How to send encrypted email from iPhone or iPad

Using Apple Mail on a Mac and a Secure E-Mail / S/MIME certificate I can an email to anyone who can verify that it came from my email address and hasn't been change on the way. A so-called private key is used to encode a checksum computed over the whole document. The recipient fetches the public key corresponding to the secret private key to decode the checksum and determine whether it corresponds to the message. If it doesn't the message has been changed along the way, or my private key wasn't used to encode it. If I want to ensure that only a particular person can read the message, I can use his/her public key to encode my message. That way, only the intended person can decode it with his/her secret key. This is called encrypting.

On my Mac I am able to send encrypted messages to people whose public keys I have. In fact, the email address for which I have the public keys are noted in the Contacts with a "squashed bug" symbol. For testing, I can encrypt messages among my three email addresses.

UNFORTUNATELY this doesn't seem to work on my iPhone or iPad. There don't seem to be any indications in Contacts which email addresses I can send encrypted mail to. Can anybody explain what's happening here? On my MacBook Pro I can send encrypted emails to one friend and all three of my email addresses. On iPhone and iPad I can't even send them to the sending address! The best I can do is sending signed mail to anyone, even though in Settings > Apps > Mail > Mail Accounts I've set in Account Settings > Advanced I've set Sign and Encrypt by Default to Yes.

Thanks