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?
12 [LEGAL] Legal questions
14 * Do I need patent licenses to use OpenSSL?
15 * Can I use OpenSSL with GPL software?
17 [USER] Questions on using the OpenSSL applications
19 * Why do I get a "PRNG not seeded" error message?
20 * How do I create certificates or certificate requests?
21 * Why can't I create certificate requests?
22 * Why does <SSL program> fail with a certificate verify error?
23 * Why can I only use weak ciphers when I connect to a server using OpenSSL?
24 * How can I create DSA certificates?
25 * Why can't I make an SSL connection using a DSA certificate?
26 * How can I remove the passphrase on a private key?
27 * Why can't I use OpenSSL certificates with SSL client authentication?
28 * Why does my browser give a warning about a mismatched hostname?
30 [BUILD] Questions about building and testing OpenSSL
32 * Why does the linker complain about undefined symbols?
33 * Why does the OpenSSL test fail with "bc: command not found"?
34 * Why does the OpenSSL test fail with "bc: 1 no implemented"?
35 * Why does the OpenSSL compilation fail on Alpha True64 Unix?
36 * Why does the OpenSSL compilation fail with "ar: command not found"?
37 * Why does the OpenSSL compilation fail on Win32 with VC++?
39 [PROG] Questions about programming with OpenSSL
41 * Is OpenSSL thread-safe?
42 * I've compiled a program under Windows and it crashes: why?
43 * How do I read or write a DER encoded buffer using the ASN1 functions?
44 * I've tried using <M_some_evil_pkcs12_macro> and I get errors why?
45 * I've called <some function> and it fails, why?
46 * I just get a load of numbers for the error output, what do they mean?
47 * Why do I get errors about unknown algorithms?
48 * Why can't the OpenSSH configure script detect OpenSSL?
49 * Can I use OpenSSL's SSL library with non-blocking I/O?
50 * Why doesn't my server application receive a client certificate?
52 ===============================================================================
54 [MISC] ========================================================================
56 * Which is the current version of OpenSSL?
58 The current version is available from <URL: http://www.openssl.org>.
59 OpenSSL 0.9.6a was released on April 5th, 2001.
61 In addition to the current stable release, you can also access daily
62 snapshots of the OpenSSL development version at <URL:
63 ftp://ftp.openssl.org/snapshot/>, or get it by anonymous CVS access.
66 * Where is the documentation?
68 OpenSSL is a library that provides cryptographic functionality to
69 applications such as secure web servers. Be sure to read the
70 documentation of the application you want to use. The INSTALL file
71 explains how to install this library.
73 OpenSSL includes a command line utility that can be used to perform a
74 variety of cryptographic functions. It is described in the openssl(1)
75 manpage. Documentation for developers is currently being written. A
76 few manual pages already are available; overviews over libcrypto and
77 libssl are given in the crypto(3) and ssl(3) manpages.
79 The OpenSSL manpages are installed in /usr/local/ssl/man/ (or a
80 different directory if you specified one as described in INSTALL).
81 In addition, you can read the most current versions at
82 <URL: http://www.openssl.org/docs/>.
84 For information on parts of libcrypto that are not yet documented, you
85 might want to read Ariel Glenn's documentation on SSLeay 0.9, OpenSSL's
86 predecessor, at <URL: http://www.columbia.edu/~ariel/ssleay/>. Much
87 of this still applies to OpenSSL.
89 There is some documentation about certificate extensions and PKCS#12
92 The original SSLeay documentation is included in OpenSSL as
93 doc/ssleay.txt. It may be useful when none of the other resources
94 help, but please note that it reflects the obsolete version SSLeay
98 * How can I contact the OpenSSL developers?
100 The README file describes how to submit bug reports and patches to
101 OpenSSL. Information on the OpenSSL mailing lists is available from
102 <URL: http://www.openssl.org>.
105 * Where can I get a compiled version of OpenSSL?
107 Some applications that use OpenSSL are distributed in binary form.
108 When using such an application, you don't need to install OpenSSL
109 yourself; the application will include the required parts (e.g. DLLs).
111 If you want to install OpenSSL on a Windows system and you don't have
112 a C compiler, read the "Mingw32" section of INSTALL.W32 for information
113 on how to obtain and install the free GNU C compiler.
