According to its documentation, on Unix, the Pharo VM's SSL plugin, libSqueakSSL.so, links into OpenSSL libraries dynamically. On my 64bit Ubuntu Trusty machine, OpenSSL is provided by the libssl1.0.0:i386 package.
(By the way, the SSH2 plugin libssh2.so.1.0.1 requires libcrypto too.)
According to packages.ubuntu.com, Trusty's libssl1.0.0 is built from openssl_1.0.1f.orig.tar.gz plus successive upstream patches.
From the OpenBSD developers, LibreSSL is "a version of the TLS/crypto stack forked from OpenSSL in 2014, with goals of modernizing the codebase, improving security, and applying best practice development processes." LibreSSL also comes with libtls, "a new TLS library, designed to make it easier to write foolproof applications".
Let's see how we go about linking libSqueakSSL.so with LibreSSL.
First, download and unpack LibreSSL. Modify the configure script at lines 2287 and 2289 so that LIBCRYPTO_VERSION and LIBSSL_VERSION both say 1:0:0 instead of 35:0:0. Then build LibreSSL:
I'm building on a 64bit OS, hence "-m32". Without "--disable-asm", the build fails. To get the assembler version, which is recommended for serious usage, either set up a 32bit build environment or muck around with autoconf/configure. I suspect the former is easier. :-)
The output files are $SRC/crypto/.libs/libcrypto.so.1.0.0 and $SRC/ssl/.libs/libssl.so.1.0.0. The shared object files have the "1.0.0" suffix because I modified configure above. Alternatively, I could've played around with autoconf, or built the shared objects with the "35.0.0" suffix and sym/hard-link them for the "1.0.0" versions. TIMTOWTDI.
Next, remove the OpenSSL package:
Finally, put the LibreSSL shared object files into the right place. Where this right place is depends on your environment. TIMTOWTDI. I choose to put them in the Pharo VM directory with its other plugins, and arrange to start Pharo with LD_LIBRARY_PATH set appropriately. Going by the output of ldd again, the following is required:
Launch the Pharo 4.0 image and run the Zodiac tests. All tests should pass. Well, except testGetPharoVersion, which looks for a file that apparently no longer exists.
Incidentally, Squeak 5.0-All-in-One's SSL plugin appears to have linked its crypto/SSL libraries in statically, so the only way to upgrade is to build a new plugin.