How could I configure
mod_ssl to use different certificates for incoming and outgoing messages? I have managed to configure
apache2 only to handle incoming messages with single certificate. Is it possible to separate incoming and outgoing flows? How could I do this?
When you use SSL/TLS, the server certificate is only used to authenticate the server. Shared keys are negotiated during the handshake, with different keys for client write and server write operations (see Key Calculation for the details).
If you're talking about "outgoing messages" in terms of Apache Httpd behaving as a reverse proxy (and thus being a client to another server), you can use the
SSLProxyMachineCertificateFile configuration directive to use a client certificate if needed.