Google Authenticator One-time Password Algorithm in C#

Question

I would like to calculate the OTP password generated via Google Authenticator or similar app in C#.

I have found some Javascript implementations and Python but not for C#:

http://blog.tinisles.com/2011/10/google-authenticator-one-time-password-algorithm-in-javascript/

https://stackoverflow.com/a/8549884/3850405

Has anyone done this in C# or should I just try to convert the javascript code to C#?

Useful links:

https://tools.ietf.org/html/rfc6238

https://tools.ietf.org/id/draft-mraihi-totp-timebased-06.html

Answer 1

Found a good library for it here:

https://github.com/kspearrin/Otp.NET

The code was pretty straight forward:

The Base32Encoding class is from this answer:

https://stackoverflow.com/a/7135008/3850405

Example program:

class Program
{
    static void Main(string[] args)
    {
        var bytes = Base32Encoding.ToBytes("JBSWY3DPEHPK3PXP");

        var totp = new Totp(bytes);

        var result = totp.ComputeTotp();
        var remainingTime = totp.RemainingSeconds();
    }
}

Totp:

public class Totp
{
    const long unixEpochTicks = 621355968000000000L;

    const long ticksToSeconds = 10000000L;

    private const int step = 30;

    private const int totpSize = 6;

    private byte[] key;

    public Totp(byte[] secretKey)
    {
        key = secretKey;
    }

    public string ComputeTotp()
    {
        var window = CalculateTimeStepFromTimestamp(DateTime.UtcNow);

        var data = GetBigEndianBytes(window);

        var hmac = new HMACSHA1();
        hmac.Key = key;
        var hmacComputedHash = hmac.ComputeHash(data);

        int offset = hmacComputedHash[hmacComputedHash.Length - 1] & 0x0F;
        var otp = (hmacComputedHash[offset] & 0x7f) << 24
               | (hmacComputedHash[offset + 1] & 0xff) << 16
               | (hmacComputedHash[offset + 2] & 0xff) << 8
               | (hmacComputedHash[offset + 3] & 0xff) % 1000000;

        var result = Digits(otp, totpSize);

        return result;
    }

    public int RemainingSeconds()
    {
        return step - (int)(((DateTime.UtcNow.Ticks - unixEpochTicks) / ticksToSeconds) % step);
    }

    private byte[] GetBigEndianBytes(long input)
    {
        // Since .net uses little endian numbers, we need to reverse the byte order to get big endian.
        var data = BitConverter.GetBytes(input);
        Array.Reverse(data);
        return data;
    }

    private long CalculateTimeStepFromTimestamp(DateTime timestamp)
    {
        var unixTimestamp = (timestamp.Ticks - unixEpochTicks) / ticksToSeconds;
        var window = unixTimestamp / (long)step;
        return window;
    }

    private string Digits(long input, int digitCount)
    {
        var truncatedValue = ((int)input % (int)Math.Pow(10, digitCount));
        return truncatedValue.ToString().PadLeft(digitCount, '0');
    }

}

Base32Encoding:

public static class Base32Encoding
{
    public static byte[] ToBytes(string input)
    {
        if (string.IsNullOrEmpty(input))
        {
            throw new ArgumentNullException("input");
        }

        input = input.TrimEnd('='); //remove padding characters
        int byteCount = input.Length * 5 / 8; //this must be TRUNCATED
        byte[] returnArray = new byte[byteCount];

        byte curByte = 0, bitsRemaining = 8;
        int mask = 0, arrayIndex = 0;

        foreach (char c in input)
        {
            int cValue = CharToValue(c);

            if (bitsRemaining > 5)
            {
                mask = cValue << (bitsRemaining - 5);
                curByte = (byte)(curByte | mask);
                bitsRemaining -= 5;
            }
            else
            {
                mask = cValue >> (5 - bitsRemaining);
                curByte = (byte)(curByte | mask);
                returnArray[arrayIndex++] = curByte;
                curByte = (byte)(cValue << (3 + bitsRemaining));
                bitsRemaining += 3;
            }
        }

        //if we didn't end with a full byte
        if (arrayIndex != byteCount)
        {
            returnArray[arrayIndex] = curByte;
        }

        return returnArray;
    }

    public static string ToString(byte[] input)
    {
        if (input == null || input.Length == 0)
        {
            throw new ArgumentNullException("input");
        }

        int charCount = (int)Math.Ceiling(input.Length / 5d) * 8;
        char[] returnArray = new char[charCount];

        byte nextChar = 0, bitsRemaining = 5;
        int arrayIndex = 0;

        foreach (byte b in input)
        {
            nextChar = (byte)(nextChar | (b >> (8 - bitsRemaining)));
            returnArray[arrayIndex++] = ValueToChar(nextChar);

            if (bitsRemaining < 4)
            {
                nextChar = (byte)((b >> (3 - bitsRemaining)) & 31);
                returnArray[arrayIndex++] = ValueToChar(nextChar);
                bitsRemaining += 5;
            }

            bitsRemaining -= 3;
            nextChar = (byte)((b << bitsRemaining) & 31);
        }

        //if we didn't end with a full char
        if (arrayIndex != charCount)
        {
            returnArray[arrayIndex++] = ValueToChar(nextChar);
            while (arrayIndex != charCount) returnArray[arrayIndex++] = '='; //padding
        }

        return new string(returnArray);
    }

    private static int CharToValue(char c)
    {
        int value = (int)c;

        //65-90 == uppercase letters
        if (value < 91 && value > 64)
        {
            return value - 65;
        }
        //50-55 == numbers 2-7
        if (value < 56 && value > 49)
        {
            return value - 24;
        }
        //97-122 == lowercase letters
        if (value < 123 && value > 96)
        {
            return value - 97;
        }

        throw new ArgumentException("Character is not a Base32 character.", "c");
    }

    private static char ValueToChar(byte b)
    {
        if (b < 26)
        {
            return (char)(b + 65);
        }

        if (b < 32)
        {
            return (char)(b + 24);
        }

        throw new ArgumentException("Byte is not a value Base32 value.", "b");
    }

}