I discovered a very simple and easy guide to configure S/MIME on osx and iphone/ipad.
I choose to create a free certificate at http://www.instantssl.com/ssl-certificate-products/free-email-certificate.html. Others are available, like versign etc.
My only problem was importing the key on my ios devices. This worked for me:
1. Follow this guide http://arstechnica.com/apple/guides/2011/10/secure-your-e-mail-under-mac-os-x-and-ios-5-with-smime.ars?comments=1&start=40#comments-bar to add the certificate to osx
2. Open Keychain Access
3. Click "My Certificates" at bottom left
4. Expand your new certificate, and choose both entries
5. Rigth click, and export 2 items as .p12
6. Send the file as email, or transfer the file securly to you ios device
7. Open the file on ios
8. Enable S/MIME for email
9. Enable S/MIME
- Open settings mail - account - account - advanced - S/MIME
- Enable signing ; choose the new cert
- Enable crypt ; choose the new cert
You are now able to send and receive encrypted email from osx and ios.