pgp: PGP/MIME signing for outgoing emails

implements PGP/MIME signing with go-pgpmail. The Sign() function of
go-pgpmail requires a private (signing) key. The signing key which matches
the senders email address (from field in email header) is looked up
in aerc's copy of the keyring.

Private keys can be exported from gpg into aerc as follows:
$ gpg --export-secret-keys  >> ~/.local/share/aerc/keyring.asc

A message is signed with the ":sign" command. The sign command sets
a bool flag in the Composer struct. Using the command repeatedly will
toggle the flag.

References: https://todo.sr.ht/~rjarry/aerc/6
Signed-off-by: Koni Marti <koni.marti@gmail.com>

commands/compose/sign.go

@@ -0,0 +1,44 @@
+package compose
+
+import (
+	"errors"
+	"time"
+
+	"git.sr.ht/~rjarry/aerc/widgets"
+)
+
+type Sign struct{}
+
+func init() {
+	register(Sign{})
+}
+
+func (Sign) Aliases() []string {
+	return []string{"sign"}
+}
+
+func (Sign) Complete(aerc *widgets.Aerc, args []string) []string {
+	return nil
+}
+
+func (Sign) Execute(aerc *widgets.Aerc, args []string) error {
+	if len(args) != 1 {
+		return errors.New("Usage: sign")
+	}
+
+	composer, _ := aerc.SelectedTab().(*widgets.Composer)
+
+	composer.SetSign(!composer.Sign())
+
+	var statusline string
+
+	if composer.Sign() {
+		statusline = "Message will be signed."
+	} else {
+		statusline = "Message will not be signed."
+	}
+
+	aerc.PushStatus(statusline, 10*time.Second)
+
+	return nil
+}

lib/keystore.go

@@ -1,6 +1,7 @@
 package lib
 
 import (
+	"fmt"
 	"io"
 	"os"
 	"path"
@@ -52,6 +53,19 @@ func UnlockKeyring() {
 	os.Remove(lockpath)
 }
 
+func GetSignerEntityByEmail(email string) (e *openpgp.Entity, err error) {
+	for _, key := range Keyring.DecryptionKeys() {
+		if key.Entity == nil {
+			continue
+		}
+		ident := key.Entity.PrimaryIdentity()
+		if ident != nil && ident.UserId.Email == email {
+			return key.Entity, nil
+		}
+	}
+	return nil, fmt.Errorf("entity not found in keyring")
+}
+
 func ImportKeys(r io.Reader) error {
 	keys, err := openpgp.ReadKeyRing(r)
 	if err != nil {

widgets/compose.go

@@ -17,6 +17,7 @@ import (
 
 	"github.com/ProtonMail/go-crypto/openpgp"
 	"github.com/emersion/go-message/mail"
+	"github.com/emersion/go-pgpmail"
 	"github.com/gdamore/tcell/v2"
 	"github.com/mattn/go-runewidth"
 	"github.com/mitchellh/go-homedir"
@@ -24,6 +25,7 @@ import (
 
 	"git.sr.ht/~rjarry/aerc/completer"
 	"git.sr.ht/~rjarry/aerc/config"
+	"git.sr.ht/~rjarry/aerc/lib"
 	"git.sr.ht/~rjarry/aerc/lib/format"
 	"git.sr.ht/~rjarry/aerc/lib/templates"
 	"git.sr.ht/~rjarry/aerc/lib/ui"
@@ -49,6 +51,7 @@ type Composer struct {
 	review      *reviewMessage
 	worker      *types.Worker
 	completer   *completer.Completer
+	sign        bool
 
 	layout    HeaderLayout
 	focusable []ui.MouseableDrawableInteractive
@@ -173,6 +176,15 @@ func (c *Composer) Sent() bool {
 	return c.sent
 }
 
+func (c *Composer) SetSign(sign bool) *Composer {
+	c.sign = sign
+	return c
+}
+
+func (c *Composer) Sign() bool {
+	return c.sign
+}
+
 // Note: this does not reload the editor. You must call this before the first
 // Draw() call.
 func (c *Composer) SetContents(reader io.Reader) *Composer {
@@ -393,34 +405,74 @@ func (c *Composer) PrepareHeader() (*mail.Header, error) {
 	return c.header, nil
 }
 
