HMAC with OpenSSL

hmac.cc

#include <iomanip>
#include <iostream>
#include <string>
#include <vector>

#include <openssl/hmac.h>

int main()
{
  // https://tools.ietf.org/html/rfc2104#section-3
  // [...] L the byte-length of hash outputs (L=16 for MD5, L=20 for SHA-1)
  // [...] The key for HMAC can be of any length (keys longer than B bytes are first hashed using H).
  // However, less than L bytes is strongly discouraged as it would decrease the security strength of the function.
  // Keys longer than L bytes are acceptable but the extra length would not significantly increase the function strength.
  const auto key = std::string{"AVERYGOODKEYAVERYGOODKEYAVERYGOODKEY"};
  const auto data = std::string{"DATA"};

  auto digest = std::vector<unsigned char>(EVP_MAX_MD_SIZE);
  auto digest_len = std::uint32_t{0};
  HMAC(
    EVP_sha1(),
    reinterpret_cast<const unsigned char*>(key.data()), key.size(),
    reinterpret_cast<const unsigned char*>(data.data()), data.size(),
    digest.data(), &digest_len
  );

  // Decimal output.
  for (auto i = 0ul; i < digest_len; ++i)
  {
    std::cout << +digest[i] << ' ';
  }
  std::cout << '\n';

  // Hexadecimal output.
  for (auto i = 0ul; i < digest_len; ++i)
  {
    std::cout << std::hex << std::setw(2) << std::setfill('0') << +digest[i] << ' ';
  }
  std::cout << '\n';

  return 0;
}