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?
51 ===============================================================================
53 [MISC] ========================================================================
55 * Which is the current version of OpenSSL?
57 The current version is available from <URL: http://www.openssl.org>.
58 OpenSSL 0.9.6a was released on April 5th, 2001.
60 In addition to the current stable release, you can also access daily
61 snapshots of the OpenSSL development version at <URL:
62 ftp://ftp.openssl.org/snapshot/>, or get it by anonymous CVS access.
65 * Where is the documentation?
67 OpenSSL is a library that provides cryptographic functionality to
68 applications such as secure web servers. Be sure to read the
69 documentation of the application you want to use. The INSTALL file
70 explains how to install this library.
72 OpenSSL includes a command line utility that can be used to perform a
73 variety of cryptographic functions. It is described in the openssl(1)
74 manpage. Documentation for developers is currently being written. A
75 few manual pages already are available; overviews over libcrypto and
76 libssl are given in the crypto(3) and ssl(3) manpages.
78 The OpenSSL manpages are installed in /usr/local/ssl/man/ (or a
79 different directory if you specified one as described in INSTALL).
80 In addition, you can read the most current versions at
81 <URL: http://www.openssl.org/docs/>.
83 For information on parts of libcrypto that are not yet documented, you
84 might want to read Ariel Glenn's documentation on SSLeay 0.9, OpenSSL's
85 predecessor, at <URL: http://www.columbia.edu/~ariel/ssleay/>. Much
86 of this still applies to OpenSSL.
88 There is some documentation about certificate extensions and PKCS#12
91 The original SSLeay documentation is included in OpenSSL as
92 doc/ssleay.txt. It may be useful when none of the other resources
93 help, but please note that it reflects the obsolete version SSLeay
97 * How can I contact the OpenSSL developers?
99 The README file describes how to submit bug reports and patches to
100 OpenSSL. Information on the OpenSSL mailing lists is available from
101 <URL: http://www.openssl.org>.
104 * Where can I get a compiled version of OpenSSL?
106 Some applications that use OpenSSL are distributed in binary form.
107 When using such an application, you don't need to install OpenSSL
108 yourself; the application will include the required parts (e.g. DLLs).
110 If you want to install OpenSSL on a Windows system and you don't have
111 a C compiler, read the "Mingw32" section of INSTALL.W32 for information
112 on how to obtain and install the free GNU C compiler.
114 A number of Linux and *BSD distributions include OpenSSL.
117 * Why aren't tools like 'autoconf' and 'libtool' used?
119 autoconf will probably be used in future OpenSSL versions. If it was
120 less Unix-centric, it might have been used much earlier.
123 [LEGAL] =======================================================================
125 * Do I need patent licenses to use OpenSSL?
127 The patents section of the README file lists patents that may apply to
128 you if you want to use OpenSSL. For information on intellectual
129 property rights, please consult a lawyer. The OpenSSL team does not
132 You can configure OpenSSL so as not to use RC5 and IDEA by using
133 ./config no-rc5 no-idea
136 * Can I use OpenSSL with GPL software?
138 On many systems including the major Linux and BSD distributions, yes (the
139 GPL does not place restrictions on using libraries that are part of the
140 normal operating system distribution).
142 On other systems, the situation is less clear. Some GPL software copyright
143 holders claim that you infringe on their rights if you use OpenSSL with
144 their software on operating systems that don't normally include OpenSSL.
146 If you develop open source software that uses OpenSSL, you may find it
147 useful to choose an other license than the GPL, or state explicitely that
148 "This program is released under the GPL with the additional exemption that
149 compiling, linking, and/or using OpenSSL is allowed." If you are using
150 GPL software developed by others, you may want to ask the copyright holder
151 for permission to use their software with OpenSSL.
154 [USER] ========================================================================
156 * Why do I get a "PRNG not seeded" error message?
158 Cryptographic software needs a source of unpredictable data to work
159 correctly. Many open source operating systems provide a "randomness
160 device" that serves this purpose. On other systems, applications have
161 to call the RAND_add() or RAND_seed() function with appropriate data
162 before generating keys or performing public key encryption.
164 Some broken applications do not do this. As of version 0.9.5, the
165 OpenSSL functions that need randomness report an error if the random
166 number generator has not been seeded with at least 128 bits of
167 randomness. If this error occurs, please contact the author of the
168 application you are using. It is likely that it never worked
169 correctly. OpenSSL 0.9.5 and later make the error visible by refusing
170 to perform potentially insecure encryption.
172 On systems without /dev/urandom, it is a good idea to use the Entropy
173 Gathering Demon; see the RAND_egd() manpage for details.
175 Most components of the openssl command line tool try to use the
176 file $HOME/.rnd (or $RANDFILE, if this environment variable is set)
177 for seeding the PRNG. If this file does not exist or is too short,
178 the "PRNG not seeded" error message may occur.
