S/MIME is the only native encryption method available for iOS and Secutor offers the only1 S/MIME solution for iOS. With just a few clicks it will generate a certificate of your own encryption keys.
and it is free to download
Secutor Certificate Wizard adds powerful email encryption to an iPhone, iPad® or iPod touch® without affecting the devices' inherent useability. It is an elegant solution and it makes no change to the way you use your phone or tablet.
In four simple steps you will create a private/public key combination that will encrypt your email messages using 2048-bit encryption, which is currently impossible and unlikely to be deciphered for a long while2.
Secutor uses the industry standard S/MIME method, which is supported by all major email clients and on all main platforms.
To support Secutor and my other works, bitcoin address for anonymous donations is 17jynzDrtF83ACjrdoa4rHjjLjGp2A8Kmj
1as of 28 October 2013 ♦ 2not before 2030