decryption using digital certificate

A client application is encrypting a text using private key and sends through socket.

My application(server written in c/c++,unix) receives the chiper-text through socket.
I have client's digital certificate.

now, how do I decrypt(may be using openssl library) this ciper-text using client's digital certificate.

The answer will very much depend on both client's enc/dec implementation and keys usage. Is it PGP one, JCE based ?