179 Pointing $RANDFILE to an Entropy Gathering Daemon socket does not work.
180 Use the "-rand" option of the OpenSSL command line tools instead.
181 The $RANDFILE environment variable and $HOME/.rnd are only used by the
182 OpenSSL command line tools. Applications using the OpenSSL library
183 provide their own configuration options to specify the entropy source,
184 please check out the documentation coming the with application.
186 [Note to OpenSSL 0.9.5 users: The command "openssl rsa" in version
187 0.9.5 does not do this and will fail on systems without /dev/urandom
188 when trying to password-encrypt an RSA key! This is a bug in the
189 library; try a later version instead.]
191 For Solaris 2.6, Tim Nibbe <tnibbe@sprint.net> and others have suggested
192 installing the SUNski package from Sun patch 105710-01 (Sparc) which
193 adds a /dev/random device and make sure it gets used, usually through
194 $RANDFILE. There are probably similar patches for the other Solaris
195 versions. However, be warned that /dev/random is usually a blocking
196 device, which may have some effects on OpenSSL.
199 * How do I create certificates or certificate requests?
201 Check out the CA.pl(1) manual page. This provides a simple wrapper round
202 the 'req', 'verify', 'ca' and 'pkcs12' utilities. For finer control check
203 out the manual pages for the individual utilities and the certificate
204 extensions documentation (currently in doc/openssl.txt).
207 * Why can't I create certificate requests?
209 You typically get the error:
211 unable to find 'distinguished_name' in config
212 problems making Certificate Request
214 This is because it can't find the configuration file. Check out the
215 DIAGNOSTICS section of req(1) for more information.
218 * Why does <SSL program> fail with a certificate verify error?
220 This problem is usually indicated by log messages saying something like
221 "unable to get local issuer certificate" or "self signed certificate".
222 When a certificate is verified its root CA must be "trusted" by OpenSSL
223 this typically means that the CA certificate must be placed in a directory
224 or file and the relevant program configured to read it. The OpenSSL program
225 'verify' behaves in a similar way and issues similar error messages: check
226 the verify(1) program manual page for more information.
229 * Why can I only use weak ciphers when I connect to a server using OpenSSL?
231 This is almost certainly because you are using an old "export grade" browser
232 which only supports weak encryption. Upgrade your browser to support 128 bit
236 * How can I create DSA certificates?
238 Check the CA.pl(1) manual page for a DSA certificate example.
241 * Why can't I make an SSL connection to a server using a DSA certificate?
243 Typically you'll see a message saying there are no shared ciphers when
244 the same setup works fine with an RSA certificate. There are two possible
245 causes. The client may not support connections to DSA servers most web
246 browsers (including Netscape and MSIE) only support connections to servers
247 supporting RSA cipher suites. The other cause is that a set of DH parameters
248 has not been supplied to the server. DH parameters can be created with the
249 dhparam(1) command and loaded using the SSL_CTX_set_tmp_dh() for example:
250 check the source to s_server in apps/s_server.c for an example.
253 * How can I remove the passphrase on a private key?
255 Firstly you should be really *really* sure you want to do this. Leaving
256 a private key unencrypted is a major security risk. If you decide that
257 you do have to do this check the EXAMPLES sections of the rsa(1) and
261 * Why can't I use OpenSSL certificates with SSL client authentication?
263 What will typically happen is that when a server requests authentication
264 it will either not include your certificate or tell you that you have
265 no client certificates (Netscape) or present you with an empty list box
266 (MSIE). The reason for this is that when a server requests a client
267 certificate it includes a list of CAs names which it will accept. Browsers
268 will only let you select certificates from the list on the grounds that
269 there is little point presenting a certificate which the server will
272 The solution is to add the relevant CA certificate to your servers "trusted
273 CA list". How you do this depends on the server sofware in uses. You can
274 print out the servers list of acceptable CAs using the OpenSSL s_client tool:
276 openssl s_client -connect www.some.host:443 -prexit
278 If your server only requests certificates on certain URLs then you may need
279 to manually issue an HTTP GET command to get the list when s_client connects:
281 GET /some/page/needing/a/certificate.html
283 If your CA does not appear in the list then this confirms the problem.
286 * Why does my browser give a warning about a mismatched hostname?
288 Browsers expect the server's hostname to match the value in the commonName
289 (CN) field of the certificate. If it does not then you get a warning.
292 [BUILD] =======================================================================
294 * Why does the linker complain about undefined symbols?
296 Maybe the compilation was interrupted, and make doesn't notice that
297 something is missing. Run "make clean; make".
