use aes_256 instead of des

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

smtprd_ng/smtprd.py

@@ -172,13 +172,7 @@ class SMTPClient(SMTP):
         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.
@@ -187,9 +181,7 @@ class SMTPClient(SMTP):
         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"))
+        s.set_cipher(SMIME.Cipher("aes_256_cbc"))
 
         # Encrypt the buffer.
         p7 = s.encrypt(buf)
@@ -197,16 +189,12 @@ class SMTPClient(SMTP):
         # Output p7 in mail-friendly format.
         out = BIO.MemoryBuffer()
 
+        # Add header
         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: