Saturday, 7 March 2015

Azure AD Cloud silent Authentication with ADAL and TokenCache encryption

The ADAL library was launched a couple of years ago, luckily people from Microsoft like Vittorio Bertocci, have been working hard to have a library capable of doing authentication between native applications, web applications and web services (Web API). ADAL.js is in beta, it is a full working library but there are some issues need to be solved before we can have a robust library (This is when I am writing my article so March 2015) but Vittorio promised me we will have something working in this side by April 2015.
Unfortunately for me, and due to a project we are releasing before April, there is not choice, so I will have to write a wrapper around my Azure Web Api’s and skip Adal.js until we have v1.0. So you will be asking yourself “what is this guy talking about?”.
Ok, let’s go to start from a "simple" scenario, so you can have an  idea of what I am talking about. I have a Web Application (MVC 5) which is talking with few web services (Web API 2’s). All of them are register in Azure AD, so Azure AD acts as a Black Box, and manage the authentication. In this case Azure AD will take care of the MVC Web Application and the Web API’s Authentication.
So where is the problem? Well if I want to be sure that the Web Application and the Web APis share the same authentication, I will have to use refresh tokens, but because the hard work done by Vittorio’s team, now we can use Silent Tokens, so we don’t have to worry about building a whole system to refresh the tokens, but the only place where we need to worry it is about about where to store the Tokens.
It is quite common to store the tokens in Sessions, but there is a small problem with this, you lose the multi-farm factor plus WebApi’s don’t have sessions, so when you have to talk to them becomes an impossible job.
So what is the solution? store the tokens in a persistent area. If we have a native application we could store the token’s in the file system, but if we have a Web Application we could store them in Blob Storage or Databases.
Let’s go to a scenario, when someone, which is not really a user, and it is trying to get our token to access to our farm or MVC application, manage to access to the database or file system. Well, I think we will be in big trouble here. That person will have at least 20 minutes (Time when our token expires) to hack our system.
So… let’s go to make the things more difficult for this person, let’s go to inject encryption in the tokens, with double key encryption, String and Byte Array.
The following example it is a simple call from a MVC Application to a Web API 2 using silent Authentication.
...//## GETTING THE TOKEN TO BE AUTHORISE string userObjectID = ClaimsPrincipal.Current.FindFirst(userSchema).Value; AuthenticationContext authContext = new AuthenticationContext(Startup.Authority, new TokenEncryptedDatabaseCache(userObjectID, "Iamakey"); ClientCredential credential = new ClientCredential(clientId, appKey); AuthenticationResult result = authContext.AcquireTokenSilent(resourceId, credential, new UserIdentifier(userObjectID, UserIdentifierType.UniqueId));string authnHeader = "Authorization: Bearer " + result.AccessToken; ...

Now… check the parameter we are passing on  AcquireTokenSilent , it is a TokenCache.

This is the model we use to store the data in the database:

public class PerUserWebCache { [Key] public int EntryId { get; set; } public string WebUserUniqueId { get; set; } public byte[] CacheBits { get; set; } public DateTime LastWrite { get; set; } }
And finally this is the beauty! TokenEncryptedDatabaseCache, which encapsulates the encryption. You will need to create a dbcontext to store the model with the data…which I am going to add as well, so you can save some time.