+func getSenderEmail(c *Composer) (string, error) {
+	// add the from: field also to the 'recipients' list
+	if c.acctConfig.From == "" {
+		return "", errors.New("No 'From' configured for this account")
+	}
+	from, err := mail.ParseAddress(c.acctConfig.From)
+	if err != nil {
+		return "", errors.Wrap(err, "ParseAddress(config.From)")
+	}
+	return from.Address, nil
+}
+
 func (c *Composer) WriteMessage(header *mail.Header, writer io.Writer) error {
 	if err := c.reloadEmail(); err != nil {
 		return err
 	}
 
-	if len(c.attachments) == 0 {
-		// don't create a multipart email if we only have text
-		return writeInlineBody(header, c.email, writer)
-	}
+	if c.sign {
 
-	// otherwise create a multipart email,
-	// with a multipart/alternative part for the text
-	w, err := mail.CreateWriter(writer, *header)
-	if err != nil {
-		return errors.Wrap(err, "CreateWriter")
-	}
-	defer w.Close()
-
-	if err := writeMultipartBody(c.email, w); err != nil {
-		return errors.Wrap(err, "writeMultipartBody")
-	}
-
-	for _, a := range c.attachments {
-		if err := writeAttachment(a, w); err != nil {
-			return errors.Wrap(err, "writeAttachment")
+		signer, err := getSigner(c)
+		if err != nil {
+			return err
 		}
-	}
 
+		var signedHeader mail.Header
+		signedHeader.SetContentType("text/plain", nil)
+
+		var buf bytes.Buffer
+		var cleartext io.WriteCloser
+
+		cleartext, err = pgpmail.Sign(&buf, header.Header.Header, signer, nil)
+		if err != nil {
+			return err
+		}
+
+		err = writeMsgImpl(c, &signedHeader, cleartext)
+		if err != nil {
+			return err
+		}
+		cleartext.Close()
+		io.Copy(writer, &buf)
+		return nil
+
+	} else {
+		return writeMsgImpl(c, header, writer)
+	}
+}
+
+func writeMsgImpl(c *Composer, header *mail.Header, writer io.Writer) error {
+	if len(c.attachments) == 0 {
+		// no attachements
+		return writeInlineBody(header, c.email, writer)
+	} else {
+		// with attachements
+		w, err := mail.CreateWriter(writer, *header)
+		if err != nil {
+			return errors.Wrap(err, "CreateWriter")
+		}
+		if err := writeMultipartBody(c.email, w); err != nil {
+			return errors.Wrap(err, "writeMultipartBody")
+		}
+		for _, a := range c.attachments {
+			if err := writeAttachment(a, w); err != nil {
+				return errors.Wrap(err, "writeAttachment")
+			}
+		}
+		w.Close()
+	}
 	return nil
 }
 
@@ -885,3 +937,30 @@ func (rm *reviewMessage) OnInvalidate(fn func(ui.Drawable)) {
 func (rm *reviewMessage) Draw(ctx *ui.Context) {
 	rm.grid.Draw(ctx)
 }
+
+func getSigner(c *Composer) (signer *openpgp.Entity, err error) {
+	signerEmail, err := getSenderEmail(c)
+	if err != nil {
+		return nil, err
+	}
+	signer, err = lib.GetSignerEntityByEmail(signerEmail)
+	if err != nil {
+		return nil, err
+	}
+
+	key, ok := signer.SigningKey(time.Now())
+	if !ok {
+		return nil, fmt.Errorf("no signing key found for %s", signerEmail)
+	}
+
+	if !key.PrivateKey.Encrypted {
+		return signer, nil
+	}
+
+	_, err = c.aerc.DecryptKeys([]openpgp.Key{key}, false)
+	if err != nil {
+		return nil, err
+	}
+
+	return signer, nil
+}