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/**
 * @brief Generate a one-time password.
 *
 * This method generates a one-time password as specified by RFC
 * 4226. It uses a pre-shared secret and a counter to generate a
 * unique code.
 *
 * @param secret The preshared secret.
 * @param secret_len The length of the preshared secret.
 * @param counter An 8-byte counter that changes after every
 *                login attempt. Should be synchronised between
 *                client and server.
 * @param length Length of the produced one-time password. Must
 *               be at least 6.
 * @param buf A buffer to store the OTP in. Must be at least
 *            length + 1 bytes.
 *
 * @return The one-time password as an ASCII string.
 */
char *mtotp_hotp(const char *secret, size_t secret_len, uint64_t counter, int length, char *buf);

char *mtotp_hotp(const char *secret, size_t secret_len, uint64_t counter, int length, char *buf) {
  assert(length >= 6);
  counter = htobe64(counter);
  uint8_t hmac[20];
  hmac_sha1((const uint8_t *)secret, secret_len, (const uint8_t *)&counter,
            sizeof(counter), hmac);

  uint32_t code = mod(dynamic_truncate(hmac), length);

  pad_otp(code, length, buf);

  return buf;
}

minitotp $ mtotp -t0 '123'
57074492
minitotp $ mtotp -t0 '456'
57074492