Add build script and fix helper for mobile (#32)

* update build and fix helper for mobile

* Update readme, changelog and script cleanup

Co-authored-by: wussler <aron@wussler.it>

helper/sign_attachment.go

@@ -0,0 +1,50 @@
+// +build !ios
+// +build !android
+
+package helper
+
+import "github.com/ProtonMail/gopenpgp/v2/crypto"
+
+// EncryptSignAttachment encrypts an attachment using a detached signature, given a publicKey, a privateKey
+// and its passphrase, the filename, and the unencrypted file data.
+// Returns keypacket, dataPacket and unarmored (!) signature separate.
+func EncryptSignAttachment(
+	publicKey, privateKey string, passphrase []byte, fileName string, plainData []byte,
+) (keyPacket, dataPacket, signature []byte, err error) {
+	var publicKeyObj, privateKeyObj, unlockedKeyObj *crypto.Key
+	var publicKeyRing, privateKeyRing *crypto.KeyRing
+	var packets *crypto.PGPSplitMessage
+	var signatureObj *crypto.PGPSignature
+
+	var binMessage = crypto.NewPlainMessage(plainData)
+
+	if publicKeyObj, err = crypto.NewKeyFromArmored(publicKey); err != nil {
+		return nil, nil, nil, err
+	}
+
+	if publicKeyRing, err = crypto.NewKeyRing(publicKeyObj); err != nil {
+		return nil, nil, nil, err
+	}
+
+	if privateKeyObj, err = crypto.NewKeyFromArmored(privateKey); err != nil {
+		return nil, nil, nil, err
+	}
+
+	if unlockedKeyObj, err = privateKeyObj.Unlock(passphrase); err != nil {
+		return nil, nil, nil, err
+	}
+
+	if privateKeyRing, err = crypto.NewKeyRing(unlockedKeyObj); err != nil {
+		return nil, nil, nil, err
+	}
+
+	if packets, err = publicKeyRing.EncryptAttachment(binMessage, fileName); err != nil {
+		return nil, nil, nil, err
+	}
+
+	if signatureObj, err = privateKeyRing.SignDetached(binMessage); err != nil {
+		return nil, nil, nil, err
+	}
+
+	return packets.GetBinaryKeyPacket(), packets.GetBinaryDataPacket(), signatureObj.GetBinary(), nil
+}