RemainingSeconds is the same for all codes in same time step.

[capegreg]

Why does generating a new code within the same time step but with a different key not have a new RemainingSeconds?

If I call GetTotp two times with a step of 9 minutes, the second call has a RemainingSeconds of the first call.

GetTotp takes a byte[] and count, 1, 2, 3, etc. the byte[] key is a hashed salt and count which generates a new code. I found that I had to make the key more unique than the previous call to GetTotp because calling GetTotp again (like when user wants a new key) within the 9 minute step just returned the same code.

Example. time step is 540 seconds.

GetTotp(salt, 1);
"309960"
RemainingSeconds 539

wait some seconds and call again...

GetTotp(salt, 2);
"718933"
RemainingSeconds 460

I expected RemainingSeconds to be closer to 539

        internal string GetTotp(byte[] salt, int count)
        {
            string saltedKey = HashSomeStringWithSalt(salt, count.ToString());
            byte[] key = Convert.FromBase64String(saltedKey);

            var totp = new Totp(key, step: 540, mode: OtpHashMode.Sha256, totpSize: 6);

            var dateTimeNow = DateTime.UtcNow;
            var totpCode = totp.ComputeTotp(dateTimeNow);
            return totpCode;
        }


        internal int GetOTPRemainingSeconds(byte[] salt, int count)
        {
            string saltedKey = HashSomeStringWithSalt(salt, count.ToString());
            byte[] key = Convert.FromBase64String(key);
            var totp = new Totp(key, step: 540, mode: OtpHashMode.Sha256, totpSize: 6);

            return totp.RemainingSeconds();
        }

[kspearrin]

See RemainingSeconds implementation https://github.com/kspearrin/Otp.NET/blob/master/src/Otp.NET/Totp.cs#L181-L200