I don\'t have a lot experience about Client Certificate Authentication. Anybody can tell me how to use it in iOS app? Thanks :)
Before using client certificates in your app (as already answered by Jake) you have to implement import of certificate within your app to your app keychain. (note you need to use PKCS#12 certificate format, but you need to register it in your app (search for exported UTIs and Document types) with different extension, other than ".p12", which is already registered by the iOS. I've used .x-p12 in my app)
Or you need to include the certificate with your app bundle.
See here: iOS Client Certificates and Mobile Device Management
and here: https://developer.apple.com/library/ios/qa/qa1745/_index.html