Initial discord remote auth support.

It works, but nothing is persisted yet as the user object needs some tweaking
especially the database side.

remoteauth/client.go

@@ -0,0 +1,109 @@
+package remoteauth
+
+import (
+	"context"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/json"
+	"net/http"
+
+	"github.com/gorilla/websocket"
+)
+
+type Client struct {
+	URL    string
+	Origin string
+
+	conn *websocket.Conn
+
+	qrChan   chan string
+	doneChan chan struct{}
+
+	user User
+	err  error
+
+	heartbeats int
+	closed     bool
+
+	privateKey *rsa.PrivateKey
+}
+
+// New creates a new Discord remote auth client. qrChan is a channel that will
+// receive the qrcode once it is available.
+func New() (*Client, error) {
+	privateKey, err := rsa.GenerateKey(rand.Reader, 2048)
+	if err != nil {
+		return nil, err
+	}
+
+	return &Client{
+		URL:        "wss://remote-auth-gateway.discord.gg/?v=1",
+		Origin:     "https://discord.com",
+		privateKey: privateKey,
+	}, nil
+}
+
+// Dialo will start the QRCode login process. ctx may be used to abandon the
+// process.
+func (c *Client) Dial(ctx context.Context, qrChan chan string, doneChan chan struct{}) error {
+	header := http.Header{
+		"Origin": []string{c.Origin},
+	}
+
+	c.qrChan = qrChan
+	c.doneChan = doneChan
+
+	conn, _, err := websocket.DefaultDialer.DialContext(ctx, c.URL, header)
+	if err != nil {
+		return err
+	}
+
+	c.conn = conn
+
+	go c.processMessages()
+
+	return nil
+}
+
+func (c *Client) Result() (User, error) {
+	return c.user, c.err
+}
+
+func (c *Client) close() error {
+	if c.closed {
+		return nil
+	}
+
+	c.conn.WriteMessage(
+		websocket.CloseMessage,
+		websocket.FormatCloseMessage(websocket.CloseNormalClosure, ""),
+	)
+
+	c.closed = true
+
+	defer close(c.doneChan)
+
+	return c.conn.Close()
+}
+
+func (c *Client) write(p clientPacket) error {
+	payload, err := json.Marshal(p)
+	if err != nil {
+		return err
+	}
+
+	return c.conn.WriteMessage(websocket.TextMessage, payload)
+}
+
+func (c *Client) decrypt(payload string) ([]byte, error) {
+	// Decode the base64 string.
+	raw, err := base64.StdEncoding.DecodeString(payload)
+	if err != nil {
+		return []byte{}, err
+	}
+
+	// Decrypt the data.
+	return rsa.DecryptOAEP(sha256.New(), nil, c.privateKey, raw, nil)
+}