Since Kryptonite is available for Android phones, it's a point at least worth highlighting.

One might think that anyone savvy enough to want an app to manage his ssh private keys would also think about his phone's security. In practice, from my observation, a lot of people who have to use ssh keys don't really understand them and would not think twice about loading an app like this onto their Android phones, alongside all manner of ad-laden apps of unknown quality or provenance.

I myself use an Android phone. But I assume it's not secure.