OpenSSL - Frequently Asked Questions -------------------------------------- [MISC] Miscellaneous questions * Which is the current version of OpenSSL? * Where is the documentation? * How can I contact the OpenSSL developers? * Where can I get a compiled version of OpenSSL? * Why aren't tools like 'autoconf' and 'libtool' used? * What is an 'engine' version? * How do I check the authenticity of the OpenSSL distribution? [LEGAL] Legal questions * Do I need patent licenses to use OpenSSL? * Can I use OpenSSL with GPL software? [USER] Questions on using the OpenSSL applications * Why do I get a "PRNG not seeded" error message? * Why do I get an "unable to write 'random state'" error message? * How do I create certificates or certificate requests? * Why can't I create certificate requests? * Why does 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 I use OpenSSL certificates with SSL client authentication? * Why does my browser give a warning about a mismatched hostname? * How do I install a CA certificate into a browser? * Why is OpenSSL x509 DN output not conformant to RFC2253? [BUILD] Questions about building and testing OpenSSL * Why does the linker complain about undefined symbols? * 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 test fail with "bc: stack empty"? * Why does the OpenSSL compilation fail on Alpha Tru64 Unix? * Why does the OpenSSL compilation fail with "ar: command not found"? * Why does the OpenSSL compilation fail on Win32 with VC++? * What is special about OpenSSL on Redhat? * Why does the OpenSSL compilation fail on MacOS X? * Why does the OpenSSL test suite fail on MacOS X? [PROG] Questions about programming with OpenSSL * Is OpenSSL thread-safe? * 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 and I get errors why? * I've called 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? * Why can't the OpenSSH configure script detect OpenSSL? * Can I use OpenSSL's SSL library with non-blocking I/O? * Why doesn't my server application receive a client certificate? * Why does compilation fail due to an undefined symbol NID_uniqueIdentifier? =============================================================================== [MISC] ======================================================================== * Which is the current version of OpenSSL? The current version is available from . OpenSSL 0.9.6h was released on December 5, 2002. In addition to the current stable release, you can also access daily snapshots of the OpenSSL development version at , 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 . 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 . 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 . * 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. * Why aren't tools like 'autoconf' and 'libtool' used? autoconf will probably be used in future OpenSSL versions. If it was less Unix-centric, it might have been used much earlier. * What is an 'engine' version? With version 0.9.6 OpenSSL was extended to interface to external crypto hardware. This was realized in a special release '0.9.6-engine'. With version 0.9.7 (not yet released) the changes were merged into the main development line, so that the special release is no longer necessary. * How do I check the authenticity of the OpenSSL distribution? We provide MD5 digests and ASC signatures of each tarball. Use MD5 to check that a tarball from a mirror site is identical: md5sum TARBALL | awk '{print $1;}' | cmp - TARBALL.md5 You can check authenticity using pgp or gpg. You need the OpenSSL team member public key used to sign it (download it from a key server). Then just do: pgp TARBALL.asc [LEGAL] ======================================================================= * 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 * Can I use OpenSSL with GPL software? On many systems including the major Linux and BSD distributions, yes (the GPL does not place restrictions on using libraries that are part of the normal operating system distribution). On other systems, the situation is less clear. Some GPL software copyright holders claim that you infringe on their rights if you use OpenSSL with their software on operating systems that don't normally include OpenSSL. If you develop open source software that uses OpenSSL, you may find it useful to choose an other license than the GPL, or state explicitly that "This program is released under the GPL with the additional exemption that compiling, linking, and/or using OpenSSL is allowed." If you are using GPL software developed by others, you may want to ask the copyright holder for permission to use their software with OpenSSL. [USER] ======================================================================== * 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. (These functions initialize the pseudo-random number generator, PRNG.) 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 and /dev/random, it is a good idea to use the Entropy Gathering Demon (EGD); see the RAND_egd() manpage for details. Starting with version 0.9.7, OpenSSL will automatically look for an EGD socket at /var/run/egd-pool, /dev/egd-pool, /etc/egd-pool and /etc/entropy. Most components of the openssl command line utility automatically try to seed the random number generator from a file. The name of the default seeding file is determined as follows: If environment variable RANDFILE is set, then it names the seeding file. Otherwise if environment variable HOME is set, then the seeding file is $HOME/.rnd. If neither RANDFILE nor HOME is set, versions up to OpenSSL 0.9.6 will use file .rnd in the current directory while OpenSSL 0.9.6a uses no default seeding file at all. OpenSSL 0.9.6b and later will behave similarly to 0.9.6a, but will use a default of "C:\" for HOME on Windows systems if the environment variable has not been set. If the default seeding file does not exist or is too short, the "PRNG not seeded" error message may occur. The openssl command line utility will write back a new state to the default seeding file (and create this file if necessary) unless there was no sufficient seeding. Pointing $RANDFILE to an Entropy Gathering Daemon socket does not work. Use the "-rand" option of the OpenSSL command line tools instead. The $RANDFILE environment variable and $HOME/.rnd are only used by the OpenSSL command line tools. Applications using the OpenSSL library provide their own configuration options to specify the entropy source, please check out the documentation coming the with application. For Solaris 2.6, Tim Nibbe 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. An official statement from Sun with respect to /dev/random support can be found at http://sunsolve.sun.com/pub-cgi/retrieve.pl?doc=fsrdb/27606&zone_32=SUNWski However, be warned that /dev/random is usually a blocking device, which may have some effects on OpenSSL. A third party /dev/random solution for Solaris is available at http://www.cosy.sbg.ac.at/~andi/ * Why do I get an "unable to write 'random state'" error message? Sometimes the openssl command line utility does not abort with a "PRNG not seeded" error message, but complains that it is "unable to write 'random state'". This message refers to the default seeding file (see previous answer). A possible reason is that no default filename is known because neither RANDFILE nor HOME is set. (Versions up to 0.9.6 used file ".rnd" in the current directory in this case, but this has changed with 0.9.6a.) * 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 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 I use OpenSSL certificates with SSL client authentication? What will typically happen is that when a server requests authentication it will either not include your certificate or tell you that you have no client certificates (Netscape) or present you with an empty list box (MSIE). The reason for this is that when a server requests a client certificate it includes a list of CAs names which it will accept. Browsers will only let you select certificates from the list on the grounds that there is little point presenting a certificate which the server will reject. The solution is to add the relevant CA certificate to your servers "trusted CA list". How you do this depends on the server software in uses. You can print out the servers list of acceptable CAs using the OpenSSL s_client tool: openssl s_client -connect www.some.host:443 -prexit If your server only requests certificates on certain URLs then you may need to manually issue an HTTP GET command to get the list when s_client connects: GET /some/page/needing/a/certificate.html If your CA does not appear in the list then this confirms the problem. * Why does my browser give a warning about a mismatched hostname? Browsers expect the server's hostname to match the value in the commonName (CN) field of the certificate. If it does not then you get a warning. * How do I install a CA certificate into a browser? The usual way is to send the DER encoded certificate to the browser as MIME type application/x-x509-ca-cert, for example by clicking on an appropriate link. On MSIE certain extensions such as .der or .cacert may also work, or you can import the certificate using the certificate import wizard. You can convert a certificate to DER form using the command: openssl x509 -in ca.pem -outform DER -out ca.der Occasionally someone suggests using a command such as: openssl pkcs12 -export -out cacert.p12 -in cacert.pem -inkey cakey.pem DO NOT DO THIS! This command will give away your CAs private key and reduces its security to zero: allowing anyone to forge certificates in whatever name they choose. * Why is OpenSSL x509 DN output not conformant to RFC2253? The ways to print out the oneline format of the DN (Distinguished Name) have been extended in version 0.9.7 of OpenSSL. Using the new X509_NAME_print_ex() interface, the "-nameopt" option could be introduded. See the manual page of the "openssl x509" commandline tool for details. The old behaviour has however been left as default for the sake of compatibility. [BUILD] ======================================================================= * 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. * 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. GNU bc (see http://www.gnu.org/software/software.html for download instructions) can be safely used, for example. * Why does the OpenSSL test fail with "bc: stack empty"? On some DG/ux versions, bc seems to have a too small stack for calculations that the OpenSSL bntest throws at it. This gets triggered when you run the test suite (using "make test"). The message returned is "bc: stack empty". The best way to deal with this is to find another implementation of bc and compile/install it. GNU bc (see http://www.gnu.org/software/software.html for download instructions) can be safely used, for example. * Why does the OpenSSL compilation fail on Alpha Tru64 Unix? On some Alpha installations running Tru64 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. * What is special about OpenSSL on Redhat? Red Hat Linux (release 7.0 and later) include a preinstalled limited version of OpenSSL. For patent reasons, support for IDEA, RC5 and MDC2 is disabled in this version. The same may apply to other Linux distributions. Users may therefore wish to install more or all of the features left out. To do this you MUST ensure that you do not overwrite the openssl that is in /usr/bin on your Red Hat machine. Several