Distributing libcurl with SSL


This guide is intended for someone that needs to make https requests from a closed-source C++ application to a limited set of hosts/sites. It only covers distributing for Linux; Windows and OS X will be in a future update.

In developing Gnomescroll, I needed a way to authenticate users. It is a multiplayer-only game, so user accounts are necessary to save progress. The original authentication system was developed in Fall 2012. It relies on an https auth server (written in Flask) and a shared-secret token based system for authenticating clients with game servers.


Libcurl source

Download the libcurl source

Compile libcurl

Libcurl uses both cmake and autoconf. We are going to use autoconf. The libcurl compilation guide does not mention this.

We are going to compile it as a shared library only, use GnuTLS for SSL, and disable everything else except cookies. We are also going to bump the library version to avoid a “no version information available” error with ldd.

GnuTLS is LGPL, and everything we need it to link to is LGPL, so it is supposedly ok for dynamic linking from a proprietary application (don’t take my word for it). OpenSSL is another option, but it is BSD 4-clause licensed, which is incompatible with your application if it is GPL.

Libcurl has several other options for SSL libraries.

Note that the various SSL libraries have slight differences in how certificates are specified and located. I will only be covering certificates with GnuTLS.

Fetching the source and generating ./configure:

git clone https://github.com/bagder/curl.git
cd curl/
aclocal; autoheader; automake; autoconf

Configuring for GnuTLS:

./configure --without-libidn --without-winidn --without-librtmp \
--without-libssh2 --without-libmetalink --without-ssl --with-gnutls \
--disable-ipv6 --disable-gopher --disable-smtp --disable-imap \
--disable-pop3 --disable-tftp --disable-telnet --disable-dict \
--disable-rtsp --disable-ldaps  --disable-ldap  --disable-file \
--disable-ftp --disable-static --disable-libcurl-option \
--disable-threaded-resolver --enable-versioned-symbols \
--enable-soname-bump --enable-cookies

Configuring for OpenSSL:

./configure --without-libidn --without-winidn --without-librtmp \
--without-libssh2 --without-libmetalink --with-ssl --disable-ipv6 \
--disable-gopher --disable-smtp --disable-imap --disable-pop3  \
--disable-tftp --disable-telnet --disable-dict --disable-rtsp  \
--disable-ldaps  --disable-ldap  --disable-file --disable-ftp \
--disable-static --disable-libcurl-option --enable-cookies \
--disable-threaded-resolver --enable-versioned-symbols \

Configuring without SSL

If you don’t want SSL, replace any instances of --with-ssl, --with-gnutls, and any other --with-<ssl_lib> lines you may have added with --without-ssl.

Other configuration options

./configure --help will show other available options if these instructions are not exactly what you need.

Cross compiling for 32-bit:

If you need to build 32-bit libcurl from a 64-bit system, add --host=i386-linux-gnu CFLAGS=-m32 to the configuration options. i386-linux-gnu is the host name for 32-bit on ubuntu/mint multilib; you may need to do --host=i686-pc-linux-gnu or similar on other systems.



Install for your project

We are not going to make install, so it does not clobber the libcurl installed by your system. However, if you provided your own non-conflicting –prefix to the ./configure script, it is not a problem. Here, we copy the libs into the lib/ subfolder of our project.

cp -P ./lib/.libs/libcurl.so* /path/to/project/lib/

We also need to copy the headers, because libcurl generates header files per-compilation, as opposed to having a universal header file like every other package you’ve encountered:

mkdir -p /path/to/project/lib/include/curl
cp ./include/curl/*.h /path/to/project/lib/include/curl/

You don’t have to put the headers in that path, but don’t let them clobber your system’s headers with a rogue make install. This is especially important if you are cross-compiling to 32-bit, because the headers generated during 64-bit and 32-bit compilation are incompatible. If you are cross-compiling, make sure to have a different lib/ and include/ path for each arch in your project.

Integrate with your build system

Update your build system to use the project- or arch-specific include and link paths for libcurl (e.g. -I./lib/include32 -L./lib/lin32).

Distributing your SSL keys

You may not wish to rely on the user to have the correct certs installed on their system. If the https sites you need to navigate to are limited and known ahead of time, you can collect and include them with your application.

Get the public keys for your site(s)

You will need to download the public keys for all of your sites, and every key in the chain. To retrieve most of the chain, optionally do:

gnutls-cli --print-cert example.com < /dev/null > example.com.pem

In my case, this retrieves all the keys in the chain except for the root key. I have no idea why so you may experience different issues with your sites. If you have problems validating your certs later, you can get them all manually.

gnutls-cli is provided by the package gnutls-bin in ubuntu/mint.

To get the final root key, or all of your keys if you skipped gnutls-cli, export them manually from the browser. With Chromium, the steps are:

  1. Navigate to https://example.com
  2. Click the lock icon next to the url in the omnibar.
  3. Click ‘Connection’ tab
  4. Click ‘Certificate Information’ link
  5. Click ‘Details’ tab
  6. Click the root authority (the top one in the hierarchy box, “Builtin Object Token”)
  7. Click ‘Export’, save to cert1.pem
  8. Repeat steps 6 and 7 for each level of the hierarchy, if you did not get the certs with gnutls-cli, or if you have problems with those certs.

Combine the public keys

Combine all the keys you fetched into one .pem file.

cat cert*.pem > example.com.pem
rm cert*.pem

Copy them into your project’s data or assets folder:

mkdir -p /path/to/project/assets/certs/
mv example.com.pem /path/to/project/assets/certs/

This method is only known to work with GnuTLS; OpenSSL handles certs slightly differently. If you are using OpenSSL, you may need to do c_rehash /path/to/project/assets/certs/ after you copy the .pem file in.

Making the SSL request from your application

Example C code for making an SSL request with the certs you created

That example includes the cookie handling example from the libcurl website, because you’ll likely need it for your authentication system. Instructions for compiling it are near the top of the file.

After compiling it, test it:

./curltest https://example.com ./path/to/certs/example.com.pem

You should see the html content displayed in the terminal.

Remaining tasks

This still isn’t ready for full distribution. We have to:

  • Include the gnutls library and its dependencies in the lib folder. Possibly compile from source to restrict its dependencies.
  • Compile libcurl for windows and OS X for a cross-platform release

Also, to finish the rest of the authentication system, you’ll need to have:

  • A web server to authenticate with
  • A way for users to enter credentials into your app
  • A system for authenticating with a 3rd party, e.g. your game server