python number handling - tiny encryption algorithm

[Carl Friedrich Bolz]

Hi!

Kinsley Turner wrote:
[snip]

> 
> 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.

Yes, python integers overflow to longs, which is a good thing for your 
use-case, since python's int type corresponds to a signed long on C 
level. To make the code do the right thing you have to keep integers in 
the range 0, 2^32 by hand by inserting something like

x = x % 2 ** 32
y = y % 2 ** 32

into the while loop.

Cheers,

Carl Friedrich