For SHA_256, I am not getting correct output.

[rdiwan2023]

Hi,
I am using below code to get the SHA_256 digest output, but I am seeing output is not correct. Can any one please tell am i missing any thing? below is the code.

#include <stdio.h>
#include <stdlib.h>
#include <openssl/err.h>
#include <openssl/bio.h>
#include <openssl/evp.h>
#include <openssl/core_names.h>

OSSL_LIB_CTX *libctx = NULL;
const char *propq = NULL;

int sha_256()
{
int ret = 0;
EVP_MD_CTX *ctx;
const EVP_MD *md;

//create md context
ctx = EVP_MD_CTX_create();
if(!ctx){
return -1;
}

// initialise the MD to use sha256
md = EVP_sha256();
if (!md) {
return -1;
}

// initialise the digest
ret = EVP_DigestInit_ex(ctx, md, NULL);
if (ret != 1) {
    return -1;
}
//std::string input = "hash function";
//char str[] = "hash function";
char str[] = "abc";
int length = 0;
length = sizeof(str);

// update digest with the given input buffer
ret = EVP_DigestUpdate(ctx, str, length);

if (ret != 1) {
    return -1;
}
uint8_t sha256_hash[32];
int sha256_hashlen = 0;

// calculate the final hash
ret = EVP_DigestFinal_ex(ctx, sha256_hash, (unsigned int *)&sha256_hashlen);
if (ret != 1) {
    return -1;
}

// dump the final hash
int i;
printf("sha256: ");
for (i = 0; i < sha256_hashlen; i ++) {
    printf("%0x", sha256_hash[i]);
}
printf("\n");

// destroy md context
EVP_MD_CTX_destroy(ctx);

}

int main(int argc, char **argv)
{
printf("inside main function====\n");
sha_256();
return 0;
}

output :
inside main function====
sha256: dc1114cd74914bd872cc1f9a23ec910ea223bc79779ab2e17da25782a624fc

Excpected output is: ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad

Thanks
Rohith

[t8m]

The length = sizeof(str); means that you're including the terminating '\0' character in the digest of the string. Is this something you really wanted?

[rdiwan2023]

sorry, I didn't get what you're asking. I am passing the length of the array to EVP_DigestUpdate(ctx, str, length);

[t8m]

Should you, in your code, use length = sizeof(str) - 1; or length = strlen(str); instead?

[nhorman]

@t8m is correct, sizeof(str) returns 4 (as it includes the NULL terminator on the char array, whereas strlen(str) would return 3. If you run this code replacing sizeof(str) with strlen(str), the expected sha256 output is produced

[rdiwan2023]

Thanks @t8m & @nhorman , by making length = sizeof(str) - 1, expected digest is produced.