cryptoutil: implement cleartext function

Implement a cleartext function in the cryptoutil package to decrypt an
encrypted message to cleartext and construct a valid rfc2822 message.

The headers from the decrypt message body will be merged with the
original headers to create a fully decrypted message.

Implements: https://todo.sr.ht/~rjarry/aerc/238
Signed-off-by: Koni Marti <koni.marti@gmail.com>
Tested-by: Jens Grassel <jens@wegtam.com>
Reviewed-by: Tim Culverhouse <tim@timculverhouse.com>
Acked-by: Robin Jarry <robin@jarry.cc>

1 files changed, 69 insertions, 0 deletions

diff --git a/lib/crypto/util/cleartext.go b/lib/crypto/util/cleartext.go
new file mode 100644
index 00000000..fe6faa89
--- /dev/null
+++ b/lib/crypto/util/cleartext.go
@@ -0,0 +1,69 @@
+package cryptoutil
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"strings"
+
+	"git.sr.ht/~rjarry/aerc/app"
+	"git.sr.ht/~rjarry/aerc/lib/rfc822"
+	"github.com/emersion/go-message/mail"
+)
+
+func Cleartext(r io.Reader, header mail.Header) ([]byte, error) {
+	msg, err := app.CryptoProvider().Decrypt(
+		rfc822.NewCRLFReader(r), app.DecryptKeys)
+	if err != nil {
+		return nil, errors.New("decrypt error")
+	}
+	full, err := createMessage(header, msg.Body)
+	if err != nil {
+		return nil, errors.New("failed to create decrypted message")
+	}
+	return full, nil
+}
+
+func createMessage(header mail.Header, body io.Reader) ([]byte, error) {
+	e, err := rfc822.ReadMessage(body)
+	if err != nil {
+		return nil, err
+	}
+
+	// copy the header values from the "decrypted body". This should set
+	// the correct content type.
+	hf := e.Header.Fields()
+	for hf.Next() {
+		header.Set(hf.Key(), hf.Value())
+	}
+
+	ctype, params, err := header.ContentType()
+	if err != nil {
+		return nil, err
+	}
+
+	// in case there remains a multipart/{encrypted,signed} content type,
+	// manually correct them to multipart/mixed as a fallback.
+	ct := strings.ToLower(ctype)
+	if strings.Contains(ct, "multipart/encrypted") ||
+		strings.Contains(ct, "multipart/signed") {
+		delete(params, "protocol")
+		delete(params, "micalg")
+		header.SetContentType("multipart/mixed", params)
+	}
+
+	// a SingleInlineWriter is sufficient since the "decrypted body"
+	// already contains the proper boundaries of the parts; we just want to
+	// combine it with the headers.
+	var message bytes.Buffer
+	w, err := mail.CreateSingleInlineWriter(&message, header)
+	if err != nil {
+		return nil, err
+	}
+	if _, err := io.Copy(w, e.Body); err != nil {
+		return nil, err
+	}
+	w.Close()
+
+	return message.Bytes(), nil
+}