connection context) */
    if(sspi_status == SEC_E_OK || sspi_status == SEC_I_RENEGOTIATE ||
       sspi_status == SEC_I_CONTEXT_EXPIRED) {
      /* check for successfully decrypted data, even before actual
         renegotiation or shutdown of the connection context */
      if(inbuf[1].BufferType == SECBUFFER_DATA) {
        SCH_DEV(infof(data, "schannel: decrypted data length: %lu",
                      inbuf[1].cbBuffer));

        /* ensure the decode buffer fits the received amount of data */
        result = ensure_decoding_size(data, &backend->decdata,
                                      inbuf[1].cbBuffer, len);
        if(result)
          goto cleanup;

        /* copy decrypted data to internal buffer */
        size = inbuf[1].cbBuffer;
        if(size) {
          memcpy(backend->decdata.buffer + backend->decdata.offset,
                 inbuf[1].pvBuffer, size);
          backend->decdata.offset += size;
        }

        SCH_DEV(infof(data, "schannel: decrypted data added: %zu", size));
        SCH_DEV(infof(data,
                      "schannel: decrypted cached: offset %zu length %zu",
                      backend->decdata.offset, backend->decdata.length));
      }

      /* check for remaining encrypted data */
      if(inbuf[3].BufferType == SECBUFFER_EXTRA && inbuf[3].cbBuffer > 0) {
        SCH_DEV(infof(data, "schannel: encrypted data length: %lu",
                      inbuf[3].cbBuffer));

        /* check if the remaining data is less than the total amount
         * and therefore begins after the already processed data
         */
        if(backend->encdata.offset > inbuf[3].cbBuffer) {
          /* move remaining encrypted data forward to the beginning of
             buffer */
          memmove(backend->encdata.buffer,
                  (backend->encdata.buffer + backend->encdata.offset) -
                  inbuf[3].cbBuffer, inbuf[3].cbBuffer);
          backend->encdata.offset = inbuf[3].cbBuffer;
        }

        SCH_DEV(infof(data,
                      "schannel: encrypted cached: offset %zu length %zu",
                      backend->encdata.offset, backend->encdata.length));
      }
      else {
        /* reset encrypted buffer offset, because there is no data remaining */
        backend->encdata.offset = 0;
      }

      /* check if server wants to renegotiate the connection context */
      if(sspi_status == SEC_I_RENEGOTIATE) {
        infof(data, "schannel: remote party requests renegotiation");
        if(result && result != CURLE_AGAIN) {
          infof(data, "schannel: cannot renegotiate, an error is pending");
          goto cleanup;
        }

        backend->recv_renegotiating = TRUE;
        result = schannel_recv_renegotiate(cf, data, SCH_RENEG_CALLER_IS_RECV);
        if(result)
          goto cleanup;

        /* now retry receiving data */
        sspi_status = SEC_E_OK;
        continue;
      }
      /* check if the server closed the connection */
      else if(sspi_status == SEC_I_CONTEXT_EXPIRED) {
        /* In Windows 2000 SEC_I_CONTEXT_EXPIRED (close_notify) is not
           returned so we have to work around that in cleanup. */
        backend->recv_sspi_close_notify = TRUE;
        if(!backend->recv_connection_closed)
          backend->recv_connection_closed = TRUE;
        /* We received the close notify fine, any error we got
         * from the lower filters afterwards (e.g. the socket), is not
         * an error on the TLS data stream. That one ended here. */
        if(result == CURLE_RECV_ERROR)
          result = CURLE_OK;
        infof(data,
              "schannel: server close notification received (close_notify)");
        goto cleanup;
      }
    }
    else if(sspi_status == SEC_E_INCOMPLETE_MESSAGE) {
      backend->encdata_is_incomplete = TRUE;
      if(!result)
        result = CURLE_AGAIN;
      SCH_DEV(infof(data, "schannel: failed to decrypt data, need more data"));
      goto cleanup;
    }
    else {
      char buffer[STRERROR_LEN];
      failf(data, "schannel: failed to read data from server: %s",
            Curl_sspi_strerror(sspi_status, buffer, sizeof(buffer)));
      result = CURLE_RECV_ERROR;
      goto cleanup;
    }
  }

