/// <summary>
/// Takes a secret key, a time step and a hash mode to compute a TOTP code
/// </summary>
/// <param name="secretKey">The secret key</param>
/// <param name="timeStep">The time step</param>
/// <param name="hashMmode">The hash mode to use</param>
/// <returns>TOTP calculated code</returns>
private string Compute(byte[] secretKey, long timeStep, OtpHashModeEnum hashMmode)
{
var bigEndianTimeStep = Utils.Utils.GetBigEndianBytes(timeStep);
var otp = this.CalculateOtp(secretKey, bigEndianTimeStep, hashMmode);
return(Utils.Utils.Digits(otp, this.totpSize));
}
/// <summary>
/// Takes a secretKey and a timestamp and then computes a TOTP(Time based one time password) value
/// </summary>
/// <param name="secretKey">The secretKey to use for the TOTP calculation</param>
/// <param name="timestamp">The timestamp to use for the TOTP calculation</param>
/// <param name="hashMode">The hash mode to use for the TOTP calculation</param>
/// <returns>a TOTP value</returns>
public string ComputeTotp(byte[] secretKey, DateTime timestamp, OtpHashModeEnum hashMode = OtpHashModeEnum.Sha1)
{
DateTime univDateTime = timestamp.ToUniversalTime();
var timeStep = CalculateTimeStepFromTimestamp(univDateTime);
return(this.Compute(secretKey, timeStep, hashMode));
}
/// <summary>
/// Uses the key to get an HMAC using the specified algorithm and time step
/// </summary>
/// <param name="secretKey">The secret key used to compute the HMAC</param>
/// <param name="timeStep">The time step used to compute the HMAC</param>
/// <param name="hashMode">The HMAC algorithm to use</param>
public static byte[] ComputeHmac(byte[] secretKey, byte[] timeStep, OtpHashModeEnum hashMode)
{
byte[] hashedValue = null;
using (HMAC hmac = CreateHmacHash(hashMode))
{
hmac.Key = secretKey;
hashedValue = hmac.ComputeHash(timeStep);
}
return(hashedValue);
}
/// <summary>
/// Helper method that calculates OTPs
/// </summary>
/// <param name="secretKey">The secret key</param>
/// <param name="data">The secret key</param>
private long CalculateOtp(byte[] secretKey, byte[] timeStep, OtpHashModeEnum mode)
{
byte[] hmacComputedHash = Utils.Utils.ComputeHmac(secretKey, timeStep, mode);
int offset = hmacComputedHash[hmacComputedHash.Length - 1] & 0x0f;
return((hmacComputedHash[offset] & 0x7f) << 24
| (hmacComputedHash[offset + 1] & 0xff) << 16
| (hmacComputedHash[offset + 2] & 0xff) << 8
| (hmacComputedHash[offset + 3] & 0xff) % 1000000);
}
public void ComputeTOTPTest(string secret, OtpHashModeEnum hashMode, long timestamp, string expectedOtp)
{
//Arrange
var timeBasedOtpService = this.ServiceProvider.GetService <ITimeBasedOtp>();
DateTime time = DateTimeOffset.FromUnixTimeSeconds(timestamp).DateTime;
//Act
var otp = timeBasedOtpService.ComputeTotp(Encoding.UTF8.GetBytes(secret), time, hashMode);
//Assert
Assert.AreEqual(otp, expectedOtp);
}
/// <summary>
/// Create an HMAC object for the specified algorithm
/// </summary>
private static HMAC CreateHmacHash(OtpHashModeEnum otpHashMode)
{
HMAC hmacAlgorithm = null;
switch (otpHashMode)
{
case OtpHashModeEnum.Sha256:
hmacAlgorithm = new HMACSHA256();
break;
case OtpHashModeEnum.Sha512:
hmacAlgorithm = new HMACSHA512();
break;
default: //case OtpHashMode.Sha1:
hmacAlgorithm = new HMACSHA1();
break;
}
return(hmacAlgorithm);
}
