Re: TOTP implementation

Shouldn't...

*hmac.Update(byteArray, sizeof(byteArraySize));*

be simply..

*hmac.Update(byteArray, byteArraySize);
*

Otherwise you passing in sizeof(size_t) rather than the size of the 
byteArray (which is sizeof(uint64_t)). Although on a 64 bit system I 
would expect them both to be 8 bytes, so maybe there is another problem.

Otherwise the function looks very similar to what I have.

- Steven

On 03/08/2023 10:51, Aravindh B wrote:
> Need to get some information how counter value is handled in this library.
>
> std::string generateTOTP(const std::string& secretKey) {
>
> using namespace CryptoPP;
> const int timeStep = 30; // Time step in seconds
> SecByteBlock key(HMAC<SHA1>::DEFAULT_KEYLENGTH);
> StringSource(secretKey, true, new Base32Decoder(new ArraySink(key, 
> key.size())));
>
> // Get the current time in 30-second intervals (TOTP time step)
> std::time_t currentTime = std::time(nullptr);
> uint64_t counter = static_cast<uint64_t>(currentTime) / timeStep;
> const size_t byteArraySize = sizeof(counter);
> byte* byteArray = new byte[byteArraySize];
>
> //Big endian representation
> for (size_t i = 0; i < byteArraySize; ++i) {
> byteArray[sizeof(counter) - 1 - i] = static_cast<byte>((counter >> (8 
> * i)) & 0xFF);
> }
>
> // Calculate the HMAC-SHA1 using the secret key and the counter bytes
> byte mac[CryptoPP::HMAC<CryptoPP::SHA1>::DIGESTSIZE];
> CryptoPP::HMAC<CryptoPP::SHA1> hmac(key, sizeof(key));
> *hmac.Update(byteArray, sizeof(byteArraySize));
> *hmac.Final(mac);
>
> // Generate the TOTP value from the last 4 bits of the HMAC-SHA1 result
> int offset = mac[CryptoPP::HMAC<CryptoPP::SHA1>::DIGESTSIZE - 1] & 0xF;
> uint32_t otpValue = (mac[offset] & 0x7F) << 24 |
> (mac[offset + 1] & 0xFF) << 16 |
> (mac[offset + 2] & 0xFF) << 8 |
> (mac[offset + 3] & 0xFF);
>
> // Convert the OTP value to a 6-digit OTP (modulo 10^6)
> otpValue %= 1000000;
>
> //Format the OTP as a 6-digit string with leading zeros if needed
> std::string otp = std::to_string(otpValue);
> otp.insert(otp.begin(), 6 - otp.size(), '0');
> delete[] byteArray;
>
> return otp;
> }
>
> This code have issues with hmac.update(). Seems problem with counter 
> value.Does anyone help to generate correct otp?
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