[Python-au] python number handling - tiny encryption algorithm

Kinsley Turner kingsleyturner at westpac.com.au
Wed Nov 30 00:08:10 CET 2005


I'm getting a bit out of my depth porting the 'tiny encryption algorithm' 
from C to python.


Specifically I don;t know how to handle a C-long block and perform the
mathmatical manipulations in python syntax.  I played with pack and unpack
(from struct module) but that didn't seem to buy me anything.

In my version, I end up with hugely long integers, which have obviously
not be constrained into the 4-byte unsigned longs that TEA is expecting.

This is the C function:

/* v is 64-bits input, w is 64-bits output, k is the 128-bit key */
void decipher(const unsigned long *const v,unsigned long *const w, const 
unsigned long * const k)
   register unsigned long 

      z -= (y << 4 ^ y >> 5) + y ^ sum + k[sum>>11 & 3];
      sum -= delta;
      y -= (z << 4 ^ z >> 5) + z ^ sum + k[sum&3];
   w[0]=y; w[1]=z;

Which gives me a (broken) python version:

def  teaDecipher(input,key):
    y = input[0]
    z = input[1]
    n = 32
    sum = 0xC6EF3720

    while (n > 0):
        n -= 1
        z -= (y << 4 ^ y >> 5) + y ^ sum + key[sum>>11 & 3];
        sum -= delta;
        y -= (z << 4 ^ z >> 5) + z ^ sum + key[sum&3];
      return y,z

That seems to return hugely-long integers (around 30? digits), whereas I'd 
them to max-out at 2^32.

Perhaps I'm not packing the input or the key properly.  My inputs are
just strings which I put into an integer with the ordinal value of each 
shifted into the correct position to make the C-long.

Something like:
    input[0] = ord(encrypted[i])<<24 + ord(encrypted[i+1])<<16 + 
ord(encrypted[i+2])<<8 + ord(encrypted[i+3])
    input[1] = ord(encrypted[i+4])<<24 + ord(encrypted[i+5])<<16 + 
ord(encrypted[i+6])<<8 + ord(encrypted[i+7])

The key is created as an array of numbers, each the ord() of the key 

Anyway, that's about it.
Any help much appreciated.


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