[w3c/webauthn] 5b026a: Fix #848: Weirdness in RP UP verification

  Branch: refs/heads/issue-848-rp-up-verification
  Home:   https://github.com/w3c/webauthn
  Commit: 5b026ad5fcab52b853995a8ca8f4959bd9f9c0b7
      https://github.com/w3c/webauthn/commit/5b026ad5fcab52b853995a8ca8f4959bd9f9c0b7
  Author: Emil Lundberg <emil@yubico.com>
  Date:   2018-03-21 (Wed, 21 Mar 2018)

  Changed paths:
    M index.bs

  Log Message:
  -----------
  Fix #848: Weirdness in RP UP verification

§7.1. Registering a new credential currently reads (and §7.2. Verifying
an authentication assertion is analogous):

>10. If user verification is required for this registration, verify that
the User Verified bit of the `flags` in _authData_ is set.
>11. If user verification is not required for this registration, verify
that the User Present bit of the `flags` in _authData_ is set.

This results in the following truth table:

    UV req.     | UP req? | UV res. | UP res. | Accept?
    ------------+---------+---------+---------+--------
    discouraged | Yes     | 0       | 0       | No
    discouraged | Yes     | 0       | 1       | Yes
    discouraged | Yes     | 1       | 0       | No
    discouraged | Yes     | 1       | 1       | Yes
    preferred   | Yes     | 0       | 0       | No
    preferred   | Yes     | 0       | 1       | Yes
    preferred   | Yes     | 1       | 0       | No
    preferred   | Yes     | 1       | 1       | Yes
    required    | No      | 0       | 0       | No
    required    | No      | 0       | 1       | No
    required    | No      | 1       | 0       | Yes
    required    | No      | 1       | 1       | Yes

Note, for example, how UV `preferred` means that a response with `(UV=1,
UP=0)` should be rejected.

It makes more sense to let the UP requirement be defined by the UV
_response_ instead of the UV _requirement_:

    UV req.     | UV res. | UP req? | UP res. | Accept?
    ------------+---------+---------+---------+--------
    discouraged | 0       | Yes     | 0       | No
    discouraged | 0       | Yes     | 1       | Yes
    discouraged | 1       | No      | 0       | Yes
    discouraged | 1       | No      | 1       | Yes
    preferred   | 0       | Yes     | 0       | No
    preferred   | 0       | Yes     | 1       | Yes
    preferred   | 1       | No      | 0       | Yes
    preferred   | 1       | No      | 1       | Yes
    required    | 0       | Yes     | 0       | No
    required    | 0       | Yes     | 1       | No
    required    | 1       | No      | 0       | Yes
    required    | 1       | No      | 1       | Yes