//-------------------------------------------------------------------------------------------------
//--- Check if a provided OTP is valid for the provided GUID (allow 3 past + 3 future ones)
//-------------------------------------------------------------------------------------------------
public static async Task <bool> IsValidOTPFromGuid(Guid TheGuid, int TheOTPToCheck, bool UseNetworkTime = false)
{
bool success = false;
int myNumberOfOTPs = 7;
Int64 myTimeStamp;
byte[] mySecret;
byte[] myHmac;
byte[] myData;
int myOffset;
int myOneTimePassword;
Int64 MyUnixTimestamp;
int[] myAllowedOTPArray = new int[myNumberOfOTPs];
mySecret = StringToBytes(TheGuid.ToString("N"));
#if WINDOWS_UWP
var myCryptprovider = MacAlgorithmProvider.OpenAlgorithm(MacAlgorithmNames.HmacSha1);
#else
HMACSHA1 myCryptprovider = new HMACSHA1(mySecret);
#endif
MyUnixTimestamp = await GetUnixTimestamp(UseNetworkTime);
myTimeStamp = Convert.ToInt64(MyUnixTimestamp / 30);
for (int i = 0; i < myNumberOfOTPs; i++)
{
//current timestamp = myTimeStamp(-0)
//So we allow the previous 3 values and future 3 values as well
myData = BitConverter.GetBytes(myTimeStamp + i - 3).Reverse().ToArray();
#if WINDOWS_UWP
var myBuffer = CryptographicBuffer.CreateFromByteArray(myData);
var myKeyBuffer = CryptographicBuffer.CreateFromByteArray(mySecret);
var myKey = myCryptprovider.CreateKey(myKeyBuffer);
var mySignedBuffer = CryptographicEngine.Sign(myKey, myBuffer);
CryptographicBuffer.CopyToByteArray(mySignedBuffer, out myHmac);
#else
myHmac = new HMACSHA1(mySecret).ComputeHash(myData);
#endif
myOffset = myHmac.Last() & 0x0F;
myOneTimePassword = (
((myHmac[myOffset + 0] & 0x7f) << 24) |
((myHmac[myOffset + 1] & 0xff) << 16) |
((myHmac[myOffset + 2] & 0xff) << 8) |
(myHmac[myOffset + 3] & 0xff)
) % 1000000;
myAllowedOTPArray[i] = myOneTimePassword;
}
//do the check
int index = 0;
while ((index < myNumberOfOTPs) & (!success))
{
success = myAllowedOTPArray[index] == TheOTPToCheck;
index++;
}
return(success);
}