299 If you used ./Configure instead of ./config, make sure that you
300 selected the right target. File formats may differ slightly between
301 OS versions (for example sparcv8/sparcv9, or a.out/elf).
303 In case you get errors about the following symbols, use the config
304 option "no-asm", as described in INSTALL:
306 BF_cbc_encrypt, BF_decrypt, BF_encrypt, CAST_cbc_encrypt,
307 CAST_decrypt, CAST_encrypt, RC4, RC5_32_cbc_encrypt, RC5_32_decrypt,
308 RC5_32_encrypt, bn_add_words, bn_div_words, bn_mul_add_words,
309 bn_mul_comba4, bn_mul_comba8, bn_mul_words, bn_sqr_comba4,
310 bn_sqr_comba8, bn_sqr_words, bn_sub_words, des_decrypt3,
311 des_ede3_cbc_encrypt, des_encrypt, des_encrypt2, des_encrypt3,
312 des_ncbc_encrypt, md5_block_asm_host_order, sha1_block_asm_data_order
314 If none of these helps, you may want to try using the current snapshot.
315 If the problem persists, please submit a bug report.
318 * Why does the OpenSSL test fail with "bc: command not found"?
320 You didn't install "bc", the Unix calculator. If you want to run the
321 tests, get GNU bc from ftp://ftp.gnu.org or from your OS distributor.
324 * Why does the OpenSSL test fail with "bc: 1 no implemented"?
326 On some SCO installations or versions, bc has a bug that gets triggered
327 when you run the test suite (using "make test"). The message returned is
328 "bc: 1 not implemented".
330 The best way to deal with this is to find another implementation of bc
331 and compile/install it. GNU bc (see http://www.gnu.org/software/software.html
332 for download instructions) can be safely used, for example.
335 * Why does the OpenSSL compilation fail on Alpha True64 Unix?
337 On some Alpha installations running True64 Unix and Compaq C, the compilation
338 of crypto/sha/sha_dgst.c fails with the message 'Fatal: Insufficient virtual
339 memory to continue compilation.' As far as the tests have shown, this may be
340 a compiler bug. What happens is that it eats up a lot of resident memory
341 to build something, probably a table. The problem is clearly in the
342 optimization code, because if one eliminates optimization completely (-O0),
343 the compilation goes through (and the compiler consumes about 2MB of resident
344 memory instead of 240MB or whatever one's limit is currently).
346 There are three options to solve this problem:
348 1. set your current data segment size soft limit higher. Experience shows
349 that about 241000 kbytes seems to be enough on an AlphaServer DS10. You do
350 this with the command 'ulimit -Sd nnnnnn', where 'nnnnnn' is the number of
351 kbytes to set the limit to.
353 2. If you have a hard limit that is lower than what you need and you can't
354 get it changed, you can compile all of OpenSSL with -O0 as optimization
355 level. This is however not a very nice thing to do for those who expect to
356 get the best result from OpenSSL. A bit more complicated solution is the
359 ----- snip:start -----
360 make DIRS=crypto SDIRS=sha "`grep '^CFLAG=' Makefile.ssl | \
361 sed -e 's/ -O[0-9] / -O0 /'`"
362 rm `ls crypto/*.o crypto/sha/*.o | grep -v 'sha_dgst\.o'`
366 This will only compile sha_dgst.c with -O0, the rest with the optimization
367 level chosen by the configuration process. When the above is done, do the
368 test and installation and you're set.
371 * Why does the OpenSSL compilation fail with "ar: command not found"?
373 Getting this message is quite usual on Solaris 2, because Sun has hidden
374 away 'ar' and other development commands in directories that aren't in
375 $PATH by default. One of those directories is '/usr/ccs/bin'. The
376 quickest way to fix this is to do the following (it assumes you use sh
377 or any sh-compatible shell):
379 ----- snip:start -----
380 PATH=${PATH}:/usr/ccs/bin; export PATH
383 and then redo the compilation. What you should really do is make sure
384 '/usr/ccs/bin' is permanently in your $PATH, for example through your
385 '.profile' (again, assuming you use a sh-compatible shell).
388 * Why does the OpenSSL compilation fail on Win32 with VC++?
390 Sometimes, you may get reports from VC++ command line (cl) that it
391 can't find standard include files like stdio.h and other weirdnesses.
392 One possible cause is that the environment isn't correctly set up.
393 To solve that problem, one should run VCVARS32.BAT which is found in
394 the 'bin' subdirectory of the VC++ installation directory (somewhere
395 under 'Program Files'). This needs to be done prior to running NMAKE,
396 and the changes are only valid for the current DOS session.
399 [PROG] ========================================================================
401 * Is OpenSSL thread-safe?
