MobileCertificate

MobileCertificate allows you to export your OS X certificates and use them for Mail signature and encryption on your iOS and OS X devices.

The generated file needs to be securely transferred to your iPhone or iPad by yourself. When you open it on your mobile device, the system will show you the contents (your selected certificates) and offer you to install it.

When it's done, you can read encrypted e-mails and send signed and encrypted e-mails as long as you've correctly set up the S/MIME options in your iOS mail settings. The configuration file created by this tool can also be used to install your certificates on OS X.