Are RSA SSL Certificates (Private and Public key) generated by makecert compatible with OpenSSL decryption ie. can I decrypt them with the openssl_* methods? I am working on two-way SSL authentication between a .NET client and a PHP service, so I want to know if I have to create OpenSSL certificates on the .NET side.
Both makecert and OpenSSL follow X.509, so they are compatible -- in theory. X.509 is extensible is various arbitrary ways, and has been extended in various arbitrary and exotic ways, so anything can happen.
Practically, OpenSSL powers a lot of existing HTTPS web sites (according to these statistics, Apache powers more than 50% of all Web sites, and when Apache does HTTPS, it does so with OpenSSL(*)) so chances are that Microsoft tried not to break compatibility in that matter. In my experience, .NET and OpenSSL talke to each other quite politely.
(*) Well, Apache can also use GnuTLS, but it seems to be rather uncommon.