  SCH_DEV(infof(data, "schannel: encrypted data buffer: offset %zu length %zu",
                backend->encdata.offset, backend->encdata.length));

  SCH_DEV(infof(data, "schannel: decrypted data buffer: offset %zu length %zu",
                backend->decdata.offset, backend->decdata.length));

cleanup:
  /* Warning- there is no guarantee the encdata state is valid at this point */
  SCH_DEV(infof(data, "schannel: schannel_recv cleanup"));

  /* Error if the connection has closed without a close_notify.

     The behavior here is a matter of debate. We do not want to be vulnerable
     to a truncation attack however there is some browser precedent for
     ignoring the close_notify for compatibility reasons.
  */
  if(len && !backend->decdata.offset && backend->recv_connection_closed &&
     !backend->recv_sspi_close_notify) {
    result = CURLE_RECV_ERROR;
    failf(data, "schannel: server closed abruptly (missing close_notify)");
  }

  /* Any error other than CURLE_AGAIN is an unrecoverable error. */
  if(result && result != CURLE_AGAIN)
    backend->recv_unrecoverable_err = result;

  size = len < backend->decdata.offset ? len : backend->decdata.offset;
  if(size) {
    memcpy(buf, backend->decdata.buffer, size);
    memmove(backend->decdata.buffer, backend->decdata.buffer + size,
            backend->decdata.offset - size);
    backend->decdata.offset -= size;
    SCH_DEV(infof(data, "schannel: decrypted data returned %zu", size));
    SCH_DEV(infof(data,
                  "schannel: decrypted data buffer: offset %zu length %zu",
                  backend->decdata.offset, backend->decdata.length));
    *pnread = size;
    return CURLE_OK;
  }

  if(!result && !backend->recv_connection_closed)
    result = CURLE_AGAIN;

  /* it is debatable what to return when !len. We could return whatever error
     we got from decryption but instead we override here so the return is
     consistent.
  */
  if(!len)
    return CURLE_OK;

  return result;
}

static bool schannel_data_pending(struct Curl_cfilter *cf,
                                  const struct Curl_easy *data)
{
  const struct ssl_connect_data *connssl = cf->ctx;
  struct schannel_ssl_backend_data *backend =
    (struct schannel_ssl_backend_data *)connssl->backend;

  (void)data;
  DEBUGASSERT(backend);

  if(backend->ctxt) /* SSL/TLS is in use */
    return backend->decdata.offset > 0 ||
           (backend->encdata.offset > 0 && !backend->encdata_is_incomplete) ||
           backend->recv_connection_closed ||
           backend->recv_sspi_close_notify ||
           backend->recv_unrecoverable_err;
  else
    return FALSE;
}

/* shut down the SSL connection and clean up related memory.
   this function can be called multiple times on the same connection including
   if the SSL connection failed (eg connection made but failed handshake). */
static CURLcode schannel_shutdown(struct Curl_cfilter *cf,
                                  struct Curl_easy *data,
                                  bool send_shutdown, bool *done)
{
  /* See https://learn.microsoft.com/windows/win32/secauthn/shutting-down-an-schannel-connection
   * Shutting Down an Schannel Connection
   */
  struct ssl_connect_data *connssl = cf->ctx;
  struct schannel_ssl_backend_data *backend =
    (struct schannel_ssl_backend_data *)connssl->backend;
  CURLcode result = CURLE_OK;

  if(cf->shutdown) {
    *done = TRUE;
    return CURLE_OK;
  }

  DEBUGASSERT(data);
  DEBUGASSERT(backend);

  /* Not supported in schannel */
  (void)send_shutdown;

