code: def _encrypt(self):
          
          from M2Crypto import BIO, SMIME, X509
  
          # Iterate through the fields and pull out the ones that have a value.
          plaintext = 'cert_id=%s\n' % self.cert_id
          for name, field in self.fields.items():
              value = None
              if name in self.initial:
                  value = self.initial[name]
              elif field.initial is not None:
                  value = field.initial
              if value is not None:
                  plaintext += u'%s=%s\n' % (name, value)
          plaintext = plaintext.encode('utf-8')
  
          # Begin crypto weirdness.
          s = SMIME.SMIME()
          s.load_key_bio(BIO.openfile(self.private_cert), BIO.openfile(self.public_cert))
          p7 = s.sign(BIO.MemoryBuffer(plaintext), flags=SMIME.PKCS7_BINARY)
          x509 = X509.load_cert_bio(BIO.openfile(self.paypal_cert))
          sk = X509.X509_Stack()
          sk.push(x509)
          s.set_x509_stack(sk)
          s.set_cipher(SMIME.Cipher('des_ede3_cbc'))
          tmp = BIO.MemoryBuffer()
          p7.write_der(tmp)
          p7 = s.encrypt(tmp, flags=SMIME.PKCS7_BINARY)
          out = BIO.MemoryBuffer()
          p7.write(out)
          return out.read().decode()
docs: Use your key thing to encrypt things.