C#•2y ago

❔ Reversing XOR of shifted numbers

Hello! I have 2 binary numbers: A= 10111 B= 11110 B is A shifted left by 2 positions C is ( A XOR B ) C= 01001 Is it possible to find out A and B by only knowing C ?

11 Replies

ero•2y ago

B is not A shifted left by 2

MODiX•2y ago

Ero#1111

REPL Result: Success

Convert.ToString(0b10111 << 2, 2)

Convert.ToString(0b10111 << 2, 2)

Result: string

Compile: 416.632ms | Execution: 22.782ms | React with ❌ to remove this embed.

HimmDawg•2y ago

I'm not too deep in this topic, but I'd say no. Especially if you have a leading 0 like C has.

SebastianOP•2y ago

So, to give more context, here is the function that does the shifting, it is in C

void SHL(uchr *c){
    char tmp = c[0];

    for(int i = 0; i < LEN-1; i++){
        c[i] = ((c[i] << 4) & 0xf0 ) | ((c[i+1] & 0xf0) >> 4);
    }
    c[LEN-1] = ((c[LEN-1] << 4) & 0xf0 ) | ((tmp & 0xf0) >> 4);
}

void SHL(uchr *c){
    char tmp = c[0];

    for(int i = 0; i < LEN-1; i++){
        c[i] = ((c[i] << 4) & 0xf0 ) | ((c[i+1] & 0xf0) >> 4);
    }
    c[LEN-1] = ((c[LEN-1] << 4) & 0xf0 ) | ((tmp & 0xf0) >> 4);
}

The original values are in hexa and I tried to make it as simple as possible but as it turns out I might not understand it completely. Sorry for that. So a hex number is passed through this function, and the result is XOR-ed with the original number. Will the XOR result be enough to find out the original hex number? I can also post the entire program if you think it helps.

HimmDawg•2y ago

I mean, what you can do is A XOR B = C C XOR A = B C XOR B = A But you cannot create A and B from C out of thin air

SebastianOP•2y ago

I understand. I was hoping that knowing the relation between A and B ( B is A shifted) would open the possibility of finding them

ChucklesTheBeard•2y ago

Yes, there's a 1:1 mapping between A and A^(A<<2) assuming this is shifting and not rotating.

MODiX•2y ago

ChucklesTheBeard#6079

REPL Result: Success

Dictionary<uint, uint> dict = new();
for (uint A = 0; A <= 0xFF; A++)
{
    uint B = A << 2;
    uint C = (A ^ B) & 0xFF;
    dict.Add(A,C);
}

Console.WriteLine(dict.Values.Distinct().Count());

Dictionary<uint, uint> dict = new();
for (uint A = 0; A <= 0xFF; A++)
{
    uint B = A << 2;
    uint C = (A ^ B) & 0xFF;
    dict.Add(A,C);
}

Console.WriteLine(dict.Values.Distinct().Count());

Console Output

Compile: 635.129ms | Execution: 51.054ms | React with ❌ to remove this embed.

SebastianOP•2y ago

So could this be used to find A and B?

ChucklesTheBeard•2y ago

Yep, 1:1 mappings are reversible.

Accord•2y ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ Reversing XOR of shifted numbers