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dexter has posted comments on this change by dexter. ( 
https://gerrit.osmocom.org/c/pysim/+/37931?usp=email )

Change subject: scp: fix key length in dek_encrypt and dek_decrypt
......................................................................


Patch Set 2:

(1 comment)

Commit Message:

https://gerrit.osmocom.org/c/pysim/+/37931/comment/74c33d5e_037b1da9?usp=email :
PS1, Line 9: 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
> did you check the spec and determie that this truncation is 
> required/intended?  IF yes, it would be  […]
Yes, I have checked it. It seems to be an implementation detail of 
pyCryptoDome. They say: "key (bytes/bytearray/memoryview) – The secret key to 
use in the symmetric cipher. It must be 8 byte long. The parity bits will be 
ignored."

In other code locations one can find the truncation as well. I have now put a 
reference to the pyCryptoDome manual into the commit message. I don't think 
that we need an extra comment in the code, since it is only a pyCryptoDome 
related.



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