ChatSecure
ChatSecure is a free and open source encrypted chat client for iOS
...Unlike other apps that keep you stuck in their walled garden, ChatSecure is fully interoperable with other clients that support OMEMO or OTR and XMPP, such as Conversations (Android), CoyIM (Desktop), and more. ChatSecure only uses well-known open-source cryptographic libraries to keep your conversations private. Other apps may make claims about "military grade" security but, without publicly auditable source code and verifiable end-to-end encryption, you cannot be truly safe.