add first WIP version of encryption

Signed-off-by: Florian Brandes <florian.brandes@posteo.de>

smtprd.py

@@ -34,6 +34,7 @@ from cryptography.hazmat.primitives.serialization import (
 )
 from cryptography.x509 import load_pem_x509_certificate
 from envelope import Envelope as EnvelopeEnvelope
+from M2Crypto import BIO, SMIME, X509
 
 
 @dataclass(frozen=True)
@@ -163,6 +164,52 @@ class SMTPClient(SMTP):
         new_message["To"] = message["To"]
         return new_message
 
+    def _encrypt(self, message: bytes, subject: str) -> bytes:
+        """Encrypt the message
+
+        Args:
+            message (bytes): message in bytes format (can/should be signed)
+
+        Returns:
+            bytes: Encrypted message
+        """
+        # Make a MemoryBuffer of the message.
+        buf = BIO.MemoryBuffer(message)
+
+        # Seed the PRNG.
+        # Rand.load_file('randpool.dat', -1)
+
+        # Instantiate an SMIME object.
+        s = SMIME.SMIME()
+
+        # Load target cert to encrypt to.
+        x509 = X509.load_cert(self._config.smime_to_cert)
+        sk = X509.X509_Stack()
+        sk.push(x509)
+        s.set_x509_stack(sk)
+
+        # Set cipher: 3-key triple-DES in CBC mode.
+        # TODO: Evaluate later
+        s.set_cipher(SMIME.Cipher("des_ede3_cbc"))
+
+        # Encrypt the buffer.
+        p7 = s.encrypt(buf)
+
+        # Output p7 in mail-friendly format.
+        out = BIO.MemoryBuffer()
+
+        out.write("From: " + self._config.sender + "\r\n")
+        out.write("To: " + ", ".join(self._config.recipients) + "\r\n")
+        out.write("Subject: " + subject + "\r\n")
+        s.write(out, p7)
+
+        # print(out.read().decode())
+
+        # Save the PRNG's state.
+        # Rand.save_file('randpool.dat')
+
+        return out.read()
+
     def _sign(self, message: Message) -> bytes:
         """Sign the message
 
@@ -187,9 +234,10 @@ class SMTPClient(SMTP):
             .sign(Encoding.SMIME, [pkcs7.PKCS7Options.DetachedSignature])
         )
         # Add correct headers
-        new = b"From: " + self._config.sender.encode() + b"\r\n" + output
-        new = b"To: " + ", ".join(self._config.recipients).encode() + b"\r\n" + new
-        new = b"Subject: " + message.get("Subject", "").encode() + b"\r\n" + new
+        # new = b"From: " + self._config.sender.encode() + b"\r\n" + output
+        # new = b"To: " + ", ".join(self._config.recipients).encode() + b"\r\n" + new
+        # new = b"Subject: " + message.get("Subject", "").encode() + b"\r\n" + new
+        new = self._encrypt(output, message.get("Subject", ""))
         return new
 
     async def _send_message(