peroxide: Sanitize the quoted printable input before parsing for encryption

Some clients add a single = at the end of the message part causing the parser
to fail.

pkg/message/encrypt.go

@@ -247,6 +247,40 @@ func (c *Base64Cleaner) Read(b []byte) (int, error) {
 	return c.r.Read(b)
 }
 
+type QuotedPrintableCleaner struct {
+	r io.Reader
+}
+
+func NewQuotedPrintableCleaner(r io.Reader) (*QuotedPrintableCleaner, error) {
+	reader := bufio.NewReader(r)
+	var data []byte
+
+	for {
+		line, err := reader.ReadBytes('\n')
+
+		if len(line) != 0 {
+			if len(line) == 1 && line[0] == '=' {
+				continue
+			}
+
+			data = append(data, line...)
+		}
+
+		if err != nil {
+			if errors.Is(err, io.EOF) {
+				break
+			}
+			return nil, err
+		}
+	}
+
+	return &QuotedPrintableCleaner{bytes.NewReader(data)}, nil
+}
+
+func (c *QuotedPrintableCleaner) Read(b []byte) (int, error) {
+	return c.r.Read(b)
+}
+
 func getTransferDecoder(r io.Reader, encoding string) (io.Reader, error) {
 	switch strings.ToLower(encoding) {
 	case "base64":
@@ -257,7 +291,11 @@ func getTransferDecoder(r io.Reader, encoding string) (io.Reader, error) {
 		return base64.NewDecoder(base64.StdEncoding, cleaner), nil
 
 	case "quoted-printable":
-		return quotedprintable.NewReader(r), nil
+		cleaner, err := NewQuotedPrintableCleaner(r)
+		if err != nil {
+			return nil, err
+		}
+		return quotedprintable.NewReader(cleaner), nil
 
 	default:
 		return r, nil