source: mod_gnutls/docs/mod_gnutls_manual.mdwn @ 351b51f

debian/masterdebian/stretch-backportsjessie-backportsupstream
Last change on this file since 351b51f was 031acac, checked in by Nikos Mavrogiannopoulos <nmav@…>, 5 years ago

Use the new (3.1.3+) GnuTLS APIs to obtain private keys.

This allows the loading a private key from a PKCS #11 or a TPM URL
(the GnuTLSPIN and GnuTLSSRKPIN variables should be set), and loading
encrypted private keys in PKCS #8, PKCS #12 or openssl format.

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1% `mod_gnutls` Manual
2
3* * * * *
4
5`mod_gnutls` is a module for the Apache web server that provides HTTPS
6(HTTP over Transport Layer Security (TLS) or the older Secure Sockets
7Layer (SSL)) using the GnuTLS library.  More information about the
8module can be found at [the project's website](https://mod.gnutls.org/).
9
10* * * * *
11
12Compilation & Installation
13==========================
14
15`mod_gnutls` uses the `./configure && make && make install` mechanism
16common to many Open Source programs.  Most of the dirty work is
17handled by either `./configure` or Apache's `apxs` utility. If you have
18built Apache modules before, there shouldn't be any surprises for you.
19
20The interesting options you can pass to configure are:
21
22`--with-apxs=PATH`
23:   This option is used to specify the location of the apxs utility that
24    was installed as part of apache. Specify the location of the
25    binary, not the directory it is located in.
26
27`--with-libgnutls=PATH`
28:   Full path to the libgnutls-config program.
29
30`--with-apr-memcache=PREFIX`
31:   Prefix to where apr\_memcache is installed.
32
33`--help`
34:   Provides a list of all available configure options.
35
36* * * * *
37
38Integration
39===========
40
41To activate `mod_gnutls` just add the following line to your httpd.conf
42and restart Apache:
43
44    LoadModule gnutls_module modules/mod_gnutls.so
45
46* * * * *
47
48Configuration Directives
49========================
50
51`GnuTLSEnable`
52--------------
53
54Enable GnuTLS for this virtual host
55
56    GnuTLSEnable [on|off]
57
58Default: *off*\
59Context: virtual host
60
61This directive enables SSL/TLS Encryption for a Virtual Host.
62
63`GnuTLSCache`
64-------------
65
66Configure SSL Session Cache
67
68    GnuTLSCache [dbm|gdbm|memcache|none] [PATH|SERVERLIST|-]
69
70Default: `GnuTLSCache none`\
71Context: server config
72
73This directive configures the SSL Session Cache for `mod_gnutls`.
74This could be shared between machines of different architectures.
75
76`dbm` (Requires Berkeley DBM)
77:   Uses the default Berkeley DB backend of APR DBM to cache SSL
78    Sessions results.  The argument is a relative or absolute path to
79    be used as the DBM Cache file. This is compatible with most
80    operating systems, but needs the Apache Runtime to be compiled
81    with Berkeley DBM support.
82
83`gdbm`
84:   Uses the GDBM backend of APR DBM to cache SSL Sessions results.
85
86    The argument is a relative or absolute path to be used as the DBM Cache
87    file.  This is the recommended option.
88
89`memcache`
90:   Uses a memcached server to cache the SSL Session.
91
92    The argument is a space separated list of servers. If no port
93    number is supplied, the default of 11211 is used.  This can be
94    used to share a session cache between all servers in a cluster.
95
96`none`
97:   Turns off all caching of SSL Sessions.
98
99    This can significantly reduce the performance of `mod_gnutls` since
100    even followup connections by a client must renegotiate parameters
101    instead of reusing old ones.  This is the default, since it
102    requires no configuration.
103
104`GnuTLSCacheTimeout`
105--------------------
106
107Timeout for SSL Session Cache expiration
108
109    GnuTLSCacheTimeout SECONDS
110
111Default: `GnuTLSCacheTimeout 300`\
112Context: server config
113
114Sets the timeout for SSL Session Cache entries expiration.  This
115directive is valid even if Session Tickets are used, and indicates the
116expiration time of the ticket in seconds.
