Documentation on using the SSL library with non-blocking I/O.

[openssl.git] / FAQ

OpenSSL  -  Frequently Asked Questions
--------------------------------------

* Which is the current version of OpenSSL?
* Where is the documentation?
* How can I contact the OpenSSL developers?
* Do I need patent licenses to use OpenSSL?
* Is OpenSSL thread-safe?
* Can I use OpenSSL's SSL library with non-blocking I/O?
* Why do I get a "PRNG not seeded" error message?
* Why does the linker complain about undefined symbols?
* Where can I get a compiled version of OpenSSL?
* I've compiled a program under Windows and it crashes: why?
* How do I read or write a DER encoded buffer using the ASN1 functions?
* I've tried using <M_some_evil_pkcs12_macro> and I get errors why?
* I've called <some function> and it fails, why?
* I just get a load of numbers for the error output, what do they mean?
* Why do I get errors about unknown algorithms?
* How do I create certificates or certificate requests?
* Why can't I create certificate requests?
* Why does <SSL program> fail with a certificate verify error?
* Why can I only use weak ciphers when I connect to a server using OpenSSL?
* How can I create DSA certificates?
* Why can't I make an SSL connection using a DSA certificate?
* How can I remove the passphrase on a private key?
* Why can't the OpenSSH configure script detect OpenSSL?
* Why does the OpenSSL test fail with "bc: command not found"?
* Why does the OpenSSL test fail with "bc: 1 no implemented"?
* Why does the OpenSSL compilation fail on Alpha True64 Unix?
* Why does the OpenSSL compilation fail with "ar: command not found"?
* Why does the OpenSSL compilation fail on Win32 with VC++?
* Why aren't tools like 'autoconf' and 'libtool' used?


* Which is the current version of OpenSSL?

The current version is available from <URL: http://www.openssl.org>.
OpenSSL 0.9.6 was released on September 24th, 2000.

In addition to the current stable release, you can also access daily
snapshots of the OpenSSL development version at <URL:
ftp://ftp.openssl.org/snapshot/>, or get it by anonymous CVS access.


* Where is the documentation?

OpenSSL is a library that provides cryptographic functionality to
applications such as secure web servers.  Be sure to read the
documentation of the application you want to use.  The INSTALL file
explains how to install this library.

OpenSSL includes a command line utility that can be used to perform a
variety of cryptographic functions.  It is described in the openssl(1)
manpage.  Documentation for developers is currently being written.  A
few manual pages already are available; overviews over libcrypto and
libssl are given in the crypto(3) and ssl(3) manpages.

The OpenSSL manpages are installed in /usr/local/ssl/man/ (or a
different directory if you specified one as described in INSTALL).
In addition, you can read the most current versions at
<URL: http://www.openssl.org/docs/>.

For information on parts of libcrypto that are not yet documented, you
might want to read Ariel Glenn's documentation on SSLeay 0.9, OpenSSL's
predecessor, at <URL: http://www.columbia.edu/~ariel/ssleay/>.  Much
of this still applies to OpenSSL.

There is some documentation about certificate extensions and PKCS#12
in doc/openssl.txt

The original SSLeay documentation is included in OpenSSL as
doc/ssleay.txt.  It may be useful when none of the other resources
help, but please note that it reflects the obsolete version SSLeay
0.6.6.


* How can I contact the OpenSSL developers?

The README file describes how to submit bug reports and patches to
OpenSSL.  Information on the OpenSSL mailing lists is available from
<URL: http://www.openssl.org>.


* Do I need patent licenses to use OpenSSL?

The patents section of the README file lists patents that may apply to
you if you want to use OpenSSL.  For information on intellectual
property rights, please consult a lawyer.  The OpenSSL team does not
offer legal advice.

You can configure OpenSSL so as not to use RC5 and IDEA by using
 ./config no-rc5 no-idea


* Is OpenSSL thread-safe?

