imap,smtp: add XOAUTH2 support

Add XOAUTH2 authentication support for IMAP and SMTP. Although XOAUTH2
is now deprecated in favor of OAuthBearer, it is the only way to connect
to Office365 since Basic Auth is now completely removed.

Since XOAUTH2 is very similar to OAuthBearer and uses the same
configuration parameters, this is basically a copy-paste of the existing
OAuthBearer code.

However, XOAUTH2 support was removed from go-sasl library, so this
change reimports the code that was removed from go-sasl and offers it
a new home in lib/xoauth2.go. Hopefully it shouldn't be too hard to
maintain, being less than 50 SLOC.

Link: https://github.com/emersion/go-sasl/commit/7bfe0ed36a21
Implements: https://todo.sr.ht/~rjarry/aerc/78
Signed-off-by: Julian Pidancet <julian.pidancet@oracle.com>
Tested-by: Inwit <inwit@sindominio.net>
Acked-by: Tim Culverhouse <tim@timculverhouse.com>

1 files changed, 88 insertions, 0 deletions

diff --git a/lib/xoauth2.go b/lib/xoauth2.go
new file mode 100644
index 00000000..83f06309
--- /dev/null
+++ b/lib/xoauth2.go
@@ -0,0 +1,88 @@
+//
+// This code is derived from the go-sasl library.
+//
+// Copyright (c) 2016 emersion
+// Copyright (c) 2022, Oracle and/or its affiliates.
+//
+// SPDX-License-Identifier: MIT
+
+package lib
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+
+	"github.com/emersion/go-imap/client"
+	"github.com/emersion/go-sasl"
+	"golang.org/x/oauth2"
+)
+
+// An XOAUTH2 error.
+type Xoauth2Error struct {
+	Status  string `json:"status"`
+	Schemes string `json:"schemes"`
+	Scope   string `json:"scope"`
+}
+
+// Implements error.
+func (err *Xoauth2Error) Error() string {
+	return fmt.Sprintf("XOAUTH2 authentication error (%v)", err.Status)
+}
+
+type xoauth2Client struct {
+	Username string
+	Token    string
+}
+
+func (a *xoauth2Client) Start() (mech string, ir []byte, err error) {
+	mech = "XOAUTH2"
+	ir = []byte("user=" + a.Username + "\x01auth=Bearer " + a.Token + "\x01\x01")
+	return
+}
+
+func (a *xoauth2Client) Next(challenge []byte) ([]byte, error) {
+	// Server sent an error response
+	xoauth2Err := &Xoauth2Error{}
+	if err := json.Unmarshal(challenge, xoauth2Err); err != nil {
+		return nil, err
+	} else {
+		return nil, xoauth2Err
+	}
+}
+
+// An implementation of the XOAUTH2 authentication mechanism, as
+// described in https://developers.google.com/gmail/xoauth2_protocol.
+func NewXoauth2Client(username, token string) sasl.Client {
+	return &xoauth2Client{username, token}
+}
+
+type Xoauth2 struct {
+	OAuth2  *oauth2.Config
+	Enabled bool
+}
+
+func (c *Xoauth2) ExchangeRefreshToken(refreshToken string) (*oauth2.Token, error) {
+	token := new(oauth2.Token)
+	token.RefreshToken = refreshToken
+	token.TokenType = "Bearer"
+	return c.OAuth2.TokenSource(context.TODO(), token).Token()
+}
+
+func (c *Xoauth2) Authenticate(username string, password string, client *client.Client) error {
+	if ok, err := client.SupportAuth("XOAUTH2"); err != nil || !ok {
+		return fmt.Errorf("Xoauth2 not supported %w", err)
+	}
+
+	if c.OAuth2.Endpoint.TokenURL != "" {
+		token, err := c.ExchangeRefreshToken(password)
+		if err != nil {
+			return err
+		}
+		password = token.AccessToken
+	}
+
+	saslClient := NewXoauth2Client(username, password)
+
+	return client.Authenticate(saslClient)
+}