117
118`GnuTLSSessionTickets`
119----------------------
120
121Enable Session Tickets for the server
122
123    GnuTLSSessionTickets [on|off]
124
125Default: `off`\
126Context: server config, virtual host
127
128To avoid storing data for TLS session resumption it is allowed to
129provide client with a ticket, to use on return.  Use for servers with
130limited storage, and don't combine with GnuTLSCache. For a pool of
131servers this option is not recommended since the tickets are unique
132for the issuing server only.
133
134
135`GnuTLSCertificateFile`
136-----------------------
137
138Set to the PEM Encoded Server Certificate
139
140    GnuTLSCertificateFile FILEPATH
141
142Default: *none*\
143Context: server config, virtual host
144
145Takes an absolute or relative path to a PEM-encoded X.509 certificate to
146use as this Server's End Entity (EE) certificate. If you need to supply
147certificates for intermediate Certificate Authorities (iCAs), they
148should be listed in sequence in the file, from EE to the iCA closest to
149the root CA. Optionally, you can also include the root CA's certificate
150as the last certificate in the list.
151
152`GnuTLSKeyFile`
153---------------
154
155Set to the PEM Encoded Server Certificate
156
157    GnuTLSCertificateFile FILEPATH
158
159Default: *none*\
160Context: server config, virtual host
161
162Takes an absolute or relative path to the Server Private Key.  This
163key cannot currently be password protected.
164
165**Security Warning:**\
166 This private key must be protected. It is read while Apache is still
167running as root, and does not need to be readable by the nobody or
168apache user.
169
170`GnuTLSPGPCertificateFile`
171--------------------------
172
173Set to a base64 Encoded Server OpenPGP Certificate
174
175    GnuTLSPGPCertificateFile FILEPATH
176
177Default: *none*\
178Context: server config, virtual host
179
180Takes an absolute or relative path to a base64 Encoded OpenPGP
181Certificate to use as this Server's Certificate.
182
183`GnuTLSPGPKeyFile`
184------------------
185
186Set to the Server OpenPGP Secret Key
187
188    GnuTLSPGPKeyFile FILEPATH
189
190Default: *none*\
191Context: server config, virtual host
192
193Takes an absolute or relative path to the Server Private Key. This key
194cannot currently be password protected.
195
196**Security Warning:**\
197 This private key must be protected. It is read while Apache is still
198running as root, and does not need to be readable by the nobody or
199apache user.
200
201`GnuTLSClientVerify`
202--------------------
203
204Enable Client Certificate Verification\
205
206    GnuTLSClientVerify [ignore|request|require]
207
208Default: `ignore`\
209Context: server config, virtual host, directory, .htaccess
210
211This directive controls the use of SSL Client Certificate
212Authentication. If used in the .htaccess context, it can force TLS
213re-negotiation.
214
215`ignore`
216:   `mod_gnutls` will ignore the contents of any SSL Client Certificates
217    sent. It will not request that the client sends a certificate.
218
219`request`
220:   The client certificate will be requested, but not required.
221    The Certificate will be validated if sent.  The output of the
222    validation status will be stored in the `SSL_CLIENT_VERIFY`
223    environment variable and can be `SUCCESS`, `FAILED` or `NONE`.
224
225`require`
226:   A Client certificate will be required. Any requests without a valid
227    client certificate will be denied.  The `SSL_CLIENT_VERIFY`
228    environment variable will only be set to `SUCCESS`.
229
230`GnuTLSClientCAFile`
231--------------------
232
233Set to the PEM Encoded Certificate Authority Certificate
234
235    GnuTLSClientCAFile FILEPATH
236
237Default: *none*
238Context: server config, virtual host
239
240Takes an absolute or relative path to a PEM Encoded Certificate to use
241as a Certificate Authority with Client Certificate Authentication.
242This file may contain a list of trusted authorities.
243
244`GnuTLSPGPKeyringFile`
245----------------------
246
247Set to a base64 Encoded key ring
248
249    GnuTLSPGPKeyringFile FILEPATH
250
251Default: *none*\
252Context: server config, virtual host
253
254Takes an absolute or relative path to a base64 Encoded Certificate
255list (key ring) to use as a means of verification of Client
256Certificates.  This file should contain a list of trusted signers.
257
258`GnuTLSDHFile`
259--------------
260
261Set to the PKCS \#3 encoded Diffie Hellman parameters
262
263    GnuTLSDHFile FILEPATH
264
265Default: *none*\
266Context: server config, virtual host
267
268Takes an absolute or relative path to a PKCS \#3 encoded DH
269parameters.Those are used when the DHE key exchange method is enabled.
