aboutsummaryrefslogtreecommitdiff
path: root/lib/curl_schannel.c
blob: 55fe92a39bf305713bdeae5d0c3b9b225d8c7ec5 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
/***************************************************************************
 *                                  _   _ ____  _
 *  Project                     ___| | | |  _ \| |
 *                             / __| | | | |_) | |
 *                            | (__| |_| |  _ <| |___
 *                             \___|\___/|_| \_\_____|
 *
 * Copyright (C) 2012, Marc Hoersken, <info@marc-hoersken.de>, et al.
 * Copyright (C) 2012, Daniel Stenberg, <daniel@haxx.se>, et al.
 *
 * This software is licensed as described in the file COPYING, which
 * you should have received as part of this distribution. The terms
 * are also available at http://curl.haxx.se/docs/copyright.html.
 *
 * You may opt to use, copy, modify, merge, publish, distribute and/or sell
 * copies of the Software, and permit persons to whom the Software is
 * furnished to do so, under the terms of the COPYING file.
 *
 * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
 * KIND, either express or implied.
 *
 ***************************************************************************/

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

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

/*
 * TODO list for TLS/SSL implementation:
 * - implement write buffering
 * - implement SSL/TLS shutdown
 * - implement client certificate authentication
 * - implement custom server certificate validation
 * - implement cipher/algorithm option
 *
 * Related articles on MSDN:
 * - Getting a Certificate for Schannel
 *   http://msdn.microsoft.com/en-us/library/windows/desktop/aa375447.aspx
 * - Specifying Schannel Ciphers and Cipher Strengths
 *   http://msdn.microsoft.com/en-us/library/windows/desktop/aa380161.aspx
 */

#include "setup.h"

#ifdef USE_SCHANNEL

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

#include "curl_sspi.h"
#include "curl_schannel.h"
#include "sslgen.h"
#include "sendf.h"
#include "connect.h" /* for the connect timeout */
#include "strerror.h"
#include "select.h" /* for the socket readyness */
#include "inet_pton.h" /* for IP addr SNI check */
#include "curl_multibyte.h"

#define _MPRINTF_REPLACE /* use our functions only */
#include <curl/mprintf.h>

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

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

static Curl_recv schannel_recv;
static Curl_send schannel_send;

static CURLcode
schannel_connect_step1(struct connectdata *conn, int sockindex)
{
  ssize_t written = -1;
  struct SessionHandle *data = conn->data;
  struct ssl_connect_data *connssl = &conn->ssl[sockindex];
  SecBuffer outbuf;
  SecBufferDesc outbuf_desc;
  SCHANNEL_CRED schannel_cred;
  SECURITY_STATUS sspi_status = SEC_E_OK;
  struct curl_schannel_cred *old_cred = NULL;
  struct in_addr addr;
#ifdef ENABLE_IPV6
  struct in6_addr addr6;
#endif
  TCHAR *host_name;

  infof(data, "schannel: connecting to %s:%hu (step 1/3)\n",
        conn->host.name, conn->remote_port);

  /* check for an existing re-usable credential handle */
  if(!Curl_ssl_getsessionid(conn, (void**)&old_cred, NULL)) {
    connssl->cred = old_cred;
    infof(data, "schannel: re-using existing credential handle\n");
  }
  else {
    /* setup Schannel API options */
    memset(&schannel_cred, 0, sizeof(schannel_cred));
    schannel_cred.dwVersion = SCHANNEL_CRED_VERSION;

    if(data->set.ssl.verifypeer) {
      schannel_cred.dwFlags = SCH_CRED_AUTO_CRED_VALIDATION |
                              SCH_CRED_REVOCATION_CHECK_CHAIN;
      infof(data, "schannel: checking server certificate revocation\n");
    }
    else {
      schannel_cred.dwFlags = SCH_CRED_MANUAL_CRED_VALIDATION |
                              SCH_CRED_IGNORE_NO_REVOCATION_CHECK |
                              SCH_CRED_IGNORE_REVOCATION_OFFLINE;
      infof(data, "schannel: disable server certificate revocation checks\n");
    }

    if(Curl_inet_pton(AF_INET, conn->host.name, &addr) ||
#ifdef ENABLE_IPV6
       Curl_inet_pton(AF_INET6, conn->host.name, &addr6) ||
#endif
       data->set.ssl.verifyhost < 2) {
      schannel_cred.dwFlags |= SCH_CRED_NO_SERVERNAME_CHECK;
      infof(data, "schannel: using IP address, disable SNI servername "
            "check\n");
    }