115 A number of Linux and *BSD distributions include OpenSSL.
118 * Why aren't tools like 'autoconf' and 'libtool' used?
120 autoconf will probably be used in future OpenSSL versions. If it was
121 less Unix-centric, it might have been used much earlier.
124 [LEGAL] =======================================================================
126 * Do I need patent licenses to use OpenSSL?
128 The patents section of the README file lists patents that may apply to
129 you if you want to use OpenSSL. For information on intellectual
130 property rights, please consult a lawyer. The OpenSSL team does not
133 You can configure OpenSSL so as not to use RC5 and IDEA by using
134 ./config no-rc5 no-idea
137 * Can I use OpenSSL with GPL software?
139 On many systems including the major Linux and BSD distributions, yes (the
140 GPL does not place restrictions on using libraries that are part of the
141 normal operating system distribution).
143 On other systems, the situation is less clear. Some GPL software copyright
144 holders claim that you infringe on their rights if you use OpenSSL with
145 their software on operating systems that don't normally include OpenSSL.
147 If you develop open source software that uses OpenSSL, you may find it
148 useful to choose an other license than the GPL, or state explicitely that
149 "This program is released under the GPL with the additional exemption that
150 compiling, linking, and/or using OpenSSL is allowed." If you are using
151 GPL software developed by others, you may want to ask the copyright holder
152 for permission to use their software with OpenSSL.
155 [USER] ========================================================================
157 * Why do I get a "PRNG not seeded" error message?
159 Cryptographic software needs a source of unpredictable data to work
160 correctly. Many open source operating systems provide a "randomness
161 device" that serves this purpose. On other systems, applications have
162 to call the RAND_add() or RAND_seed() function with appropriate data
163 before generating keys or performing public key encryption.
165 Some broken applications do not do this. As of version 0.9.5, the
166 OpenSSL functions that need randomness report an error if the random
167 number generator has not been seeded with at least 128 bits of
168 randomness. If this error occurs, please contact the author of the
169 application you are using. It is likely that it never worked
170 correctly. OpenSSL 0.9.5 and later make the error visible by refusing
171 to perform potentially insecure encryption.
173 On systems without /dev/urandom and /dev/random, it is a good idea to
174 use the Entropy Gathering Demon (EGD); see the RAND_egd() manpage for
175 details. Starting with version 0.9.7, OpenSSL will automatically look
176 for an EGD socket at /var/run/egd-pool, /dev/egd-pool, /etc/egd-pool and
179 Most components of the openssl command line tool try to use the
180 file $HOME/.rnd (or $RANDFILE, if this environment variable is set)
181 for seeding the PRNG. If this file does not exist or is too short,
182 the "PRNG not seeded" error message may occur.
183 Pointing $RANDFILE to an Entropy Gathering Daemon socket does not work.
184 Use the "-rand" option of the OpenSSL command line tools instead.
185 The $RANDFILE environment variable and $HOME/.rnd are only used by the
186 OpenSSL command line tools. Applications using the OpenSSL library
187 provide their own configuration options to specify the entropy source,
188 please check out the documentation coming the with application.
190 For Solaris 2.6, Tim Nibbe <tnibbe@sprint.net> and others have suggested
191 installing the SUNski package from Sun patch 105710-01 (Sparc) which
192 adds a /dev/random device and make sure it gets used, usually through
193 $RANDFILE. There are probably similar patches for the other Solaris
194 versions. However, be warned that /dev/random is usually a blocking
195 device, which may have some effects on OpenSSL.
198 * How do I create certificates or certificate requests?
200 Check out the CA.pl(1) manual page. This provides a simple wrapper round
201 the 'req', 'verify', 'ca' and 'pkcs12' utilities. For finer control check
202 out the manual pages for the individual utilities and the certificate
203 extensions documentation (currently in doc/openssl.txt).
206 * Why can't I create certificate requests?
208 You typically get the error:
210 unable to find 'distinguished_name' in config
211 problems making Certificate Request
213 This is because it can't find the configuration file. Check out the
214 DIAGNOSTICS section of req(1) for more information.
217 * Why does <SSL program> fail with a certificate verify error?
