1 OpenSSL - Frequently Asked Questions
2 --------------------------------------
4 [MISC] Miscellaneous questions
6 * Which is the current version of OpenSSL?
7 * Where is the documentation?
8 * How can I contact the OpenSSL developers?
9 * Where can I get a compiled version of OpenSSL?
10 * Why aren't tools like 'autoconf' and 'libtool' used?
11 * What is an 'engine' version?
13 [LEGAL] Legal questions
15 * Do I need patent licenses to use OpenSSL?
16 * Can I use OpenSSL with GPL software?
18 [USER] Questions on using the OpenSSL applications
20 * Why do I get a "PRNG not seeded" error message?
21 * Why do I get an "unable to write 'random state'" error message?
22 * How do I create certificates or certificate requests?
23 * Why can't I create certificate requests?
24 * Why does <SSL program> fail with a certificate verify error?
25 * Why can I only use weak ciphers when I connect to a server using OpenSSL?
26 * How can I create DSA certificates?
27 * Why can't I make an SSL connection using a DSA certificate?
28 * How can I remove the passphrase on a private key?
29 * Why can't I use OpenSSL certificates with SSL client authentication?
30 * Why does my browser give a warning about a mismatched hostname?
31 * How do I install a CA certificate into a browser?
33 [BUILD] Questions about building and testing OpenSSL
35 * Why does the linker complain about undefined symbols?
36 * Why does the OpenSSL test fail with "bc: command not found"?
37 * Why does the OpenSSL test fail with "bc: 1 no implemented"?
38 * Why does the OpenSSL compilation fail on Alpha Tru64 Unix?
39 * Why does the OpenSSL compilation fail with "ar: command not found"?
40 * Why does the OpenSSL compilation fail on Win32 with VC++?
41 * What is special about OpenSSL on Redhat?
42 * Why does the OpenSSL test suite fail on MacOS X?
44 [PROG] Questions about programming with OpenSSL
46 * Is OpenSSL thread-safe?
47 * I've compiled a program under Windows and it crashes: why?
48 * How do I read or write a DER encoded buffer using the ASN1 functions?
49 * I've tried using <M_some_evil_pkcs12_macro> and I get errors why?
50 * I've called <some function> and it fails, why?
51 * I just get a load of numbers for the error output, what do they mean?
52 * Why do I get errors about unknown algorithms?
53 * Why can't the OpenSSH configure script detect OpenSSL?
54 * Can I use OpenSSL's SSL library with non-blocking I/O?
55 * Why doesn't my server application receive a client certificate?
57 ===============================================================================
59 [MISC] ========================================================================
61 * Which is the current version of OpenSSL?
63 The current version is available from <URL: http://www.openssl.org>.
64 OpenSSL 0.9.6g was released on 9 August 2002.
66 In addition to the current stable release, you can also access daily
67 snapshots of the OpenSSL development version at <URL:
68 ftp://ftp.openssl.org/snapshot/>, or get it by anonymous CVS access.
71 * Where is the documentation?
73 OpenSSL is a library that provides cryptographic functionality to
74 applications such as secure web servers. Be sure to read the
75 documentation of the application you want to use. The INSTALL file
76 explains how to install this library.
78 OpenSSL includes a command line utility that can be used to perform a
79 variety of cryptographic functions. It is described in the openssl(1)
80 manpage. Documentation for developers is currently being written. A
81 few manual pages already are available; overviews over libcrypto and
82 libssl are given in the crypto(3) and ssl(3) manpages.
84 The OpenSSL manpages are installed in /usr/local/ssl/man/ (or a
85 different directory if you specified one as described in INSTALL).
86 In addition, you can read the most current versions at
87 <URL: http://www.openssl.org/docs/>.
89 For information on parts of libcrypto that are not yet documented, you
90 might want to read Ariel Glenn's documentation on SSLeay 0.9, OpenSSL's
91 predecessor, at <URL: http://www.columbia.edu/~ariel/ssleay/>. Much
92 of this still applies to OpenSSL.
