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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. 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

Related: OS#6531
Change-Id: I92e0dc6a6196b532bd8b53fca7b9e78070d6903f
---
M pySim/global_platform/scp.py
1 file changed, 2 insertions(+), 2 deletions(-)

git pull ssh://gerrit.osmocom.org:29418/pysim refs/changes/31/37931/1

diff --git a/pySim/global_platform/scp.py b/pySim/global_platform/scp.py
index f68bcf9..17463ff 100644
--- a/pySim/global_platform/scp.py
+++ b/pySim/global_platform/scp.py
@@ -230,11 +230,11 @@
         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):

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