  *done = FALSE;
  if(backend->ctxt) {
    infof(data, "schannel: shutting down SSL/TLS connection with %s port %d",
          connssl->peer.hostname, connssl->peer.port);
  }

  if(!backend->ctxt || cf->shutdown) {
    *done = TRUE;
    goto out;
  }

  if(backend->cred && backend->ctxt && !backend->sent_shutdown) {
    SecBufferDesc BuffDesc;
    SecBuffer Buffer;
    SECURITY_STATUS sspi_status;
    SecBuffer outbuf;
    SecBufferDesc outbuf_desc;
    DWORD dwshut = SCHANNEL_SHUTDOWN;

    InitSecBuffer(&Buffer, SECBUFFER_TOKEN, &dwshut, sizeof(dwshut));
    InitSecBufferDesc(&BuffDesc, &Buffer, 1);

    sspi_status = Curl_pSecFn->ApplyControlToken(&backend->ctxt->ctxt_handle,
                                                 &BuffDesc);

    if(sspi_status != SEC_E_OK) {
      char buffer[STRERROR_LEN];
      failf(data, "schannel: ApplyControlToken failure: %s",
            Curl_sspi_strerror(sspi_status, buffer, sizeof(buffer)));
      result = CURLE_SEND_ERROR;
      goto out;
    }

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

    sspi_status = Curl_pSecFn->InitializeSecurityContext(
      &backend->cred->cred_handle,
      &backend->ctxt->ctxt_handle,
      backend->cred->sni_hostname,
      backend->req_flags,
      0,
      0,
      NULL,
      0,
      &backend->ctxt->ctxt_handle,
      &outbuf_desc,
      &backend->ret_flags, NULL);

    if((sspi_status == SEC_E_OK) || (sspi_status == SEC_I_CONTEXT_EXPIRED)) {
      /* send close message which is in output buffer */
      size_t written;

      result = Curl_conn_cf_send(cf->next, data,
                                 (const uint8_t *)outbuf.pvBuffer,
                                 outbuf.cbBuffer,
                                 FALSE, &written);
      Curl_pSecFn->FreeContextBuffer(outbuf.pvBuffer);
      if(!result) {
        if(written < outbuf.cbBuffer) {
          result = CURLE_SEND_ERROR;
          failf(data, "schannel: failed to send close msg: %s"
                " (bytes written: %zu)", curl_easy_strerror(result), written);
          goto out;
        }
        backend->sent_shutdown = TRUE;
        *done = TRUE;
      }
      else if(result == CURLE_AGAIN) {
        connssl->io_need = CURL_SSL_IO_NEED_SEND;
        result = CURLE_OK;
        goto out;
      }
      else {
        if(!backend->recv_connection_closed) {
          result = CURLE_SEND_ERROR;
          failf(data, "schannel: error sending close msg: %d", result);
          goto out;
        }
        /* Looks like server already closed the connection.
         * An error to send our close notify is not a failure. */
        *done = TRUE;
        result = CURLE_OK;
      }
    }
  }

  /* If the connection seems open and we have not seen the close notify
   * from the server yet, try to receive it. */
  if(backend->cred && backend->ctxt &&
     !backend->recv_sspi_close_notify && !backend->recv_connection_closed) {
    char buffer[1024];
    size_t nread;

    result = schannel_recv(cf, data, buffer, sizeof(buffer), &nread);
    if(result == CURLE_AGAIN) {
      connssl->io_need = CURL_SSL_IO_NEED_RECV;
    }
    else if(result) {
      CURL_TRC_CF(data, cf, "SSL shutdown, error %d", result);
      result = CURLE_RECV_ERROR;
    }
    else if(nread == 0) {
      /* We got the close notify alert and are done. */
      backend->recv_connection_closed = TRUE;
      *done = TRUE;
    }
    else {
      /* still data coming in? */
    }
  }

out:
  cf->shutdown = (result || *done);
  return result;
}

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

  DEBUGASSERT(data);
  DEBUGASSERT(backend);

