C++17 implementation for one-time passwords. It provides HMAC-based and time-based implementations of one-time passwords.

generate-otp.cpp

// Compile: g++ generate-otp.cpp --std=c++17 -Wall -Wextra -o otp_generator
// Run: ./otp_generator

#ifndef ONE_TIME_PASSWORD_H
#define ONE_TIME_PASSWORD_H

#include <string>
#include <vector>

class OneTimePassword
{
private:
    std::string secret_key;
    static const std::string base32_chars;
    static const uint32_t SHA1_K[];
    static const size_t SHA1_BLOCK_SIZE;

protected:
    std::vector<uint8_t> base32_decode(const std::string &input);
    void sha1_transform(uint32_t *state, const uint8_t *data);
    std::vector<uint8_t> sha1(const std::vector<uint8_t> &data);
    std::vector<uint8_t> hmac_sha1(const std::vector<uint8_t> &key, const std::vector<uint8_t> &data);
    uint32_t truncate(const std::vector<uint8_t> &hash);

    OneTimePassword();
    OneTimePassword(const std::string &key);
    virtual ~OneTimePassword() = default;

public:
    void set_secret_key(const std::string &key);
    std::string get_secret_key() const;
};

#include <chrono>
#include <iomanip>
#include <sstream>

const std::string OneTimePassword::base32_chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567";
const uint32_t OneTimePassword::SHA1_K[] = {0x5A827999, 0x6ED9EBA1, 0x8F1BBCDC, 0xCA62C1D6};
const size_t OneTimePassword::SHA1_BLOCK_SIZE = 64; // SHA-1 processes in 512-bit blocks (64 bytes)

// SHA-1 helper functions
#define ROTL(n, x) (((x) << (n)) | ((x) >> (32 - (n))))
#define F0(b, c, d) (((b) & (c)) | (~(b) & (d)))
#define F1(b, c, d) ((b) ^ (c) ^ (d))
#define F2(b, c, d) (((b) & (c)) | ((b) & (d)) | ((c) & (d)))
#define F3(b, c, d) ((b) ^ (c) ^ (d))

std::vector<uint8_t> OneTimePassword::base32_decode(const std::string &input)
{
    std::vector<uint8_t> output;
    size_t estimated_size = (input.length() * 5) / 8 + 1; // +1 for rounding up if necessary
    output.reserve(estimated_size);
    int val = 0;
    int valb = -8;

    for (unsigned char c : input)
    {
        if (base32_chars.find(c) == std::string::npos)
            continue;
        val = (val << 5) + base32_chars.find(c);
        valb += 5;
        if (valb >= 0)
        {
            output.push_back((val >> valb) & 0xFF);
            valb -= 8;
        }
    }

    return output;
}

void OneTimePassword::sha1_transform(uint32_t *state, const uint8_t *data)
{
    uint32_t W[80];
    uint32_t A, B, C, D, E;
    uint32_t temp;

    for (int i = 0; i < 16; ++i)
    {
        W[i] = (data[i * 4] << 24) | (data[i * 4 + 1] << 16) | (data[i * 4 + 2] << 8) | (data[i * 4 + 3]);
    }

    for (int i = 16; i < 80; ++i)
    {
        W[i] = ROTL(1, W[i - 3] ^ W[i - 8] ^ W[i - 14] ^ W[i - 16]);
    }

    A = state[0];
    B = state[1];
    C = state[2];
    D = state[3];
    E = state[4];

    for (int i = 0; i < 80; ++i)
    {
        if (i < 20)
        {
            temp = ROTL(5, A) + F0(B, C, D) + E + W[i] + SHA1_K[0];
        }
        else if (i < 40)
        {
            temp = ROTL(5, A) + F1(B, C, D) + E + W[i] + SHA1_K[1];
        }
        else if (i < 60)
        {
            temp = ROTL(5, A) + F2(B, C, D) + E + W[i] + SHA1_K[2];
        }
        else
        {
            temp = ROTL(5, A) + F3(B, C, D) + E + W[i] + SHA1_K[3];
        }