270You can generate this file using `certtool --generate-dh-params --bits
2712048`.  If not set `mod_gnutls` will use the included parameters.
272
273`GnuTLSSRPPasswdFile`
274---------------------
275
276Set to the SRP password file for SRP ciphersuites
277
278    GnuTLSSRPPasswdFile FILEPATH
279
280Default: *none*\
281Context: server config, virtual host
282
283Takes an absolute or relative path to an SRP password file. This is
284the same format as used in libsrp.  You can generate such file using
285the command `srptool --passwd /etc/tpasswd --passwd-conf
286/etc/tpasswd.conf -u test` to set a password for user test.  This
287password file holds the username, a password verifier and the
288dependency to the SRP parameters.
289
290`GnuTLSSRPPasswdConfFile`
291-------------------------
292
293Set to the SRP password.conf file for SRP ciphersuites
294
295    GnuTLSSRPPasswdConfFile FILEPATH
296
297Default: *none*\
298Context: server config, virtual host
299
300Takes an absolute or relative path to an SRP password.conf file. This
301is the same format as used in `libsrp`.  You can generate such file
302using the command `srptool --create-conf /etc/tpasswd.conf`.  This
303file holds the SRP parameters and is associate with the password file
304(the verifiers depends on these parameters).
305
306`GnuTLSPriorities`
307------------------
308
309Set the allowed ciphers, key exchange algorithms, MACs and compression
310methods
311
312    GnuTLSPriorities NORMAL:+CIPHER_0:+CIPHER_1:...:+CIPHER_N
313
314Default: *none*\
315Context: server config, virtual host
316
317Takes a semi-colon separated list of ciphers, key exchange methods
318Message authentication codes and compression methods to enable.
319The allowed keywords are specified in the `gnutls_priority_init()`
320function of GnuTLS.
321
322Full details can be found at [the GnuTLS documentation](http://gnutls.org/manual/html_node/Priority-Strings.html#Priority-Strings).
323In brief you can specify a set of ciphersuites from the choices:
324
325`NONE`
326:   The empty list.
327
328`EXPORT`
329:   A list with all the supported cipher combinations
330    including the `EXPORT` strength algorithms.
331
332`PERFORMANCE`
333:   A list with all the secure cipher combinations sorted in terms of performance.
334
335`NORMAL`
336:   A list with all the secure cipher combinations sorted
337    with respect to security margin (subjective term).
338
339`SECURE`
340:   A list with all the secure cipher combinations including
341    the 256-bit ciphers sorted with respect to security margin.
342
343Additionally you can add or remove algorithms using the `+` and `!`
344prefixes respectively.
345
346For example, in order to disable the `ARCFOUR` cipher from the `NORMAL` set
347you can use the string `NORMAL:!ARCFOUR-128`
348
349Other options such as the protocol version and the compression method
350can be specified using the `VERS-` and `COMP-` prefixes.
351
352So in order to remove or add a specific TLS version from the `NORMAL`
353set, use `NORMAL:!VERS-SSL3.0`.  And to enable zlib compression use
354`NORMAL:+COMP-DEFLATE`.
355
356
357However it is recommended not to add compression at this level.  With
358the `NONE` set, in order to be usable, you have to specify a complete
359set of combinations of protocol versions, cipher algorithms
360(`AES-128-CBC`), key exchange algorithms (`RSA`), message
361authentication codes (`SHA1`) and compression methods (`COMP-NULL`).
362
363You can find a list of all supported Ciphers, Versions, MACs, etc.  by
364running `gnutls-cli --list`.
365
366The special keyword `%COMPAT` will disable some security features such
367as protection against statistical attacks to ciphertext data in order to
368achieve maximum compatibility (some broken mobile clients need this).
369
370`GnuTLSPIN`
371------------------
372
373Set the PIN to be used to access encrypted key files or PKCS #11 objects.
374
375    GnuTLSPIN XXXXXX
376
377Default: *none*\
378Context: server config, virtual host
379
380Takes a string to be used as a PIN for the protected objects in
381a security module, or as a key to be used to decrypt PKCS #8, PKCS #12,
382or openssl encrypted keys.
383
384`GnuTLSSRKPIN`
385------------------
386
387Set the SRK PIN to be used to unlaccess the TPM.
