Hash of Multiple BIGNUM

Question

I need to do the hash of several BIGNUM in C/C++ by using OpenSSL.

If I assume that I have 10 BIGNUM, i.e. n1, n2, ..., n10 and I want to compute the hash of them as H(n1||n2||...||n10), is there a fast way to do it?

I would like to avoid to convert one by one into string, copy in a buffer and after that do the hash. Any suggestion/recommendation?

Answer 1

Instead of converting the bignums to strings, you can get a byte representation with BN_bn2bin(), and feed those byte arrays to the hash function sequentially to get the final hash of all values.

Example:

#include <stdio.h>
#include <stdlib.h>

#include <openssl/bn.h>
#include <openssl/evp.h>

int main(void) {
  BIGNUM *bns[3];
  bns[0] = BN_new();
  bns[1] = BN_new();
  bns[2] = BN_new();
  BN_set_word(bns[0], 10);
  BN_set_word(bns[1], 50);
  BN_set_word(bns[2], 150);

  EVP_MD_CTX *hash_ctx = EVP_MD_CTX_new();
  EVP_DigestInit_ex(hash_ctx, EVP_sha256(), NULL);

  unsigned char *bytes = NULL;
  for (int n = 0; n < 3; n++) {
    size_t size = BN_num_bytes(bns[n]);
    bytes = realloc(bytes, size);
    BN_bn2bin(bns[n], bytes);
    EVP_DigestUpdate(hash_ctx, bytes, size);
  }
  free(bytes);

  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);

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

  return 0;
}

(Real code should include error checking, of course.)