Simple Linux PAM module for OTP authentication

pam_dotp.c

#include <security/pam_modules.h>
#include <security/pam_appl.h>
#include <sys/types.h>
#include <gcrypt.h>
#include <stdio.h>
#include <string.h>
#include <time.h>
#define SERVER_SECRET "MY_SECRET"

void gen_otp (char **otp) {
  time_t unixtime;
  time(&unixtime);
  int otp_time = (int)unixtime - (int)unixtime % 60;

  char buffer[1024];
  snprintf(buffer, 1024, "%d+%s", otp_time, SERVER_SECRET);

  gcry_md_hd_t md_handle = NULL;
  gcry_md_open(&md_handle, GCRY_MD_SHA256, GCRY_MD_FLAG_SECURE);
  gcry_md_write(md_handle, buffer, strlen(buffer));

  const unsigned char *digest = gcry_md_read(md_handle, GCRY_MD_SHA256);
  int len = gcry_md_get_algo_dlen(GCRY_MD_SHA256);

  *otp = malloc(sizeof(2*len)+8);
  int i;
  for (i = 0; i < len; i++)
    snprintf(*otp+2*i, 3, "%02x", digest[i]);
}

PAM_EXTERN int pam_sm_authenticate (pam_handle_t *pamh, int flags, int argc, const char **argv) {
  int pam_ret = PAM_AUTH_ERR;

  char *password = NULL;
  pam_get_authtok(pamh, PAM_AUTHTOK, (const void **) &password), NULL);
  password = strdup(password);

  char *otp = NULL;
  gen_otp(&otp);

  return (strncmp(password, otp, 5) == 0) ? PAM_SUCCESS : PAM_AUTH_ERR;
}

PAM_EXTERN int pam_sm_setcred (pam_handle_t *pamh, int flags, int argc, const char **argv) {
  return PAM_SUCCESS;
}