在32位数组上复制16位哈希

Question

I have a problem with my use of sprintf and strcat . Here's my code :

unsigned char hashResults[8][16];
unsigned char tmp[2];
unsigned char hash[8][32];

transformation("toto", 4, hashResults);

for (int k = 0; k < 8; ++k)
{
    for (int i = 0; i < 16; ++i)
    {
        sprintf(tmp,"%2.2x",hashResults[k][i]);
        strcat(hash[k],tmp);
    }
    printf("%d \n%s\n", strlen(hash[k]), hash[k]);
}
printf("Test : %s\n", hash[3]);

The function transformation() gives me 8 hashes on 16 bits.

I use sprintf and strcat to get hash on 32 bits. When I try to read all hash[k] , strlen(hash[8]) returns to me 32 (it's correct), and the string is correct too. But, when I try to read hash[3] outside the loop it contains all values after it.

For example, the output of my program :

32 - 4a18e332afba75b9734e875323f452f8

32 - b96833277faf31a5915c769f44634506

32 - f89f6dd8cd5aee79de3b2c0c27cafe2e

32 - c9f629472c862c1e7542f4cb2835d02b

32 - 09fc12cfb0a81a38513dbd5edff19e52

32 - 35564354793555a3ae1382f647044445

Test : b96833277faf31a5915c769f44634506f89f6dd8cd5aee79de3b2c0c27cafe2ec9f629472c862c1e7542f4cb2835d02b09fc12cfb0a81a38513dbd5edff19e5235564354793555a3ae1382f647044445

Does anyone see the problem? I wish to use these hashes to compare them to others hashes.

Answer 1

You should increase unsigned char tmp[2] to unsigned char tmp[3] and unsigned char hash[8][32] to unsigned char hash[8][33];

One problem is sprintf(tmp,"%2.2x",hashResults[k][i]) , because it writes two characters + '\\0' which takes three elements in the array tmp .

But the biggest problem is strcat(hash[k],tmp); . At the end of every inner for loop you have written in the hash[k] arrays 33 characters(32 chars + '\\0'). What happens is when you populate one of the hash[k] arrays in the inner for loop you also write \\0' to the first element in the next array, that's why printf("%d \\n%s\\n", strlen(hash[k]), hash[k]); prints correct results. That misleads you to believing you have null terminated the hash[k] arrays. Now when you enter the next inner for loop you override the null you have written to this array the previous time you exited the inner for loop, no longer terminating the previous array.

So you null terminate your hash[k] array by writing zero to the first element of the hash[k+1] array on the exit of the inner for loop. Then you override this null value every time you enter the inner for loop.

In the end you have no terminating nulls in your arrays and only the final null on the last array is there.

I wonder how do you even get this to work every time, because you write the terminating null beyond the array size which leads to undefined behavior.

Making tmp[3] and hash[8][33] should fix your problem.

In order for the strcat() function to work properly you have to have at least one null in the array you are concatenating to, because otherwise it wouldn't know where to concatenate to. You have to add hash[k][0] = 0; before entering every inner loop:

for (int k = 0; k < 8; ++k)
{
    hash[k][0] = 0;
    for (int i = 0; i < 16; ++i)
    {
        sprintf(tmp,"%2.2x",hashResults[k][i]);
        strcat(hash[k],tmp);
    }
    printf("%d \n%s\n", strlen(hash[k]), hash[k]);
}