aboutsummaryrefslogtreecommitdiffstats
path: root/lib/ssl/doc/src/ssl.xml
blob: e88407dd6d3efc6778ff931b75bd1aad18d7a21e (plain) (blame)
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
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>1999</year><year>2017</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.

    </legalnotice>
    <title>ssl</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>ssl.xml</file>
  </header>
  <module>ssl</module>
  <modulesummary>Interface Functions for Secure Socket Layer</modulesummary>
  <description>
    <p>
      This module contains interface functions for the SSL/TLS/DTLS protocol.
      For detailed information about the supported standards see 
      <seealso marker="ssl_app">ssl(6)</seealso>.
    </p>
  </description>
    
  <section>
    <title>DATA TYPES</title>
    <p>The following data types are used in the functions for SSL/TLS/DTLS:</p>

    <taglist>

      <tag><c>boolean() =</c></tag>
      <item><p><c>true | false</c></p></item>

      <tag><c>option() =</c></tag>
      <item><p><c>socketoption() | ssl_option() | transport_option()</c></p>
      </item>

      <tag><c>socketoption() =</c></tag>
      <item><p><c>proplists:property()</c></p>
      <p>The default socket options are
      <c>[{mode,list},{packet, 0},{header, 0},{active, true}]</c>.</p>
      <p>For valid options, see the
      <seealso marker="kernel:inet">inet(3)</seealso>,
      <seealso marker="kernel:gen_tcp">gen_tcp(3)</seealso> and
      <seealso marker="kernel:gen_tcp">gen_udp(3)</seealso> 
      manual pages
      in Kernel. Note that stream oriented options such as packet are only relevant for SSL/TLS and not DTLS</p></item>

      <tag><marker id="type-ssloption"/><c>ssl_option() =</c></tag>
      <item>
	<p><c>{verify, verify_type()}</c></p>
	<p><c>| {verify_fun, {fun(), term()}}</c></p>
	<p><c>| {fail_if_no_peer_cert, boolean()}</c></p>
	<p><c>| {depth, integer()}</c></p>
	<p><c>| {cert, public_key:der_encoded()}</c></p>
	<p><c>| {certfile, path()}</c></p>
	<p><c>| {key, {'RSAPrivateKey'| 'DSAPrivateKey' | 'ECPrivateKey' 
	| 'PrivateKeyInfo', public_key:der_encoded()} |
	#{algorithm := rsa | dss | ecdsa,
	engine := crypto:engine_ref(), key_id := crypto:key_id(), password => crypto:password()}</c></p>
	<p><c>| {keyfile, path()}</c></p>
	<p><c>| {password, string()}</c></p>
	<p><c>| {cacerts, [public_key:der_encoded()]}</c></p>
	<p><c>| {cacertfile, path()}</c></p>
	<p><c>| {dh, public_key:der_encoded()}</c></p>
	<p><c>| {dhfile, path()}</c></p>
	<p><c>| {ciphers, ciphers()}</c></p>
	<p><c>| {user_lookup_fun, {fun(), term()}}, {psk_identity, string()},
	{srp_identity, {string(), string()}}</c></p>
	<p><c>| {reuse_sessions, boolean()}</c></p>
	<p><c>| {reuse_session, fun()} {next_protocols_advertised, [binary()]}</c></p>
	<p><c>| {client_preferred_next_protocols, {client | server,
	[binary()]} | {client | server, [binary()], binary()}}</c></p>
	<p><c>| {log_alert, boolean()}</c></p>
	<p><c>| {server_name_indication, hostname() | disable}</c></p>
	<p><c>| {sni_hosts, [{hostname(), [ssl_option()]}]}</c></p>
	<p><c>| {sni_fun, SNIfun::fun()}</c></p>
      </item>
      
      <tag><c>transport_option() =</c></tag>
      <item><p><c>{cb_info, {CallbackModule::atom(), DataTag::atom(),

      ClosedTag::atom(), ErrTag:atom()}}</c></p>
      <p>Defaults to <c>{gen_tcp, tcp, tcp_closed, tcp_error}</c> for TLS
      and <c>{gen_udp, udp, udp_closed, udp_error}</c> for DTLS. Can be used
      to customize the transport layer. For TLS the callback module must implement a
      reliable transport protocol, behave as <c>gen_tcp</c>, and have functions
      corresponding to <c>inet:setopts/2</c>, <c>inet:getopts/2</c>,
      <c>inet:peername/1</c>, <c>inet:sockname/1</c>, and <c>inet:port/1</c>.
      The callback <c>gen_tcp</c> is treated specially and calls <c>inet</c>
      directly. For DTLS this feature must be considered exprimental.</p>
      <taglist>
	<tag><c>CallbackModule =</c></tag>
	<item><p><c>atom()</c></p></item>
	<tag><c>DataTag =</c></tag>
	<item><p><c>atom()</c></p>
	<p>Used in socket data message.</p></item>
	<tag><c>ClosedTag =</c></tag>
	<item><p><c>atom()</c></p>
	<p>Used in socket close message.</p></item>
      </taglist>
      </item>

      <tag><c>verify_type() =</c></tag>
      <item><p><c>verify_none | verify_peer</c></p></item>

      <tag><c>path() =</c></tag>
      <item><p><c>string()</c></p>
      <p>Represents a file path.</p></item>

      <tag><c>public_key:der_encoded() =</c></tag>
      <item><p><c>binary()</c></p>
      <p>ASN.1 DER-encoded entity as an Erlang binary.</p></item>

      <tag><c>host() =</c></tag>
      <item><p><c>hostname() | ipaddress()</c></p></item>

      <tag><c>hostname() =</c></tag>
      <item><p><c>string() - DNS hostname</c></p></item>

      <tag><c>ip_address() =</c></tag>
      <item><p><c>{N1,N2,N3,N4} % IPv4 | {K1,K2,K3,K4,K5,K6,K7,K8} % IPv6
      </c></p></item>

      <tag><c>sslsocket() =</c></tag>
      <item><p>opaque()</p></item>
      
      <tag><marker id="type-protocol"/><c> protocol_version() =</c></tag>
      <item><p><c> ssl_tls_protocol() |  dtls_protocol() </c></p></item>
      
      <item><p><c>sslv3 | tlsv1 | 'tlsv1.1' | 'tlsv1.2'</c></p></item>

      <tag><marker id="type-protocol"/><c> dtls_protocol() =</c></tag>
      <item><p><c>'dtlsv1' | 'dtlsv1.2'</c></p></item>

      <tag><c>ciphers() =</c></tag>
      <item><p><c>= [ciphersuite()]</c></p>
      <p>Tuples and string formats accepted by versions
      before ssl-8.2.4 will be converted for backwards compatibility</p></item>

      <tag><c>ciphersuite() =</c></tag>
      <item><p><c>
      #{key_exchange := key_exchange(),
        cipher := cipher(),
        mac := MAC::hash() | aead,
        prf := PRF::hash() | default_prf} </c></p></item>

      <tag><c>key_exchange()=</c></tag>
      <item><p><c>rsa | dhe_dss | dhe_rsa | dh_anon | psk | dhe_psk
      | rsa_psk | srp_anon | srp_dss | srp_rsa | ecdh_anon | ecdh_ecdsa
      | ecdhe_ecdsa | ecdh_rsa | ecdhe_rsa</c></p></item>

      <tag><c>cipher() =</c></tag>
      <item><p><c>rc4_128 | des_cbc | '3des_ede_cbc'
      | aes_128_cbc | aes_256_cbc | aes_128_gcm | aes_256_gcm | chacha20_poly1305</c></p></item>

      <tag><c>hash() =</c></tag>
      <item><p><c>md5 | sha | sha224 | sha256 | sha348 | sha512</c></p></item>

      <tag><c>prf_random() =</c></tag>
      <item><p><c>client_random | server_random</c></p></item>

      <tag><c>cipher_filters() =</c></tag>
      <item><p><c> [{key_exchange | cipher | mac | prf, algo_filter()}])</c></p></item>

      <tag><c>algo_filter() =</c></tag>
      <item><p>fun(key_exchange() | cipher() | hash() | aead | default_prf) -> true | false </p></item>

      <tag><c>srp_param_type() =</c></tag>
      <item><p><c>srp_1024 | srp_1536 | srp_2048 | srp_3072
      | srp_4096 | srp_6144 | srp_8192</c></p></item>

      <tag><c>SNIfun::fun()</c></tag>
      <item><p><c>= fun(ServerName :: string()) -> [ssl_option()]</c></p></item>

