Which hash algorithm is used when using S/MIME by default?

I just installed my S/MIME certificates on my iPhone 6 for both, signing and encryption.

Using a mail client on computers I can configure which hash algorithm and encryption is used. As for example Outlook per default uses SHA1 this should be changed to at least SHA256 for various reason.

Such an option is not available in the iOS settings for my mail app. I can only decide if I want to sign or not.

Which hash algorithm is used under iOS 8.1 ?