scp: fix key length in dek_encrypt and dek_decrypt

When creating the DES cipher object with DES.new, we use the property
card_keys.dek. This property may hold a 16 byte key, but DES uses
an 8 byte key (56 bit + 8 bit integrity). Pycryptodome does not
automatically ignore excess key bytes. Instead it throws an
exception. This means we need to make sure to supply only the first
8 bytes of card_keys.dek

See also: https://pycryptodome.readthedocs.io/en/latest/src/cipher/des.html

Related: OS#6531
Change-Id: I92e0dc6a6196b532bd8b53fca7b9e78070d6903f

pySim/global_platform/scp.py

@@ -230,11 +230,11 @@ class SCP02(SCP):
         super().__init__(*args, **kwargs)
 
     def dek_encrypt(self, plaintext:bytes) -> bytes:
-        cipher = DES.new(self.card_keys.dek, 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:
-        cipher = DES.new(self.card_keys.dek, 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):