CODEWITHSUNDEEP
c++opensslhmac
Meric Ozcan
Meric Ozcan

Reputation: 838

C++ OPEN SSL Library HMAC funtion return value is not same everytime I run?

I am trying HMAC SHA256 hash function from Open SSL Library with C++, However when I print the result hex values I saw that my output differs everytime I run the code. What could be the problem

#include <iostream>
#include <string>
#include <openssl/hmac.h>

int main(){
std::cout <<  "Generating key for RRC" << std::endl;

std::array <char, 32> test = {0x69, 0x01, 0x01,  0x00, 0x01, 0x01, 0x01,  0x00, 0x01, 0x01,
                             0x00, 0x01, 0x01, 0x01,  0x00, 0x01, 0x01, 0x01,  0x00, 0x01,
                             0x00, 0x01, 0x01, 0x01,  0x00, 0x01, 0x01, 0x01,  0x00, 0x01,
                             0x00, 0x01 };

std::array <char, 5> string = = {0x69, 0x03, 0x01,  0x02, 0x01 };

unsigned int lengthResult;

unsigned char result[EVP_MAX_MD_SIZE];

HMAC(EVP_sha256(), (unsigned char*)test.data(), test.size(),
                            (unsigned char*)string.data(), string.size(),
                             result, &lengthResult);

for (auto i:result)
    std::cout  << i + 0 <<" " ;

}

Upvotes: 1

Views: 506

Answers (1)

Botje
Botje

Reputation: 31020

HMAC_SHA256 produces a SHA256 sum at the end, which is 32 bytes. EVP_MAX_MD_SIZE is 64 bytes, initialized with random memory garbage. If I compile and run your code, the first 32 bytes are always the same.

Upvotes: 1

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