    switch(data->set.ssl.version) {
      case CURL_SSLVERSION_TLSv1:
        schannel_cred.grbitEnabledProtocols = SP_PROT_TLS1_0_CLIENT |
                                              SP_PROT_TLS1_1_CLIENT |
                                              SP_PROT_TLS1_2_CLIENT;
        break;
      case CURL_SSLVERSION_SSLv3:
        schannel_cred.grbitEnabledProtocols = SP_PROT_SSL3_CLIENT;
        break;
      case CURL_SSLVERSION_SSLv2:
        schannel_cred.grbitEnabledProtocols = SP_PROT_SSL2_CLIENT;
        break;
    }

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

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

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

  /* setup output buffer */
  outbuf.pvBuffer = NULL;
  outbuf.cbBuffer = 0;
  outbuf.BufferType = SECBUFFER_EMPTY;

  outbuf_desc.pBuffers = &outbuf;
  outbuf_desc.cBuffers = 1;
  outbuf_desc.ulVersion = SECBUFFER_VERSION;

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

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

#ifdef UNICODE
  host_name = Curl_convert_UTF8_to_wchar(conn->host.name);
  if(!host_name)
    return CURLE_OUT_OF_MEMORY;
#else
  host_name = conn->host.name;
#endif

  /* http://msdn.microsoft.com/en-us/library/windows/desktop/aa375924.aspx */

  sspi_status = s_pSecFn->InitializeSecurityContext(
    &connssl->cred->cred_handle, NULL, host_name,
    connssl->req_flags, 0, 0, NULL, 0, &connssl->ctxt->ctxt_handle,
    &outbuf_desc, &connssl->ret_flags, &connssl->ctxt->time_stamp);

#ifdef UNICODE
  free(host_name);
#endif

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

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

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

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

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

  return CURLE_OK;
}

static CURLcode
schannel_connect_step2(struct connectdata *conn, int sockindex)
{
  int i;
  ssize_t nread = -1, written = -1;
  struct SessionHandle *data = conn->data;
  struct ssl_connect_data *connssl = &conn->ssl[sockindex];
  SecBuffer outbuf[2];
  SecBufferDesc outbuf_desc;
  SecBuffer inbuf[2];
  SecBufferDesc inbuf_desc;
  SECURITY_STATUS sspi_status = SEC_E_OK;
  TCHAR *host_name;

  infof(data, "schannel: connecting to %s:%hu (step 2/3)\n",
        conn->host.name, conn->remote_port);

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

  /* read encrypted handshake data from socket */
  nread = sread(conn->sock[sockindex],
                connssl->encdata_buffer + connssl->encdata_offset,
                connssl->encdata_length - connssl->encdata_offset);
  if(nread > 0) {
    /* increase encrypted data buffer offset */
    connssl->encdata_offset += nread;
  }
  else if(connssl->connecting_state != ssl_connect_2_writing) {
    if(nread < 0) {
      connssl->connecting_state = ssl_connect_2_reading;
      infof(data, "schannel: failed to receive handshake, need more data\n");
      return CURLE_OK;
    }
    else if(nread == 0) {
      failf(data, "schannel: failed to receive handshake, connection failed");
      return CURLE_SSL_CONNECT_ERROR;
    }
  }

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

  /* setup input buffers */
  inbuf[0].pvBuffer = malloc(connssl->encdata_offset);
  inbuf[0].cbBuffer = connssl->encdata_offset;
  inbuf[0].BufferType = SECBUFFER_TOKEN;

  inbuf[1].pvBuffer = NULL;
  inbuf[1].cbBuffer = 0;
  inbuf[1].BufferType = SECBUFFER_EMPTY;

  inbuf_desc.pBuffers = &inbuf[0];
  inbuf_desc.cBuffers = 2;
  inbuf_desc.ulVersion = SECBUFFER_VERSION;

  /* setup output buffers */
  outbuf[0].pvBuffer = NULL;
  outbuf[0].cbBuffer = 0;
  outbuf[0].BufferType = SECBUFFER_TOKEN;

  outbuf[1].pvBuffer = NULL;
  outbuf[1].cbBuffer = 0;
  outbuf[1].BufferType = SECBUFFER_ALERT;

  outbuf_desc.pBuffers = &outbuf[0];
  outbuf_desc.cBuffers = 2;
  outbuf_desc.ulVersion = SECBUFFER_VERSION;

