I can't find other tools on Android that can do this it seems only Openkeychain app is the only app available I don't know for IOS it seems it does not yet exist.
I heard this from one of my friends last month talking about encryption and decryption capabilities apps.
I think this is similar to Openkeychain, you can find it on the Google Play Store.
It's K-9 mail, once installed, you can integrate it with OpenKeychain.
As long as it's an open-source app maybe we can trust it once we verify it on our own.
Because IMO, everything you store on the internet is prone to hack.