chore(GODT-2916): Split Decryption from Message Building

This helps the export tool to deal with problems arising from message
assembly after everything has been successfully encrypted.

The original behavior is still available under `DecryptAndBuildRFC822`.

pkg/message/decrypt.go

@@ -0,0 +1,88 @@
+// Copyright (c) 2023 Proton AG
+//
+// This file is part of Proton Mail Bridge.
+//
+// Proton Mail Bridge is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Proton Mail Bridge is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Proton Mail Bridge. If not, see <https://www.gnu.org/licenses/>.
+
+package message
+
+import (
+	"bytes"
+	"encoding/base64"
+	"io"
+
+	"github.com/ProtonMail/go-proton-api"
+	"github.com/ProtonMail/gopenpgp/v2/crypto"
+	"github.com/pkg/errors"
+)
+
+type DecryptedAttachment struct {
+	Packet    []byte
+	Encrypted []byte
+	Data      bytes.Buffer
+	Err       error
+}
+
+type DecryptedMessage struct {
+	Msg         proton.Message
+	Body        bytes.Buffer
+	BodyErr     error
+	Attachments []DecryptedAttachment
+}
+
+var ErrInvalidAttachmentPacket = errors.New("invalid attachment packet")
+
+func DecryptMessage(kr *crypto.KeyRing, msg proton.Message, attData [][]byte) DecryptedMessage {
+	result := DecryptedMessage{
+		Msg: msg,
+	}
+
+	result.Body.Grow(len(msg.Body))
+
+	if err := msg.DecryptInto(kr, &result.Body); err != nil {
+		result.BodyErr = errors.Wrap(ErrDecryptionFailed, err.Error())
+	}
+
+	result.Attachments = make([]DecryptedAttachment, len(msg.Attachments))
+
+	for i, attachment := range msg.Attachments {
+		result.Attachments[i].Encrypted = attData[i]
+
+		kps, err := base64.StdEncoding.DecodeString(attachment.KeyPackets)
+		if err != nil {
+			result.Attachments[i].Err = errors.Wrap(ErrInvalidAttachmentPacket, err.Error())
+			continue
+		}
+
+		result.Attachments[i].Packet = kps
+
+		// Use io.Multi
+		attachmentReader := io.MultiReader(bytes.NewReader(kps), bytes.NewReader(attData[i]))
+
+		stream, err := kr.DecryptStream(attachmentReader, nil, crypto.GetUnixTime())
+		if err != nil {
+			result.Attachments[i].Err = errors.Wrap(ErrDecryptionFailed, err.Error())
+			continue
+		}
+
+		result.Attachments[i].Data.Grow(len(kps) + len(attData))
+
+		if _, err := result.Attachments[i].Data.ReadFrom(stream); err != nil {
+			result.Attachments[i].Err = errors.Wrap(ErrDecryptionFailed, err.Error())
+			continue
+		}
+	}
+
+	return result
+}