94 There is some documentation about certificate extensions and PKCS#12
97 The original SSLeay documentation is included in OpenSSL as
98 doc/ssleay.txt. It may be useful when none of the other resources
99 help, but please note that it reflects the obsolete version SSLeay
103 * How can I contact the OpenSSL developers?
105 The README file describes how to submit bug reports and patches to
106 OpenSSL. Information on the OpenSSL mailing lists is available from
107 <URL: http://www.openssl.org>.
110 * Where can I get a compiled version of OpenSSL?
112 Some applications that use OpenSSL are distributed in binary form.
113 When using such an application, you don't need to install OpenSSL
114 yourself; the application will include the required parts (e.g. DLLs).
116 If you want to install OpenSSL on a Windows system and you don't have
117 a C compiler, read the "Mingw32" section of INSTALL.W32 for information
118 on how to obtain and install the free GNU C compiler.
120 A number of Linux and *BSD distributions include OpenSSL.
123 * Why aren't tools like 'autoconf' and 'libtool' used?
125 autoconf will probably be used in future OpenSSL versions. If it was
126 less Unix-centric, it might have been used much earlier.
128 * What is an 'engine' version?
130 With version 0.9.6 OpenSSL was extended to interface to external crypto
131 hardware. This was realized in a special release '0.9.6-engine'. With
132 version 0.9.7 (not yet released) the changes were merged into the main
133 development line, so that the special release is no longer necessary.
135 [LEGAL] =======================================================================
137 * Do I need patent licenses to use OpenSSL?
139 The patents section of the README file lists patents that may apply to
140 you if you want to use OpenSSL. For information on intellectual
141 property rights, please consult a lawyer. The OpenSSL team does not
144 You can configure OpenSSL so as not to use RC5 and IDEA by using
145 ./config no-rc5 no-idea
148 * Can I use OpenSSL with GPL software?
150 On many systems including the major Linux and BSD distributions, yes (the
151 GPL does not place restrictions on using libraries that are part of the
152 normal operating system distribution).
154 On other systems, the situation is less clear. Some GPL software copyright
155 holders claim that you infringe on their rights if you use OpenSSL with
156 their software on operating systems that don't normally include OpenSSL.
158 If you develop open source software that uses OpenSSL, you may find it
159 useful to choose an other license than the GPL, or state explicitly that
160 "This program is released under the GPL with the additional exemption that
161 compiling, linking, and/or using OpenSSL is allowed." If you are using
162 GPL software developed by others, you may want to ask the copyright holder
163 for permission to use their software with OpenSSL.
166 [USER] ========================================================================
168 * Why do I get a "PRNG not seeded" error message?
170 Cryptographic software needs a source of unpredictable data to work
171 correctly. Many open source operating systems provide a "randomness
172 device" that serves this purpose. On other systems, applications have
173 to call the RAND_add() or RAND_seed() function with appropriate data
174 before generating keys or performing public key encryption.
175 (These functions initialize the pseudo-random number generator, PRNG.)
177 Some broken applications do not do this. As of version 0.9.5, the
178 OpenSSL functions that need randomness report an error if the random
179 number generator has not been seeded with at least 128 bits of
180 randomness. If this error occurs, please contact the author of the
181 application you are using. It is likely that it never worked
182 correctly. OpenSSL 0.9.5 and later make the error visible by refusing
183 to perform potentially insecure encryption.
185 On systems without /dev/urandom and /dev/random, it is a good idea to
186 use the Entropy Gathering Demon (EGD); see the RAND_egd() manpage for
187 details. Starting with version 0.9.7, OpenSSL will automatically look
188 for an EGD socket at /var/run/egd-pool, /dev/egd-pool, /etc/egd-pool and
191 Most components of the openssl command line utility automatically try
192 to seed the random number generator from a file. The name of the
193 default seeding file is determined as follows: If environment variable
194 RANDFILE is set, then it names the seeding file. Otherwise if
195 environment variable HOME is set, then the seeding file is $HOME/.rnd.
