/// <summary>
/// Uses the procedure defined in RFC 4226 section 7.5 to derive a key from the master key
/// </summary>
/// <param name="masterKey">The master key from which to derive a device specific key</param>
/// <param name="publicIdentifier">The public identifier that is unique to the authenticating device</param>
/// <param name="mode">The hash mode to use. This will determine the resulting key lenght. The default is sha-1 (as per the RFC) which is 20 bytes</param>
/// <returns>Derived key</returns>
public static byte[] DeriveKeyFromMaster(IKeyProvider masterKey, byte[] publicIdentifier, OtpHashMode mode = OtpHashMode.Sha1)
{
if (masterKey == null)
{
throw new ArgumentNullException("masterKey");
}
return(masterKey.ComputeHmac(mode, publicIdentifier));
}
/// <summary>
/// Helper method that calculates OTPs
/// </summary>
protected internal long CalculateOtp(byte[] data, OtpHashMode mode)
{
var hmacComputedHash = SecretKey.ComputeHmac(mode, data);
// The RFC has a hard coded index 19 in this value.
// This is the same thing but also accomodates SHA256 and SHA512
// hmacComputedHash[19] => hmacComputedHash[hmacComputedHash.Length - 1]
var 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);
}
/// <summary>
/// Uses the procedure defined in RFC 4226 section 7.5 to derive a key from the master key
/// </summary>
/// <param name="masterKey">The master key from which to derive a device specific key</param>
/// <param name="publicIdentifier">The public identifier that is unique to the authenticating device</param>
/// <param name="mode">The hash mode to use. This will determine the resulting key lenght. The default is sha-1 (as per the RFC) which is 20 bytes</param>
/// <returns>Derived key</returns>
public static byte[] DeriveKeyFromMaster(IKeyProvider masterKey, byte[] publicIdentifier, OtpHashMode mode = OtpHashMode.Sha1)
{
if (masterKey == null)
throw new ArgumentNullException("masterKey");
return masterKey.ComputeHmac(mode, publicIdentifier);
}