getsalt

https://keybase.io/_/api/1.0/getsalt.json

email_or_username: "chris"

{
  "status": {
    "code": 0,
    "name": "OK"
  },
  "salt": "32355c2e7843513463263...",
  "csrf_token": "lgHZIDAxNzM1NzR...",
  "login_session": "lgHZIDhlY2I0..."
}

"BAD_SIGNUP_USERNAME_TAKEN"
"BAD_SIGNUP_EMAIL_TAKEN"
"BAD_SIGNUP_USERNAME_DELETED"
"INPUT_ERROR"

Round 1 of the 2-Round Login Protocol

The Keybase login protocol is two rounds, to prevent replay attacks, and to allow strong randomized salting of passwords.

In the first round of the login protocol, the user presents a username or email address to the server, and the server responds with the salt the user uploaded during signup, and a short-lived random challenge. The salt doesn't change for a given user, but the random challege will always be different, and expires after a few minutes.

The random challege is called login_session. It's not a trully random token, but an unforgeable token cryptographically tied to client's user ID (or email) and a timestamp. This construction saves the server from having to keep state, but the client can treat the login_session as an opaque random token.

If the user or email address does not exist, getsalt will reply with a deterministic salt, a function of the parameters. This is so /getsalt can't be used to test for user membership. (An unnecessary precaution with usernames, since Keybase is a public directory...)