using System;using System.Collections.Generic;using System.Data.Entity;using System.Linq;using System.Web;using System.Web.Caching;using Microsoft.IdentityModel.Clients.ActiveDirectory;using System.Security.Cryptography;using System.IO; public class TokenEncryptedDatabaseCache : TokenCache { private TokenCacheDataContext db = new TokenCacheDataContext(); string User; private PerUserWebCache Cache; private string Key; /// <summary> /// Contsructor for the TokenEncryptedDatabaseCache class /// </summary> /// <param name="user">Current User</param> /// <param name="key">Key for the encription</param> public TokenEncryptedDatabaseCache(string user, string key) { Key = key; User = user; this.AfterAccess = AfterAccessNotification; this.BeforeAccess = BeforeAccessNotification; this.BeforeWrite = BeforeWriteNotification; //## We check if the user is in our database Cache = db.PerUserCacheList.FirstOrDefault(c => c.WebUserUniqueId == User); //## If that is the case we keep it in memory //## We decrypt the token this.Deserialize((Cache == null) ? null : Cache.CacheBits!=null?Decrypt(Cache.CacheBits):null); } /// <summary> /// Method to clean the database /// </summary> public override void Clear() { base.Clear(); foreach (var cacheEntry in db.PerUserCacheList) db.PerUserCacheList.Remove(cacheEntry); db.SaveChanges(); } /// <summary> /// ADAL raise a notification before acces to the cache. /// Notification raised before ADAL accesses the cache. /// This is your chance to update the in-memory copy from the DB, /// if the in-memory version is stale. The token is decrypted. /// </summary> /// <param name="args"></param> void BeforeAccessNotification(TokenCacheNotificationArgs args) { if (Cache == null) { // first time access Cache = db.PerUserCacheList.FirstOrDefault(c => c.WebUserUniqueId == User); } else { // retrieve last write from the DB var status = from e in db.PerUserCacheList where (e.WebUserUniqueId == User) select new { LastWrite = e.LastWrite }; // if the in-memory copy is older than the persistent copy if (status.First().LastWrite > Cache.LastWrite) //// read from from storage, update in-memory copy { Cache = db.PerUserCacheList.FirstOrDefault(c => c.WebUserUniqueId == User); } } this.Deserialize((Cache == null) ? null : Cache.CacheBits!=null?Decrypt(Cache.CacheBits):null); } /// <summary> ///Notification raised after ADAL accessed the cache. ///If the HasStateChanged flag is set, ADAL changed the content of the cache, ///At this time we encrypt the token and save it. /// </summary> /// <param name="args"></param> void AfterAccessNotification(TokenCacheNotificationArgs args) { // if state changed if (this.HasStateChanged) { Cache = new PerUserWebCache { WebUserUniqueId = User, CacheBits = Encrypt(this.Serialize()), LastWrite = DateTime.Now }; //// update the DB and the lastwrite db.Entry(Cache).State = Cache.EntryId == 0 ? EntityState.Added : EntityState.Modified; db.SaveChanges(); this.HasStateChanged = false; } } void BeforeWriteNotification(TokenCacheNotificationArgs args) { // if you want to ensure that no concurrent write take place, use this notification to place a lock on the entry } /// <summary> /// Encription of the token. /// </summary> /// <param name="DataToEncrypt">Data to be encrypted</param> /// <returns>Data encrypted.</returns> private byte[] Encrypt(byte[] DataToEncrypt) { PasswordDeriveBytes passwordDeriveBytes = new PasswordDeriveBytes(Key, new byte[] { 0x43, 0x87, 0x23, 0x72 }); MemoryStream memoryStream = new MemoryStream(); Aes aes = new AesManaged(); aes.Key = passwordDeriveBytes.GetBytes(aes.KeySize / 8); aes.IV = passwordDeriveBytes.GetBytes(aes.BlockSize / 8); CryptoStream cryptoStream = new CryptoStream(memoryStream,aes.CreateEncryptor(), CryptoStreamMode.Write); cryptoStream.Write(DataToEncrypt, 0, DataToEncrypt.Length); cryptoStream.Close(); return memoryStream.ToArray(); } /// <summary> /// Decryption of the token. /// </summary> /// <param name="DataToDecrypt">Data to be decrypted</param> /// <returns>Data decrypted</returns> private byte[] Decrypt(byte[] DataToDecrypt) { PasswordDeriveBytes passwordDeriveBytes = new PasswordDeriveBytes(Key, new byte[] { 0x43, 0x87, 0x23, 0x72 }); MemoryStream memoryStream = new MemoryStream(); Aes aes = new AesManaged(); aes.Key = passwordDeriveBytes.GetBytes(aes.KeySize / 8); aes.IV = passwordDeriveBytes.GetBytes(aes.BlockSize / 8); CryptoStream cryptoStream = new CryptoStream(memoryStream, aes.CreateDecryptor(), CryptoStreamMode.Write); cryptoStream.Write(DataToDecrypt, 0, DataToDecrypt.Length); cryptoStream.Close(); return memoryStream.ToArray(); } }

This is the class where we have our DBContext

using System;using System.Collections.Generic;using System.ComponentModel.DataAnnotations;using System.Data.Entity;using System.Data.Entity.ModelConfiguration.Conventions;using System.Linq;using System.Text;using System.Threading.Tasks; public class TokenCacheDataContext : DbContext { public TokenCacheDataContext() : base("TokenCacheDataContext") { } public DbSet<PerUserWebCache> PerUserCacheList { get; set; } protected override void OnModelCreating(DbModelBuilder modelBuilder) { modelBuilder.Conventions.Remove<PluralizingTableNameConvention>(); } }


using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.Threading.Tasks; public class TokenCacheInitializer : System.Data.Entity.DropCreateDatabaseIfModelChanges<TokenCacheDataContext> { }

Well, I hope you enjoy, any questions let me know.


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