Yes (with limitations: an SSL connection may not concurrently be used
by multiple threads).  On Windows and many Unix systems, OpenSSL
automatically uses the multi-threaded versions of the standard
libraries.  If your platform is not one of these, consult the INSTALL
file.

Multi-threaded applications must provide two callback functions to
OpenSSL.  This is described in the threads(3) manpage.


* Can I use OpenSSL's SSL library with non-blocking I/O?

Yes; make sure to read the SSL_get_error(3) manual page!

A pitfall to avoid: Don't assume that SSL_read() will just read from
the underlying transport or that SSL_write() will just write to it --
it is also possible that SSL_write() cannot do any useful work until
there is data to read, or that SSL_read() cannot do anything until it
is possible to send data.  One reason for this is that the peer may
request a new TLS/SSL handshake at any time during the protocol,
requiring a bi-directional message exchange; both SSL_read() and
SSL_write() will try to continue any pending handshake.


* Why do I get a "PRNG not seeded" error message?

Cryptographic software needs a source of unpredictable data to work
correctly.  Many open source operating systems provide a "randomness
device" that serves this purpose.  On other systems, applications have
to call the RAND_add() or RAND_seed() function with appropriate data
before generating keys or performing public key encryption.

Some broken applications do not do this.  As of version 0.9.5, the
OpenSSL functions that need randomness report an error if the random
number generator has not been seeded with at least 128 bits of
randomness.  If this error occurs, please contact the author of the
application you are using.  It is likely that it never worked
correctly.  OpenSSL 0.9.5 and later make the error visible by refusing
to perform potentially insecure encryption.

On systems without /dev/urandom, it is a good idea to use the Entropy
Gathering Demon; see the RAND_egd() manpage for details.

Most components of the openssl command line tool try to use the
file $HOME/.rnd (or $RANDFILE, if this environment variable is set)
for seeding the PRNG.  If this file does not exist or is too short,
the "PRNG not seeded" error message may occur.

[Note to OpenSSL 0.9.5 users: The command "openssl rsa" in version
0.9.5 does not do this and will fail on systems without /dev/urandom
when trying to password-encrypt an RSA key!  This is a bug in the
library; try a later version instead.]

For Solaris 2.6, Tim Nibbe <tnibbe@sprint.net> and others have suggested
installing the SUNski package from Sun patch 105710-01 (Sparc) which
adds a /dev/random device and make sure it gets used, usually through
$RANDFILE.  There are probably similar patches for the other Solaris
versions.  However, be warned that /dev/random is usually a blocking
device, which may have some effects on OpenSSL.


* Why does the linker complain about undefined symbols?

Maybe the compilation was interrupted, and make doesn't notice that
something is missing.  Run "make clean; make".

If you used ./Configure instead of ./config, make sure that you
selected the right target.  File formats may differ slightly between
OS versions (for example sparcv8/sparcv9, or a.out/elf).

In case you get errors about the following symbols, use the config
option "no-asm", as described in INSTALL:

 BF_cbc_encrypt, BF_decrypt, BF_encrypt, CAST_cbc_encrypt,
 CAST_decrypt, CAST_encrypt, RC4, RC5_32_cbc_encrypt, RC5_32_decrypt,
 RC5_32_encrypt, bn_add_words, bn_div_words, bn_mul_add_words,
 bn_mul_comba4, bn_mul_comba8, bn_mul_words, bn_sqr_comba4,
 bn_sqr_comba8, bn_sqr_words, bn_sub_words, des_decrypt3,
 des_ede3_cbc_encrypt, des_encrypt, des_encrypt2, des_encrypt3,
 des_ncbc_encrypt, md5_block_asm_host_order, sha1_block_asm_data_order

If none of these helps, you may want to try using the current snapshot.
If the problem persists, please submit a bug report.


* Where can I get a compiled version of OpenSSL?

