/*
 *
 *  TOTP: Time-Based One-Time Password Algorithm
 *  Copyright (c) 2017, fmount <fmount9@autistici.org>
 *
 *  This software is distributed under MIT License
 *
 *  Compute the hmac using openssl library.
 *  SHA-1 engine is used by default, but you can pass another one,
 *
 *  e.g EVP_md5(), EVP_sha224, EVP_sha512, etc
 *
 */

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>

#include "hmac.h"

char currentHmacResult[HMAC_SHA1_HASH_SIZE];

uint8_t *hmac(unsigned char *key, int kl, uint64_t interval)
{
	hmac_sha1(key, kl, &interval, sizeof(interval), &currentHmacResult);
	return currentHmacResult;
}

uint32_t DT(uint8_t *digest)
{
    uint64_t offset;
    uint32_t bin_code;

#ifdef DEBUG
    char mdString[40];
    for (int i = 0; i < 20; i++)
        sprintf(&mdString[i*2], "%02x", (unsigned int)digest[i]);
    printf("HMAC digest: %s\n", mdString);
#endif

    // dynamically truncates hash
    offset   = digest[19] & 0x0f;

    bin_code = (digest[offset]  & 0x7f) << 24
        | (digest[offset+1] & 0xff) << 16
        | (digest[offset+2] & 0xff) <<  8
        | (digest[offset+3] & 0xff);

    // truncates code to 6 digits
#ifdef DEBUG
    printf("OFFSET: %d\n", offset);
    printf("\nDBC1: %d\n", bin_code);
#endif

    return bin_code;
}

uint32_t mod_hotp(uint32_t bin_code, int digits)
{
    int power = pow(10, digits);
    uint32_t otp = bin_code % power;
    return otp;
}

uint32_t HOTP(uint8_t *key, size_t kl, uint64_t interval, int digits)
{
    uint8_t *digest;
    uint32_t result;
    uint32_t endianness;

#ifdef DEBUG
    printf("KEY IS: %s\n", key);
    printf("KEY LEN IS: %d\n", kl);
    printf("COUNTER IS: %d\n", interval);
#endif

    endianness = 0xdeadbeef;
    if ((*(const uint8_t *)&endianness) == 0xef) {
        interval = ((interval & 0x00000000ffffffff) << 32) | ((interval & 0xffffffff00000000) >> 32);
        interval = ((interval & 0x0000ffff0000ffff) << 16) | ((interval & 0xffff0000ffff0000) >> 16);
        interval = ((interval & 0x00ff00ff00ff00ff) <<  8) | ((interval & 0xff00ff00ff00ff00) >>  8);
    };

    //First Phase, get the digest of the message using the provided key ...
    digest = (uint8_t *)hmac(key, kl, interval);

    //Second Phase, get the dbc from the algorithm
    uint32_t dbc = DT(digest);

    //Third Phase: calculate the mod_k of the dbc to get the correct number
    result = mod_hotp(dbc, digits);

    return result;
}