  /* free SSPI Schannel API security context handle */
  if(backend->ctxt) {
    DEBUGF(infof(data, "schannel: clear security context handle"));
    Curl_pSecFn->DeleteSecurityContext(&backend->ctxt->ctxt_handle);
    Curl_safefree(backend->ctxt);
  }

  /* free SSPI Schannel API credential handle */
  if(backend->cred) {
    Curl_ssl_scache_lock(data);
    schannel_session_free(backend->cred);
    Curl_ssl_scache_unlock(data);
    backend->cred = NULL;
  }

  /* free internal buffer for received encrypted data */
  if(backend->encdata.buffer) {
    Curl_safefree(backend->encdata.buffer);
    backend->encdata.length = 0;
    backend->encdata.offset = 0;
    backend->encdata_is_incomplete = FALSE;
  }

  /* free internal buffer for received decrypted data */
  if(backend->decdata.buffer) {
    Curl_safefree(backend->decdata.buffer);
    backend->decdata.length = 0;
    backend->decdata.offset = 0;
  }
}

static int schannel_init(void)
{
#ifdef HAS_ALPN_SCHANNEL
  typedef const char *(APIENTRY *WINE_GET_VERSION_FN)(void);
#if defined(__clang__) && __clang_major__ >= 16
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wcast-function-type-strict"
#endif
  WINE_GET_VERSION_FN p_wine_get_version =
    CURLX_FUNCTION_CAST(WINE_GET_VERSION_FN,
      GetProcAddress(GetModuleHandle(TEXT("ntdll")), "wine_get_version"));
#if defined(__clang__) && __clang_major__ >= 16
#pragma clang diagnostic pop
#endif
  if(p_wine_get_version) {  /* WINE detected */
    curl_off_t ver = 0;
    const char *wine_version = p_wine_get_version();  /* e.g. "6.0.2" */
    /* Assume ALPN support with WINE 6.0 or upper */
    if(wine_version)
      curlx_str_number(&wine_version, &ver, 20);
    s_win_has_alpn = (ver >= 6);
  }
  else {
    /* ALPN is supported on Windows 8.1 / Server 2012 R2 and above. */
    s_win_has_alpn = curlx_verify_windows_version(6, 3, 0, PLATFORM_WINNT,
                                                  VERSION_GREATER_THAN_EQUAL);
  }
#endif /* HAS_ALPN_SCHANNEL */

  return Curl_sspi_global_init() == CURLE_OK ? 1 : 0;
}

static void schannel_cleanup(void)
{
  Curl_sspi_global_cleanup();
}

static size_t schannel_version(char *buffer, size_t size)
{
  return curl_msnprintf(buffer, size, "Schannel");
}

static CURLcode schannel_random(struct Curl_easy *data,
                                unsigned char *entropy, size_t length)
{
  (void)data;

  return Curl_win32_random(entropy, length);
}

static void schannel_checksum(const unsigned char *input,
                              size_t inputlen,
                              unsigned char *checksum,
                              size_t checksumlen,
                              DWORD provType,
                              const unsigned int algId)
{
  HCRYPTPROV hProv = 0;
  HCRYPTHASH hHash = 0;

  /* since this can fail in multiple ways, zero memory first so we never
   * return old data
   */
  memset(checksum, 0, checksumlen);

  if(!CryptAcquireContext(&hProv, NULL, NULL, provType,
                          CRYPT_VERIFYCONTEXT | CRYPT_SILENT))
    return; /* failed */

  do {
    DWORD cbHashSize = 0;
    DWORD dwHashSizeLen = (DWORD)sizeof(cbHashSize);
    DWORD dwChecksumLen = (DWORD)checksumlen;

    if(!CryptCreateHash(hProv, algId, 0, 0, &hHash))
      break; /* failed */

    if(!CryptHashData(hHash, input, (DWORD)inputlen, 0))
      break; /* failed */

    /* get hash size */
    if(!CryptGetHashParam(hHash, HP_HASHSIZE, (BYTE *)&cbHashSize,
                          &dwHashSizeLen, 0))
      break; /* failed */