196 If neither RANDFILE nor HOME is set, versions up to OpenSSL 0.9.6 will
197 use file .rnd in the current directory while OpenSSL 0.9.6a uses no
198 default seeding file at all. OpenSSL 0.9.6b and later will behave
199 similarly to 0.9.6a, but will use a default of "C:\" for HOME on
200 Windows systems if the environment variable has not been set.
202 If the default seeding file does not exist or is too short, the "PRNG
203 not seeded" error message may occur.
205 The openssl command line utility will write back a new state to the
206 default seeding file (and create this file if necessary) unless
207 there was no sufficient seeding.
209 Pointing $RANDFILE to an Entropy Gathering Daemon socket does not work.
210 Use the "-rand" option of the OpenSSL command line tools instead.
211 The $RANDFILE environment variable and $HOME/.rnd are only used by the
212 OpenSSL command line tools. Applications using the OpenSSL library
213 provide their own configuration options to specify the entropy source,
214 please check out the documentation coming the with application.
216 For Solaris 2.6, Tim Nibbe <tnibbe@sprint.net> and others have suggested
217 installing the SUNski package from Sun patch 105710-01 (Sparc) which
218 adds a /dev/random device and make sure it gets used, usually through
219 $RANDFILE. There are probably similar patches for the other Solaris
220 versions. An official statement from Sun with respect to /dev/random
221 support can be found at
222 http://sunsolve.sun.com/pub-cgi/retrieve.pl?doc=fsrdb/27606&zone_32=SUNWski
223 However, be warned that /dev/random is usually a blocking device, which
224 may have some effects on OpenSSL.
227 * Why do I get an "unable to write 'random state'" error message?
230 Sometimes the openssl command line utility does not abort with
231 a "PRNG not seeded" error message, but complains that it is
232 "unable to write 'random state'". This message refers to the
233 default seeding file (see previous answer). A possible reason
234 is that no default filename is known because neither RANDFILE
235 nor HOME is set. (Versions up to 0.9.6 used file ".rnd" in the
236 current directory in this case, but this has changed with 0.9.6a.)
239 * How do I create certificates or certificate requests?
241 Check out the CA.pl(1) manual page. This provides a simple wrapper round
242 the 'req', 'verify', 'ca' and 'pkcs12' utilities. For finer control check
243 out the manual pages for the individual utilities and the certificate
244 extensions documentation (currently in doc/openssl.txt).
247 * Why can't I create certificate requests?
249 You typically get the error:
251 unable to find 'distinguished_name' in config
252 problems making Certificate Request
254 This is because it can't find the configuration file. Check out the
255 DIAGNOSTICS section of req(1) for more information.
258 * Why does <SSL program> fail with a certificate verify error?
260 This problem is usually indicated by log messages saying something like
261 "unable to get local issuer certificate" or "self signed certificate".
262 When a certificate is verified its root CA must be "trusted" by OpenSSL
263 this typically means that the CA certificate must be placed in a directory
264 or file and the relevant program configured to read it. The OpenSSL program
265 'verify' behaves in a similar way and issues similar error messages: check
266 the verify(1) program manual page for more information.
269 * Why can I only use weak ciphers when I connect to a server using OpenSSL?
271 This is almost certainly because you are using an old "export grade" browser
272 which only supports weak encryption. Upgrade your browser to support 128 bit
276 * How can I create DSA certificates?
278 Check the CA.pl(1) manual page for a DSA certificate example.
281 * Why can't I make an SSL connection to a server using a DSA certificate?
283 Typically you'll see a message saying there are no shared ciphers when
284 the same setup works fine with an RSA certificate. There are two possible
285 causes. The client may not support connections to DSA servers most web
286 browsers (including Netscape and MSIE) only support connections to servers
287 supporting RSA cipher suites. The other cause is that a set of DH parameters
288 has not been supplied to the server. DH parameters can be created with the
289 dhparam(1) command and loaded using the SSL_CTX_set_tmp_dh() for example:
290 check the source to s_server in apps/s_server.c for an example.
