Re: Well you cannot make this secure
" you cannot enter complex alphanumeric passphrases on a touchscreen"
Err, why not? I can enter almost all the characters on my phone that I can on my keyboard.
My most important passphrase has about 77 bits of entropy (I can be that precise because of the way I generated it). I enter it on my phone. (It actually only consists of lower-case ASCII, but length is more important than character set, and Password123! is not a secure password.)