Some applications that use OpenSSL are distributed in binary form.
When using such an application, you don't need to install OpenSSL
yourself; the application will include the required parts (e.g. DLLs).

If you want to install OpenSSL on a Windows system and you don't have
a C compiler, read the "Mingw32" section of INSTALL.W32 for information
on how to obtain and install the free GNU C compiler.

A number of Linux and *BSD distributions include OpenSSL.


* I've compiled a program under Windows and it crashes: why?

This is usually because you've missed the comment in INSTALL.W32. You
must link with the multithreaded DLL version of the VC++ runtime library
otherwise the conflict will cause a program to crash: typically on the
first BIO related read or write operation.


* How do I read or write a DER encoded buffer using the ASN1 functions?

You have two options. You can either use a memory BIO in conjunction
with the i2d_XXX_bio() or d2i_XXX_bio() functions or you can use the
i2d_XXX(), d2i_XXX() functions directly. Since these are often the
cause of grief here are some code fragments using PKCS7 as an example:

unsigned char *buf, *p;
int len;

len = i2d_PKCS7(p7, NULL);
buf = OPENSSL_malloc(len); /* or Malloc, error checking omitted */
p = buf;
i2d_PKCS7(p7, &p);

At this point buf contains the len bytes of the DER encoding of
p7.

The opposite assumes we already have len bytes in buf:

unsigned char *p;
p = buf;
p7 = d2i_PKCS7(NULL, &p, len);

At this point p7 contains a valid PKCS7 structure of NULL if an error
occurred. If an error occurred ERR_print_errors(bio) should give more
information.

The reason for the temporary variable 'p' is that the ASN1 functions
increment the passed pointer so it is ready to read or write the next
structure. This is often a cause of problems: without the temporary
variable the buffer pointer is changed to point just after the data
that has been read or written. This may well be uninitialized data
and attempts to free the buffer will have unpredictable results
because it no longer points to the same address.


* I've tried using <M_some_evil_pkcs12_macro> and I get errors why?

This usually happens when you try compiling something using the PKCS#12
macros with a C++ compiler. There is hardly ever any need to use the
PKCS#12 macros in a program, it is much easier to parse and create
PKCS#12 files using the PKCS12_parse() and PKCS12_create() functions
documented in doc/openssl.txt and with examples in demos/pkcs12. The
'pkcs12' application has to use the macros because it prints out 
debugging information.


* I've called <some function> and it fails, why?

Before submitting a report or asking in one of the mailing lists, you
should try to determine the cause. In particular, you should call
ERR_print_errors() or ERR_print_errors_fp() after the failed call
and see if the message helps. Note that the problem may occur earlier
than you think -- you should check for errors after every call where
it is possible, otherwise the actual problem may be hidden because
some OpenSSL functions clear the error state.


* I just get a load of numbers for the error output, what do they mean?

The actual format is described in the ERR_print_errors() manual page.
You should call the function ERR_load_crypto_strings() before hand and
the message will be output in text form. If you can't do this (for example
it is a pre-compiled binary) you can use the errstr utility on the error
code itself (the hex digits after the second colon).


* Why do I get errors about unknown algorithms?

This can happen under several circumstances such as reading in an
encrypted private key or attempting to decrypt a PKCS#12 file. The cause
is forgetting to load OpenSSL's table of algorithms with
OpenSSL_add_all_algorithms(). See the manual page for more information.


* How do I create certificates or certificate requests?

Check out the CA.pl(1) manual page. This provides a simple wrapper round
the 'req', 'verify', 'ca' and 'pkcs12' utilities. For finer control check
out the manual pages for the individual utilities and the certificate
extensions documentation (currently in doc/openssl.txt).


* Why can't I create certificate requests?

You typically get the error:

        unable to find 'distinguished_name' in config
        problems making Certificate Request

This is because it can't find the configuration file. Check out the
DIAGNOSTICS section of req(1) for more information.


