Make passphrases optional

The whole point of Keybase is to connect cryptographic credentials to a federated identity, correct? That being the case, why do we need to provide a passphrase specific to Keybase at all?

I propose you allow users to sign in with other forms of cryptographic proof. For instance, you could provide them a nonce value, which they must sign with their previously-provided public key. Upon registration, users could simply provide a public key instead of setting a passphrase in the first place.

The issue proposes replacing or supplementing passphrase sign-in with nonce-based proof using a previously provided public key. No files, tests, or entry points are named; begin by locating the registration and authentication flows in the keybase client and website. Done would require an agreed design and corresponding implementation and tests for registration, sign-in, and key-based proof.

Written by the indexing model from the issue text.