move email composing to utils

utils/email.go

@@ -1,5 +1,15 @@
 package utils
 
+import (
+	"crypto"
+	"crypto/x509"
+	"encoding/pem"
+	"strings"
+	"time"
+
+	"github.com/emersion/go-msgauth/dkim"
+)
+
 // MTA is mail transfer agent
 type MTA interface {
 	Send(from, to, data string) error
@@ -7,6 +17,9 @@ type MTA interface {
 
 // Email object
 type Email struct {
+	data strings.Builder
+
+	Date      string
 	MessageID string
 	InReplyTo string
 	From      string
@@ -20,6 +33,7 @@ type Email struct {
 // NewEmail constructs Email object
 func NewEmail(messageID, inReplyTo, subject, from, to, text, html string, files []*File) *Email {
 	email := &Email{
+		Date:      time.Now().UTC().Format(time.RFC1123Z),
 		MessageID: messageID,
 		InReplyTo: inReplyTo,
 		From:      from,
@@ -40,3 +54,71 @@ func NewEmail(messageID, inReplyTo, subject, from, to, text, html string, files
 
 	return email
 }
+
+// Compose converts email object to string and (optionally) signs it
+func (e *Email) Compose(privkey string) string {
+	domain := strings.SplitN(e.From, "@", 2)[0]
+
+	e.data.WriteString("From: ")
+	e.data.WriteString(e.From)
+	e.data.WriteString("\r\n")
+
+	e.data.WriteString("To: ")
+	e.data.WriteString(e.To)
+	e.data.WriteString("\r\n")
+
+	e.data.WriteString("Message-Id: ")
+	e.data.WriteString(e.MessageID)
+	e.data.WriteString("\r\n")
+
+	e.data.WriteString("Date: ")
+	e.data.WriteString(e.Date)
+	e.data.WriteString("\r\n")
+
+	if e.InReplyTo != "" {
+		e.data.WriteString("In-Reply-To: ")
+		e.data.WriteString(e.InReplyTo)
+		e.data.WriteString("\r\n")
+	}
+
+	e.data.WriteString("Subject: ")
+	e.data.WriteString(e.Subject)
+	e.data.WriteString("\r\n")
+
+	e.data.WriteString("\r\n")
+
+	e.data.WriteString(e.Text)
+	e.data.WriteString("\r\n")
+
+	e.sign(domain, privkey)
+	return e.data.String()
+}
+
+func (e *Email) sign(domain, privkey string) {
+	if privkey == "" {
+		return
+	}
+	pemblock, _ := pem.Decode([]byte(privkey))
+	if pemblock == nil {
+		return
+	}
+	parsedkey, err := x509.ParsePKCS8PrivateKey(pemblock.Bytes)
+	if err != nil {
+		return
+	}
+	signer := parsedkey.(crypto.Signer)
+
+	options := &dkim.SignOptions{
+		Domain:   domain,
+		Selector: "postmoogle",
+		Signer:   signer,
+	}
+
+	var msg strings.Builder
+	err = dkim.Sign(&msg, strings.NewReader(e.data.String()), options)
+	if err != nil {
+		return
+	}
+
+	e.data = msg
+}