python number handling - tiny encryption algorithm

[Bernhard Mulder]

One systematic, if maybe clumsy way, is to mimic the C arithmetic 
operations more closely in Python. If you do, for example, a left shift 
in C, the result is always returned in a certain precision, 64 bits in 
the example below. In python, no bits are lost, so you have to force the 
result into the 64 bits back to obtain the same result.

Instead of (y << 4) you can write something like (untested!) (y << 4) & 
0xFF...FF.

Once you understand what the algorithm is doing, you may want to go back 
and express the algorithm in a more pythonic way (not by programming C 
in  Python).

Kinsley Turner wrote:
> 
> Hey-ho,
> 
> I'm getting a bit out of my depth porting the 'tiny encryption algorithm' 
> from C to python.
> 
> Ref: 
> http://en.wikipedia.org/wiki/Tiny_Encryption_Algorithm
> http://www.simonshepherd.supanet.com/source.htm#new_ansi
> 
> 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 
> y=v[0],z=v[1],sum=0xC6EF3720,delta=0x9E3779B9,n=32;
> 
>    while(n-->0)
>   {
>       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
>     delta=0x9E3779B9 
> 
>     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 
> expect
> 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 
> character
> 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 
> character...
> 
> 
> Anyway, that's about it.
> Any help much appreciated.
> 
> 
> thanks,
> -kt
> 
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