packages depend on this file, including sendmail and ssh. /usr/local/bin is a good alternative choice. The libraries that come with Red Hat 7.0 onwards have different names and so are not affected. (eg For Red Hat 7.2 they are /lib/libssl.so.0.9.6b and /lib/libcrypto.so.0.9.6b with symlinks /lib/libssl.so.2 and /lib/libcrypto.so.2 respectively). Please note that we have been advised by Red Hat attempting to recompile the openssl rpm with all the cryptography enabled will not work. All other packages depend on the original Red Hat supplied openssl package. It is also worth noting that due to the way Red Hat supplies its packages, updates to openssl on each distribution never change the package version, only the build number. For example, on Red Hat 7.1, the latest openssl package has version number 0.9.6 and build number 9 even though it contains all the relevant updates in packages up to and including 0.9.6b. A possible way around this is to persuade Red Hat to produce a non-US version of Red Hat Linux. FYI: Patent numbers and expiry dates of US patents: MDC-2: 4,908,861 13/03/2007 IDEA: 5,214,703 25/05/2010 RC5: 5,724,428 03/03/2015 * Why does the OpenSSL compilation fail on MacOS X? If the failure happens when trying to build the "openssl" binary, with a large number of undefined symbols, it's very probable that you have OpenSSL 0.9.6b delivered with the operating system (you can find out by running '/usr/bin/openssl version') and that you were trying to build OpenSSL 0.9.7 or newer. The problem is that the loader ('ld') in MacOS X has a misfeature that's quite difficult to go around. Look in the file PROBLEMS for a more detailed explanation and for possible solutions. * Why does the OpenSSL test suite fail on MacOS X? If the failure happens when running 'make test' and the RC4 test fails, it's very probable that you have OpenSSL 0.9.6b delivered with the operating system (you can find out by running '/usr/bin/openssl version') and that you were trying to build OpenSSL 0.9.6d. The problem is that the loader ('ld') in MacOS X has a misfeature that's quite difficult to go around and has linked the programs "openssl" and the test programs with /usr/lib/libcrypto.dylib and /usr/lib/libssl.dylib instead of the libraries you just built. Look in the file PROBLEMS for a more detailed explanation and for possible solutions. [PROG] ======================================================================== * 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. * I've compiled a program under Windows and it crashes: why? This is usually because you've missed the comment in INSTALL.W32. Your application must link against the same version of the Win32 C-Runtime against which your openssl libraries were linked. The default version for OpenSSL is /MD - "Multithreaded DLL". If you are using Microsoft Visual C++'s IDE (Visual Studio), in many cases, your new project most likely defaulted to "Debug Singlethreaded" - /ML. This is NOT interchangeable with /MD and your program will crash, typically on the first BIO related read or write operation. For each of the six possible link stage configurations within Win32, your application must link against the same by which OpenSSL was built. If you are using MS Visual C++ (Studio) this can be changed by: 1. Select Settings... from the Project Menu. 2. Select the C/C++ Tab. 3. Select "Code Generation from the "Category" drop down list box 4. Select the Appropriate library (see table below) from the "Use run-time library" drop down list box. Perform this step for both your debug and release versions of your application (look at the top left of the settings panel to change between the two) Single Threaded /ML - MS VC++ often defaults to this for the release version of a new project. Debug Single Threaded /MLd - MS VC++ often defaults to this for the debug version of a new project. Multithreaded /MT Debug Multithreaded /MTd Multithreaded DLL /MD - OpenSSL defaults to this. Debug Multithreaded DLL /MDd Note that debug and release libraries are NOT interchangeable. If you built OpenSSL with /MD your application must use /MD and cannot use /MDd. * 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 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 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. * Why can't the OpenSSH configure script detect OpenSSL? Several reasons for problems with the automatic detection exist. OpenSSH requires at least version 0.9.5a of the OpenSSL libraries. Sometimes the distribution has installed an older version in the system locations that is detected instead of a new one installed. The OpenSSL library might have been compiled for another CPU or another mode (32/64 bits). Permissions might be wrong. The general answer is to check the config.log file generated when running the OpenSSH configure script. It should contain the detailed information on why the OpenSSL library was not detected or considered incompatible. * 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 doesn't my server application receive a client certificate? Due to the TLS protocol definition, a client will only send a certificate, if explicitly asked by the server. Use the SSL_VERIFY_PEER flag of the SSL_CTX_set_verify() function to enable the use of client certificates. * Why does compilation fail due to an undefined symbol NID_uniqueIdentifier? For OpenSSL 0.9.7 the OID table was extended and corrected. In earlier versions, uniqueIdentifier was incorrectly used for X.509 certificates. The correct name according to RFC2256 (LDAP) is x500UniqueIdentifier. Change your code to use the new name when compiling against OpenSSL 0.9.7. ===============================================================================