      <tag><c>named_curve() =</c></tag>
      <item><p><c>sect571r1 | sect571k1 | secp521r1 | brainpoolP512r1
       | sect409k1 | sect409r1 | brainpoolP384r1 | secp384r1
       | sect283k1 | sect283r1 | brainpoolP256r1 | secp256k1 | secp256r1
       | sect239k1 | sect233k1 | sect233r1 | secp224k1 | secp224r1
       | sect193r1 | sect193r2 | secp192k1 | secp192r1 | sect163k1
       | sect163r1 | sect163r2 | secp160k1 | secp160r1 | secp160r2</c></p></item>

       <tag><c>hello_extensions() =</c></tag>
       <item><p><c>#{renegotiation_info => 
                   signature_algs => [{hash(), ecsda| rsa| dsa}] | undefined
		   alpn => binary() | undefined,
		   next_protocol_negotiation,
		   srp => string() | undefined,
		   ec_point_formats ,
		   elliptic_curves = [oid] | undefined
		   sni = string()} 
       }</c></p></item>
      

    </taglist>
  </section>

  <section>
    <title>TLS/DTLS OPTION DESCRIPTIONS - COMMON for SERVER and CLIENT</title>

    <p>The following options have the same meaning in the client and 
    the server:</p>
    
    <taglist>

      <tag><c>{protocol, tls | dtls}</c></tag>
      <item><p>Choose TLS or DTLS protocol for the transport layer security.
      Defaults to <c>tls</c> Introduced in OTP 20, DTLS support is considered
      experimental in this release. Other transports than UDP are not yet supported.</p></item>

      <tag><c>{handshake, hello | full}</c></tag>
      <item><p> Defaults to <c>full</c>. If hello is specified the handshake will
      pause after the hello message and give the user a possibility make decisions
      based on hello extensions before continuing or aborting the handshake by calling
      <seealso marker="#handshake_continue-3"> handshake_continue/3</seealso> or
      <seealso marker="#handshake_cancel-1"> handshake_cancel/1</seealso>
      </p></item>
      
      <tag><c>{cert, public_key:der_encoded()}</c></tag>
      <item><p>The DER-encoded users certificate. If this option
      is supplied, it overrides option <c>certfile</c>.</p></item>
      
      <tag><c>{certfile, path()}</c></tag>
      <item><p>Path to a file containing the user certificate.</p></item>
      
      <tag>
	<marker id="key_option_def"/>
	<c>{key, {'RSAPrivateKey'| 'DSAPrivateKey' | 'ECPrivateKey'
      |'PrivateKeyInfo', public_key:der_encoded()} |  #{algorithm := rsa | dss | ecdsa,
	engine := crypto:engine_ref(), key_id :=  crypto:key_id(), password => crypto:password()}</c></tag>
      <item><p>The DER-encoded user's private key or a map refering to a crypto
      engine and its key reference that optionally can be password protected,
      seealso <seealso marker="crypto:crypto#engine_load-4"> crypto:engine_load/4
      </seealso> and  <seealso marker="crypto:engine_load"> Crypto's Users Guide</seealso>. If this option 
      is supplied, it overrides option <c>keyfile</c>.</p></item>
      
      <tag><c>{keyfile, path()}</c></tag>
      <item><p>Path to the file containing the user's
      private PEM-encoded key. As PEM-files can contain several
      entries, this option defaults to the same file as given by
      option <c>certfile</c>.</p></item>

      <tag><c>{password, string()}</c></tag>
      <item><p>String containing the user's password. Only used if the 
      private keyfile is password-protected.</p></item>

      <tag><c>{ciphers, ciphers()}</c></tag>
      <item><p>Supported cipher suites. The function
      <c>cipher_suites/0</c> can be used to find all ciphers that are
      supported by default. <c>cipher_suites(all)</c> can be called
      to find all available cipher suites. Pre-Shared Key 
      (<url href="http://www.ietf.org/rfc/rfc4279.txt">RFC 4279</url> and
      <url href="http://www.ietf.org/rfc/rfc5487.txt">RFC 5487</url>), 
      Secure Remote Password 
      (<url href="http://www.ietf.org/rfc/rfc5054.txt">RFC 5054</url>), RC4 cipher suites,
      and anonymous cipher suites only work if explicitly enabled by
      this option; they are supported/enabled by the peer also.
      Anonymous cipher suites are supported for testing purposes
      only and are not be used when security matters.</p></item>

      <tag><c>{eccs, [named_curve()]}</c></tag>
      <item><p> Allows to specify the order of preference for named curves
      and to restrict their usage when using a cipher suite supporting them.
      </p></item>

      <tag><c>{secure_renegotiate, boolean()}</c></tag>
      <item><p>Specifies if to reject renegotiation attempt that does
      not live up to 
      <url href="http://www.ietf.org/rfc/rfc5746.txt">RFC 5746</url>. 
      By default <c>secure_renegotiate</c> is set to <c>true</c>, 
      that is, secure renegotiation is enforced. If set to <c>false</c> secure renegotiation
      will still be used if possible,
      but it falls back to insecure renegotiation if the peer
      does not support 
      <url href="http://www.ietf.org/rfc/rfc5746.txt">RFC 5746</url>.</p>
      </item>

      <tag><c>{depth, integer()}</c></tag>
      <item><p>Maximum number of non-self-issued
      intermediate certificates that can follow the peer certificate 
      in a valid certification path. So, if depth is 0 the PEER must 
      be signed by the trusted ROOT-CA directly; if 1 the path can 
      be PEER, CA, ROOT-CA; if 2 the path can be PEER, CA, CA, 
      ROOT-CA, and so on. The default value is 1.</p></item>

      <tag><marker id="verify_fun"/><c>{verify_fun, {Verifyfun :: fun(), InitialUserState ::
      term()}}</c></tag>
      <item><p>The verification fun is to be defined as follows:</p>

	<code>
fun(OtpCert :: #'OTPCertificate'{}, Event :: {bad_cert, Reason :: atom() | {revoked,
atom()}} |
	     {extension, #'Extension'{}}, InitialUserState :: term()) ->
	{valid, UserState :: term()} | {valid_peer, UserState :: term()} |
	{fail, Reason :: term()} | {unknown, UserState :: term()}.
	</code>

	<p>The verification fun is called during the X509-path
	validation when an error or an extension unknown to the SSL
	application is encountered. It is also called
	when a certificate is considered valid by the path validation
	to allow access to each certificate in the path to the user
	application. It differentiates between the peer
	certificate and the CA certificates by using <c>valid_peer</c> or
	<c>valid</c> as second argument to the verification fun. See the
	<seealso marker="public_key:public_key_records">public_key User's
	Guide</seealso> for definition of <c>#'OTPCertificate'{}</c> and
	<c>#'Extension'{}</c>.</p>

	<list type="bulleted">
	  <item><p>If the verify callback fun returns <c>{fail, Reason}</c>,
	  the verification process is immediately stopped, an alert is
	  sent to the peer, and the TLS/DTLS handshake terminates.</p></item>
	  <item><p>If the verify callback fun returns <c>{valid, UserState}</c>,
	  the verification process continues.</p></item> 
	  <item><p>If the verify callback fun always returns
	  <c>{valid, UserState}</c>, the TLS/DTLS handshake does not
	  terminate regarding verification failures and the connection is
	  established.</p></item>
	  <item><p>If called with an extension unknown to the user application,
	  return value <c>{unknown, UserState}</c> is to be used.</p>

	  <p>Note that if the fun returns <c>unknown</c> for an extension marked
	  as critical, validation will fail.</p>
	  </item>
	</list>

	<p>Default option <c>verify_fun</c> in <c>verify_peer mode</c>:</p>

      <code>
{fun(_,{bad_cert, _} = Reason, _) ->
	 {fail, Reason};
    (_,{extension, _}, UserState) ->
	 {unknown, UserState};
    (_, valid, UserState) ->
	 {valid, UserState};
    (_, valid_peer, UserState) ->
         {valid, UserState}
 end, []}
      </code>

      <p>Default option <c>verify_fun</c> in mode <c>verify_none</c>:</p>

       <code>
{fun(_,{bad_cert, _}, UserState) ->
	 {valid, UserState};
    (_,{extension, #'Extension'{critical = true}}, UserState) ->
	 {valid, UserState};
    (_,{extension, _}, UserState) ->
	 {unknown, UserState};
    (_, valid, UserState) ->
	 {valid, UserState};
    (_, valid_peer, UserState) ->
         {valid, UserState}
 end, []}
      </code>