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

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

#ifdef UNICODE
  host_name = Curl_convert_UTF8_to_wchar(conn->host.name);
  if(!host_name)
    return CURLE_OUT_OF_MEMORY;
#else
  host_name = conn->host.name;
#endif

  /* http://msdn.microsoft.com/en-us/library/windows/desktop/aa375924.aspx */

  sspi_status = s_pSecFn->InitializeSecurityContext(
    &connssl->cred->cred_handle, &connssl->ctxt->ctxt_handle,
    host_name, connssl->req_flags, 0, 0, &inbuf_desc, 0, NULL,
    &outbuf_desc, &connssl->ret_flags, &connssl->ctxt->time_stamp);

#ifdef UNICODE
  free(host_name);
#endif

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

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

  /* check if the handshake needs to be continued */
  if(sspi_status == SEC_I_CONTINUE_NEEDED || sspi_status == SEC_E_OK) {
    for(i = 0; i < 2; i++) {
      /* search for handshake tokens that need to be send */
      if(outbuf[i].BufferType == SECBUFFER_TOKEN && outbuf[i].cbBuffer > 0) {
        infof(data, "schannel: sending next handshake data: "
              "sending %lu bytes...\n", outbuf[i].cbBuffer);

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

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

  /* check if there was additional remaining encrypted data */
  if(inbuf[1].BufferType == SECBUFFER_EXTRA && inbuf[1].cbBuffer > 0) {
    infof(data, "schannel: encrypted data length: %lu\n", inbuf[1].cbBuffer);

    /* check if the remaining data is less than the total amount
     * and therefore begins after the already processed data
     */
    if(connssl->encdata_offset > inbuf[1].cbBuffer) {
      memmove(connssl->encdata_buffer,
              (connssl->encdata_buffer + connssl->encdata_offset) -
                inbuf[1].cbBuffer, inbuf[1].cbBuffer);
      connssl->encdata_offset = inbuf[1].cbBuffer;
    }
  }
  else {
    connssl->encdata_offset = 0;
  }

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

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

  return CURLE_OK;
}

static CURLcode
schannel_connect_step3(struct connectdata *conn, int sockindex)
{
  CURLcode retcode = CURLE_OK;
  struct SessionHandle *data = conn->data;
  struct ssl_connect_data *connssl = &conn->ssl[sockindex];
  struct curl_schannel_cred *old_cred = NULL;
  int incache;

  DEBUGASSERT(ssl_connect_3 == connssl->connecting_state);

  infof(data, "schannel: connecting to %s:%hu (step 3/3)\n",
        conn->host.name, conn->remote_port);

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

  /* save the current session data for possible re-use */
  incache = !(Curl_ssl_getsessionid(conn, (void**)&old_cred, NULL));
  if(incache) {
    if(old_cred != connssl->cred) {
      infof(data, "schannel: old credential handle is stale, removing\n");
      Curl_ssl_delsessionid(conn, (void*)old_cred);
      incache = FALSE;
    }
  }
  if(!incache) {
    retcode = Curl_ssl_addsessionid(conn, (void*)connssl->cred,
                                    sizeof(struct curl_schannel_cred));
    if(retcode) {
      failf(data, "schannel: failed to store credential handle");
      return retcode;
    }
    else {
      infof(data, "schannel: stored crendential handle\n");
    }
  }

  connssl->connecting_state = ssl_connect_done;

  return CURLE_OK;
}

static CURLcode
schannel_connect_common(struct connectdata *conn, int sockindex,
                        bool nonblocking, bool *done)
{
  CURLcode retcode;
  struct SessionHandle *data = conn->data;
  struct ssl_connect_data *connssl = &conn->ssl[sockindex];
  curl_socket_t sockfd = conn->sock[sockindex];
  long timeout_ms;
  int what;

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

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

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

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

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

    /* check out how much more time we're allowed */
    timeout_ms = Curl_timeleft(data, NULL, TRUE);

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

    /* if ssl is expecting something, check if it's available. */
    if(connssl->connecting_state == ssl_connect_2_reading
       || connssl->connecting_state == ssl_connect_2_writing) {

      curl_socket_t writefd = ssl_connect_2_writing ==
        connssl->connecting_state ? sockfd : CURL_SOCKET_BAD;
      curl_socket_t readfd = ssl_connect_2_reading ==
        connssl->connecting_state ? sockfd : CURL_SOCKET_BAD;

