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Rfc6238AuthenticationService.cs
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Rfc6238AuthenticationService.cs
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// Copyright (c) .NET Foundation. All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
using System;
using System.Diagnostics;
using System.Net;
using System.Security.Cryptography;
using System.Text;
namespace Microsoft.AspNetCore.Identity
{
internal static class Rfc6238AuthenticationService
{
private static readonly TimeSpan _timestep = TimeSpan.FromMinutes(3);
private static readonly Encoding _encoding = new UTF8Encoding(false, true);
#if NETSTANDARD2_0 || NET461
private static readonly DateTime _unixEpoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
private static readonly RandomNumberGenerator _rng = RandomNumberGenerator.Create();
#endif
// Generates a new 80-bit security token
public static byte[] GenerateRandomKey()
{
byte[] bytes = new byte[20];
#if NETSTANDARD2_0 || NET461
_rng.GetBytes(bytes);
#else
RandomNumberGenerator.Fill(bytes);
#endif
return bytes;
}
internal static int ComputeTotp(HashAlgorithm hashAlgorithm, ulong timestepNumber, string modifier)
{
// # of 0's = length of pin
const int Mod = 1000000;
// See https://tools.ietf.org/html/rfc4226
// We can add an optional modifier
var timestepAsBytes = BitConverter.GetBytes(IPAddress.HostToNetworkOrder((long)timestepNumber));
var hash = hashAlgorithm.ComputeHash(ApplyModifier(timestepAsBytes, modifier));
// Generate DT string
var offset = hash[hash.Length - 1] & 0xf;
Debug.Assert(offset + 4 < hash.Length);
var binaryCode = (hash[offset] & 0x7f) << 24
| (hash[offset + 1] & 0xff) << 16
| (hash[offset + 2] & 0xff) << 8
| (hash[offset + 3] & 0xff);
return binaryCode % Mod;
}
private static byte[] ApplyModifier(byte[] input, string modifier)
{
if (String.IsNullOrEmpty(modifier))
{
return input;
}
var modifierBytes = _encoding.GetBytes(modifier);
var combined = new byte[checked(input.Length + modifierBytes.Length)];
Buffer.BlockCopy(input, 0, combined, 0, input.Length);
Buffer.BlockCopy(modifierBytes, 0, combined, input.Length, modifierBytes.Length);
return combined;
}
// More info: https://tools.ietf.org/html/rfc6238#section-4
private static ulong GetCurrentTimeStepNumber()
{
#if NETSTANDARD2_0 || NET461
var delta = DateTime.UtcNow - _unixEpoch;
#else
var delta = DateTimeOffset.UtcNow - DateTimeOffset.UnixEpoch;
#endif
return (ulong)(delta.Ticks / _timestep.Ticks);
}
public static int GenerateCode(byte[] securityToken, string modifier = null)
{
if (securityToken == null)
{
throw new ArgumentNullException(nameof(securityToken));
}
// Allow a variance of no greater than 9 minutes in either direction
var currentTimeStep = GetCurrentTimeStepNumber();
using (var hashAlgorithm = new HMACSHA1(securityToken))
{
return ComputeTotp(hashAlgorithm, currentTimeStep, modifier);
}
}
public static bool ValidateCode(byte[] securityToken, int code, string modifier = null)
{
if (securityToken == null)
{
throw new ArgumentNullException(nameof(securityToken));
}
// Allow a variance of no greater than 9 minutes in either direction
var currentTimeStep = GetCurrentTimeStepNumber();
using (var hashAlgorithm = new HMACSHA1(securityToken))
{
for (var i = -2; i <= 2; i++)
{
var computedTotp = ComputeTotp(hashAlgorithm, (ulong)((long)currentTimeStep + i), modifier);
if (computedTotp == code)
{
return true;
}
}
}
// No match
return false;
}
}
}