      <p>The possible path validation errors are given on form
      <c>{bad_cert, Reason}</c> where <c>Reason</c> is:</p>

      <taglist>
	<tag><c>unknown_ca</c></tag>
	<item><p>No trusted CA was found in the trusted store. The trusted CA is
	normally a so called ROOT CA, which is a self-signed certificate. Trust can
	be claimed for an intermediate CA (trusted anchor does not have to be
	self-signed according to X-509) by using option <c>partial_chain</c>.</p>
	</item>

	<tag><c>selfsigned_peer</c></tag>
	<item><p>The chain consisted only of one self-signed certificate.</p></item>

	<tag><c>PKIX X-509-path validation error</c></tag>
	<item><p>For possible reasons, see <seealso
marker="public_key:public_key#pkix_path_validation-3">public_key:pkix_path_validation/3</seealso>
	</p></item>
      </taglist>
      </item>

      <tag><c>{crl_check, boolean() | peer | best_effort }</c></tag>
      <item>
	<p>Perform CRL (Certificate Revocation List) verification
	<seealso marker="public_key:public_key#pkix_crls_validate-3">
	(public_key:pkix_crls_validate/3)</seealso> on all the certificates during the path validation
	<seealso
	    marker="public_key:public_key#pkix_path_validation-3">(public_key:pkix_path_validation/3)
	</seealso>
	of the certificate chain. Defaults to <c>false</c>.</p>
	
	<taglist>
	  <tag><c>peer</c></tag>
	  <item>check is only performed on the peer certificate.</item>

	  <tag><c>best_effort</c></tag>
	  <item>if certificate revocation status can not be determined
	  it will be accepted as valid.</item>
	</taglist>

	<p>The CA certificates specified for the connection will be used to 
	construct the certificate chain validating the CRLs.</p>
 	
	<p>The CRLs will be fetched from a local or external cache. See
	<seealso marker="ssl:ssl_crl_cache_api">ssl_crl_cache_api(3)</seealso>.</p>
      </item>

      <tag><c>{crl_cache, {Module :: atom(), {DbHandle :: internal | term(), Args :: list()}}}</c></tag>
      <item>
	<p>Specify how to perform lookup and caching of certificate revocation lists.
	<c>Module</c> defaults to <seealso marker="ssl:ssl_crl_cache">ssl_crl_cache</seealso>
	with <c> DbHandle </c> being <c>internal</c> and an
	empty argument list.</p>

	<p>There are two implementations available:</p>

	<taglist>
	  <tag><c>ssl_crl_cache</c></tag>
	  <item>
	    <p>This module maintains a cache of CRLs.  CRLs can be
	    added to the cache using the function <seealso
	    marker="ssl:ssl_crl_cache#insert-1">ssl_crl_cache:insert/1</seealso>,
	    and optionally automatically fetched through HTTP if the
	    following argument is specified:</p>

	    <taglist>
	      <tag><c>{http, timeout()}</c></tag>
	      <item><p>
		Enables fetching of CRLs specified as http URIs in<seealso
		marker="public_key:public_key_records">X509 certificate extensions</seealso>.
		Requires the OTP inets application.</p>
	      </item>
	    </taglist>
	  </item>

	  <tag><c>ssl_crl_hash_dir</c></tag>
	  <item>
	    <p>This module makes use of a directory where CRLs are
	    stored in files named by the hash of the issuer name.</p>

	    <p>The file names consist of eight hexadecimal digits
	    followed by <c>.rN</c>, where <c>N</c> is an integer,
	    e.g. <c>1a2b3c4d.r0</c>.  For the first version of the
	    CRL, <c>N</c> starts at zero, and for each new version,
	    <c>N</c> is incremented by one.  The OpenSSL utility
	    <c>c_rehash</c> creates symlinks according to this
	    pattern.</p>

	    <p>For a given hash value, this module finds all
	    consecutive <c>.r*</c> files starting from zero, and those
	    files taken together make up the revocation list.  CRL
	    files whose <c>nextUpdate</c> fields are in the past, or
	    that are issued by a different CA that happens to have the
	    same name hash, are excluded.</p>

	    <p>The following argument is required:</p>

	    <taglist>
	      <tag><c>{dir, string()}</c></tag>
	      <item><p>Specifies the directory in which the CRLs can be found.</p></item>
	    </taglist>

	  </item>

	  <tag><c>max_handshake_size</c></tag>
	  <item>
	    <p>Integer (24 bits unsigned). Used to limit the size of
	    valid TLS handshake packets to avoid DoS attacks.
	    Defaults to 256*1024.</p>
          </item>
          
	</taglist>

      </item>

      <tag><c>{partial_chain, fun(Chain::[DerCert]) -> {trusted_ca, DerCert} |
      unknown_ca }</c></tag>
      <item><p>Claim an intermediate CA in the chain as trusted. TLS then
      performs <seealso
      marker="public_key:public_key#pkix_path_validation-3">public_key:pkix_path_validation/3</seealso>
      with the selected CA as trusted anchor and the rest of the chain.</p></item>

      <tag><c>{versions, [protocol_version()]}</c></tag>

      <item><p>TLS protocol versions supported by started clients and servers.
      This option overrides the application environment option
      <c>protocol_version</c> and  <c>dtls_protocol_version</c>. If the environment option is not set, it defaults
      to all versions, except SSL-3.0, supported by the SSL application.
      See also <seealso marker="ssl:ssl_app">ssl(6).</seealso></p></item>

      <tag><c>{hibernate_after, integer()|undefined}</c></tag>
      <item><p>When an integer-value is specified, <c>TLS/DTLS-connection</c>
      goes into hibernation after the specified number of milliseconds
      of inactivity, thus reducing its memory footprint. When
      <c>undefined</c> is specified (this is the default), the process
      never goes into hibernation.</p></item>

      <tag><c>{user_lookup_fun, {Lookupfun :: fun(), UserState :: term()}}</c></tag>
      <item><p>The lookup fun is to defined as follows:</p>

	<code>
fun(psk, PSKIdentity ::string(), UserState :: term()) ->
	{ok, SharedSecret :: binary()} | error;
fun(srp, Username :: string(), UserState :: term()) ->
	{ok, {SRPParams :: srp_param_type(), Salt :: binary(), DerivedKey :: binary()}} | error.
	</code>

	<p>For Pre-Shared Key (PSK) cipher suites, the lookup fun is
	called by the client and server to determine the shared
	secret. When called by the client, <c>PSKIdentity</c> is set to the
	hint presented by the server or to undefined. When called by the
	server, <c>PSKIdentity</c> is the identity presented by the client.</p>

	<p>For Secure Remote Password (SRP), the fun is only used by the server to
	obtain parameters that it uses to generate its session keys.
	<c>DerivedKey</c> is to be derived according to
	<url href="http://tools.ietf.org/html/rfc2945#section-3"> RFC 2945</url> and
	<url href="http://tools.ietf.org/html/rfc5054#section-2.4"> RFC 5054</url>:
	<c>crypto:sha([Salt, crypto:sha([Username, &lt;&lt;$:&gt;&gt;, Password])])</c>
	</p>
      </item>

      <tag><c>{padding_check, boolean()}</c></tag>
      <item><p>Affects TLS-1.0 connections only.
      If set to <c>false</c>, it disables the block cipher padding check
      to be able to interoperate with legacy software.</p>
      <warning><p>Using <c>{padding_check, boolean()}</c> makes TLS
	vulnerable to the Poodle attack.</p></warning>
      </item>

      

      <tag><c>{beast_mitigation, one_n_minus_one | zero_n | disabled}</c></tag>
      <item><p>Affects SSL-3.0 and TLS-1.0 connections only. Used to change the BEAST
       mitigation strategy to interoperate with legacy software.
       Defaults to <c>one_n_minus_one</c>.</p>

      <p><c>one_n_minus_one</c> - Perform 1/n-1 BEAST mitigation.</p>

      <p><c>zero_n</c> - Perform 0/n BEAST mitigation.</p>

      <p><c>disabled</c> - Disable BEAST mitigation.</p>

      <warning><p>Using <c>{beast_mitigation, disabled}</c> makes SSL or TLS
        vulnerable to the BEAST attack.</p></warning>
      </item>
      </taglist>

  </section>
  
  <section>
    <title>TLS/DTLS OPTION DESCRIPTIONS - CLIENT SIDE</title>

    <p>The following options are client-specific or have a slightly different
    meaning in the client than in the server:</p>