    /* check hash size */
    if(checksumlen < cbHashSize)
      break; /* failed */

    if(CryptGetHashParam(hHash, HP_HASHVAL, checksum, &dwChecksumLen, 0))
      break; /* failed */
  } while(0);

  if(hHash)
    CryptDestroyHash(hHash);

  if(hProv)
    CryptReleaseContext(hProv, 0);
}

static CURLcode schannel_sha256sum(const unsigned char *input,
                                   size_t inputlen,
                                   unsigned char *sha256sum,
                                   size_t sha256len)
{
  schannel_checksum(input, inputlen, sha256sum, sha256len,
                    PROV_RSA_AES, CALG_SHA_256);
  return CURLE_OK;
}

static void *schannel_get_internals(struct ssl_connect_data *connssl,
                                    CURLINFO info)
{
  struct schannel_ssl_backend_data *backend =
    (struct schannel_ssl_backend_data *)connssl->backend;
  (void)info;
  DEBUGASSERT(backend);
  return &backend->ctxt->ctxt_handle;
}

HCERTSTORE Curl_schannel_get_cached_cert_store(struct Curl_cfilter *cf,
                                               struct Curl_easy *data)
{
  struct ssl_primary_config *conn_config = Curl_ssl_cf_get_primary_config(cf);
  struct Curl_multi *multi = data->multi;
  const struct curl_blob *ca_info_blob = conn_config->ca_info_blob;
  struct schannel_cert_share *share;
  const struct ssl_general_config *cfg = &data->set.general_ssl;
  timediff_t timeout_ms;
  unsigned char info_blob_digest[CURL_SHA256_DIGEST_LENGTH];

  DEBUGASSERT(multi);

  if(!multi) {
    return NULL;
  }

  share = Curl_hash_pick(&multi->proto_hash,
                         CURL_UNCONST(MPROTO_SCHANNEL_CERT_SHARE_KEY),
                         sizeof(MPROTO_SCHANNEL_CERT_SHARE_KEY) - 1);
  if(!share || !share->cert_store) {
    return NULL;
  }

  /* zero ca_cache_timeout completely disables caching */
  if(!cfg->ca_cache_timeout) {
    return NULL;
  }

  /* check for cache timeout by using the cached_x509_store_expired timediff
     calculation pattern from openssl.c.
     negative timeout means retain forever. */
  timeout_ms = cfg->ca_cache_timeout * (timediff_t)1000;
  if(timeout_ms >= 0) {
    timediff_t elapsed_ms =
      curlx_ptimediff_ms(Curl_pgrs_now(data), &share->time);
    if(elapsed_ms >= timeout_ms) {
      return NULL;
    }
  }

  if(ca_info_blob) {
    if(share->CAinfo_blob_size != ca_info_blob->len) {
      return NULL;
    }
    schannel_sha256sum((const unsigned char *)ca_info_blob->data,
                       ca_info_blob->len,
                       info_blob_digest,
                       CURL_SHA256_DIGEST_LENGTH);
    if(memcmp(share->CAinfo_blob_digest, info_blob_digest,
              CURL_SHA256_DIGEST_LENGTH)) {
      return NULL;
    }
  }
  else {
    if(!conn_config->CAfile || !share->CAfile ||
       strcmp(share->CAfile, conn_config->CAfile)) {
      return NULL;
    }
  }