388
389    GnuTLSSRKPIN XXXXXX
390
391Default: *none*\
392Context: server config, virtual host
393
394Takes a string to be used as a PIN for the protected objects in
395the TPM module.
396
397`GnuTLSExportCertificates`
398--------------------------
399
400Export the PEM encoded certificates to CGIs
401
402    GnuTLSExportCertificates [off|on|SIZE]
403
404Default: `off`\
405Context: server config, virtual host
406
407This directive configures exporting the full certificates of the
408server and the client to CGI scripts via the `SSL_SERVER_CERT` and
409`SSL_CLIENT_CERT` environment variables. The exported certificates
410will be PEM-encoded (if X.509) or ASCII-armored (if OpenPGP) up to the
411size given.  The type of the certificate will be exported in
412`SSL_SERVER_CERT_TYPE` and `SSL_CLIENT_CERT_TYPE`.
413
414SIZE should be an integer number of bytes, or may be written with a
415trailing `K` to indicate kibibytes.  `off` means the same thing as
416`0`, in which case the certificates will not be exported to the
417environment.  `on` is an alias for `16K`.  If a non-zero size is
418specified for this directive, but a certificate is too large to fit in
419the buffer, then the corresponding environment variable will contain
420the fixed string `GNUTLS_CERTIFICATE_SIZE_LIMIT_EXCEEDED`.
421
422With GnuTLSExportCertificates enabled, `mod_gnutls` exports the same
423environment variables to the CGI process as `mod_ssl`.
424
425* * * * *
426
427Configuration Examples
428======================
429
430Simple Standard SSL Example
431---------------------------
432
433The following is an example of standard SSL Hosting, using one IP
434Addresses for each virtual host
435
436     # Load the module into Apache.
437     LoadModule gnutls_module modules/mod_gnutls.so
438     GnuTLSCache gdbm /var/cache/www-tls-cache
439     GnuTLSCacheTimeout 500
440     # With normal SSL Websites, you need one IP Address per-site.
441     Listen 1.2.3.1:443
442     Listen 1.2.3.2:443
443     Listen 1.2.3.3:443
444     Listen 1.2.3.4:443
445     <VirtualHost 1.2.3.1:443>
446     GnuTLSEnable on
447     GnuTLSPriorities NONE:+AES-128-CBC:+3DES-CBC:+ARCFOUR-128:+RSA:+DHE-RSA:+DHE-DSS:+SHA1:+MD5:+COMP-NULL
448     DocumentRoot /www/site1.example.com/html
449     ServerName site1.example.com:443
450     GnuTLSCertificateFile conf/ssl/site1.crt
451     GnuTLSKeyFile conf/ss/site1.key
452     </VirtualHost>
453     <VirtualHost 1.2.3.2:443>
454     # This virtual host enables SRP authentication
455     GnuTLSEnable on
456     GnuTLSPriorities NORMAL:+SRP
457     DocumentRoot /www/site2.example.com/html
458     ServerName site2.example.com:443
459     GnuTLSSRPPasswdFile conf/ssl/tpasswd.site2
460     GnuTLSSRPPasswdConfFile conf/ssl/tpasswd.site2.conf
461     </VirtualHost>
462     <VirtualHost 1.2.3.3:443>
463     # This server enables SRP, OpenPGP and X.509 authentication.
464     GnuTLSEnable on
465     GnuTLSPriorities NORMAL:+SRP:+SRP-RSA:+SRP-DSS
466     DocumentRoot /www/site3.example.com/html
467     ServerName site3.example.com:443
468     GnuTLSCertificateFile conf/ssl/site3.crt
469     GnuTLSKeyFile conf/ss/site3.key
470     GnuTLSClientVerify ignore
471     GnuTLSPGPCertificateFile conf/ss/site3.pub.asc
472     GnuTLSPGPKeyFile conf/ss/site3.sec.asc
473     GnuTLSSRPPasswdFile conf/ssl/tpasswd.site3
474     GnuTLSSRPPasswdConfFile conf/ssl/tpasswd.site3.conf
475     </VirtualHost>
476     <VirtualHost 1.2.3.4:443>
477     GnuTLSEnable on
478     # %COMPAT disables some security features to enable maximum compatibility with clients.
