implement SASL OAUTHBEARER and draft/bearer (#2122)

* implement SASL OAUTHBEARER and draft/bearer
* Upgrade JWT lib
* Fix an edge case in SASL EXTERNAL
* Accept longer SASL responses
* review fix: allow multiple token definitions
* enhance tests
* use SASL utilities from irc-go
* test expired tokens

irc/oauth2/oauth2.go

@@ -0,0 +1,108 @@
+// Copyright 2022-2023 Simon Ser <contact@emersion.fr>
+// Derived from https://git.sr.ht/~emersion/soju/tree/36d6cb19a4f90d217d55afb0b15318321baaad09/item/auth/oauth2.go
+// Originally released under the AGPLv3, relicensed to the Ergo project under the MIT license
+// Modifications copyright 2024 Shivaram Lingamneni <slingamn@cs.stanford.edu>
+// Released under the MIT license
+
+package oauth2
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/url"
+	"strings"
+	"time"
+)
+
+var (
+	ErrAuthDisabled = fmt.Errorf("OAuth 2.0 authentication is disabled")
+
+	// all cases where the infrastructure is working correctly, but we determined
+	// that the user supplied an invalid token
+	ErrInvalidToken = fmt.Errorf("OAuth 2.0 bearer token invalid")
+)
+
+type OAuth2BearerConfig struct {
+	Enabled              bool          `yaml:"enabled"`
+	Autocreate           bool          `yaml:"autocreate"`
+	AuthScript           bool          `yaml:"auth-script"`
+	IntrospectionURL     string        `yaml:"introspection-url"`
+	IntrospectionTimeout time.Duration `yaml:"introspection-timeout"`
+	// omit for `none`, required for `client_secret_basic`
+	ClientID     string `yaml:"client-id"`
+	ClientSecret string `yaml:"client-secret"`
+}
+
+func (o *OAuth2BearerConfig) Postprocess() error {
+	if !o.Enabled {
+		return nil
+	}
+
+	if o.IntrospectionTimeout == 0 {
+		return fmt.Errorf("a nonzero oauthbearer introspection timeout is required (try 10s)")
+	}
+
+	if _, err := url.Parse(o.IntrospectionURL); err != nil {
+		return fmt.Errorf("invalid introspection-url: %w", err)
+	}
+
+	return nil
+}
+
+func (o *OAuth2BearerConfig) Introspect(ctx context.Context, token string) (username string, err error) {
+	if !o.Enabled {
+		return "", ErrAuthDisabled
+	}
+
+	ctx, cancel := context.WithTimeout(ctx, o.IntrospectionTimeout)
+	defer cancel()
+
+	reqValues := make(url.Values)
+	reqValues.Set("token", token)
+
+	reqBody := strings.NewReader(reqValues.Encode())
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, o.IntrospectionURL, reqBody)
+	if err != nil {
+		return "", fmt.Errorf("failed to create OAuth 2.0 introspection request: %w", err)
+	}
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	req.Header.Set("Accept", "application/json")
+
+	if o.ClientID != "" {
+		req.SetBasicAuth(url.QueryEscape(o.ClientID), url.QueryEscape(o.ClientSecret))
+	}
+
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return "", fmt.Errorf("failed to send OAuth 2.0 introspection request: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return "", fmt.Errorf("OAuth 2.0 introspection error: %v", resp.Status)
+	}
+
+	var data oauth2Introspection
+	if err := json.NewDecoder(resp.Body).Decode(&data); err != nil {
+		return "", fmt.Errorf("failed to decode OAuth 2.0 introspection response: %v", err)
+	}
+
+	if !data.Active {
+		return "", ErrInvalidToken
+	}
+	if data.Username == "" {
+		// We really need the username here, otherwise an OAuth 2.0 user can
+		// impersonate any other user.
+		return "", fmt.Errorf("missing username in OAuth 2.0 introspection response")
+	}
+
+	return data.Username, nil
+}
+
+type oauth2Introspection struct {
+	Active   bool   `json:"active"`
+	Username string `json:"username"`
+}