        E = D;
        D = C;
        C = ROTL(30, B);
        B = A;
        A = temp;
    }

    state[0] += A;
    state[1] += B;
    state[2] += C;
    state[3] += D;
    state[4] += E;
}

std::vector<uint8_t> OneTimePassword::sha1(const std::vector<uint8_t> &data)
{
    uint32_t state[5] = {0x67452301,
                         0xEFCDAB89,
                         0x98BADCFE,
                         0x10325476,
                         0xC3D2E1F0};

    size_t original_size = data.size() * 8;
    size_t padded_size = data.size() + 1;

    if ((padded_size + 8) % SHA1_BLOCK_SIZE != 0)
    {
        padded_size += (SHA1_BLOCK_SIZE - ((padded_size + 8) % SHA1_BLOCK_SIZE));
    }

    std::vector<uint8_t> padded_data(padded_size + 8);

    std::copy(data.begin(), data.end(), padded_data.begin());
    padded_data[data.size()] = 0x80;

    std::fill(padded_data.begin() + data.size() + 1, padded_data.end() - 8, 0x00);

    for (int i = 0; i < 8; ++i)
    {
        padded_data[padded_size + i] = (original_size >> (56 - i * 8)) & 0xFF;
    }

    for (size_t i = 0; i < padded_data.size(); i += SHA1_BLOCK_SIZE)
    {
        sha1_transform(state, padded_data.data() + i);
    }

    std::vector<uint8_t> hash(20);
    for (int i = 0; i < 5; ++i)
    {
        hash[i * 4] = (state[i] >> 24) & 0xFF;
        hash[i * 4 + 1] = (state[i] >> 16) & 0xFF;
        hash[i * 4 + 2] = (state[i] >> 8) & 0xFF;
        hash[i * 4 + 3] = state[i] & 0xFF;
    }

    return hash;
}

std::vector<uint8_t> OneTimePassword::hmac_sha1(const std::vector<uint8_t> &key, const std::vector<uint8_t> &data)
{
    std::vector<uint8_t> key_padded(SHA1_BLOCK_SIZE, 0x00);

    if (key.size() > SHA1_BLOCK_SIZE)
    {
        std::vector<uint8_t> key_hash = sha1(key);
        std::copy(key_hash.begin(), key_hash.end(), key_padded.begin());
    }
    else
    {
        std::copy(key.begin(), key.end(), key_padded.begin());
    }

    std::vector<uint8_t> o_key_pad(SHA1_BLOCK_SIZE);
    std::vector<uint8_t> i_key_pad(SHA1_BLOCK_SIZE);

    for (size_t i = 0; i < SHA1_BLOCK_SIZE; ++i)
    {
        o_key_pad[i] = key_padded[i] ^ 0x5C;
        i_key_pad[i] = key_padded[i] ^ 0x36;
    }

    std::vector<uint8_t> inner_hash_input;
    inner_hash_input.reserve(SHA1_BLOCK_SIZE + data.size());
    inner_hash_input.insert(inner_hash_input.end(), i_key_pad.begin(), i_key_pad.end());
    inner_hash_input.insert(inner_hash_input.end(), data.begin(), data.end());

    std::vector<uint8_t> inner_hash = sha1(inner_hash_input);

    std::vector<uint8_t> outer_hash_input;
    outer_hash_input.reserve(SHA1_BLOCK_SIZE + inner_hash.size());
    outer_hash_input.insert(outer_hash_input.end(), o_key_pad.begin(), o_key_pad.end());
    outer_hash_input.insert(outer_hash_input.end(), inner_hash.begin(), inner_hash.end());

    // Calculate final hash
    return sha1(outer_hash_input);
}

uint32_t OneTimePassword::truncate(const std::vector<uint8_t> &hash)
{
    int offset = hash.back() & 0x0F;
    uint32_t binary = ((hash[offset] & 0x7F) << 24) |
                      ((hash[offset + 1] & 0xFF) << 16) |
                      ((hash[offset + 2] & 0xFF) << 8) |
                      (hash[offset + 3] & 0xFF);
    return binary;
}

void OneTimePassword::set_secret_key(const std::string &key)
{
    secret_key = key;
}

std::string OneTimePassword::get_secret_key() const
{
    return secret_key;
}

OneTimePassword::OneTimePassword() : secret_key("") {}

OneTimePassword::OneTimePassword(const std::string &key) : secret_key(key) {}

#endif // !ONE_TIME_PASSWORD_H

#ifndef HMAC_BASED_ONE_TIME_PASSWORD_H
#define HMAC_BASED_ONE_TIME_PASSWORD_H

class HMACbasedOneTimePassword : public OneTimePassword
{
private:
    uint64_t counter;

public:
    std::string generate_hotp();
    void set_counter(uint64_t counter);
    uint64_t get_counter() const;