403 Yes (with limitations: an SSL connection may not concurrently be used
404 by multiple threads). On Windows and many Unix systems, OpenSSL
405 automatically uses the multi-threaded versions of the standard
406 libraries. If your platform is not one of these, consult the INSTALL
409 Multi-threaded applications must provide two callback functions to
410 OpenSSL. This is described in the threads(3) manpage.
413 * I've compiled a program under Windows and it crashes: why?
415 This is usually because you've missed the comment in INSTALL.W32. You
416 must link with the multithreaded DLL version of the VC++ runtime library
417 otherwise the conflict will cause a program to crash: typically on the
418 first BIO related read or write operation.
421 * How do I read or write a DER encoded buffer using the ASN1 functions?
423 You have two options. You can either use a memory BIO in conjunction
424 with the i2d_XXX_bio() or d2i_XXX_bio() functions or you can use the
425 i2d_XXX(), d2i_XXX() functions directly. Since these are often the
426 cause of grief here are some code fragments using PKCS7 as an example:
428 unsigned char *buf, *p;
431 len = i2d_PKCS7(p7, NULL);
432 buf = OPENSSL_malloc(len); /* or Malloc, error checking omitted */
436 At this point buf contains the len bytes of the DER encoding of
439 The opposite assumes we already have len bytes in buf:
443 p7 = d2i_PKCS7(NULL, &p, len);
445 At this point p7 contains a valid PKCS7 structure of NULL if an error
446 occurred. If an error occurred ERR_print_errors(bio) should give more
449 The reason for the temporary variable 'p' is that the ASN1 functions
450 increment the passed pointer so it is ready to read or write the next
451 structure. This is often a cause of problems: without the temporary
452 variable the buffer pointer is changed to point just after the data
453 that has been read or written. This may well be uninitialized data
454 and attempts to free the buffer will have unpredictable results
455 because it no longer points to the same address.
458 * I've tried using <M_some_evil_pkcs12_macro> and I get errors why?
460 This usually happens when you try compiling something using the PKCS#12
461 macros with a C++ compiler. There is hardly ever any need to use the
462 PKCS#12 macros in a program, it is much easier to parse and create
463 PKCS#12 files using the PKCS12_parse() and PKCS12_create() functions
464 documented in doc/openssl.txt and with examples in demos/pkcs12. The
465 'pkcs12' application has to use the macros because it prints out
466 debugging information.
469 * I've called <some function> and it fails, why?
471 Before submitting a report or asking in one of the mailing lists, you
472 should try to determine the cause. In particular, you should call
473 ERR_print_errors() or ERR_print_errors_fp() after the failed call
474 and see if the message helps. Note that the problem may occur earlier
475 than you think -- you should check for errors after every call where
476 it is possible, otherwise the actual problem may be hidden because
477 some OpenSSL functions clear the error state.
480 * I just get a load of numbers for the error output, what do they mean?
482 The actual format is described in the ERR_print_errors() manual page.
483 You should call the function ERR_load_crypto_strings() before hand and
484 the message will be output in text form. If you can't do this (for example
485 it is a pre-compiled binary) you can use the errstr utility on the error
486 code itself (the hex digits after the second colon).
489 * Why do I get errors about unknown algorithms?
491 This can happen under several circumstances such as reading in an
492 encrypted private key or attempting to decrypt a PKCS#12 file. The cause
493 is forgetting to load OpenSSL's table of algorithms with
494 OpenSSL_add_all_algorithms(). See the manual page for more information.
497 * Why can't the OpenSSH configure script detect OpenSSL?
499 Several reasons for problems with the automatic detection exist.
500 OpenSSH requires at least version 0.9.5a of the OpenSSL libraries.
501 Sometimes the distribution has installed an older version in the system
502 locations that is detected instead of a new one installed. The OpenSSL
503 library might have been compiled for another CPU or another mode (32/64 bits).
504 Permissions might be wrong.
506 The general answer is to check the config.log file generated when running
507 the OpenSSH configure script. It should contain the detailed information
508 on why the OpenSSL library was not detected or considered incompatible.
510 * Can I use OpenSSL's SSL library with non-blocking I/O?
512 Yes; make sure to read the SSL_get_error(3) manual page!
514 A pitfall to avoid: Don't assume that SSL_read() will just read from
515 the underlying transport or that SSL_write() will just write to it --
516 it is also possible that SSL_write() cannot do any useful work until
517 there is data to read, or that SSL_read() cannot do anything until it
518 is possible to send data. One reason for this is that the peer may
519 request a new TLS/SSL handshake at any time during the protocol,
520 requiring a bi-directional message exchange; both SSL_read() and
521 SSL_write() will try to continue any pending handshake.
524 ===============================================================================