479     GnuTLSPriorities NONE:+AES-128-CBC:+ARCFOUR-128:+RSA:+SHA1:+MD5:+COMP-NULL:%COMPAT
480     DocumentRoot /www/site4.example.com/html
481     ServerName site4.example.com:443
482     GnuTLSCertificateFile conf/ssl/site4.crt
483     GnuTLSKeyFile conf/ss/site4.key
484     </VirtualHost>
485
486Server Name Indication Example
487------------------------------
488
489`mod_gnutls` can also use "Server Name Indication", as specified in
490RFC 3546.  This allows hosting many SSL Websites, with a Single IP
491Address.  Currently all the recent browsers support this
492standard. Here is an example, using SNI: ` `
493
494
495     # Load the module into Apache.
496     LoadModule gnutls_module modules/mod_gnutls.so
497     # With normal SSL Websites, you need one IP Address per-site.
498     Listen 1.2.3.1:443
499     # This could also be 'Listen *:443',
500     # just like '*:80' is common for non-https
501     # No caching. Enable session tickets. Timeout is still used for
502     # ticket expiration.
503     GnuTLSCacheTimeout 600
504     # This tells apache, that for this IP/Port combination, we want to use
505     # Name Based Virtual Hosting. In the case of Server Name Indication,
506     # it lets mod_gnutls pick the correct Server Certificate.
507     NameVirtualHost 1.2.3.1:443
508     <VirtualHost 1.2.3.1:443>
509     GnuTLSEnable on
510     GnuTLSSessionTickets on
511     GnuTLSPriorities NORMAL
512     DocumentRoot /www/site1.example.com/html
513     ServerName site1.example.com:443
514     GnuTLSCertificateFile conf/ssl/site1.crt
515     GnuTLSKeyFile conf/ss/site1.key
516     </VirtualHost>
517     <VirtualHost 1.2.3.1:443>
518     GnuTLSEnable on
519     GnuTLSPriorities NORMAL
520     DocumentRoot /www/site2.example.com/html
521     ServerName site2.example.com:443
522     GnuTLSCertificateFile conf/ssl/site2.crt
523     GnuTLSKeyFile conf/ss/site2.key
524     </VirtualHost>
525     <VirtualHost 1.2.3.1:443>
526     GnuTLSEnable on
527     GnuTLSPriorities NORMAL
528     DocumentRoot /www/site3.example.com/html
529     ServerName site3.example.com:443
530     GnuTLSCertificateFile conf/ssl/site3.crt
531     GnuTLSKeyFile conf/ss/site3.key
532     </VirtualHost>
533     <VirtualHost 1.2.3.1:443>
534     GnuTLSEnable on
535     GnuTLSPriorities NORMAL
536     DocumentRoot /www/site4.example.com/html
537     ServerName site4.example.com:443
538     GnuTLSCertificateFile conf/ssl/site4.crt
539     GnuTLSKeyFile conf/ss/site4.key
540     </VirtualHost>
541
542
543* * * * *
544
545Performance Issues
546==================
547
548`mod_gnutls` by default uses conservative settings for the server.
549You can fine tune the configuration to reduce the load on a busy
550server.  The following examples do exactly this:
551
552
553     # Load the module into Apache.
554     LoadModule gnutls_module modules/mod_gnutls.so
555     # Using 4 memcache servers to distribute the SSL Session Cache.
556     GnuTLSCache memcache "mc1.example.com mc2.example.com mc3.example.com mc4.example.com"
557     GnuTLSCacheTimeout 600
558     Listen 1.2.3.1:443
559     NameVirtualHost 1.2.3.1:443
560     <VirtualHost 1.2.3.1:443>
561     GnuTLSEnable on
562     # Here we disable the Perfect forward secrecy ciphersuites (DHE)
563     # and disallow AES-256 since AES-128 is just fine.
564     GnuTLSPriorities NORMAL:!DHE-RSA:!DHE-DSS:!AES-256-CBC:%COMPAT
565     DocumentRoot /www/site1.example.com/html
566     ServerName site1.example.com:443
567     GnuTLSCertificateFile conf/ssl/site1.crt
568     GnuTLSKeyFile conf/ss/site1.key
569     </VirtualHost>
570     <VirtualHost 1.2.3.1:443>
571     GnuTLSEnable on
572     # Here we instead of disabling the DHE ciphersuites we use
573     # Diffie Hellman parameters of smaller size than the default (2048 bits).
