xor incongruences

[John Machin]

On Oct 17, 7:02 am, Michele <mich... at nectarine.it> wrote:
> Hi,
> I write a simple encoder in python and Java; they do the same
> computations, with the same inputs

No they don't.

> however they won't produce the same
> output.
> Let me explain with code.

You have a strange understanding of the word "explain".

>
> First of all, you need a test file for input:
> $ dd if=/dev/urandom of=test.img bs=1048576 count=1
>
> I have attached the code.
> As you can see we have xor-red the same inputs (bitlists are the same,
> thus the selected blocks to xor are the same - you can easily see it,
> whenever a block is xor-red both programs will print out its hash).
> But here comes the strange: the random_block that the
> create_random_block function returns is not the same: in Java this block
> has an hash which is different from the Python one.
> Why?
>
> Thank you
>
> [test.py1K ]
>         random_block = ['0']*blocksize

This initialises each element to '0'. ord('0') is 48, not 0. Either
I'm hallucinating, or I pointed this out to you in response to your
previous posting (within the last few days).

>                     random_block[j] = chr(ord(random_block[j]) ^ ord(block[j]))
>
> [Test.java2K ]

>                 byte[] random_block = new byte[16384];

Presumably java initialises each element to 0 (or maybe random
gibberish); it is highly unlikely to be 48!

>                                         random_block[j] = (byte) (random_block[j] ^ block[j]);