  return share->cert_store;
}

static void schannel_cert_share_free(void *key, size_t key_len, void *p)
{
  struct schannel_cert_share *share = p;
  DEBUGASSERT(key_len == (sizeof(MPROTO_SCHANNEL_CERT_SHARE_KEY) - 1));
  DEBUGASSERT(!memcmp(MPROTO_SCHANNEL_CERT_SHARE_KEY, key, key_len));
  (void)key;
  (void)key_len;
  if(share->cert_store) {
    CertCloseStore(share->cert_store, 0);
  }
  curlx_free(share->CAfile);
  curlx_free(share);
}

bool Curl_schannel_set_cached_cert_store(struct Curl_cfilter *cf,
                                         struct Curl_easy *data,
                                         HCERTSTORE cert_store)
{
  struct ssl_primary_config *conn_config = Curl_ssl_cf_get_primary_config(cf);
  struct Curl_multi *multi = data->multi;
  const struct curl_blob *ca_info_blob = conn_config->ca_info_blob;
  struct schannel_cert_share *share;
  size_t CAinfo_blob_size = 0;
  char *CAfile = NULL;

  DEBUGASSERT(multi);

  if(!multi) {
    return FALSE;
  }

  share = Curl_hash_pick(&multi->proto_hash,
                         CURL_UNCONST(MPROTO_SCHANNEL_CERT_SHARE_KEY),
                         sizeof(MPROTO_SCHANNEL_CERT_SHARE_KEY) - 1);
  if(!share) {
    share = curlx_calloc(1, sizeof(*share));
    if(!share) {
      return FALSE;
    }
    if(!Curl_hash_add2(&multi->proto_hash,
                       CURL_UNCONST(MPROTO_SCHANNEL_CERT_SHARE_KEY),
                       sizeof(MPROTO_SCHANNEL_CERT_SHARE_KEY) - 1,
                       share, schannel_cert_share_free)) {
      curlx_free(share);
      return FALSE;
    }
  }

  if(ca_info_blob) {
    schannel_sha256sum((const unsigned char *)ca_info_blob->data,
                       ca_info_blob->len,
                       share->CAinfo_blob_digest,
                       CURL_SHA256_DIGEST_LENGTH);
    CAinfo_blob_size = ca_info_blob->len;
  }
  else {
    if(conn_config->CAfile) {
      CAfile = curlx_strdup(conn_config->CAfile);
      if(!CAfile) {
        return FALSE;
      }
    }
  }

  /* free old cache data */
  if(share->cert_store) {
    CertCloseStore(share->cert_store, 0);
  }
  curlx_free(share->CAfile);

  share->time = curlx_now();
  share->cert_store = cert_store;
  share->CAinfo_blob_size = CAinfo_blob_size;
  share->CAfile = CAfile;
  return TRUE;
}

const struct Curl_ssl Curl_ssl_schannel = {
  { CURLSSLBACKEND_SCHANNEL, "schannel" }, /* info */

  SSLSUPP_CERTINFO |
  SSLSUPP_CAINFO_BLOB |
  SSLSUPP_PINNEDPUBKEY |
  SSLSUPP_CA_CACHE |
  SSLSUPP_HTTPS_PROXY |
  SSLSUPP_CIPHER_LIST,

  sizeof(struct schannel_ssl_backend_data),

  schannel_init,                     /* init */
  schannel_cleanup,                  /* cleanup */
  schannel_version,                  /* version */
  schannel_shutdown,                 /* shutdown */
  schannel_data_pending,             /* data_pending */
  schannel_random,                   /* random */
  NULL,                              /* cert_status_request */
  schannel_connect,                  /* connect */
  Curl_ssl_adjust_pollset,           /* adjust_pollset */
  schannel_get_internals,            /* get_internals */
  schannel_close,                    /* close_one */
  NULL,                              /* close_all */
  NULL,                              /* set_engine */
  NULL,                              /* set_engine_default */
  NULL,                              /* engines_list */
  schannel_sha256sum,                /* sha256sum */
  schannel_recv,                     /* recv decrypted data */
  schannel_send,                     /* send data to encrypt */
  NULL,                              /* get_channel_binding */
};

#endif /* USE_SCHANNEL */
