Add support for incoming emails (#22056)

closes #13585
fixes #9067
fixes #2386
ref #6226
ref #6219
fixes #745

This PR adds support to process incoming emails to perform actions.
Currently I added handling of replies and unsubscribing from
issues/pulls. In contrast to #13585 the IMAP IDLE command is used
instead of polling which results (in my opinion 😉) in cleaner code.

Procedure:
- When sending an issue/pull reply email, a token is generated which is
present in the Reply-To and References header.
- IMAP IDLE waits until a new email arrives
- The token tells which action should be performed

A possible signature and/or reply gets stripped from the content.

I added a new service to the drone pipeline to test the receiving of
incoming mails. If we keep this in, we may test our outgoing emails too
in future.

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

services/mailer/token/token.go

@@ -0,0 +1,128 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package token
+
+import (
+	"context"
+	crypto_hmac "crypto/hmac"
+	"crypto/sha256"
+	"encoding/base32"
+	"fmt"
+	"time"
+
+	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/util"
+)
+
+// A token is a verifiable container describing an action.
+//
+// A token has a dynamic length depending on the contained data and has the following structure:
+// | Token Version | User ID | HMAC | Payload |
+//
+// The payload is verifiable by the generated HMAC using the user secret. It contains:
+// | Timestamp | Action/Handler Type | Action/Handler Data |
+
+const (
+	tokenVersion1        byte = 1
+	tokenLifetimeInYears int  = 1
+)
+
+type HandlerType byte
+
+const (
+	UnknownHandlerType HandlerType = iota
+	ReplyHandlerType
+	UnsubscribeHandlerType
+)
+
+var encodingWithoutPadding = base32.StdEncoding.WithPadding(base32.NoPadding)
+
+type ErrToken struct {
+	context string
+}
+
+func (err *ErrToken) Error() string {
+	return "invalid email token: " + err.context
+}
+
+func (err *ErrToken) Unwrap() error {
+	return util.ErrInvalidArgument
+}
+
+// CreateToken creates a token for the action/user tuple
+func CreateToken(ht HandlerType, user *user_model.User, data []byte) (string, error) {
+	payload, err := util.PackData(
+		time.Now().AddDate(tokenLifetimeInYears, 0, 0).Unix(),
+		ht,
+		data,
+	)
+	if err != nil {
+		return "", err
+	}
+
+	packagedData, err := util.PackData(
+		user.ID,
+		generateHmac([]byte(user.Rands), payload),
+		payload,
+	)
+	if err != nil {
+		return "", err
+	}
+
+	return encodingWithoutPadding.EncodeToString(append([]byte{tokenVersion1}, packagedData...)), nil
+}
+
+// ExtractToken extracts the action/user tuple from the token and verifies the content
+func ExtractToken(ctx context.Context, token string) (HandlerType, *user_model.User, []byte, error) {
+	data, err := encodingWithoutPadding.DecodeString(token)
+	if err != nil {
+		return UnknownHandlerType, nil, nil, err
+	}
+
+	if len(data) < 1 {
+		return UnknownHandlerType, nil, nil, &ErrToken{"no data"}
+	}
+
+	if data[0] != tokenVersion1 {
+		return UnknownHandlerType, nil, nil, &ErrToken{fmt.Sprintf("unsupported token version: %v", data[0])}
+	}
+
+	var userID int64
+	var hmac []byte
+	var payload []byte
+	if err := util.UnpackData(data[1:], &userID, &hmac, &payload); err != nil {
+		return UnknownHandlerType, nil, nil, err
+	}
+
+	user, err := user_model.GetUserByID(ctx, userID)
+	if err != nil {
+		return UnknownHandlerType, nil, nil, err
+	}
+
+	if !crypto_hmac.Equal(hmac, generateHmac([]byte(user.Rands), payload)) {
+		return UnknownHandlerType, nil, nil, &ErrToken{"verification failed"}
+	}
+
+	var expiresUnix int64
+	var handlerType HandlerType
+	var innerPayload []byte
+	if err := util.UnpackData(payload, &expiresUnix, &handlerType, &innerPayload); err != nil {
+		return UnknownHandlerType, nil, nil, err
+	}
+
+	if time.Unix(expiresUnix, 0).Before(time.Now()) {
+		return UnknownHandlerType, nil, nil, &ErrToken{"token expired"}
+	}
+
+	return handlerType, user, innerPayload, nil
+}
+
+// generateHmac creates a trunkated HMAC for the given payload
+func generateHmac(secret, payload []byte) []byte {
+	mac := crypto_hmac.New(sha256.New, secret)
+	mac.Write(payload)
+	hmac := mac.Sum(nil)
+
+	return hmac[:10] // RFC2104 recommends not using less then 80 bits
+}