Fix gomobile compilation ssues (#81)

Co-authored-by: marin thiercelin <marin.thiercelin@pm.me>

helper/helper.go

@@ -184,6 +184,28 @@ func DecryptBinaryMessageArmored(privateKey string, passphrase []byte, ciphertex
 	return message.GetBinary(), nil
 }
 
+// encryptSignArmoredDetached takes a public key for encryption,
+// a private key and its passphrase for signature, and the plaintext data
+// Returns an armored ciphertext and a detached armored signature.
+func encryptSignArmoredDetached(
+	publicKey, privateKey string,
+	passphrase, plainData []byte,
+) (ciphertext, signature string, err error) {
+	var message *crypto.PlainMessage = crypto.NewPlainMessage(plainData)
+
+	// We encrypt the message
+	if ciphertext, err = encryptMessageArmored(publicKey, message); err != nil {
+		return "", "", err
+	}
+
+	// We sign the message
+	if signature, err = signDetachedArmored(privateKey, passphrase, message); err != nil {
+		return "", "", err
+	}
+
+	return ciphertext, signature, nil
+}
+
 // DecryptVerifyArmoredDetached decrypts an armored pgp message
 // and verify a detached armored signature
 // given a publicKey, and a privateKey with its passphrase.