    <taglist>

      <tag><c>{verify, verify_type()}</c></tag>
      <item><p>In mode <c>verify_none</c> the default behavior is to allow
      all x509-path validation errors. See also option <c>verify_fun</c>.</p>
      </item>

      <tag><c>{reuse_sessions, boolean()}</c></tag>
      <item><p>Specifies if the client is to try to reuse sessions
      when possible.</p></item>

      <tag><c>{cacerts, [public_key:der_encoded()]}</c></tag>
      <item><p>The DER-encoded trusted certificates. If this option
      is supplied it overrides option <c>cacertfile</c>.</p></item>
      
      <tag><c>{cacertfile, path()}</c></tag>
      <item><p>Path to a file containing PEM-encoded CA certificates. The CA
      certificates are used during server authentication and when building the
      client certificate chain.</p>
    </item>

      <tag><c>{alpn_advertised_protocols, [binary()]}</c></tag>
      <item>
      <p>The list of protocols supported by the client to be sent to the
      server to be used for an Application-Layer Protocol Negotiation (ALPN).
      If the server supports ALPN then it will choose a protocol from this
      list; otherwise it will fail the connection with a "no_application_protocol"
      alert. A server that does not support ALPN will ignore this value.</p>

      <p>The list of protocols must not contain an empty binary.</p>

      <p>The negotiated protocol can be retrieved using the <c>negotiated_protocol/1</c> function.</p>
      </item>

      <tag><c>{client_preferred_next_protocols, {Precedence :: server | client, ClientPrefs :: [binary()]}}</c><br/>
      <c>{client_preferred_next_protocols, {Precedence :: server | client, ClientPrefs :: [binary()], Default :: binary()}}</c></tag>
	   <item>
	   <p>Indicates that the client is to try to perform Next Protocol
	   Negotiation.</p>

	   <p>If precedence is server, the negotiated protocol is the
	   first protocol to be shown on the server advertised list, which is
	   also on the client preference list.</p>

	   <p>If precedence is client, the negotiated protocol is the
	   first protocol to be shown on the client preference list, which is
	   also on the server advertised list.</p>

	   <p>If the client does not support any of the server advertised
	   protocols or the server does not advertise any protocols, the
	   client falls back to the first protocol in its list or to the
	   default protocol (if a default is supplied). If the
	   server does not support Next Protocol Negotiation, the
	   connection terminates if no default protocol is supplied.</p>
	   </item>

      <tag><c>{psk_identity, string()}</c></tag>
      <item><p>Specifies the identity the client presents to the server.
      The matching secret is found by calling <c>user_lookup_fun</c>.</p>
      </item>

      <tag><c>{srp_identity, {Username :: string(), Password :: string()}
      </c></tag>
      <item><p>Specifies the username and password to use to authenticate
      to the server.</p></item>

      <tag><c>{server_name_indication, HostName :: hostname()}</c></tag>
      <item><p>Specify the hostname to be used in TLS Server Name Indication extension.
      If not specified it will default to the <c>Host</c> argument of <seealso marker="#connect-3">connect/[3,4]</seealso>
      unless it is of type inet:ipaddress().</p>
      <p>
	The <c>HostName</c> will also be used in the hostname verification of the peer certificate using
	<seealso marker="public_key:public_key#pkix_verify_hostname-2">public_key:pkix_verify_hostname/2</seealso>.
      </p>
      </item>
      <tag><c>{server_name_indication, disable}</c></tag>
	<item>
          <p> Prevents the Server Name Indication extension from being sent and
	  disables the hostname verification check
	  <seealso marker="public_key:public_key#pkix_verify_hostname-2">public_key:pkix_verify_hostname/2</seealso> </p>
	</item>
      <tag><c>{fallback, boolean()}</c></tag>
      <item>
	<p> Send special cipher suite TLS_FALLBACK_SCSV to avoid undesired TLS version downgrade.
	Defaults to false</p>
	<warning><p>Note this option is not needed in normal TLS usage and should not be used
	to implement new clients. But legacy clients that retries connections in the following manner</p>

	<p><c> ssl:connect(Host, Port, [...{versions, ['tlsv2', 'tlsv1.1', 'tlsv1', 'sslv3']}])</c></p>
	<p><c>  ssl:connect(Host, Port, [...{versions, [tlsv1.1', 'tlsv1', 'sslv3']}, {fallback, true}])</c></p>
	<p><c>  ssl:connect(Host, Port, [...{versions, ['tlsv1', 'sslv3']}, {fallback, true}]) </c></p>
	<p><c>  ssl:connect(Host, Port, [...{versions, ['sslv3']}, {fallback, true}]) </c></p>
	 
	 <p>may use it to avoid undesired TLS version downgrade. Note that TLS_FALLBACK_SCSV must also
	 be supported by the server for the prevention to work.
	</p></warning>
      </item>
      <tag><marker id="client_signature_algs"/><c>{signature_algs, [{hash(), ecdsa | rsa | dsa}]}</c></tag>
	<item>
	<p>In addition to the algorithms negotiated by the cipher
	suite used for key exchange, payload encryption, message
	authentication and pseudo random calculation, the TLS signature
	algorithm extension <url
	href="http://www.ietf.org/rfc/rfc5246.txt">Section 7.4.1.4.1 in RFC 5246</url> may be
	used, from TLS 1.2, to negotiate which signature algorithm to use during the
	TLS handshake. If no lower TLS versions than 1.2 are supported,
	the client will send a TLS signature algorithm extension
	with the algorithms specified by this option.
	Defaults to</p>
	
	<code>[
%% SHA2
{sha512, ecdsa},
{sha512, rsa},
{sha384, ecdsa},
{sha384, rsa},
{sha256, ecdsa},
{sha256, rsa},
{sha224, ecdsa},
{sha224, rsa},
%% SHA
{sha, ecdsa},
{sha, rsa},
{sha, dsa},
]</code>
<p>
	The algorithms should be in the preferred order.
	Selected signature algorithm can restrict which hash functions
	that may be selected. Default support for {md5, rsa} removed in ssl-8.0
	</p>
	</item>
      </taglist>
   </section>
   
  <section>
    <title>TLS/DTLS OPTION DESCRIPTIONS - SERVER SIDE</title>

    <p>The following options are server-specific or have a slightly different
    meaning in the server than in the client:</p>

    <taglist>

      <tag><c>{cacerts, [public_key:der_encoded()]}</c></tag>
      <item><p>The DER-encoded trusted certificates. If this option
      is supplied it overrides option <c>cacertfile</c>.</p></item>
      
      <tag><c>{cacertfile, path()}</c></tag>
      <item><p>Path to a file containing PEM-encoded CA
      certificates. The CA certificates are used to build the server
      certificate chain and for client authentication. The CAs are
      also used in the list of acceptable client CAs passed to the
      client when a certificate is requested. Can be omitted if there
      is no need to verify the client and if there are no
      intermediate CAs for the server certificate.</p></item>
  
      <tag><c>{dh, public_key:der_encoded()}</c></tag>
      <item><p>The DER-encoded Diffie-Hellman parameters. If specified,
      it overrides option <c>dhfile</c>.</p></item>

      <tag><c>{dhfile, path()}</c></tag>
      <item><p>Path to a file containing PEM-encoded Diffie Hellman parameters
      to be used by the server if a cipher suite using Diffie Hellman key
      exchange is negotiated. If not specified, default parameters are used.
      </p></item>

      <tag><c>{verify, verify_type()}</c></tag>
      <item><p>A server only does x509-path validation in mode <c>verify_peer</c>,
      as it then sends a certificate request to the client
      (this message is not sent if the verify option is <c>verify_none</c>).
      You can then also want to specify option <c>fail_if_no_peer_cert</c>.
      </p></item>

      <tag><c>{fail_if_no_peer_cert, boolean()}</c></tag>
      <item><p>Used together with <c>{verify, verify_peer}</c> by an TLS/DTLS server.
      If set to <c>true</c>, the server fails if the client does not have
      a certificate to send, that is, sends an empty certificate. If set to
      <c>false</c>, it fails only if the client sends an invalid
      certificate (an empty certificate is considered valid). Defaults to false.</p>
      </item>

      <tag><c>{reuse_sessions, boolean()}</c></tag>
      <item><p>Specifies if the server is to agree to reuse sessions
      when requested by the clients. See also option <c>reuse_session</c>.
      </p></item>