    HMACbasedOneTimePassword();
    HMACbasedOneTimePassword(const std::string &key);
    HMACbasedOneTimePassword(const std::string &key, uint64_t starting_point);
};

std::string HMACbasedOneTimePassword::generate_hotp()
{
    std::vector<uint8_t> key = base32_decode(get_secret_key());
    uint64_t counter_value = get_counter();

    std::vector<uint8_t> counter_data(8);
    for (int i = 0; i < 8; ++i)
    {
        counter_data[7 - i] = counter_value & 0xFF;
        counter_value >>= 8;
    }

    std::vector<uint8_t> hash = hmac_sha1(key, counter_data);
    uint32_t code = truncate(hash) % 1000000;

    std::ostringstream otp;
    otp << std::setw(6) << std::setfill('0') << code;

    set_counter(get_counter() + 1);

    return otp.str();
}

void HMACbasedOneTimePassword::set_counter(uint64_t counter)
{
    this->counter = counter;
}

uint64_t HMACbasedOneTimePassword::get_counter() const
{
    return counter;
}

HMACbasedOneTimePassword::HMACbasedOneTimePassword() : OneTimePassword(""), counter(0) {}
HMACbasedOneTimePassword::HMACbasedOneTimePassword(const std::string &key) : OneTimePassword(key), counter(0) {}
HMACbasedOneTimePassword::HMACbasedOneTimePassword(const std::string &key, uint64_t starting_point) : OneTimePassword(key), counter(starting_point) {}

#endif //! HMAC_BASED_ONE_TIME_PASSWORD_H

#ifndef TIME_BASE_ONE_TIME_PASSWORD_H
#define TIME_BASE_ONE_TIME_PASSWORD_H
class TimebasedOneTimePassword : protected HMACbasedOneTimePassword
{
public:
    std::string generate_totp();
    std::vector<std::string> generate_totps(int window = 10);

    TimebasedOneTimePassword();
    TimebasedOneTimePassword(const std::string &key);
};

std::string TimebasedOneTimePassword::generate_totp()
{
    auto time_step = std::chrono::system_clock::now().time_since_epoch();
    long current_time = std::chrono::duration_cast<std::chrono::seconds>(time_step).count() / 30;
    set_counter(current_time);
    return generate_hotp();
}

std::vector<std::string> TimebasedOneTimePassword::generate_totps(int window)
{
    auto time_step = std::chrono::system_clock::now().time_since_epoch();
    long current_time = std::chrono::duration_cast<std::chrono::seconds>(time_step).count() / 30;
    std::vector<std::string> results;
    results.reserve(window);

    for (int i = -window; i <= window; ++i)
    {
        long time_interval = current_time + i;
        set_counter(time_interval);
        std::string otp_str = generate_hotp();
        results.push_back(otp_str);
    }

    return results;
}
TimebasedOneTimePassword::TimebasedOneTimePassword() : HMACbasedOneTimePassword("") {}

TimebasedOneTimePassword::TimebasedOneTimePassword(const std::string &key) : HMACbasedOneTimePassword(key) {}

#endif //! TIME_BASE_ONE_TIME_PASSWORD_H

#include <iostream>

int main()
{
    // Example secret key, Use it with Google Authenticator, Microsoft Authenticator, Authy, ..
    std::string secret = "JBSWY3DPEHPK3PXP";
    {
        std::cout << "===============================================" << std::endl;
        std::cout << "Time-based one-time password (TOTP)" << std::endl;
        const int WINDOW = 10;
        TimebasedOneTimePassword otp(secret);

        std::vector<std::string> passwords = otp.generate_totps(WINDOW);
        for (auto password : passwords)
        {
            std::cout << "Generated OTP: " << password << std::endl;
        }
    }

    {
        std::cout << "===============================================" << std::endl;
        std::cout << "HMAC-based One-Time Password (HOTP)" << std::endl;
        const int MAX_OTP = 10;
        const uint64_t COUNTER = 0;
        HMACbasedOneTimePassword otp(secret, COUNTER);
        for (size_t i = 0; i != MAX_OTP; ++i)
        {
            std::string password = otp.generate_hotp();
            std::cout << "Generated OTP: " << password << std::endl;
        }
    }
    return 0;
}

C++17 implementation for one-time passwords. It provides HMAC-based and time-based implementations of one-time passwords. To use it, just compile it with:

// Compile: g++ generate-otp.cpp --std=c++17 -Wall -Wextra -o otp_generator
// Run: ./otp_generator

Insert the secret key into the authenticator apps (Google, Microsoft, Authy) to use it.