I am trying to reverse a seed/key algorithm that has a constant value inside it. and there is different const value for different device that use this algorithm. i can give some sample from each device so i have seed/key of devices. the algorithm is :

int SeedKey_Algorithm(int seed){ // sample input: 0x01010101
for (int i = 0; i < 0x23; i++)
    if ((seed & 0x80000000) == 0x80000000)
        seed = ( x ^ seed); // x is constant value
    seed = seed << 1;
return seed;
//out = 0xFFAA5550

then if when inject the 0x01010101 as input we get 0xFFAA5550 as output. so how i can find this constant value. is there any mathematics algorithm for find it? is it needed more sample for reverse this?

so i check another device that work with this algorithm and i find 12 true value for 0x01010101.
0x0d7c76ff, 0x1049164d, 0x37749eba, 0x6071e476, 0x6cced1e7, 0x7657a4aa, 0x8d7c76ff, 0x9049164d, 0xb7749eba, 0xe071e476, 0xecced1e7, 0xf657a4aa

but for 0x02020202 i can't find any right value :(
is this possible? or I made a mistake?

1 Answer 1


The keyspace for x is only 2**32. This can be easily bruteforced. Also you can use something like z3 a SAT solver to model the equations. For your given pair these were the possible 16 values for x

0x153f11fa, 0x953f11fa
0x24c66e44, 0xa4c66e44
0x3bcc14c8, 0xbbcc14c8
0x3c2918cb, 0xbc2918cb
0x477bb478, 0xc77bb478
0x662a1eac, 0xe62a1eac
0x71539c35, 0xf1539c35
0x76a55966, 0xf6a55966

With an additional keypair the candidates can be boiled down to 2 values


Both of these can be used interchangeably as the 31st bit is inconclusive.

  • thanks @sudhacker, so i need more sample to achieve unique answer. is this ture?
    – Unicornux
    Feb 15, 2020 at 10:13
  • Can you provide additional samples?
    – sudhackar
    Feb 15, 2020 at 10:31
  • absolutely yes.
    – Unicornux
    Feb 15, 2020 at 10:38
  • Please add as many as possible pairs you can.
    – sudhackar
    Feb 15, 2020 at 10:46
  • 1
    o0o0om. . .I didn't understand what you mean. but i have some sample for new device that worked with this algorithm but different counter or different constant value. for example: 0x01010101 ==> 0xDFBB4565 and 0x02020202 ==> 0x21028781 and 0x02010101 ==> 0xB1C7ED2B and 0x08010101 ==> 0xFB9718DE
    – Unicornux
    Feb 15, 2020 at 11:33

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