219 This problem is usually indicated by log messages saying something like
220 "unable to get local issuer certificate" or "self signed certificate".
221 When a certificate is verified its root CA must be "trusted" by OpenSSL
222 this typically means that the CA certificate must be placed in a directory
223 or file and the relevant program configured to read it. The OpenSSL program
224 'verify' behaves in a similar way and issues similar error messages: check
225 the verify(1) program manual page for more information.
228 * Why can I only use weak ciphers when I connect to a server using OpenSSL?
230 This is almost certainly because you are using an old "export grade" browser
231 which only supports weak encryption. Upgrade your browser to support 128 bit
235 * How can I create DSA certificates?
237 Check the CA.pl(1) manual page for a DSA certificate example.
240 * Why can't I make an SSL connection to a server using a DSA certificate?
242 Typically you'll see a message saying there are no shared ciphers when
243 the same setup works fine with an RSA certificate. There are two possible
244 causes. The client may not support connections to DSA servers most web
245 browsers (including Netscape and MSIE) only support connections to servers
246 supporting RSA cipher suites. The other cause is that a set of DH parameters
247 has not been supplied to the server. DH parameters can be created with the
248 dhparam(1) command and loaded using the SSL_CTX_set_tmp_dh() for example:
249 check the source to s_server in apps/s_server.c for an example.
252 * How can I remove the passphrase on a private key?
254 Firstly you should be really *really* sure you want to do this. Leaving
255 a private key unencrypted is a major security risk. If you decide that
256 you do have to do this check the EXAMPLES sections of the rsa(1) and
260 * Why can't I use OpenSSL certificates with SSL client authentication?
262 What will typically happen is that when a server requests authentication
263 it will either not include your certificate or tell you that you have
264 no client certificates (Netscape) or present you with an empty list box
265 (MSIE). The reason for this is that when a server requests a client
266 certificate it includes a list of CAs names which it will accept. Browsers
267 will only let you select certificates from the list on the grounds that
268 there is little point presenting a certificate which the server will
271 The solution is to add the relevant CA certificate to your servers "trusted
272 CA list". How you do this depends on the server sofware in uses. You can
273 print out the servers list of acceptable CAs using the OpenSSL s_client tool:
275 openssl s_client -connect www.some.host:443 -prexit
277 If your server only requests certificates on certain URLs then you may need
278 to manually issue an HTTP GET command to get the list when s_client connects:
280 GET /some/page/needing/a/certificate.html
282 If your CA does not appear in the list then this confirms the problem.
285 * Why does my browser give a warning about a mismatched hostname?
287 Browsers expect the server's hostname to match the value in the commonName
288 (CN) field of the certificate. If it does not then you get a warning.
291 [BUILD] =======================================================================
293 * Why does the linker complain about undefined symbols?
295 Maybe the compilation was interrupted, and make doesn't notice that
296 something is missing. Run "make clean; make".
298 If you used ./Configure instead of ./config, make sure that you
299 selected the right target. File formats may differ slightly between
300 OS versions (for example sparcv8/sparcv9, or a.out/elf).
302 In case you get errors about the following symbols, use the config
303 option "no-asm", as described in INSTALL:
305 BF_cbc_encrypt, BF_decrypt, BF_encrypt, CAST_cbc_encrypt,
306 CAST_decrypt, CAST_encrypt, RC4, RC5_32_cbc_encrypt, RC5_32_decrypt,
307 RC5_32_encrypt, bn_add_words, bn_div_words, bn_mul_add_words,
308 bn_mul_comba4, bn_mul_comba8, bn_mul_words, bn_sqr_comba4,
309 bn_sqr_comba8, bn_sqr_words, bn_sub_words, des_decrypt3,
310 des_ede3_cbc_encrypt, des_encrypt, des_encrypt2, des_encrypt3,
311 des_ncbc_encrypt, md5_block_asm_host_order, sha1_block_asm_data_order
313 If none of these helps, you may want to try using the current snapshot.
314 If the problem persists, please submit a bug report.
317 * Why does the OpenSSL test fail with "bc: command not found"?
319 You didn't install "bc", the Unix calculator. If you want to run the
320 tests, get GNU bc from ftp://ftp.gnu.org or from your OS distributor.
