I have just successfully tested my Nitrokey Pro stick with
- a OnePlus 3T mobile phone
- using an USB OTG adapter cable
- running LineageOS14.1 (Android 7.1.x)
- K-9 Mail 5.203
- OpenKeychain 4.2.4.
I can decrypt emails received with the K-9 email client.
The key problem to solve was to import a reference to the private key into OpenKeychain:
Solution: Export the “secret key” (that contains only a reference to the Nitrokey but
does not contain the private key) from a desktop computer where I have already installed
the NitrokeyPro and import that “secret key” file after plugging the Nitrokey Pro into the mobile phone.
Use “Manage my keys > Use security token > Import key file” to do this once.
A port of the Nitrokey App to Android is therefore not required - at least not for email decryption (as indicated in the wiki https://github.com/Nitrokey/wiki/wiki/Ideas#port-nitrokey-app-to-android:
“OpenKeychain is an GPG-like Android app which works with Yubikey NFC already. It should be relatively easy to extend it to work with the Nitrokey via USB.”).