AT Protocol OAuth 2.0 authorization flow.

Implements the authorization code flow with PAR (Pushed Authorization Requests), PKCE and DPoP as described in https://atproto.com/specs/oauth, as an alternative to app password authentication (ProtoRune.login/3).

Only public clients are supported: the client authenticates with a client_id URL pointing to its client metadata document, and every authorization server request carries a DPoP proof signed with an ES256 key generated locally (see ProtoRune.Atproto.OAuth.DPoP). No external JWT dependency is required.

Flow overview

alias ProtoRune.Atproto.OAuth
alias ProtoRune.Atproto.OAuth.Client

{:ok, client} =
  Client.new(
    client_id: "https://myapp.example.com/oauth/client-metadata.json",
    redirect_uri: "https://myapp.example.com/oauth/callback"
  )

# 1. Send the user to the authorization server
{:ok, url, pending} = OAuth.authorization_url(client, "alice.bsky.social")
# redirect the user to `url`, persist `pending` (e.g. in the session)

# 2. Handle the callback with the received query params
{:ok, session} = OAuth.exchange_code(client, pending, callback_params)

# 3. Refresh when the access token expires
{:ok, fresh_session} = OAuth.refresh(client, session)

The pending map holds the flow state (PKCE verifier, state, DPoP nonce) and must be kept between steps 1 and 2. It contains the DPoP private key, so persist it securely.

Scope of this implementation

Issued access tokens are DPoP-bound. Using an OAuth session for XRPC calls requires the XRPC layer to attach DPoP proofs, which is not wired yet: exchange_code/3 and refresh/2 cover the authorization flow itself. Confidential clients (private_key_jwt) and client metadata document serving are out of scope.