323 * Why does the OpenSSL test fail with "bc: 1 no implemented"?
325 On some SCO installations or versions, bc has a bug that gets triggered
326 when you run the test suite (using "make test"). The message returned is
327 "bc: 1 not implemented".
329 The best way to deal with this is to find another implementation of bc
330 and compile/install it. GNU bc (see http://www.gnu.org/software/software.html
331 for download instructions) can be safely used, for example.
334 * Why does the OpenSSL compilation fail on Alpha True64 Unix?
336 On some Alpha installations running True64 Unix and Compaq C, the compilation
337 of crypto/sha/sha_dgst.c fails with the message 'Fatal: Insufficient virtual
338 memory to continue compilation.' As far as the tests have shown, this may be
339 a compiler bug. What happens is that it eats up a lot of resident memory
340 to build something, probably a table. The problem is clearly in the
341 optimization code, because if one eliminates optimization completely (-O0),
342 the compilation goes through (and the compiler consumes about 2MB of resident
343 memory instead of 240MB or whatever one's limit is currently).
345 There are three options to solve this problem:
347 1. set your current data segment size soft limit higher. Experience shows
348 that about 241000 kbytes seems to be enough on an AlphaServer DS10. You do
349 this with the command 'ulimit -Sd nnnnnn', where 'nnnnnn' is the number of
350 kbytes to set the limit to.
352 2. If you have a hard limit that is lower than what you need and you can't
353 get it changed, you can compile all of OpenSSL with -O0 as optimization
354 level. This is however not a very nice thing to do for those who expect to
355 get the best result from OpenSSL. A bit more complicated solution is the
358 ----- snip:start -----
359 make DIRS=crypto SDIRS=sha "`grep '^CFLAG=' Makefile.ssl | \
360 sed -e 's/ -O[0-9] / -O0 /'`"
361 rm `ls crypto/*.o crypto/sha/*.o | grep -v 'sha_dgst\.o'`
365 This will only compile sha_dgst.c with -O0, the rest with the optimization
366 level chosen by the configuration process. When the above is done, do the
367 test and installation and you're set.
370 * Why does the OpenSSL compilation fail with "ar: command not found"?
372 Getting this message is quite usual on Solaris 2, because Sun has hidden
373 away 'ar' and other development commands in directories that aren't in
374 $PATH by default. One of those directories is '/usr/ccs/bin'. The
375 quickest way to fix this is to do the following (it assumes you use sh
376 or any sh-compatible shell):
378 ----- snip:start -----
379 PATH=${PATH}:/usr/ccs/bin; export PATH
382 and then redo the compilation. What you should really do is make sure
383 '/usr/ccs/bin' is permanently in your $PATH, for example through your
384 '.profile' (again, assuming you use a sh-compatible shell).
387 * Why does the OpenSSL compilation fail on Win32 with VC++?
389 Sometimes, you may get reports from VC++ command line (cl) that it
390 can't find standard include files like stdio.h and other weirdnesses.
391 One possible cause is that the environment isn't correctly set up.
392 To solve that problem, one should run VCVARS32.BAT which is found in
393 the 'bin' subdirectory of the VC++ installation directory (somewhere
394 under 'Program Files'). This needs to be done prior to running NMAKE,
395 and the changes are only valid for the current DOS session.
398 [PROG] ========================================================================
400 * Is OpenSSL thread-safe?
402 Yes (with limitations: an SSL connection may not concurrently be used
403 by multiple threads). On Windows and many Unix systems, OpenSSL
404 automatically uses the multi-threaded versions of the standard
405 libraries. If your platform is not one of these, consult the INSTALL
408 Multi-threaded applications must provide two callback functions to
409 OpenSSL. This is described in the threads(3) manpage.
412 * I've compiled a program under Windows and it crashes: why?
414 This is usually because you've missed the comment in INSTALL.W32. You
415 must link with the multithreaded DLL version of the VC++ runtime library
416 otherwise the conflict will cause a program to crash: typically on the
417 first BIO related read or write operation.
420 * How do I read or write a DER encoded buffer using the ASN1 functions?
