Hash to Bignum - unsigned char[] to char *

Question

I need to export the current hash value into a BIGNUM. I am using OpenSSL in C. When I try to do this operation and I print the content of h with BN_print I have 0 as output. As following the code snippet:

BIGNUM *h;
h = BN_new();
unsigned char hash[EVP_MAX_MD_SIZE];
unsigned hash_size = EVP_MAX_MD_SIZE;
EVP_DigestFinal_ex(hash_ctx, hash, &hash_size);
EVP_MD_CTX_free(hash_ctx);

// printing
for (unsigned n = 0; n < hash_size; n++) {
    printf("%02hhx", hash[n]);
}

BN_hex2bn(&h, hash);
printf("\n");
BN_print(bio_out, h);

The function BN_hex2bn(BIGNUM **a, const char *str); accepts in input a pointer to a BIGNUM and a pointer to the a const char. Do I need to convert it? If, yes, what could be the best approach?

Answer 1

BN_hex2bn takes the textual representation of the value. You must convert the hash to a textual representation in hex:

unsigned char hash[EVP_MAX_MD_SIZE];
unsigned hash_size = EVP_MAX_MD_SIZE;
EVP_DigestFinal_ex(hash_ctx, hash, &hash_size);
EVP_MD_CTX_free(hash_ctx);

// hex conversion
unsigned char hash_text[2 * EVP_MAX_MD_SIZE + 1];
static char const hexdigits[16] = "0123456789abcdef";
for (unsigned n = 0; n < hash_size; n++) {
    hash_text[2 * n + 0] = hexdigits[hash[n] >> 4];
    hash_text[2 * n + 1] = hexdigits[hash[n] & 15];
}
hash_text[2 * hash_size] = '\0';

// printing
printf("%s\n", hash_text);

// BIGNUM conversion
BIGNUM *h = BN_new();
BN_hex2bn(&h, hash);
BN_print(bio_out, h);

You can also initialize the BIGNUM directly from big endian binary data with BN_bin2bn:

// BIGNUM conversion
BIGNUM *h = BN_new();
BN_bin2bn(hash, hash_size, h);
BN_print(bio_out, h);

Or simpler:

// BIGNUM conversion
BIGNUM *h = BN_bin2bn(hash, hash_size, NULL);
BN_print(bio_out, h);