This is a still unripe idea, but I'll mention it here, just in case somebody can improve it to make it really useful.

I was trying to create a private key backup and was worried to let the file with the private key wander through insecure communications channels and systems like Windows computers.

So I found a ZIP archiver that can encrypt and zipped and encrypted the file right on the phone. Now I can move and store the file through insecure channels and on insecure machines.

An example could be that you have a safe phone and a safe computer with a printer, but no safe connection between the two. Another example is safe storage of the backup on an unsafe computer. You would never decrypt your backup on that unsafe computer, but you could move it back to a safe phone and decrypt it only there. (A safe phone could be a factory-reset phone with only Mycelium and a ZIP archiver app on it.)

Remaining problems are, obviously:

• You have to remember the encryption key safely.
• The ZIP archiver program could steal the key.

I used ZArchiver, which is apparently the most powerful ZIP archiver for Android. My reasoning is that nobody writes a powerful first-class archiver only to embed a virus in it. But, of course, a residual risk always remains.

In theory, Mycelium itself could contain an encrypting ZIP archiver or some other encrypting software, if the idea holds any water.

I'm putting this out here for discussion, in case there is any interest and I have not overlooked some fundamental counter-indication.