* Why does <SSL program> fail with a certificate verify error?

This problem is usually indicated by log messages saying something like
"unable to get local issuer certificate" or "self signed certificate".
When a certificate is verified its root CA must be "trusted" by OpenSSL
this typically means that the CA certificate must be placed in a directory
or file and the relevant program configured to read it. The OpenSSL program
'verify' behaves in a similar way and issues similar error messages: check
the verify(1) program manual page for more information.


* Why can I only use weak ciphers when I connect to a server using OpenSSL?

This is almost certainly because you are using an old "export grade" browser
which only supports weak encryption. Upgrade your browser to support 128 bit
ciphers.


* How can I create DSA certificates?

Check the CA.pl(1) manual page for a DSA certificate example.


* Why can't I make an SSL connection to a server using a DSA certificate?

Typically you'll see a message saying there are no shared ciphers when
the same setup works fine with an RSA certificate. There are two possible
causes. The client may not support connections to DSA servers most web
browsers (including Netscape and MSIE) only support connections to servers
supporting RSA cipher suites. The other cause is that a set of DH parameters
has not been supplied to the server. DH parameters can be created with the
dhparam(1) command and loaded using the SSL_CTX_set_tmp_dh() for example:
check the source to s_server in apps/s_server.c for an example.


* How can I remove the passphrase on a private key?

Firstly you should be really *really* sure you want to do this. Leaving
a private key unencrypted is a major security risk. If you decide that
you do have to do this check the EXAMPLES sections of the rsa(1) and
dsa(1) manual pages.


* Why can't the OpenSSH configure script detect OpenSSL?

There is a problem with OpenSSH 1.2.2p1, in that the configure script
can't find the installed OpenSSL libraries.  The problem is actually
a small glitch that is easily solved with the following patch to be
applied to the OpenSSH distribution:

----- snip:start -----
--- openssh-1.2.2p1/configure.in.orig   Thu Mar 23 18:56:58 2000
+++ openssh-1.2.2p1/configure.in        Thu Mar 23 18:55:05 2000
@@ -152,10 +152,10 @@
 AC_MSG_CHECKING([for OpenSSL/SSLeay directory])
 for ssldir in "" $tryssldir /usr /usr/local/openssl /usr/lib/openssl /usr/local/ssl /usr/lib/ssl /usr/local /usr/pkg /opt /opt/openssl ; do
        if test ! -z "$ssldir" ; then
-               LIBS="$saved_LIBS -L$ssldir"
+               LIBS="$saved_LIBS -L$ssldir/lib"
                CFLAGS="$CFLAGS -I$ssldir/include"
                if test "x$need_dash_r" = "x1" ; then
-                       LIBS="$LIBS -R$ssldir"
+                       LIBS="$LIBS -R$ssldir/lib"
                fi
        fi
        LIBS="$LIBS -lcrypto"
--- openssh-1.2.2p1/configure.orig      Thu Mar 23 18:55:02 2000
+++ openssh-1.2.2p1/configure   Thu Mar 23 18:57:08 2000
@@ -1890,10 +1890,10 @@
 echo "configure:1891: checking for OpenSSL/SSLeay directory" >&5
 for ssldir in "" $tryssldir /usr /usr/local/openssl /usr/lib/openssl /usr/local/ssl /usr/lib/ssl /usr/local /usr/pkg /opt /opt/openssl ; do
        if test ! -z "$ssldir" ; then
-               LIBS="$saved_LIBS -L$ssldir"
+               LIBS="$saved_LIBS -L$ssldir/lib"
                CFLAGS="$CFLAGS -I$ssldir/include"
                if test "x$need_dash_r" = "x1" ; then
-                       LIBS="$LIBS -R$ssldir"
+                       LIBS="$LIBS -R$ssldir/lib"
                fi
        fi
        LIBS="$LIBS -lcrypto"
----- snip:end -----


* Why does the OpenSSL test fail with "bc: command not found"?