422 You have two options. You can either use a memory BIO in conjunction
423 with the i2d_XXX_bio() or d2i_XXX_bio() functions or you can use the
424 i2d_XXX(), d2i_XXX() functions directly. Since these are often the
425 cause of grief here are some code fragments using PKCS7 as an example:
427 unsigned char *buf, *p;
430 len = i2d_PKCS7(p7, NULL);
431 buf = OPENSSL_malloc(len); /* or Malloc, error checking omitted */
435 At this point buf contains the len bytes of the DER encoding of
438 The opposite assumes we already have len bytes in buf:
442 p7 = d2i_PKCS7(NULL, &p, len);
444 At this point p7 contains a valid PKCS7 structure of NULL if an error
445 occurred. If an error occurred ERR_print_errors(bio) should give more
448 The reason for the temporary variable 'p' is that the ASN1 functions
449 increment the passed pointer so it is ready to read or write the next
450 structure. This is often a cause of problems: without the temporary
451 variable the buffer pointer is changed to point just after the data
452 that has been read or written. This may well be uninitialized data
453 and attempts to free the buffer will have unpredictable results
454 because it no longer points to the same address.
457 * I've tried using <M_some_evil_pkcs12_macro> and I get errors why?
459 This usually happens when you try compiling something using the PKCS#12
460 macros with a C++ compiler. There is hardly ever any need to use the
461 PKCS#12 macros in a program, it is much easier to parse and create
462 PKCS#12 files using the PKCS12_parse() and PKCS12_create() functions
463 documented in doc/openssl.txt and with examples in demos/pkcs12. The
464 'pkcs12' application has to use the macros because it prints out
465 debugging information.
468 * I've called <some function> and it fails, why?
470 Before submitting a report or asking in one of the mailing lists, you
471 should try to determine the cause. In particular, you should call
472 ERR_print_errors() or ERR_print_errors_fp() after the failed call
473 and see if the message helps. Note that the problem may occur earlier
474 than you think -- you should check for errors after every call where
475 it is possible, otherwise the actual problem may be hidden because
476 some OpenSSL functions clear the error state.
479 * I just get a load of numbers for the error output, what do they mean?
481 The actual format is described in the ERR_print_errors() manual page.
482 You should call the function ERR_load_crypto_strings() before hand and
483 the message will be output in text form. If you can't do this (for example
484 it is a pre-compiled binary) you can use the errstr utility on the error
485 code itself (the hex digits after the second colon).
488 * Why do I get errors about unknown algorithms?
490 This can happen under several circumstances such as reading in an
491 encrypted private key or attempting to decrypt a PKCS#12 file. The cause
492 is forgetting to load OpenSSL's table of algorithms with
493 OpenSSL_add_all_algorithms(). See the manual page for more information.
496 * Why can't the OpenSSH configure script detect OpenSSL?
498 Several reasons for problems with the automatic detection exist.
499 OpenSSH requires at least version 0.9.5a of the OpenSSL libraries.
500 Sometimes the distribution has installed an older version in the system
501 locations that is detected instead of a new one installed. The OpenSSL
502 library might have been compiled for another CPU or another mode (32/64 bits).
503 Permissions might be wrong.
505 The general answer is to check the config.log file generated when running
506 the OpenSSH configure script. It should contain the detailed information
507 on why the OpenSSL library was not detected or considered incompatible.
509 * Can I use OpenSSL's SSL library with non-blocking I/O?
511 Yes; make sure to read the SSL_get_error(3) manual page!
513 A pitfall to avoid: Don't assume that SSL_read() will just read from
514 the underlying transport or that SSL_write() will just write to it --
515 it is also possible that SSL_write() cannot do any useful work until
516 there is data to read, or that SSL_read() cannot do anything until it
517 is possible to send data. One reason for this is that the peer may
518 request a new TLS/SSL handshake at any time during the protocol,
519 requiring a bi-directional message exchange; both SSL_read() and
520 SSL_write() will try to continue any pending handshake.
523 * Why doesn't my server application receive a client certificate?
525 Due to the TLS protocol definition, a client will only send a certificate,
526 if explicitely asked by the server. Use the SSL_VERIFY_PEER flag of the
527 SSL_CTX_set_verify() function to enable the use of client certificates.
530 ===============================================================================