Modifying hash function

Question

I need to modify hash code,so that instead of 256 hash value,I get 64 bit hash value following the rule that instead of 256 bit fromula.

h=A||B||C||D||E||F||G||H

where A, B, D, E, F, G, H is 32 bit words, || concat operation. I must receive 64 bit hash with formula

 h=B xor D xor F xor H || A xor C xor E xor G.

I am using following Sha256 implementation but I am unable to find part where this has to be done- http://www.zedwood.com/article/cpp-sha256-function

Where sha should be something like

SHA-256("10301231030456") = 0xe4a6aade78b1c5ad21a76adca6beb75634c1ff45e7aba9d18f861211d43d69e1

the goal is to receive:

SHA-256-mod("10301231030456") = 0xed99b2cb7e462d56

Thank you

So what stops you from modifying this hash function? Read the code and then modify it appropriately. — ForceBru, Jan 12 '16 at 15:11
So what's your question? How to xor few `uint32_t`s? Rather than modifying that function, call it and convert the output to what you need. — bereal, Jan 12 '16 at 15:11

Mykola · Accepted Answer · 2016-01-13T12:05:48.127

2

Try to cast your 256 bit array to unsigned long mod_args[8];

std::string sha = sha256("10301231030456");

char hash[32];

for (int i = 0, j = 0; i < sha.length(); i += 2, j++)
{
    std::string sub = sha.substr(i, 2);
    hash[j] = strtoul(sub.c_str(), NULL, 16);
}

unsigned long mod_args[8];
memcpy(mod_args, hash, 32);

than getting 2 pieces of 64 bit:

unsigned long a = mod_args[1] ^ mod_args[3] ^ mod_args[5] ^ mod_args[7];
unsigned long b = mod_args[0] ^ mod_args[2] ^ mod_args[4] ^ mod_args[6];

than getting result by concat that two pieces

unsigned long long result = (((unsigned long long)a) << 32) | b;

or

unsigned long long result = (((unsigned long long)b) << 32) | a;

considering what piece of hash must be older a or b

the complete solution is:

#define B_OLDER_THAN_A

int main()
{
    std::string sha = sha256("10301231030456");

    char hash[32];

    for (int i = 0, j = 0; i < sha.length(); i += 2, j++)
    {
        std::string sub = sha.substr(i, 2);
        hash[j] = strtoul(sub.c_str(), NULL, 16);
    }

    unsigned long mod_args[8];
    memcpy(mod_args, hash, 32);

    unsigned long a = mod_args[1] ^ mod_args[3] ^ mod_args[5] ^ mod_args[7];
    unsigned long b = mod_args[0] ^ mod_args[2] ^ mod_args[4] ^ mod_args[6];

#ifdef B_OLDER_THAN_A
    unsigned long long result = (((unsigned long long)b) << 32) | ((unsigned long long)a);
#else
    unsigned long long result = (((unsigned long long)a) << 32) | ((unsigned long long)b);
#endif

    unsigned char output[8] = { 0 };

    memcpy(output, (char*)(&result), 8);

    for (int i = 0; i < 8; i++)
        std::cout << setfill('0') << setw(2) << hex << (unsigned int)(output[i]);

    std::cout << endl;


    return 0;
}

edited Jan 13 '16 at 12:05

answered Jan 12 '16 at 15:26

Mykola

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A violation of strict aliasing rule is punishable by two years of coding in Java. – SergeyA Jan 12 '16 at 15:42
@SergeyA: What you mean? – Mykola Jan 12 '16 at 15:46
I mean that your code violates strict aliasing rule. And we do not want such code, we do not! – SergeyA Jan 12 '16 at 15:46
Right here: `unsigned long* mod_args = reinterpret_cast(sha_result);` – SergeyA Jan 12 '16 at 15:51
@SergeyA: And what did you suggest? – Mykola Jan 12 '16 at 15:54
1

I suggest not to violate such a rule. – SergeyA Jan 12 '16 at 15:54
@SergeyA: How I can do it? – Mykola Jan 12 '16 at 15:55
1

Well, you are trying to answer a question, not ask one, right? If you want to **ask** a question, feel free to do so by clicking 'Ask Question' in the top left corner of the screen. – SergeyA Jan 12 '16 at 16:04
-> http://stackoverflow.com/questions/98650/what-is-the-strict-aliasing-rule – Naios Jan 12 '16 at 16:07
@SergeyA: Did my answer satisfy your standards now ? – Mykola Jan 12 '16 at 16:33
Well, you took my answer and copied-pasted it into yours. Do not expect an upvote from me for that. – SergeyA Jan 12 '16 at 16:36
@SergeyA: This was the different question. – Mykola Jan 12 '16 at 16:37
@SergeyA: And thank you for help! :) – Mykola Jan 12 '16 at 16:41
no it dodnot work for me,but I got an idea form you I made 8 substrings using .substr...and now problem is using xor operator on strings – njok Jan 12 '16 at 19:31
@njok: so you also whant to diplay result value as string? – Mykola Jan 12 '16 at 19:32
yes.. let say that string A=123fb string B=234ac I need string C that would be 31757 – njok Jan 12 '16 at 19:34
@njok: did you try to print result value? in hexadecimal mode – Mykola Jan 12 '16 at 19:35
given number is in hex mode,what come out of hash function only it does not have 0x at beggining... I tried all kinds of string to hex and than xor..but nothing seems to work – njok Jan 12 '16 at 19:40
@njok: i have found a typo in my code so a update it. – Mykola Jan 12 '16 at 19:54
@njok: Are you sure that modified function must return `SHA-256-mod("10301231030456") = 0xed99b2cb7e462d56`? – Mykola Jan 12 '16 at 20:11
I solve it on my on way,but without you I would'nt have made it Thanks once more :D – njok Jan 12 '16 at 21:33
@njok: Glad can help you! Good luck!) – Mykola Jan 12 '16 at 21:42

Modifying hash function

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