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

    /* Run transaction, and return to the caller if it failed or if
     * this connection is part of a multi handle and this loop would
     * execute again. This permits the owner of a multi handle to
     * abort a connection attempt before step2 has completed while
     * ensuring that a client using select() or epoll() will always
     * have a valid fdset to wait on.
     */
    retcode = schannel_connect_step2(conn, sockindex);
    if(retcode || (nonblocking &&
                   (ssl_connect_2 == connssl->connecting_state ||
                    ssl_connect_2_reading == connssl->connecting_state ||
                    ssl_connect_2_writing == connssl->connecting_state)))
      return retcode;

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

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

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

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

  return CURLE_OK;
}

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

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

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

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

  /* setup output buffers (header, data, trailer, empty) */
  outbuf[0].pvBuffer = data;
  outbuf[0].cbBuffer = connssl->stream_sizes.cbHeader;
  outbuf[0].BufferType = SECBUFFER_STREAM_HEADER;

  outbuf[1].pvBuffer = data + connssl->stream_sizes.cbHeader;
  outbuf[1].cbBuffer = len;
  outbuf[1].BufferType = SECBUFFER_DATA;

  outbuf[2].pvBuffer = data + connssl->stream_sizes.cbHeader + len;
  outbuf[2].cbBuffer = connssl->stream_sizes.cbTrailer;
  outbuf[2].BufferType = SECBUFFER_STREAM_TRAILER;

  outbuf[3].pvBuffer = NULL;
  outbuf[3].cbBuffer = 0;
  outbuf[3].BufferType = SECBUFFER_EMPTY;

  outbuf_desc.pBuffers = &outbuf[0];
  outbuf_desc.cBuffers = 4;
  outbuf_desc.ulVersion = SECBUFFER_VERSION;

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

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

  /* check if the message was encrypted */
  if(sspi_status == SEC_E_OK) {
    /* send the encrypted message including header, data and trailer */
    len = outbuf[0].cbBuffer + outbuf[1].cbBuffer + outbuf[2].cbBuffer;
    written = swrite(conn->sock[sockindex], data, len);
    /* TODO: implement write buffering */
  }
  else if(sspi_status == SEC_E_INSUFFICIENT_MEMORY) {
    *err = CURLE_OUT_OF_MEMORY;
  }
  else{
    *err = CURLE_SEND_ERROR;
  }

  free(data);

  return written;
}

static ssize_t
schannel_recv(struct connectdata *conn, int sockindex,
              char *buf, size_t len, CURLcode *err)
{
  size_t size = 0;
  ssize_t nread = 0, ret = -1;
  CURLcode retcode;
  struct SessionHandle *data = conn->data;
  struct ssl_connect_data *connssl = &conn->ssl[sockindex];
  bool done = FALSE;
  SecBuffer inbuf[4];
  SecBufferDesc inbuf_desc;
  SECURITY_STATUS sspi_status = SEC_E_OK;

  infof(data, "schannel: client wants to read %zu bytes\n", len);
  *err = CURLE_OK;

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

  /* increase buffer in order to fit the requested amount of data */
  while(connssl->encdata_length - connssl->encdata_offset <
        CURL_SCHANNEL_BUFFER_STEP_SIZE || connssl->encdata_length < len) {
    /* increase internal encrypted data buffer */
    connssl->encdata_length += CURL_SCHANNEL_BUFFER_STEP_SIZE;
    connssl->encdata_buffer = realloc(connssl->encdata_buffer,
                                      connssl->encdata_length);
    if(connssl->encdata_buffer == NULL) {
      failf(data, "schannel: unable to re-allocate memory");
      *err = CURLE_OUT_OF_MEMORY;
      return -1;
    }
  }

  /* read encrypted data from socket */
  infof(data, "schannel: encrypted data buffer: offset %zu length %zu\n",
        connssl->encdata_offset, connssl->encdata_length);
  size = connssl->encdata_length - connssl->encdata_offset;
  if(size > 0) {
    nread = sread(conn->sock[sockindex],
                  connssl->encdata_buffer + connssl->encdata_offset, size);
    infof(data, "schannel: encrypted data got %zd\n", nread);

    /* check for received data */
    if(nread > 0) {
      /* increase encrypted data buffer offset */
      connssl->encdata_offset += nread;
    }
    else if(connssl->encdata_offset == 0) {
      if(nread == 0)
        ret = 0;
      else
        *err = CURLE_AGAIN;
    }
  }

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

  /* check if we still have some data in our buffers */
  while(connssl->encdata_offset > 0 && sspi_status == SEC_E_OK) {
    /* prepare data buffer for DecryptMessage call */
    inbuf[0].pvBuffer = connssl->encdata_buffer;
    inbuf[0].cbBuffer = connssl->encdata_offset;
    inbuf[0].BufferType = SECBUFFER_DATA;

