GP: fix kcb for non block aligned keys

how encrypt_key() pads a kcv:
len(key) % blocksize bytes
what it should do to actually do it right:
blocksize - len(key) % blocksize
so the plaintext handed to the cipher was only block aligned by luck as
long as the key length happened to be a multiple of half the block size.

And of course decrypt_key() did not invert encrypt_key() at all,
the clear text length of GP CardSpec v2.3 Table 11-70 precedes the
ENCRYPTED kcv, but it was parsed out of the DECRYPTED data, and the
length byte itself was fed to the cipher along with the cryptogram.

Fix this up with a helper and tests so it is actually usable.

Change-Id: I02b4f2ed948c31e1741e40f0226fb49757fa2570

pySim/global_platform/scp.py

@@ -215,11 +215,20 @@ class SCP(SecureChannel, abc.ABC):
     def gen_ext_auth_apdu(self, security_level: int = 0x01) -> bytes:
         pass
 
+    def pad_to_blocksize(self, data: bytes) -> bytes:
+        """Right pad the data with zero bytes to a multiple of the DEK cipher block size."""
+        if len(data) % self.sk.blocksize:
+            # not '+=' which would mutate the callers bytearray in place..
+            data = data + b'\x00' * (self.sk.blocksize - len(data) % self.sk.blocksize)
+        return data
+
     def encrypt_key(self, key: bytes) -> bytes:
         """Encrypt a key with the DEK."""
-        num_pad = len(key) % self.sk.blocksize
-        if num_pad:
-            return bertlv_encode_len(len(key)) + self.dek_encrypt(key + b'\x00'*num_pad)
+        if len(key) % self.sk.blocksize:
+            # The kcv is right padded before encryption and the kcb
+            # is formatted as described in Table 11-70: preceded by the actual length of the
+            # clear text kcv.
+            return bertlv_encode_len(len(key)) + self.dek_encrypt(self.pad_to_blocksize(key))
         return self.dek_encrypt(key)
 
     def decrypt_key(self, encrypted_key:bytes) -> bytes:
@@ -232,9 +241,8 @@ class SCP(SecureChannel, abc.ABC):
             # Block provides the actual length of the key component value, which allows recovering the
             # clear-text key component value after decryption of the encrypted key component value and removal
             # of padding bytes.
-            decrypted = self.dek_decrypt(encrypted_key)
-            key_len, remainder = bertlv_parse_len(decrypted)
-            return remainder[:key_len]
+            key_len, remainder = bertlv_parse_len(encrypted_key)
+            return self.dek_decrypt(remainder)[:key_len]
         else:
             # If the length of the Key Component Block is a multiple of the block size of the encryption
             # algorithm (i.e.  8 bytes for DES, 16 bytes for AES), then it shall be assumed that no padding