How to hash twice a string C openssl-CodePudding

my goal is to hash with sha256 a string twice, but I don't have a good result Here's the code :

void double_hash(char *string, char output[65]) {
    unsigned char out[32];
    char hash[65];
    unsigned char scnd_hash[65];
    SHA256_CTX sha256;
    SHA256_Init(&sha256);
    SHA256_Update(&sha256, string, strlen(string));
    SHA256_Final(out, &sha256);
    for (int i = 0; i < SHA256_DIGEST_LENGTH; i  ) {
        sprintf(hash   (i * 2), "x", out[i]);
    }
    SHA256_CTX sha256_2;
    SHA256_Init(&sha256_2);
    SHA256_Update(&sha256_2, hash, strlen(hash));
    SHA256_Final(scnd_hash, &sha256_2);
    for (int i = 0; i < SHA256_DIGEST_LENGTH; i  ) {
        sprintf(output   (i * 2), "x", scnd_hash[i]);
    }
}

With the string "hello" it does :

hello
2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824
d7914fe546b684688bb95f4f888a92dfc680603a75f23eb823658031fff766d9

But I want :

hello
2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824
9595c9df90075148eb06860365df33584b75bff782a510c6cd4883a419833d50

I've tried with this code but it don't produce the good result :

#include <openssl/sha.h>
#include <stdio.h>
#include <string.h>

void double_hash(char *string, char output[65]) {
    unsigned char out[32];
    unsigned char scnd_hash[65];
    SHA256_CTX sha256;
    SHA256_Init(&sha256);
    SHA256_Update(&sha256, string, strlen(string));
    SHA256_Final(out, &sha256);
    SHA256_CTX sha256_2;
    SHA256_Init(&sha256_2);
    SHA256_Update(&sha256_2, out, strlen((char *)out));
    SHA256_Final(scnd_hash, &sha256_2);
    for (int i = 0; i < SHA256_DIGEST_LENGTH; i  ) {
        sprintf(output   (i * 2), "x", scnd_hash[i]);
    }
}

int main() {
    char outp[65];
    char *hello = "hello";
    double_hash(hello, outp);
    printf("%s", outp);
}

It produce 852d59ce6fb3099c9d563870d298f0108eff079e1dde63e66c68d2e0b40353c1

Thank you

CodePudding user response：

To end up with 9595c9df9007..., perform the 2nd hash on the 32-byte binary data, not on the string.

// SHA256_Update(&sha256_2, hash, strlen(hash));
SHA256_Update(&sha256_2, out, 32);

Ref: SHA256