Python Translation of C# DES Encryption

Andreas Pauley andreasp+python at qbcon.com
Mon May 15 13:01:34 CEST 2006


Edward Elliott wrote:

>You need to find the exact algorithm the DESCryptoServiceProvider uses.  
>CBC and CTR are common modes with IVs.  Besides mode of operation, there's
>the issue of how it pads the data (in the cipher, not base64).  There could
>also be differences in things like the key scheduling routines, but that's
>less likely.  Until you know exactly what the original code does, your only
>option is trial and error.
>
>  
>
I read some of the C# documentation on DESCryptoServiceProvider, but it 
was completely useless for
my purpose. The documentation basically only tells you how to use the 
class, not what it does.

I managed to *decrypt* a string encrypted by the C# method using CBC 
mode, as suggested by Gary (thanks).
The python CBC encryption results in a different encrypted string, but 
decrypting both these strings
returns the original plaintext value.

As for padding, the C# method uses characters '\x01' to '\x07' for padding.
If there are 3 padding characters needed (eg with a password of 5 
chars), then three '\x03' chars will be used.
Similarly, a 2-char password will be padded with '\x06' * 6

Regards,
Andreas Pauley

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