293 * How can I remove the passphrase on a private key?
295 Firstly you should be really *really* sure you want to do this. Leaving
296 a private key unencrypted is a major security risk. If you decide that
297 you do have to do this check the EXAMPLES sections of the rsa(1) and
301 * Why can't I use OpenSSL certificates with SSL client authentication?
303 What will typically happen is that when a server requests authentication
304 it will either not include your certificate or tell you that you have
305 no client certificates (Netscape) or present you with an empty list box
306 (MSIE). The reason for this is that when a server requests a client
307 certificate it includes a list of CAs names which it will accept. Browsers
308 will only let you select certificates from the list on the grounds that
309 there is little point presenting a certificate which the server will
312 The solution is to add the relevant CA certificate to your servers "trusted
313 CA list". How you do this depends on the server software in uses. You can
314 print out the servers list of acceptable CAs using the OpenSSL s_client tool:
316 openssl s_client -connect www.some.host:443 -prexit
318 If your server only requests certificates on certain URLs then you may need
319 to manually issue an HTTP GET command to get the list when s_client connects:
321 GET /some/page/needing/a/certificate.html
323 If your CA does not appear in the list then this confirms the problem.
326 * Why does my browser give a warning about a mismatched hostname?
328 Browsers expect the server's hostname to match the value in the commonName
329 (CN) field of the certificate. If it does not then you get a warning.
332 * How do I install a CA certificate into a browser?
334 The usual way is to send the DER encoded certificate to the browser as
335 MIME type application/x-x509-ca-cert, for example by clicking on an appropriate
336 link. On MSIE certain extensions such as .der or .cacert may also work, or you
337 can import the certificate using the certificate import wizard.
339 You can convert a certificate to DER form using the command:
341 openssl x509 -in ca.pem -outform DER -out ca.der
343 Occasionally someone suggests using a command such as:
345 openssl pkcs12 -export -out cacert.p12 -in cacert.pem -inkey cakey.pem
347 DO NOT DO THIS! This command will give away your CAs private key and
348 reduces its security to zero: allowing anyone to forge certificates in
349 whatever name they choose.
352 [BUILD] =======================================================================
354 * Why does the linker complain about undefined symbols?
356 Maybe the compilation was interrupted, and make doesn't notice that
357 something is missing. Run "make clean; make".
359 If you used ./Configure instead of ./config, make sure that you
360 selected the right target. File formats may differ slightly between
361 OS versions (for example sparcv8/sparcv9, or a.out/elf).
363 In case you get errors about the following symbols, use the config
364 option "no-asm", as described in INSTALL:
366 BF_cbc_encrypt, BF_decrypt, BF_encrypt, CAST_cbc_encrypt,
367 CAST_decrypt, CAST_encrypt, RC4, RC5_32_cbc_encrypt, RC5_32_decrypt,
368 RC5_32_encrypt, bn_add_words, bn_div_words, bn_mul_add_words,
369 bn_mul_comba4, bn_mul_comba8, bn_mul_words, bn_sqr_comba4,
370 bn_sqr_comba8, bn_sqr_words, bn_sub_words, des_decrypt3,
371 des_ede3_cbc_encrypt, des_encrypt, des_encrypt2, des_encrypt3,
372 des_ncbc_encrypt, md5_block_asm_host_order, sha1_block_asm_data_order
374 If none of these helps, you may want to try using the current snapshot.
375 If the problem persists, please submit a bug report.
378 * Why does the OpenSSL test fail with "bc: command not found"?
380 You didn't install "bc", the Unix calculator. If you want to run the
381 tests, get GNU bc from ftp://ftp.gnu.org or from your OS distributor.
384 * Why does the OpenSSL test fail with "bc: 1 no implemented"?
