docs/smpp-ota-tool: Add documentation/tutorial

We already have documentation that explains how to run pySim-smpp2sim.
With smpp-ota-tool we now have a counterpart for pySim-smpp2sim, so
let's add documentation for this tool as well.

Related: SYS#7881
Change-Id: If0d18a263f5a6dc035b90f5c5c6a942d46bbba49

pySim/global_platform/scp.py

@@ -266,13 +266,11 @@ class SCP02(SCP):
         super().__init__(*args, **kwargs)
 
     def dek_encrypt(self, plaintext:bytes) -> bytes:
-        # See also GPC section B.1.1.2, E.4.7, and E.4.1
-        cipher = DES3.new(self.sk.data_enc, DES.MODE_ECB)
+        cipher = DES.new(self.card_keys.dek[:8], DES.MODE_ECB)
         return cipher.encrypt(plaintext)
 
     def dek_decrypt(self, ciphertext:bytes) -> bytes:
-        # See also GPC section B.1.1.2, E.4.7, and E.4.1
-        cipher = DES3.new(self.sk.data_enc, DES.MODE_ECB)
+        cipher = DES.new(self.card_keys.dek[:8], DES.MODE_ECB)
         return cipher.decrypt(ciphertext)
 
     def _compute_cryptograms(self, card_challenge: bytes, host_challenge: bytes):