      <tag><c>{reuse_session, fun(SuggestedSessionId,
      PeerCert, Compression, CipherSuite) -> boolean()}</c></tag>
      <item><p>Enables the TLS/DTLS server to have a local policy
      for deciding if a session is to be reused or not.
      Meaningful only if <c>reuse_sessions</c> is set to <c>true</c>.
      <c>SuggestedSessionId</c> is a <c>binary()</c>, <c>PeerCert</c> is
      a DER-encoded certificate, <c>Compression</c> is an enumeration integer,
      and <c>CipherSuite</c> is of type <c>ciphersuite()</c>.</p></item>

      <tag><c>{alpn_preferred_protocols, [binary()]}</c></tag>
      <item>
      <p>Indicates the server will try to perform Application-Layer
      Protocol Negotiation (ALPN).</p>

      <p>The list of protocols is in order of preference. The protocol
      negotiated will be the first in the list that matches one of the
      protocols advertised by the client. If no protocol matches, the
      server will fail the connection with a "no_application_protocol" alert.</p>

      <p>The negotiated protocol can be retrieved using the <c>negotiated_protocol/1</c> function.</p>
      </item>

      <tag><c>{next_protocols_advertised, Protocols :: [binary()]}</c></tag>
      <item><p>List of protocols to send to the client if the client indicates that
      it supports the Next Protocol extension. The client can select a protocol
      that is not on this list. The list of protocols must not contain an empty
      binary. If the server negotiates a Next Protocol, it can be accessed
      using the <c>negotiated_next_protocol/1</c> method.</p></item>

      <tag><c>{psk_identity, string()}</c></tag>
      <item><p>Specifies the server identity hint, which the server presents to
      the client.</p></item>

      <tag><c>{log_alert, boolean()}</c></tag>
      <item><p>If set to <c>false</c>, error reports are not displayed.</p></item>

      <tag><c>{honor_cipher_order, boolean()}</c></tag>
      <item><p>If set to <c>true</c>, use the server preference for cipher
      selection. If set to <c>false</c> (the default), use the client
      preference.</p></item>

      <tag><c>{sni_hosts, [{hostname(), [ssl_option()]}]}</c></tag>
      <item><p>If the server receives a SNI (Server Name Indication) from the client
      matching a host listed in the <c>sni_hosts</c> option, the specific options for
      that host will override previously specified options.

      The option <c>sni_fun</c>, and <c>sni_hosts</c> are mutually exclusive.</p></item>

      <tag><c>{sni_fun, SNIfun::fun()}</c></tag>
      <item><p>If the server receives a SNI (Server Name Indication) from the client,
      the given function will be called to retrieve <c>[ssl_option()]</c> for the indicated server.
      These options will be merged into predefined <c>[ssl_option()]</c>.

      The function should be defined as:
        <c>fun(ServerName :: string()) -> [ssl_option()]</c>
      and can be specified as a fun or as named <c>fun module:function/1</c>

      The option <c>sni_fun</c>, and <c>sni_hosts</c> are mutually exclusive.</p></item>

      <tag><c>{client_renegotiation, boolean()}</c></tag>
      <item>In protocols that support client-initiated renegotiation, the cost
      of resources of such an operation is higher for the server than the
      client. This can act as a vector for denial of service attacks. The SSL
      application already takes measures to counter-act such attempts,
      but client-initiated renegotiation can be strictly disabled by setting
      this option to <c>false</c>. The default value is <c>true</c>.
      Note that disabling renegotiation can result in long-lived connections
      becoming unusable due to limits on the number of messages the underlying
      cipher suite can encipher.
      </item>

      <tag><c>{honor_cipher_order, boolean()}</c></tag>
      <item>If true, use the server's preference for cipher selection. If false
      (the default), use the client's preference.
      </item>
      
      <tag><c>{honor_ecc_order, boolean()}</c></tag>
      <item>If true, use the server's preference for ECC curve selection. If false
      (the default), use the client's preference.
      </item>

      <tag><c>{signature_algs, [{hash(), ecdsa | rsa | dsa}]}</c></tag>
      <item><p> The algorithms specified by
      this option will be the ones accepted by the server in a signature algorithm
      negotiation, introduced in TLS-1.2. The algorithms will also be offered to the client if a
      client certificate is requested. For more details see the <seealso marker="#client_signature_algs">corresponding client option</seealso>.
      </p> </item>

    </taglist>
  </section>
  
  <section>
    <title>General</title>
      
    <p>When an TLS/DTLS socket is in active mode (the default), data from the
      socket is delivered to the owner of the socket in the form of
      messages:</p>

    <list type="bulleted">
      <item><p><c>{ssl, Socket, Data}</c></p></item>
      <item><p><c>{ssl_closed, Socket}</c></p></item>
      <item><p><c>{ssl_error, Socket, Reason}</c></p></item>
    </list>

    <p>A <c>Timeout</c> argument specifies a time-out in milliseconds. The
      default value for argument <c>Timeout</c> is <c>infinity</c>.</p>
  </section>
  
  <funcs>

    <func>
      <name>append_cipher_suites(Deferred, Suites) -> ciphers() </name>
      <fsummary></fsummary>
      <type>
           <v>Deferred = ciphers() | cipher_filters() </v>
	   <v>Suites  = ciphers() </v>
      </type>
      <desc><p>Make <c>Deferred</c> suites become the least preferred
      suites, that is put them at the end of the cipher suite list
      <c>Suites</c> after removing them from <c>Suites</c> if
      present. <c>Deferred</c> may be a list of cipher suits or a
      list of filters in which case the filters are use on  <c>Suites</c> to
      extract the Deferred cipher list.</p> 
      </desc>
    </func>
    
    <func>
      <name>cipher_suites() -></name>
      <name>cipher_suites(Type) -> old_ciphers()</name>
      <fsummary>Returns a list of supported cipher suites.</fsummary>
      <type>
        <v>Type = erlang | openssl | all</v>
      </type>
      <desc>
	<p>Returns a list of supported cipher suites.
	This function will become deprecated in OTP 21, and replaced
	by <seealso marker="#cipher_suites-2">ssl:cipher-suites/2</seealso>
	<c>cipher_suites()</c> is equivalent to <c>cipher_suites(erlang).</c>
	Type <c>openssl</c> is provided for backwards compatibility with the
	old SSL, which used OpenSSL. <c>cipher_suites(all)</c> returns
	all available cipher suites. The cipher suites not present
	in <c>cipher_suites(erlang)</c> but included in
	<c>cipher_suites(all)</c> are not used unless explicitly configured
	by the user.</p>
      </desc>
    </func>
    
   <func>
      <name>cipher_suites(Supported, Version) ->  ciphers()</name>
      <fsummary>Returns a list of all default or
      all supported cipher suites.</fsummary>
      <type>
        <v> Supported = default | all | anonymous </v>
	<v> Version = protocol_version() </v>
      </type>
      <desc><p>Returns all default or all supported (except anonymous),
      or all anonymous cipher suites for a
      TLS version</p>
    </desc>
    </func>

    <func>
      <name>eccs() -></name>
      <name>eccs(protocol_version()) -> [named_curve()]</name>
      <fsummary>Returns a list of supported ECCs.</fsummary>

      <desc><p>Returns a list of supported ECCs. <c>eccs()</c>
      is equivalent to calling <c>eccs(Protocol)</c> with all
      supported protocols and then deduplicating the output.</p>
      </desc>
    </func>
    
    <func>
      <name>clear_pem_cache() -> ok </name>
      <fsummary> Clears the pem cache</fsummary>

      <desc><p>PEM files, used by ssl API-functions, are cached. The
      cache is regularly checked to see if any cache entries should be
      invalidated, however this function provides a way to
      unconditionally clear the whole cache.
      </p>
      </desc>
    </func>
   
    <func>
      <name>connect(Socket, SslOptions) -> </name>
      <name>connect(Socket, SslOptions, Timeout) -> {ok, TLSSocket} | {ok, TLSSocket, Ext}
	| {error, Reason}</name>
      <fsummary>Upgrades a <c>gen_tcp</c>, or
	equivalent, connected socket to an TLS socket.</fsummary>
      <type>
	<v>Socket = socket()</v>
	<v>SslOptions = [{handshake, hello| full} | ssl_option()]</v>
	<v>Timeout = integer() | infinity</v>
	<v>TLSSocket = sslsocket()</v>
	<v>Ext = hello_extensions()</v>
	<v>Reason = term()</v>
      </type>
      <desc><p>Upgrades a <c>gen_tcp</c>, or equivalent,
	  connected socket to an TLS socket, that is, performs the
	  client-side TLS handshake.</p>
	  
	  <note><p>If the option <c>verify</c> is set to <c>verify_peer</c>
	  the option <c>server_name_indication</c> shall also be specified,
	  if it is not no Server Name Indication extension will be sent,
	  and <seealso marker="public_key:public_key#pkix_verify_hostname-2">public_key:pkix_verify_hostname/2</seealso>
	  will be called with the IP-address of the connection as <c>ReferenceID</c>, which is proably not what you want.</p>
	  </note>

	  <p> If the option <c>{handshake, hello}</c> is used the
	  handshake is paused after receiving the server hello message
	  and the success response is <c>{ok, TLSSocket, Ext}</c>
	  instead of <c>{ok, TLSSocket}</c>. Thereafter the handshake is continued or
	  canceled by calling <seealso marker="#handshake_continue-3">
	  <c>handshake_continue/3</c></seealso> or <seealso
	  marker="#handshake_cancel-1"><c>handshake_cancel/1</c></seealso>.	 
	  </p>
	  
    </desc>
    </func>

    <func>
      <name>connect(Host, Port, Options) -></name>
      <name>connect(Host, Port, Options, Timeout) ->
	  {ok, SslSocket}| {ok, TLSSocket, Ext} | {error, Reason}</name>
      <fsummary>Opens an TLS/DTLS connection to <c>Host</c>, <c>Port</c>.</fsummary>
      <type>
	  <v>Host = host()</v>
	  <v>Port = integer()</v>
	  <v>Options = [option()]</v>
	  <v>Timeout = integer() | infinity</v>
	  <v>SslSocket = sslsocket()</v>
	  <v>Reason = term()</v>
      </type>
      <desc><p>Opens an TLS/DTLS connection to <c>Host</c>, <c>Port</c>.</p>
      
      <p> When the option <c>verify</c> is set to <c>verify_peer</c> the check 
      <seealso marker="public_key:public_key#pkix_verify_hostname-2">public_key:pkix_verify_hostname/2</seealso> 
      will be performed in addition to the usual x509-path validation checks. If the check fails the error {bad_cert, hostname_check_failed} will
      be propagated to the path validation fun <seealso marker="#verify_fun">verify_fun</seealso>, where it is possible to do customized
      checks by using the full possibilities of the <seealso marker="public_key:public_key#pkix_verify_hostname-3">public_key:pkix_verify_hostname/3</seealso> API.
      
      When the option <c>server_name_indication</c> is provided, its value (the DNS name) will be used as <c>ReferenceID</c>
      to <seealso marker="public_key:public_key#pkix_verify_hostname-2">public_key:pkix_verify_hostname/2</seealso>.
      When no <c>server_name_indication</c> option is given, the <c>Host</c> argument will be used as
      Server Name Indication extension. The <c>Host</c> argument will also be used for the
      <seealso marker="public_key:public_key#pkix_verify_hostname-2">public_key:pkix_verify_hostname/2</seealso> check and if the <c>Host</c>
      argument is an <c>inet:ip_address()</c> the <c>ReferenceID</c> used for the check will be <c>{ip, Host}</c> otherwise
      <c>dns_id</c> will be assumed with a fallback to <c>ip</c> if that fails. </p>
      <note><p>According to good practices certificates should not use IP-addresses as "server names". It would
      be very surprising if this happen outside a closed network. </p></note> 


      <p> If the option <c>{handshake, hello}</c> is used the
	  handshake is paused after receiving the server hello message
	  and the success response is <c>{ok, TLSSocket, Ext}</c>
	  instead of <c>{ok, TLSSocket}</c>. Thereafter the handshake is continued or
	  canceled by calling <seealso marker="#handshake_continue-3">
	  <c>handshake_continue/3</c></seealso> or <seealso
	  marker="#handshake_cancel-1"><c>handshake_cancel/1</c></seealso>.
	  </p>
      </desc>
    </func>

    <func>
      <name>close(SslSocket) -> ok | {error, Reason}</name>
      <fsummary>Closes an TLS/DTLS connection.</fsummary>
      <type>
	  <v>SslSocket = sslsocket()</v>
	  <v>Reason = term()</v>
      </type>
      <desc><p>Closes an TLS/DTLS connection.</p>
      </desc>
    </func>

    <func>
      <name>close(SslSocket, How) -> ok | {ok, port()} | {error, Reason}</name>
      <fsummary>Closes an TLS connection.</fsummary>
      <type>
	  <v>SslSocket = sslsocket()</v>
	  <v>How =  timeout() | {NewController::pid(), timeout()} </v>
	  <v>Reason = term()</v>
      </type>
      <desc><p>Closes or downgrades an TLS connection. In the latter case the transport
      connection will be handed over to the <c>NewController</c> process after receiving
      the TLS close alert from the peer. The returned transport socket will have
      the following options set: <c>[{active, false}, {packet, 0}, {mode, binary}]</c></p>
      </desc>
    </func>
    
    <func>
      <name>controlling_process(SslSocket, NewOwner) ->
	ok | {error, Reason}</name>
	<fsummary>Assigns a new controlling process to the
	TLS/DTLS socket.</fsummary>
	<type>
	  <v>SslSocket = sslsocket()</v>
	  <v>NewOwner = pid()</v>
	  <v>Reason = term()</v>
	</type>
	<desc><p>Assigns a new controlling process to the SSL socket. A
	controlling process is the owner of an SSL socket, and receives
	all messages from the socket.</p>
      </desc>
    </func>

    <func>
      <name>connection_information(SslSocket) ->
        {ok, Result} |  {error, Reason} </name>
      <fsummary>Returns all the connection information.
      </fsummary>
      <type>
        <v>Item = protocol | cipher_suite | sni_hostname | ecc | session_id | atom()</v>
	<d>Meaningful atoms, not specified above, are the ssl option names.</d>
	<v>Result = [{Item::atom(), Value::term()}]</v>
        <v>Reason = term()</v>
      </type>
      <desc><p>Returns the most relevant information about the connection, ssl options that
      are undefined will be filtered out. Note that values that affect the security of the
      connection will only be returned if explicitly requested by connection_information/2.</p>
      </desc>
    </func>

    <func>
      <name>connection_information(SslSocket, Items) ->
        {ok, Result} |  {error, Reason} </name>
      <fsummary>Returns the requested connection information.
      </fsummary>
      <type>
	<v>Items = [Item]</v>
	<v>Item = protocol | cipher_suite | sni_hostname | ecc | session_id | client_random 
          | server_random  | master_secret | atom()</v>
	<d>Note that client_random, server_random  and master_secret are values
        that affect the security of connection. Meaningful atoms, not specified above, are the ssl option names.</d>
	<v>Result = [{Item::atom(), Value::term()}]</v>
        <v>Reason = term()</v>
      </type>
      <desc><p>Returns the requested information items about the connection,
      if they are defined.</p>
      <note><p>If only undefined options are requested the
      resulting list can be empty.</p></note>
      </desc>
    </func>

      <func>
      <name>filter_cipher_suites(Suites, Filters) -> ciphers()</name>
      <fsummary></fsummary>
      <type>
	<v> Suites = ciphers()</v>
        <v> Filters = cipher_filters()</v>
      </type>
      <desc><p>Removes cipher suites if any of the filter functions
      returns false for any part of the cipher suite. This function
      also calls default filter functions to make sure the cipher
      suites are supported by crypto. If no filter function is supplied for some
      part the default behaviour is fun(Algorithm) -> true.</p>
    </desc>
    </func>
    
    <func>
      <name>format_error(Reason) -> string()</name>
      <fsummary>Returns an error string.</fsummary>
      <type>
        <v>Reason = term()</v>
      </type>
      <desc>
        <p>Presents the error returned by an SSL function as a printable string.</p>
      </desc>
    </func>
   
    <func>
      <name>getopts(Socket, OptionNames) ->
	{ok, [socketoption()]} | {error, Reason}</name>
      <fsummary>Gets the values of the specified options.</fsummary>
      <type>
	<v>Socket = sslsocket()</v>
	<v>OptionNames = [atom()]</v>
      </type>
      <desc>
	<p>Gets the values of the specified socket options.
	</p>
      </desc>
    </func>

    <func>
      <name>getstat(Socket) ->
        {ok, OptionValues} | {error, inet:posix()}</name>
      <name>getstat(Socket, OptionNames) ->
        {ok, OptionValues} | {error, inet:posix()}</name>
      <fsummary>Get one or more statistic options for a socket</fsummary>
      <type>
    <v>Socket = sslsocket()</v>
    <v>OptionNames = [atom()]</v>
        <v>OptionValues = [{inet:stat_option(), integer()}]</v>
      </type>
      <desc>
        <p>Gets one or more statistic options for the underlying TCP socket.</p>
        <p>See inet:getstat/2 for statistic options description.</p>
      </desc>
    </func>

