------------------------------------------------------------------
== PKCE parameters (RFC 7636)
------------------------------------------------------------------
code_verifier         RnM5322FJWXI9zUZDUkmRmK-x_rjKT6HHFQyIhSEqjtVkb527uqjZmfQCwrNmavk   (64 chars)
code_challenge        rzSywj6JKmWWVVBuZsD26hYR4_3PK0zZRCXrn_4sU9M
code_challenge_method S256

The verifier never leaves the client until the token request. The challenge is
all the authorization request carries, and it is a one-way hash of the verifier.

------------------------------------------------------------------
== 1. Log in to the authorization server (browser session)
------------------------------------------------------------------
$ curl -c jar -d username=alice -d password=password -d _csrf=<token> http://localhost:9000/login
HTTP/1.1 302
Location: http://localhost:9000/

------------------------------------------------------------------
== 2. GET /oauth2/authorize  (client=demo-spa)
------------------------------------------------------------------
NO_CHALLENGE=1: the authorization request carries no code_challenge.

$ curl -b jar 'http://localhost:9000/oauth2/authorize?response_type=code&client_id=demo-spa&redirect_uri=http%3A%2F%2F127.0.0.1%3A8080%2Fauthorized&scope=openid%20orders.read&state=xyz123'
-> 302 http://127.0.0.1:8080/authorized?code=ydxIVNbSbrjWRlj1YxX6LS4cnXgDdzxTIyZgqoqlmX6cD8q-AYnL_Sdy6m5gqheZ-yd8qKUgSrrgMtg331sYHjblO3IYqGb1i94OxMypdyLr6RMqm_O_XuVZX2g1FSeD&state=xyz123

------------------------------------------------------------------
== 3. No consent page
------------------------------------------------------------------
The authorization endpoint went straight back to the client. Either consent is
off for this client, or every requested scope was already approved.

------------------------------------------------------------------
== 5. The authorization code
------------------------------------------------------------------
code  = ydxIVNbSbrjWRlj1YxX6LS4cnXgDdzxTIyZgqoqlmX6cD8q-AYnL_Sdy6m5gqheZ-yd8qKUgSrrgMtg331sYHjblO3IYqGb1i94OxMypdyLr6RMqm_O_XuVZX2g1FSeD
state = xyz123   (the client's own value, returned untouched - compare it)

------------------------------------------------------------------
== 6a. Exchange the code WITHOUT the verifier
------------------------------------------------------------------
This is the request an attacker who stole the code can make.
HTTP 401
(empty response body)

>>> Rejected. invalid_grant is deliberately vague: the server will not tell
>>> a caller whether the code was wrong, expired, already used, or missing a
>>> verifier, because each of those is information an attacker can use.
Note: this consumed the code. Authorization codes are single-use, so the
successful exchange below needs a fresh one.

------------------------------------------------------------------
== 6b. A fresh code, exchanged properly
------------------------------------------------------------------
fresh code = uvr0H49ByWte7NfWDoDnZ61-11rMP0wxYFJMY8eM73Lm2dcc2Edl-oFdW3DwJNSqUBMyIRX8rY9JqPDZIvwnEo4grUU-D5Z-GTwYYV1B8Xvosdbg-vAjiss1wbRsvD03

$ curl -d grant_type=authorization_code -d code=... http://localhost:9000/oauth2/token
  (no code_verifier - there was no challenge to verify against)
HTTP 401
(empty response body)

>>> No token, even though the client is registered with requireProofKey(false)
>>> and the authorization request carried no challenge. The reason is that a
>>> public client has no other way to authenticate at the token endpoint:
>>> PublicClientAuthenticationProvider delegates entirely to
>>> CodeVerifierAuthenticator, and raises invalid_client when there is nothing
>>> to verify. requireProofKey(false) relaxes the AUTHORIZATION endpoint only.