386 On some SCO installations or versions, bc has a bug that gets triggered
387 when you run the test suite (using "make test"). The message returned is
388 "bc: 1 not implemented".
390 The best way to deal with this is to find another implementation of bc
391 and compile/install it. GNU bc (see http://www.gnu.org/software/software.html
392 for download instructions) can be safely used, for example.
395 * Why does the OpenSSL compilation fail on Alpha Tru64 Unix?
397 On some Alpha installations running Tru64 Unix and Compaq C, the compilation
398 of crypto/sha/sha_dgst.c fails with the message 'Fatal: Insufficient virtual
399 memory to continue compilation.' As far as the tests have shown, this may be
400 a compiler bug. What happens is that it eats up a lot of resident memory
401 to build something, probably a table. The problem is clearly in the
402 optimization code, because if one eliminates optimization completely (-O0),
403 the compilation goes through (and the compiler consumes about 2MB of resident
404 memory instead of 240MB or whatever one's limit is currently).
406 There are three options to solve this problem:
408 1. set your current data segment size soft limit higher. Experience shows
409 that about 241000 kbytes seems to be enough on an AlphaServer DS10. You do
410 this with the command 'ulimit -Sd nnnnnn', where 'nnnnnn' is the number of
411 kbytes to set the limit to.
413 2. If you have a hard limit that is lower than what you need and you can't
414 get it changed, you can compile all of OpenSSL with -O0 as optimization
415 level. This is however not a very nice thing to do for those who expect to
416 get the best result from OpenSSL. A bit more complicated solution is the
419 ----- snip:start -----
420 make DIRS=crypto SDIRS=sha "`grep '^CFLAG=' Makefile.ssl | \
421 sed -e 's/ -O[0-9] / -O0 /'`"
422 rm `ls crypto/*.o crypto/sha/*.o | grep -v 'sha_dgst\.o'`
426 This will only compile sha_dgst.c with -O0, the rest with the optimization
427 level chosen by the configuration process. When the above is done, do the
428 test and installation and you're set.
431 * Why does the OpenSSL compilation fail with "ar: command not found"?
433 Getting this message is quite usual on Solaris 2, because Sun has hidden
434 away 'ar' and other development commands in directories that aren't in
435 $PATH by default. One of those directories is '/usr/ccs/bin'. The
436 quickest way to fix this is to do the following (it assumes you use sh
437 or any sh-compatible shell):
439 ----- snip:start -----
440 PATH=${PATH}:/usr/ccs/bin; export PATH
443 and then redo the compilation. What you should really do is make sure
444 '/usr/ccs/bin' is permanently in your $PATH, for example through your
445 '.profile' (again, assuming you use a sh-compatible shell).
448 * Why does the OpenSSL compilation fail on Win32 with VC++?
450 Sometimes, you may get reports from VC++ command line (cl) that it
451 can't find standard include files like stdio.h and other weirdnesses.
452 One possible cause is that the environment isn't correctly set up.
453 To solve that problem, one should run VCVARS32.BAT which is found in
454 the 'bin' subdirectory of the VC++ installation directory (somewhere
455 under 'Program Files'). This needs to be done prior to running NMAKE,
456 and the changes are only valid for the current DOS session.
459 * What is special about OpenSSL on Redhat?
461 Red Hat Linux (release 7.0 and later) include a preinstalled limited
462 version of OpenSSL. For patent reasons, support for IDEA, RC5 and MDC2
463 is disabled in this version. The same may apply to other Linux distributions.
464 Users may therefore wish to install more or all of the features left out.
466 To do this you MUST ensure that you do not overwrite the openssl that is in
467 /usr/bin on your Red Hat machine. Several packages depend on this file,
468 including sendmail and ssh. /usr/local/bin is a good alternative choice. The
469 libraries that come with Red Hat 7.0 onwards have different names and so are
470 not affected. (eg For Red Hat 7.2 they are /lib/libssl.so.0.9.6b and
471 /lib/libcrypto.so.0.9.6b with symlinks /lib/libssl.so.2 and
472 /lib/libcrypto.so.2 respectively).
