I was looking for an encryption/decryption tool with a very simple GUI that is available on all platforms (including mobile phones) and uses an open format so that I can decrypt my file via other standard tools too. Another key requirement was that I had to trust the tool.
The main use case was the protection of those few files I do not want to upload to a cloud drive unencrypted.
Having checked and discarded multiple Android apps, I ended up writing a tool on my own. Please find it here:
Feel free to use it! :)