    <func>
      <name>handshake(Socket) -> </name>
      <name>handshake(Socket, Timeout) -> {ok, Socket}  | {error, Reason}</name>
      <fsummary>Performs server-side SSL/TLS handshake.</fsummary>
      <type>
        <v>Socket = sslsocket()</v>
        <v>Timeout = integer()</v>
        <v>Reason = term()</v>
      </type>
      <desc>
        <p>Performs the SSL/TLS/DTLS server-side handshake.</p>
	<p><c>Socket</c> is a socket as returned by
	<seealso marker="#transport_accept-2">ssl:transport_accept/[1,2]</seealso>.
	</p>
      </desc>
    </func>

    <func>
      <name>handshake(Socket, SslOptions) -> </name>
      <name>handshake(Socket, SslOptions, Timeout) -> {ok, Socket} | {ok, Socket, Ext} | {error, Reason}</name>
      <fsummary>Performs server-side SSL/TLS/DTLS handshake.</fsummary>
      <type>
        <v>Socket = socket() | sslsocket() </v>
	<v>Ext = hello_extensions()</v>
	<v>SslOptions = [{handshake, hello| full} | ssl_option()]</v>
        <v>Timeout = integer()</v>
        <v>Reason = term()</v>
      </type>
      <desc>
        <p>If <c>Socket</c> is a ordinary <c>socket()</c>: upgrades a <c>gen_tcp</c>,
	or equivalent, socket to an SSL socket, that is, performs
        the SSL/TLS server-side handshake and returns the SSL socket.</p>

	<warning><p>The Socket shall be in passive mode ({active,
	false}) before calling this function or the handshake can fail
	due to a race condition.</p></warning>
	
	<p>If <c>Socket</c> is an <c>sslsocket()</c>: provides extra SSL/TLS/DTLS
	options to those specified in
	<seealso marker="#listen-2">ssl:listen/2 </seealso> and then performs
	the SSL/TLS/DTLS handshake.</p>

	<p>
	  If option <c>{handshake, hello}</c> is specified the handshake is
	  paused after receiving the client hello message and the
	  sucess response is <c>{ok, TLSSocket, Ext}</c>  instead of <c>{ok,
	  TLSSocket}</c>. Thereafter the handshake is continued or
	  canceled by calling <seealso marker="#handshake_continue-3">
	  <c>handshake_continue/3</c></seealso> or <seealso
	  marker="#handshake_cancel-1"><c>handshake_cancel/1</c></seealso>.
	</p>
      </desc>
    </func>

    <func>
      <name>handshake_cancel(Socket) -> ok </name>
      <fsummary>Cancel handshake with a fatal alert</fsummary>
      <type>
        <v>Socket = sslsocket()</v>
      </type>
      <desc>
        <p>Cancel the handshake with a fatal <c>USER_CANCELED</c> alert.</p>
      </desc>
    </func>

    <func>
      <name>handshake_continue(Socket, SSLOptions, Timeout) -> 	{ok, Socket} | {error, Reason}</name>
      <fsummary>Continue the SSL/TLS handshake.</fsummary>
      <type>
        <v>Socket = sslsocket()</v>
	<v>SslOptions = [ssl_option()]</v>
	<v>Timeout = integer()</v>
	<v>Reason = term()</v>
      </type>
      <desc>
        <p>Continue the SSL/TLS handshake possiby with new, additional or changed options.</p>
      </desc>
    </func>
    
    <func>
      <name>listen(Port, Options) ->
	{ok, ListenSocket} | {error, Reason}</name>
      <fsummary>Creates an SSL listen socket.</fsummary>
      <type>
	<v>Port = integer()</v>
	<v>Options = options()</v>
	<v>ListenSocket = sslsocket()</v>
      </type>
      <desc>
	<p>Creates an SSL listen socket.</p>
      </desc>
    </func>

    <func>
      <name>negotiated_protocol(Socket) -> {ok, Protocol} | {error, protocol_not_negotiated}</name>
      <fsummary>Returns the protocol negotiated through ALPN or NPN extensions.</fsummary>
      <type>
        <v>Socket = sslsocket()</v>
        <v>Protocol = binary()</v>
      </type>
      <desc>
        <p>
          Returns the protocol negotiated through ALPN or NPN extensions.
        </p>
      </desc>
    </func>
    
    <func>
      <name>peercert(Socket) -> {ok, Cert} | {error, Reason}</name>
      <fsummary>Returns the peer certificate.</fsummary>
     <type>
        <v>Socket = sslsocket()</v>
        <v>Cert = binary()</v>
      </type>
      <desc>
        <p>The peer certificate is returned as a DER-encoded binary.
	  The certificate can be decoded with
	  <c>public_key:pkix_decode_cert/2</c>.</p>
      </desc>
    </func>

    <func>
      <name>peername(Socket) -> {ok, {Address, Port}} |
	{error, Reason}</name>
      <fsummary>Returns the peer address and port.</fsummary>
      <type>
        <v>Socket = sslsocket()</v>
        <v>Address = ipaddress()</v>
        <v>Port = integer()</v>
      </type>
      <desc>
        <p>Returns the address and port number of the peer.</p>
      </desc>
    </func>
    
      <func>
      <name>prepend_cipher_suites(Preferred, Suites) -> ciphers()</name>
      <fsummary></fsummary>
      <type>
        <v>Preferred = ciphers() | cipher_filters() </v>
	<v>Suites = ciphers() </v>
      </type>
      <desc><p>Make <c>Preferred</c> suites become the most preferred
      suites that is put them at the head of the cipher suite list
      <c>Suites</c> after removing them from <c>Suites</c> if
      present. <c>Preferred</c> may be a list of cipher suits or a
      list of filters in which case the filters are use on <c>Suites</c> to
      extract the preferred cipher list. </p>
      </desc>
    </func>

    <func>
      <name>prf(Socket, Secret, Label, Seed, WantedLength) -> {ok, binary()} | {error, reason()}</name>
      <fsummary>Uses a session Pseudo-Random Function to generate key material.</fsummary>
      <type>
	<v>Socket = sslsocket()</v>
	<v>Secret = binary() | master_secret</v>
	<v>Label = binary()</v>
	<v>Seed = [binary() | prf_random()]</v>
	<v>WantedLength = non_neg_integer()</v>
      </type>
      <desc>
        <p>Uses the Pseudo-Random Function (PRF) of a TLS session to generate
	  extra key material. It either takes user-generated values for
	  <c>Secret</c> and <c>Seed</c> or atoms directing it to use a specific
	  value from the session security parameters.</p>
        <p>Can only be used with TLS/DTLS connections; <c>{error, undefined}</c>
	  is returned for SSLv3 connections.</p>
      </desc>
    </func>
    
    <func>
      <name>recv(Socket, Length) -> </name>
      <name>recv(Socket, Length, Timeout) -> {ok, Data} | {error,
	Reason}</name>
      <fsummary>Receives data on a socket.</fsummary>
      <type>
        <v>Socket = sslsocket()</v>
        <v>Length = integer()</v>
        <v>Timeout = integer()</v>
        <v>Data = [char()] | binary()</v>
      </type>
      <desc>
        <p>Receives a packet from a socket in passive
          mode. A closed socket is indicated by return value
          <c>{error, closed}</c>.</p>
        <p>Argument <c>Length</c> is meaningful only when
          the socket is in mode <c>raw</c> and denotes the number of
          bytes to read. If <c>Length</c> = 0, all available bytes are
          returned. If <c>Length</c> &gt; 0, exactly <c>Length</c>
          bytes are returned, or an error; possibly discarding less
          than <c>Length</c> bytes of data when the socket gets closed
          from the other side.</p>
        <p>Optional argument <c>Timeout</c> specifies a time-out in
          milliseconds. The default value is <c>infinity</c>.</p>
      </desc>
    </func>
    
    <func>
      <name>renegotiate(Socket) -> ok | {error, Reason}</name>
      <fsummary>Initiates a new handshake.</fsummary>
      <type>
	<v>Socket = sslsocket()</v>
      </type>
      <desc><p>Initiates a new handshake. A notable return value is
      <c>{error, renegotiation_rejected}</c> indicating that the peer
      refused to go through with the renegotiation, but the connection
      is still active using the previously negotiated session.</p>
      </desc>
    </func>
    