474 Please note that we have been advised by Red Hat attempting to recompile the
475 openssl rpm with all the cryptography enabled will not work. All other
476 packages depend on the original Red Hat supplied openssl package. It is also
477 worth noting that due to the way Red Hat supplies its packages, updates to
478 openssl on each distribution never change the package version, only the
479 build number. For example, on Red Hat 7.1, the latest openssl package has
480 version number 0.9.6 and build number 9 even though it contains all the
481 relevant updates in packages up to and including 0.9.6b.
483 A possible way around this is to persuade Red Hat to produce a non-US
484 version of Red Hat Linux.
486 FYI: Patent numbers and expiry dates of US patents:
487 MDC-2: 4,908,861 13/03/2007
488 IDEA: 5,214,703 25/05/2010
489 RC5: 5,724,428 03/03/2015
492 * Why does the OpenSSL test suite fail on MacOS X?
494 If the failure happens when running 'make test' and the RC4 test fails,
495 it's very probable that you have OpenSSL 0.9.6b delivered with the
496 operating system (you can find out by running '/usr/bin/openssl version')
497 and that you were trying to build OpenSSL 0.9.6d. The problem is that
498 the loader ('ld') in MacOS X has a misfeature that's quite difficult to
499 go around and has linked the programs "openssl" and the test programs
500 with /usr/lib/libcrypto.dylib and /usr/lib/libssl.dylib instead of the
501 libraries you just built.
502 Look in the file PROBLEMS for a more detailed explanation and for possible
505 [PROG] ========================================================================
507 * Is OpenSSL thread-safe?
509 Yes (with limitations: an SSL connection may not concurrently be used
510 by multiple threads). On Windows and many Unix systems, OpenSSL
511 automatically uses the multi-threaded versions of the standard
512 libraries. If your platform is not one of these, consult the INSTALL
515 Multi-threaded applications must provide two callback functions to
516 OpenSSL. This is described in the threads(3) manpage.
519 * I've compiled a program under Windows and it crashes: why?
521 This is usually because you've missed the comment in INSTALL.W32.
522 Your application must link against the same version of the Win32
523 C-Runtime against which your openssl libraries were linked. The
524 default version for OpenSSL is /MD - "Multithreaded DLL".
526 If you are using Microsoft Visual C++'s IDE (Visual Studio), in
527 many cases, your new project most likely defaulted to "Debug
528 Singlethreaded" - /ML. This is NOT interchangeable with /MD and your
529 program will crash, typically on the first BIO related read or write
532 For each of the six possible link stage configurations within Win32,
533 your application must link against the same by which OpenSSL was
534 built. If you are using MS Visual C++ (Studio) this can be changed
537 1. Select Settings... from the Project Menu.
538 2. Select the C/C++ Tab.
539 3. Select "Code Generation from the "Category" drop down list box
540 4. Select the Appropriate library (see table below) from the "Use
541 run-time library" drop down list box. Perform this step for both
542 your debug and release versions of your application (look at the
543 top left of the settings panel to change between the two)
545 Single Threaded /ML - MS VC++ often defaults to
547 version of a new project.
548 Debug Single Threaded /MLd - MS VC++ often defaults to
549 this for the debug version
552 Debug Multithreaded /MTd
553 Multithreaded DLL /MD - OpenSSL defaults to this.
554 Debug Multithreaded DLL /MDd
556 Note that debug and release libraries are NOT interchangeable. If you
557 built OpenSSL with /MD your application must use /MD and cannot use /MDd.
560 * How do I read or write a DER encoded buffer using the ASN1 functions?
