Compatibility with Krypton mobile secret key app
Krypton (https://Krypt.co) provide a way to replace your usual id_rsa identity with a secret key that never leaves your mobile and requires you to allow connections (once, for 3 hours or always for that host as preferred).
It's practical because, like your phone you always have it with you, and the key is never moved to a sensitive place, it stays in IOS Secure Enclave and there is no risk you wrongly manipulate the file and make it available to unwanted access. (There is rather easy way to switch from a device to another though if you ever need to)
We are working on the iOS Secure Enclave integration.