    <func>
      <name>send(Socket, Data) -> ok | {error, Reason}</name>
      <fsummary>Writes data to a socket.</fsummary>
      <type>
        <v>Socket = sslsocket()</v>
        <v>Data = iodata()</v>
      </type>
      <desc>
        <p>Writes <c>Data</c> to <c>Socket</c>.</p>
        <p>A notable return value is <c>{error, closed}</c> indicating that
          the socket is closed.</p>
      </desc>
    </func>

    <func>
      <name>setopts(Socket, Options) -> ok | {error, Reason}</name>
      <fsummary>Sets socket options.</fsummary>
      <type>
        <v>Socket = sslsocket()</v>
        <v>Options = [socketoption]()</v>
      </type>
      <desc>
        <p>Sets options according to <c>Options</c> for socket
          <c>Socket</c>.</p>
      </desc>
    </func>

    <func>
      <name>shutdown(Socket, How) -> ok | {error, Reason}</name>
      <fsummary>Immediately closes a socket.</fsummary>
      <type>
        <v>Socket = sslsocket()</v>
        <v>How = read | write | read_write</v>
        <v>Reason = reason()</v>
      </type>
      <desc>
        <p>Immediately closes a socket in one or two directions.</p>
        <p><c>How == write</c> means closing the socket for writing,
          reading from it is still possible.</p>
        <p>To be able to handle that the peer has done a shutdown on
          the write side, option <c>{exit_on_close, false}</c>
          is useful.</p>
      </desc>
    </func>
    
    <func>
      <name>ssl_accept(Socket) -> </name>
      <name>ssl_accept(Socket, Timeout) -> ok | {error, Reason}</name>
      <fsummary>Performs server-side SSL/TLS handshake.</fsummary>
      <type>
        <v>Socket = sslsocket()</v>
        <v>Timeout = integer()</v>
        <v>Reason = term()</v>
      </type>
      <desc>
        <p>Performs the SSL/TLS/DTLS server-side handshake.</p>
	<p><c>Socket</c> is a socket as returned by
	<seealso marker="#transport_accept-2">ssl:transport_accept/[1,2]</seealso>
	</p>
      </desc>
    </func>

    <func>
      <name>ssl_accept(Socket, SslOptions) -> </name>
      <name>ssl_accept(Socket, SslOptions, Timeout) -> {ok, Socket} | ok | {error, Reason}</name>
      <fsummary>Performs server-side SSL/TLS/DTLS handshake.</fsummary>
      <type>
        <v>Socket = socket() | sslsocket() </v>
	<v>SslOptions = [ssl_option()]</v>
        <v>Timeout = integer()</v>
        <v>Reason = term()</v>
      </type>
      <desc>
        <p>If <c>Socket</c> is a <c>socket()</c>: upgrades a <c>gen_tcp</c>,
	or equivalent, socket to an SSL socket, that is, performs
        the SSL/TLS server-side handshake and returns the SSL socket.</p>
	
	<warning><p>The listen socket is to be in mode <c>{active, false}</c>
	before telling the client that the server is ready to upgrade
	by calling this function, else the upgrade succeeds or does not
	succeed depending on timing.</p></warning>
	
	<p>If <c>Socket</c> is an <c>sslsocket()</c>: provides extra SSL/TLS/DTLS
	options to those specified in
	<seealso marker="#listen-2">ssl:listen/2 </seealso> and then performs
	the SSL/TLS/DTLS handshake.
	</p>
      </desc>
    </func>
    
    <func>
      <name>sockname(Socket) -> {ok, {Address, Port}} |
	{error, Reason}</name>
      <fsummary>Returns the local address and port.</fsummary>
      <type>
        <v>Socket = sslsocket()</v>
        <v>Address = ipaddress()</v>
        <v>Port = integer()</v>
      </type>
      <desc>
        <p>Returns the local address and port number of socket
          <c>Socket</c>.</p>
      </desc>
    </func>
    
    <func>
      <name>start() -> </name>
      <name>start(Type) -> ok | {error, Reason}</name>
      <fsummary>Starts the SSL application.</fsummary>
      <type>
        <v>Type = permanent | transient | temporary</v>
      </type>
      <desc>
        <p>Starts the SSL application. Default type
          is <c>temporary</c>.</p>
      </desc>
    </func>

    <func>
      <name>stop() -> ok </name>
      <fsummary>Stops the SSL application.</fsummary>
      <desc>
        <p>Stops the SSL application.</p>
      </desc>
    </func>

    <func>
      <name>suite_to_str(CipherSuite) -> String</name>
      <fsummary>Returns the string representation of a cipher suite.</fsummary>
      <type>
        <v>CipherSuite = erl_cipher_suite()</v>
	<v>String = string()</v>
      </type>
      <desc>
        <p>Returns the string representation of a cipher suite.</p>
      </desc>
    </func>

    <func>
      <name>transport_accept(ListenSocket) -></name>
      <name>transport_accept(ListenSocket, Timeout) ->
	{ok, NewSocket} | {error, Reason}</name>
      <fsummary>Accepts an incoming connection and
	prepares for <c>ssl_accept</c>.</fsummary>
      <type>
        <v>ListenSocket = NewSocket = sslsocket()</v>
        <v>Timeout = integer()</v>
        <v>Reason = reason()</v>
      </type>
      <desc>
        <p>Accepts an incoming connection request on a listen socket.
	<c>ListenSocket</c> must be a socket returned from
	<seealso marker="#listen-2"> ssl:listen/2</seealso>.
	The socket returned is to be passed to
	<seealso marker="#ssl_accept-2"> ssl:ssl_accept[2,3]</seealso>
	to complete handshaking, that is,
	establishing the SSL/TLS/DTLS connection.</p>
        <warning>
          <p>The socket returned can only be used with
	  <seealso marker="#ssl_accept-2"> ssl:ssl_accept[2,3]</seealso>.
	  No traffic can be sent or received before that call.</p>
        </warning>
        <p>The accepted socket inherits the options set for
	<c>ListenSocket</c> in
	<seealso marker="#listen-2"> ssl:listen/2</seealso>.</p>
	<p>The default
	value for <c>Timeout</c> is <c>infinity</c>. If
	<c>Timeout</c> is specified and no connection is accepted
	within the given time, <c>{error, timeout}</c> is
	returned.</p>
      </desc>
    </func>
    
    <func>
      <name>versions() -> [versions_info()]</name>
      <fsummary>Returns version information relevant for the
	SSL application.</fsummary>
      <type>
	<v>versions_info() = {app_vsn, string()} | {supported | available, [ssl_tls_protocol()]} |
	{supported_dtls | available_dtls, [dtls_protocol()]}	</v>
      </type>
      <desc>
	<p>Returns version information relevant for the SSL
	application.</p>
	<taglist>
	  <tag><c>app_vsn</c></tag>
	  <item>The application version of the SSL application.</item>

	  <tag><c>supported</c></tag>
	  <item>SSL/TLS versions supported by default.
	  Overridden by a version option on
	  <seealso marker="#connect-2"> connect/[2,3,4]</seealso>,
	  <seealso marker="#listen-2"> listen/2</seealso>, and <seealso
	  marker="#ssl_accept-2">ssl_accept/[1,2,3]</seealso>.
	  For the negotiated SSL/TLS version, see <seealso
	  marker="#connection_information-1">ssl:connection_information/1
	  </seealso>.</item>
	  
	  <tag><c>supported_dtls</c></tag>
	  <item>DTLS versions supported by default.
	  Overridden by a version option on
	  <seealso marker="#connect-2"> connect/[2,3,4]</seealso>,
	  <seealso marker="#listen-2"> listen/2</seealso>, and <seealso
	  marker="#ssl_accept-2">ssl_accept/[1,2,3]</seealso>.
	  For the negotiated DTLS version, see <seealso
	  marker="#connection_information-1">ssl:connection_information/1
	  </seealso>.</item>

	  <tag><c>available</c></tag>
	  <item>All SSL/TLS versions supported by the SSL application.
	  TLS 1.2 requires sufficient support from the Crypto
	  application.</item>

	  <tag><c>available_dtls</c></tag>
	  <item>All DTLS versions supported by the SSL application.
	  DTLS 1.2 requires sufficient support from the Crypto
	  application.</item>
	  
	</taglist>
      </desc>
    </func>
   
  </funcs> 

  <section>
    <title>SEE ALSO</title>
    <p><seealso marker="kernel:inet">inet(3)</seealso> and
      <seealso marker="kernel:gen_tcp">gen_tcp(3)</seealso>
      <seealso marker="kernel:gen_udp">gen_udp(3)</seealso>
    </p>
  </section>

</erlref>