    /* we need 3 more empty input buffers for possible output */
    inbuf[1].pvBuffer = NULL;
    inbuf[1].cbBuffer = 0;
    inbuf[1].BufferType = SECBUFFER_EMPTY;

    inbuf[2].pvBuffer = NULL;
    inbuf[2].cbBuffer = 0;
    inbuf[2].BufferType = SECBUFFER_EMPTY;

    inbuf[3].pvBuffer = NULL;
    inbuf[3].cbBuffer = 0;
    inbuf[3].BufferType = SECBUFFER_EMPTY;

    inbuf_desc.pBuffers = &inbuf[0];
    inbuf_desc.cBuffers = 4;
    inbuf_desc.ulVersion = SECBUFFER_VERSION;

    /* http://msdn.microsoft.com/en-us/library/windows/desktop/aa375348.aspx */
    sspi_status = s_pSecFn->DecryptMessage(&connssl->ctxt->ctxt_handle,
                                           &inbuf_desc, 0, NULL);

    /* check if we need more data */
    if(sspi_status == SEC_E_INCOMPLETE_MESSAGE) {
      infof(data, "schannel: failed to decrypt data, need more data\n");
      *err = CURLE_AGAIN;
      return -1;
    }

    /* check if everything went fine (server may want to renegotiate
       context) */
    if(sspi_status == SEC_E_OK || sspi_status == SEC_I_RENEGOTIATE ||
                                  sspi_status == SEC_I_CONTEXT_EXPIRED) {
      /* check for successfully decrypted data */
      if(inbuf[1].BufferType == SECBUFFER_DATA) {
        infof(data, "schannel: decrypted data length: %lu\n",
              inbuf[1].cbBuffer);

        /* increase buffer in order to fit the received amount of data */
        size = inbuf[1].cbBuffer > CURL_SCHANNEL_BUFFER_STEP_SIZE ?
               inbuf[1].cbBuffer : CURL_SCHANNEL_BUFFER_STEP_SIZE;
        while(connssl->decdata_length - connssl->decdata_offset < size ||
              connssl->decdata_length < len) {
          /* increase internal decrypted data buffer */
          connssl->decdata_length += size;
          connssl->decdata_buffer = realloc(connssl->decdata_buffer,
                                            connssl->decdata_length);
          if(connssl->decdata_buffer == NULL) {
            failf(data, "schannel: unable to re-allocate memory");
            *err = CURLE_OUT_OF_MEMORY;
            return -1;
          }
        }

        /* copy decrypted data to internal buffer */
        size = inbuf[1].cbBuffer;
        if(size > 0) {
          memcpy(connssl->decdata_buffer + connssl->decdata_offset,
                 inbuf[1].pvBuffer, size);
          connssl->decdata_offset += size;
        }

        infof(data, "schannel: decrypted data added: %zu\n", size);
        infof(data, "schannel: decrypted data cached: offset %zu length %zu\n",
              connssl->decdata_offset, connssl->decdata_length);
      }

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

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

        infof(data, "schannel: encrypted data cached: offset %zu length %zu\n",
              connssl->encdata_offset, connssl->encdata_length);
      }
      else{
        /* reset encrypted buffer offset, because there is no data remaining */
        connssl->encdata_offset = 0;
      }
    }

    /* check if server wants to renegotiate the connection context */
    if(sspi_status == SEC_I_RENEGOTIATE) {
      infof(data, "schannel: client needs to renegotiate with server\n");

      /* begin renegotiation */
      connssl->state = ssl_connection_negotiating;
      connssl->connecting_state = ssl_connect_2_writing;
      retcode = schannel_connect_common(conn, sockindex, FALSE, &done);
      if(retcode)
        *err = retcode;
      else /* now retry receiving data */
        return schannel_recv(conn, sockindex, buf, len, err);
    }
  }

  /* copy requested decrypted data to supplied buffer */
  size = len < connssl->decdata_offset ? len : connssl->decdata_offset;
  if(size > 0) {
    memcpy(buf, connssl->decdata_buffer, size);
    ret = size;

    /* move remaining decrypted data forward to the beginning of buffer */
    memmove(connssl->decdata_buffer, connssl->decdata_buffer + size,
            connssl->decdata_offset - size);
    connssl->decdata_offset -= size;
  }

