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/mobile.go

@@ -0,0 +1,67 @@
+package helper
+
+import (
+	"github.com/ProtonMail/gopenpgp/v2/crypto"
+)
+
+type ExplicitVerifyMessage struct {
+	Message                    *crypto.PlainMessage
+	SignatureVerificationError *crypto.SignatureVerificationError
+}
+
+// DecryptVerifyMessageArmored decrypts an armored PGP message given a private key and its passphrase
+// and verifies the embedded signature.
+// Returns the plain data or an error on signature verification failure.
+func DecryptExplicitVerify(
+	pgpMessage *crypto.PGPMessage,
+	privateKeyRing, publicKeyRing *crypto.KeyRing,
+	verifyTime int64,
+) (*ExplicitVerifyMessage, error) {
+	var explicitVerify *ExplicitVerifyMessage
+
+	message, err := privateKeyRing.Decrypt(pgpMessage, publicKeyRing, verifyTime)
+
+	if err != nil {
+		castedErr, isType := err.(crypto.SignatureVerificationError)
+		if !isType {
+			return nil, err
+		}
+
+		explicitVerify = &ExplicitVerifyMessage{
+			Message:                    message,
+			SignatureVerificationError: &castedErr,
+		}
+	} else {
+		explicitVerify = &ExplicitVerifyMessage{
+			Message:                    message,
+			SignatureVerificationError: nil,
+		}
+	}
+
+	return explicitVerify, nil
+}
+
+// DecryptAttachment takes a keypacket and datpacket
+// and returns a decrypted PlainMessage
+// Specifically designed for attachments rather than text messages.
+func DecryptAttachment(keyPacket []byte, dataPacket []byte, keyRing *crypto.KeyRing) (*crypto.PlainMessage, error) {
+	splitMessage := crypto.NewPGPSplitMessage(keyPacket, dataPacket)
+
+	decrypted, err := keyRing.DecryptAttachment(splitMessage)
+	if err != nil {
+		return nil, err
+	}
+	return decrypted, nil
+}
+
+// EncryptAttachment encrypts a file given a plainData and a fileName.
+// Returns a PGPSplitMessage containing a session key packet and symmetrically encrypted data.
+// Specifically designed for attachments rather than text messages.
+func EncryptAttachment(plainData []byte, fileName string, keyRing *crypto.KeyRing) (*crypto.PGPSplitMessage, error) {
+	plainMessage := crypto.NewPlainMessage(plainData)
+	decrypted, err := keyRing.EncryptAttachment(plainMessage, fileName)
+	if err != nil {
+		return nil, err
+	}
+	return decrypted, nil
+}