Apple stopped including the OpenSSL development headers on recent versions of OSX, trying to get people to move away from the old 0.9.8 version that’s been deprecated for a very long time. Making people stop using this shared library is a Good Thing to be sure but you may come across older software that you want to build for yourself.

If you try to compile a newer version of OpenSSL you will likely find that programs will fail to build against more recent versions because a lot of data structures have been hidden. You may see errors such as:

error: variable has incomplete type 'EVP_PKEY' (aka 'struct evp_pkey_st')

        EVP_PKEY pk;

                 ^

/usr/local/include/openssl/ossl_typ.h:92:16: note: forward declaration of 'struct evp_pkey_st'

typedef struct evp_pkey_st EVP_PKEY;

If you want to get such code to compile there’s a quick and easy solution! OSX still ships with the 0.9.8 library, you just need to provide the headers. Remove any newer versions of OpenSSL, grab the 0.9.8 sources, and copy over the headers:

$ sudo cp -r include/openssl /usr/local/include/

And then you’re all set.