  /* reduce internal buffer length to reduce memory usage */
  if(connssl->encdata_length > CURL_SCHANNEL_BUFFER_INIT_SIZE) {
    connssl->encdata_length =
      connssl->encdata_offset > CURL_SCHANNEL_BUFFER_INIT_SIZE ?
      connssl->encdata_offset : CURL_SCHANNEL_BUFFER_INIT_SIZE;
    connssl->encdata_buffer = realloc(connssl->encdata_buffer,
                                      connssl->encdata_length);
  }
  if(connssl->decdata_length > CURL_SCHANNEL_BUFFER_INIT_SIZE) {
    connssl->decdata_length =
      connssl->decdata_offset > CURL_SCHANNEL_BUFFER_INIT_SIZE ?
      connssl->decdata_offset : CURL_SCHANNEL_BUFFER_INIT_SIZE;
    connssl->decdata_buffer = realloc(connssl->decdata_buffer,
                                      connssl->decdata_length);
  }

  /* check if the server closed the connection */
  if(ret <= 0 && ( /* special check for Windows 2000 Professional */
      sspi_status == SEC_I_CONTEXT_EXPIRED || (sspi_status == SEC_E_OK &&
        connssl->encdata_offset > 0 && connssl->encdata_buffer[0] == 0x15))) {
    infof(data, "schannel: server closed the connection\n");
    *err = CURLE_OK;
    return 0;
  }

  /* check if something went wrong and we need to return an error */
  if(ret < 0 && sspi_status != SEC_E_OK) {
    infof(data, "schannel: failed to read data from server: %s\n",
          Curl_sspi_strerror(conn, sspi_status));
    *err = CURLE_RECV_ERROR;
    return -1;
  }

  return ret;
}

CURLcode
Curl_schannel_connect_nonblocking(struct connectdata *conn, int sockindex,
                                  bool *done)
{
  return schannel_connect_common(conn, sockindex, TRUE, done);
}

CURLcode
Curl_schannel_connect(struct connectdata *conn, int sockindex)
{
  CURLcode retcode;
  bool done = FALSE;

  retcode = schannel_connect_common(conn, sockindex, FALSE, &done);
  if(retcode)
    return retcode;

  DEBUGASSERT(done);

  return CURLE_OK;
}

bool Curl_schannel_data_pending(const struct connectdata *conn, int sockindex)
{
  const struct ssl_connect_data *connssl = &conn->ssl[sockindex];

  if(connssl->use) /* SSL is in use */
    return (connssl->encdata_offset > 0 ||
            connssl->decdata_offset > 0 ) ? TRUE : FALSE;
  else
    return FALSE;
}

void Curl_schannel_close(struct connectdata *conn, int sockindex)
{
  struct SessionHandle *data = conn->data;
  struct ssl_connect_data *connssl = &conn->ssl[sockindex];

  infof(data, "schannel: Closing connection with %s:%hu\n",
        conn->host.name, conn->remote_port);

  /* free SSPI Schannel API security context handle */
  if(connssl->ctxt) {
    s_pSecFn->DeleteSecurityContext(&connssl->ctxt->ctxt_handle);
    free(connssl->ctxt);
    connssl->ctxt = NULL;
  }

  /* free internal buffer for received encrypted data */
  if(connssl->encdata_buffer != NULL) {
    free(connssl->encdata_buffer);
    connssl->encdata_buffer = NULL;
    connssl->encdata_length = 0;
    connssl->encdata_offset = 0;
  }

  /* free internal buffer for received decrypted data */
  if(connssl->decdata_buffer != NULL) {
    free(connssl->decdata_buffer);
    connssl->decdata_buffer = NULL;
    connssl->decdata_length = 0;
    connssl->decdata_offset = 0;
  }
}

int Curl_schannel_shutdown(struct connectdata *conn, int sockindex)
{
  (void)conn;
  (void)sockindex;
  return CURLE_NOT_BUILT_IN; /* TODO: implement SSL/TLS shutdown */
}

void Curl_schannel_session_free(void *ptr)
{
  struct curl_schannel_cred *cred = ptr;

  if(cred) {
    s_pSecFn->FreeCredentialsHandle(&cred->cred_handle);
    free(cred);
  }
}

int Curl_schannel_init(void)
{
  return (Curl_sspi_global_init() == CURLE_OK ? 1 : 0);
}

void Curl_schannel_cleanup(void)
{
  Curl_sspi_global_cleanup();
}

size_t Curl_schannel_version(char *buffer, size_t size)
{
  size = snprintf(buffer, size, "SSL-Windows-native");

  return size;
}

#endif /* USE_SCHANNEL */