I actually quite like the idea of on the fly key derivation from a passphrase, I just don't know that people could memorize a password with 256 bits of entropy in it. "I wonder how much entropy is in a sentence like the one I am typing right now, or the ease with which I could remember said sentence. Well, I could remember it, but I do not even know if it is entropic enough in itself to be used for private asymmetric key generation!!!" hm that sentence is 270 characters, generally safe to assume 1 bit of entropy per keystroke of an English sentence, so a passphrase of that magnitude may be adequate for on the fly password based key derivation of a 256 bit ECC key.