You didn't install "bc", the Unix calculator.  If you want to run the
tests, get GNU bc from ftp://ftp.gnu.org or from your OS distributor.


* Why does the OpenSSL test fail with "bc: 1 no implemented"?

On some SCO installations or versions, bc has a bug that gets triggered when
you run the test suite (using "make test").  The message returned is "bc:
1 not implemented".  The best way to deal with this is to find another
implementation of bc and compile/install it.  For example, GNU bc (see
http://www.gnu.org/software/software.html for download instructions) can
be safely used.


* Why does the OpenSSL compilation fail on Alpha True64 Unix?

On some Alpha installations running True64 Unix and Compaq C, the compilation
of crypto/sha/sha_dgst.c fails with the message 'Fatal:  Insufficient virtual
memory to continue compilation.'  As far as the tests have shown, this may be
a compiler bug.  What happens is that it eats up a lot of resident memory
to build something, probably a table.  The problem is clearly in the
optimization code, because if one eliminates optimization completely (-O0),
the compilation goes through (and the compiler consumes about 2MB of resident
memory instead of 240MB or whatever one's limit is currently).

There are three options to solve this problem:

1. set your current data segment size soft limit higher.  Experience shows
that about 241000 kbytes seems to be enough on an AlphaServer DS10.  You do
this with the command 'ulimit -Sd nnnnnn', where 'nnnnnn' is the number of
kbytes to set the limit to.

2. If you have a hard limit that is lower than what you need and you can't
get it changed, you can compile all of OpenSSL with -O0 as optimization
level.  This is however not a very nice thing to do for those who expect to
get the best result from OpenSSL.  A bit more complicated solution is the
following:

----- snip:start -----
  make DIRS=crypto SDIRS=sha "`grep '^CFLAG=' Makefile.ssl | \
       sed -e 's/ -O[0-9] / -O0 /'`"
  rm `ls crypto/*.o crypto/sha/*.o | grep -v 'sha_dgst\.o'`
  make
----- snip:end -----

This will only compile sha_dgst.c with -O0, the rest with the optimization
level chosen by the configuration process.  When the above is done, do the
test and installation and you're set.


* Why does the OpenSSL compilation fail with "ar: command not found"?

Getting this message is quite usual on Solaris 2, because Sun has hidden
away 'ar' and other development commands in directories that aren't in
$PATH by default.  One of those directories is '/usr/ccs/bin'.  The
quickest way to fix this is to do the following (it assumes you use sh
or any sh-compatible shell):

----- snip:start -----
  PATH=${PATH}:/usr/ccs/bin; export PATH
----- snip:end -----

and then redo the compilation.  What you should really do is make sure
'/usr/ccs/bin' is permanently in your $PATH, for example through your
'.profile' (again, assuming you use a sh-compatible shell).


* Why does the OpenSSL compilation fail on Win32 with VC++?

Sometimes, you may get reports from VC++ command line (cl) that it
can't find standard include files like stdio.h and other weirdnesses.
One possible cause is that the environment isn't correctly set up.
To solve that problem, one should run VCVARS32.BAT which is found in
the 'bin' subdirectory of the VC++ installation directory (somewhere
under 'Program Files').  This needs to be done prior to running NMAKE,
and the changes are only valid for the current DOS session.


* Why aren't tools like 'autoconf' and 'libtool' used?

autoconf is a nice tool, but is unfortunately very Unix-centric.
Although one can come up with solution to have ports keep in track,
there's also some work needed for that, and can be quite painful at
times.  If there was a 'autoconf'-like tool that generated perl
scripts or something similarly general, it would probably be used
in OpenSSL much earlier.

libtool has repeatadly been reported by some members of the OpenSSL
development and others to be a pain to use.  So far, those in the
development team who have said anything about this have expressed
a wish to avoid libtool for that reason.