574     # Using small numbers from 768 to 1024 bits should be ok once they are
575     # regenerated every few hours.
576     # Use "certtool --generate-dh-params --bits 1024" to get those
577     GnuTLSDHFile /etc/apache2/dh.params
578     GnuTLSPriorities NORMAL:!AES-256-CBC:%COMPAT
579     DocumentRoot /www/site2.example.com/html
580     ServerName site2.example.com:443
581     GnuTLSCertificateFile conf/ssl/site2.crt
582     GnuTLSKeyFile conf/ss/site2.key
583     </VirtualHost>
584
585* * * * *
586
587Environment Variables
588=====================
589
590`mod_gnutls` exports the following environment variables to scripts.
591These are compatible with `mod_ssl`.
592
593`HTTPS`
594-------
595
596Can be `on` or `off`
597
598`SSL_VERSION_LIBRARY`
599---------------------
600
601The version of the GnuTLS library
602
603`SSL_VERSION_INTERFACE`
604-----------------------
605
606The version of this module
607
608`SSL_PROTOCOL`
609--------------
610
611The SSL or TLS protocol name (such as `TLS 1.0` etc.)
612
613`SSL_CIPHER`
614------------
615
616The SSL or TLS cipher suite name
617
618`SSL_COMPRESS_METHOD`
619---------------------
620
621The negotiated compression method (`NULL` or `DEFLATE`)
622
623`SSL_SRP_USER`
624--------------
625
626The SRP username used for authentication (only set when
627`GnuTLSSRPPasswdFile` and `GnuTLSSRPPasswdConfFile` are configured).
628
629`SSL_CIPHER_USEKEYSIZE` & `SSL_CIPHER_ALGKEYSIZE`
630-------------------------------------------------
631
632The number if bits used in the used cipher algorithm.
633
634This does not fully reflect the security level since the size of
635RSA or DHE key exchange parameters affect the security level too.
636
637`SSL_DH_PRIME_BITS`
638-------------------
639
640The number if bits in the modulus for the DH group, if DHE or static
641DH is used.
642
643This will not be set if DH is not used.
644
645`SSL_CIPHER_EXPORT`
646-------------------
647
648`True` or `False`. Whether the cipher suite negotiated is an export one.
649
650`SSL_SESSION_ID`
651----------------
652
653The session ID negotiated in this session. Can be the same during client
654reloads.
655
656`SSL_CLIENT_V_REMAIN`
657---------------------
658
659The number of days until the client's certificate is expired.
660
661`SSL_CLIENT_V_START`
662--------------------
663
664The activation time of client's certificate.
665
666`SSL_CLIENT_V_END`
667------------------
668
669The expiration time of client's certificate.
670
671`SSL_CLIENT_S_DN`
672-----------------
673
674The distinguished name of client's certificate in RFC2253 format.
675
676`SSL_CLIENT_I_DN`
677-----------------
678
679The SSL or TLS cipher suite name
680
681`SSL_CLIENT_S_AN%`
682------------------
683
684These will contain the alternative names of the client certificate (`%` is
685a number starting from zero).
686
687The values will be prepended by `DNSNAME:`, `RFC822NAME:` or `URI:`
688depending on the type.
689
690If it is not supported the value `UNSUPPORTED` will be set.
691
692`SSL_SERVER_M_SERIAL`
693---------------------
694
695The serial number of the server's certificate.
696
697`SSL_SERVER_M_VERSION`
698----------------------
699
700The version of the server's certificate.
701
702`SSL_SERVER_A_SIG`
703------------------
704
705The algorithm used for the signature in server's certificate.
706
707`SSL_SERVER_A_KEY`
708------------------
709
710The public key algorithm in server's certificate.
711
712`SSL_SERVER_CERT`
713------------------
714
715The PEM-encoded (X.509) or ASCII-armored (OpenPGP) server certificate
716(see the `GnuTLSExportCertificates` directive).
717
718`SSL_SERVER_CERT_TYPE`
719----------------------
720
721The certificate type can be `X.509` or `OPENPGP`.
722
723`SSL_CLIENT_CERT`
724------------------
725
726The PEM-encoded (X.509) or ASCII-armored (OpenPGP) client certificate
727(see the `GnuTLSExportCertificates` directive).
728
729`SSL_CLIENT_CERT_TYPE`
730----------------------
731
732The certificate type can be `X.509` or `OPENPGP`.
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