how do i find collision for a simple hash algorithm

问题

I have the following hash algorithm:

    unsigned long specialNum=0x4E67C6A7;
    unsigned int ch;
    char inputVal[]="                        AAPB2GXG";


    for(int i=0;i<strlen(inputVal);i++)
    {
        ch=inputVal[i];

        ch=ch+(specialNum*32);
        ch=ch+(specialNum/4);

        specialNum=bitXor(specialNum,ch);
    }

    unsigned int outputVal=specialNum;

The bitXor simply does the Xor operation:

int bitXor(int a,int b)
{
    return (a & ~b) | (~a & b);
}

Now I want to find an Algorithm that can generate an "inputVal" when the outputVal is given.(The generated inputVal may not be necessarily be same as the original inputVal.That's why I want to find collision). This means that I need to find an algorithm that generates a solution that when fed into the above algorithm results same as specified "outputVal". The length of solution to be generated should be less than or equal to 32.

回答1:

Method 1: Brute force. Not a big deal, because your "specialNum" is always in the range of an int, so after trying on average a few billion input values, you find the right one. Should be done in a few seconds.

Method 2: Brute force, but clever.

Consider the specialNum value before the last ch is processed. You first calculate (specialNum * 32) + (specialNum / 4) + ch. Since -128 <= ch < 128 or 0 <= ch < 256 depending on the signedness of char, you know the highest 23 bits of the result, independent of ch. After xor'ing ch with specialNum, you also know the highest 23 bits (if ch is signed, there are two possible values for the highest 23 bits). You check whether those 23 bits match the desired output, and if they don't, you have excluded all 256 values of ch in one go. So the brute force method will end on average after 16 million steps.

Now consider the specialNum value before the last two ch are processed. Again, you can determine the highest possible 14 bits of the result (if ch is signed with four alternatives) without examining the last two characters at all. If the highest 14 bits don't match, you are done.

Method 3: This is how you do it. Consider in turn all strings s of length 0, 1, 2, etc. (however, your algorithm will most likely find a solution much quicker). Calculate specialNum after processing the string s. Following your algorithm, and allowing for char to be signed, find the up to 4 different values that the highest 14 bits of specialNum might have after processing two further characters. If any of those matches the desired output, then examine the value of specialNum after processing each of the 256 possible values of the next character, and find the up to 2 different values that the highest 23 bits of specialNum might have after examining another char. If one of those matches the highest 23 bits of the desired output then examine what specialNum would be after processing each of the 256 possible next characters and look for a match.

This should work below a millisecond. If char is unsigned, it is faster.

来源：https://stackoverflow.com/questions/30594470/how-do-i-find-collision-for-a-simple-hash-algorithm