Question

I have the following hash function, and I’m trying to get my way to reverse it, so that I can find the key from a hashed value.

uint Hash(string s)
{
    uint result = 0;
    for (int i = 0; i < s.Length; i++)
    {
        result = ((result << 5) + result) + s[i];
    }
    return result;
}

The code is in C# but I assume it is clear.

I am aware that for one hashed value, there can be more than one key, but my intent is not to find them all, just one that satisfies the hash function suffices.

EDIT :

The string that the function accepts is formed only from digits 0 to 9 and the chars ‘*’ and ‘#’ hence the Unhash function must respect this criteria too.

Any ideas? Thank you.

Answer 1

Brute force should work if uint is 32 bits. Try at least 2^32 strings and one of them is likely to hash to the same value. Should only take a few minutes on a modern pc.

You have 12 possible characters, and 12^9 is about 2^32, so if you try 9 character strings you’re likely to find your target hash. I’ll do 10 character strings just to be safe.

(simple recursive implementation in C++, don’t know C# that well)

#define NUM_VALID_CHARS 12
#define STRING_LENGTH 10
const char valid_chars[NUM_VALID_CHARS] = {'0', ..., '#' ,'*'};

void unhash(uint hash_value, char *string, int nchars) {
  if (nchars == STRING_LENGTH) {
    string[STRING_LENGTH] = 0;
    if (Hash(string) == hash_value) { printf("%s\n", string); }
  } else {
    for (int i = 0; i < NUM_VALID_CHARS; i++) {
      string[nchars] = valid_chars[i];
      unhash(hash_value, string, nchars + 1);
    }
  }
}

Then call it with:

char string[STRING_LENGTH + 1];
unhash(hash_value, string, 0);