using System; using System.Linq; using System.Security.Cryptography; namespace InABox.Core { public static class Authenticator { public static readonly int CODE_LENGTH = 6; private static readonly int CODE_MODULO = (int)Math.Pow(10, CODE_LENGTH); private static byte[] FromHexString(string hex) { if (hex.Length % 2 == 1) throw new Exception("The binary key cannot have an odd number of digits"); byte[] arr = new byte[hex.Length >> 1]; for (int i = 0; i < hex.Length >> 1; ++i) { arr[i] = (byte)((GetHexVal(hex[i << 1]) << 4) + (GetHexVal(hex[(i << 1) + 1]))); } return arr; } public static int GetHexVal(char hex) { int val = (int)hex; //For uppercase A-F letters: //return val - (val < 58 ? 48 : 55); //For lowercase a-f letters: //return val - (val < 58 ? 48 : 87); //Or the two combined, but a bit slower: return val - (val < 58 ? 48 : (val < 97 ? 55 : 87)); } public static string GenerateGoogleAuthenticatorCode(byte[] key) { var _time = DateTimeOffset.Now.ToUnixTimeSeconds(); return GenerateGoogleAuthenticatorCode(_time, key); } private static string GenerateGoogleAuthenticatorCode(long time, byte[] key) { var _window = time / 30; var _hex = _window.ToString("x"); if (_hex.Length < 16) { _hex = _hex.PadLeft(16, '0'); } var _bytes = FromHexString(_hex); var _hash = new HMACSHA1(key).ComputeHash(_bytes); var _offset = _hash.Last() & 0xf; var _selected = new byte[4]; Buffer.BlockCopy(_hash, _offset, _selected, 0, 4); if (BitConverter.IsLittleEndian) { Array.Reverse(_selected); } var _integer = BitConverter.ToInt32(_selected, 0); var _truncated = _integer & 0x7fffffff; return (_truncated % CODE_MODULO).ToString().PadLeft(CODE_LENGTH, '0'); } public static bool CheckAuthenticationCode(byte[] key, string code) { var _time = DateTimeOffset.Now.ToUnixTimeSeconds(); for (long _l = _time - 30; _l <= _time; _l += 30) { if(GenerateGoogleAuthenticatorCode(_l, key) == code) { return true; } } return false; } } }