562 You have two options. You can either use a memory BIO in conjunction
563 with the i2d_XXX_bio() or d2i_XXX_bio() functions or you can use the
564 i2d_XXX(), d2i_XXX() functions directly. Since these are often the
565 cause of grief here are some code fragments using PKCS7 as an example:
567 unsigned char *buf, *p;
570 len = i2d_PKCS7(p7, NULL);
571 buf = OPENSSL_malloc(len); /* or Malloc, error checking omitted */
575 At this point buf contains the len bytes of the DER encoding of
578 The opposite assumes we already have len bytes in buf:
582 p7 = d2i_PKCS7(NULL, &p, len);
584 At this point p7 contains a valid PKCS7 structure of NULL if an error
585 occurred. If an error occurred ERR_print_errors(bio) should give more
588 The reason for the temporary variable 'p' is that the ASN1 functions
589 increment the passed pointer so it is ready to read or write the next
590 structure. This is often a cause of problems: without the temporary
591 variable the buffer pointer is changed to point just after the data
592 that has been read or written. This may well be uninitialized data
593 and attempts to free the buffer will have unpredictable results
594 because it no longer points to the same address.
597 * I've tried using <M_some_evil_pkcs12_macro> and I get errors why?
599 This usually happens when you try compiling something using the PKCS#12
600 macros with a C++ compiler. There is hardly ever any need to use the
601 PKCS#12 macros in a program, it is much easier to parse and create
602 PKCS#12 files using the PKCS12_parse() and PKCS12_create() functions
603 documented in doc/openssl.txt and with examples in demos/pkcs12. The
604 'pkcs12' application has to use the macros because it prints out
605 debugging information.
608 * I've called <some function> and it fails, why?
610 Before submitting a report or asking in one of the mailing lists, you
611 should try to determine the cause. In particular, you should call
612 ERR_print_errors() or ERR_print_errors_fp() after the failed call
613 and see if the message helps. Note that the problem may occur earlier
614 than you think -- you should check for errors after every call where
615 it is possible, otherwise the actual problem may be hidden because
616 some OpenSSL functions clear the error state.
619 * I just get a load of numbers for the error output, what do they mean?
621 The actual format is described in the ERR_print_errors() manual page.
622 You should call the function ERR_load_crypto_strings() before hand and
623 the message will be output in text form. If you can't do this (for example
624 it is a pre-compiled binary) you can use the errstr utility on the error
625 code itself (the hex digits after the second colon).
628 * Why do I get errors about unknown algorithms?
630 This can happen under several circumstances such as reading in an
631 encrypted private key or attempting to decrypt a PKCS#12 file. The cause
632 is forgetting to load OpenSSL's table of algorithms with
633 OpenSSL_add_all_algorithms(). See the manual page for more information.
636 * Why can't the OpenSSH configure script detect OpenSSL?
638 Several reasons for problems with the automatic detection exist.
639 OpenSSH requires at least version 0.9.5a of the OpenSSL libraries.
640 Sometimes the distribution has installed an older version in the system
641 locations that is detected instead of a new one installed. The OpenSSL
642 library might have been compiled for another CPU or another mode (32/64 bits).
643 Permissions might be wrong.
645 The general answer is to check the config.log file generated when running
646 the OpenSSH configure script. It should contain the detailed information
647 on why the OpenSSL library was not detected or considered incompatible.
649 * Can I use OpenSSL's SSL library with non-blocking I/O?
651 Yes; make sure to read the SSL_get_error(3) manual page!
653 A pitfall to avoid: Don't assume that SSL_read() will just read from
654 the underlying transport or that SSL_write() will just write to it --
655 it is also possible that SSL_write() cannot do any useful work until
656 there is data to read, or that SSL_read() cannot do anything until it
657 is possible to send data. One reason for this is that the peer may
658 request a new TLS/SSL handshake at any time during the protocol,
659 requiring a bi-directional message exchange; both SSL_read() and
660 SSL_write() will try to continue any pending handshake.
663 * Why doesn't my server application receive a client certificate?
665 Due to the TLS protocol definition, a client will only send a certificate,
666 if explicitly asked by the server. Use the SSL_VERIFY_PEER flag of the
667 SSL_CTX_set_verify() function to enable the use of client certificates.
670 ===============================================================================