[PM-15608] Create more KDF defaults for prelogin (#5122)

* kdf defaults on null map to email hash

* cleanup code. add some randomness as well

* remove null check

* fix test

* move to private method

* remove random options

* tests for random defaults

* SetDefaultKdfHmacKey for old test

src/Core/Settings/GlobalSettings.cs

@@ -82,6 +82,7 @@ public class GlobalSettings : IGlobalSettings
     public virtual ILaunchDarklySettings LaunchDarkly { get; set; } = new LaunchDarklySettings();
     public virtual string DevelopmentDirectory { get; set; }
     public virtual bool EnableEmailVerification { get; set; }
+    public virtual string KdfDefaultHashKey { get; set; }
     public virtual string PricingUri { get; set; }
 
     public string BuildExternalUri(string explicitValue, string name)

src/Identity/Controllers/AccountsController.cs

@@ -1,4 +1,5 @@
 using System.Diagnostics;
+using System.Text;
 using Bit.Core;
 using Bit.Core.Auth.Enums;
 using Bit.Core.Auth.Models.Api.Request.Accounts;
@@ -15,6 +16,7 @@ using Bit.Core.Exceptions;
 using Bit.Core.Models.Data;
 using Bit.Core.Repositories;
 using Bit.Core.Services;
+using Bit.Core.Settings;
 using Bit.Core.Tokens;
 using Bit.Core.Tools.Enums;
 using Bit.Core.Tools.Models.Business;
@@ -44,6 +46,41 @@ public class AccountsController : Controller
     private readonly IFeatureService _featureService;
     private readonly IDataProtectorTokenFactory<RegistrationEmailVerificationTokenable> _registrationEmailVerificationTokenDataFactory;
 
+    private readonly byte[] _defaultKdfHmacKey = null;
+    private static readonly List<UserKdfInformation> _defaultKdfResults =
+    [
+        // The first result (index 0) should always return the "normal" default.
+        new()
+        {
+            Kdf = KdfType.PBKDF2_SHA256,
+            KdfIterations = AuthConstants.PBKDF2_ITERATIONS.Default,
+        },
+        // We want more weight for this default, so add it again
+        new()
+        {
+            Kdf = KdfType.PBKDF2_SHA256,
+            KdfIterations = AuthConstants.PBKDF2_ITERATIONS.Default,
+        },
+        // Add some other possible defaults...
+        new()
+        {
+            Kdf = KdfType.PBKDF2_SHA256,
+            KdfIterations = 100_000,
+        },
+        new()
+        {
+            Kdf = KdfType.PBKDF2_SHA256,
+            KdfIterations = 5_000,
+        },
+        new()
+        {
+            Kdf = KdfType.Argon2id,
+            KdfIterations = AuthConstants.ARGON2_ITERATIONS.Default,
+            KdfMemory = AuthConstants.ARGON2_MEMORY.Default,
+            KdfParallelism = AuthConstants.ARGON2_PARALLELISM.Default,
+        }
+    ];
+
     public AccountsController(
         ICurrentContext currentContext,
         ILogger<AccountsController> logger,
@@ -55,7 +92,8 @@ public class AccountsController : Controller
         ISendVerificationEmailForRegistrationCommand sendVerificationEmailForRegistrationCommand,
         IReferenceEventService referenceEventService,
         IFeatureService featureService,
-        IDataProtectorTokenFactory<RegistrationEmailVerificationTokenable> registrationEmailVerificationTokenDataFactory
+        IDataProtectorTokenFactory<RegistrationEmailVerificationTokenable> registrationEmailVerificationTokenDataFactory,
+        GlobalSettings globalSettings
         )
     {
         _currentContext = currentContext;
@@ -69,6 +107,11 @@ public class AccountsController : Controller
         _referenceEventService = referenceEventService;
         _featureService = featureService;
         _registrationEmailVerificationTokenDataFactory = registrationEmailVerificationTokenDataFactory;
+
+        if (CoreHelpers.SettingHasValue(globalSettings.KdfDefaultHashKey))
+        {
+            _defaultKdfHmacKey = Encoding.UTF8.GetBytes(globalSettings.KdfDefaultHashKey);
+        }
     }
 
     [HttpPost("register")]
@@ -217,11 +260,7 @@ public class AccountsController : Controller
         var kdfInformation = await _userRepository.GetKdfInformationByEmailAsync(model.Email);
         if (kdfInformation == null)
         {
-            kdfInformation = new UserKdfInformation
-            {
-                Kdf = KdfType.PBKDF2_SHA256,
-                KdfIterations = AuthConstants.PBKDF2_ITERATIONS.Default,
-            };
+            kdfInformation = GetDefaultKdf(model.Email);
         }
         return new PreloginResponseModel(kdfInformation);
     }
@@ -240,4 +279,26 @@ public class AccountsController : Controller
             Token = token
         };
     }
+
+    private UserKdfInformation GetDefaultKdf(string email)
+    {
+        if (_defaultKdfHmacKey == null)
+        {
+            return _defaultKdfResults[0];
+        }
+        else
+        {
+            // Compute the HMAC hash of the email
+            var hmacMessage = Encoding.UTF8.GetBytes(email.Trim().ToLowerInvariant());
+            using var hmac = new System.Security.Cryptography.HMACSHA256(_defaultKdfHmacKey);
+            var hmacHash = hmac.ComputeHash(hmacMessage);
+            // Convert the hash to a number
+            var hashHex = BitConverter.ToString(hmacHash).Replace("-", string.Empty).ToLowerInvariant();
+            var hashFirst8Bytes = hashHex.Substring(0, 16);
+            var hashNumber = long.Parse(hashFirst8Bytes, System.Globalization.NumberStyles.HexNumber);
+            // Find the default KDF value for this hash number
+            var hashIndex = (int)(Math.Abs(hashNumber) % _defaultKdfResults.Count);
+            return _defaultKdfResults[hashIndex];
+        }
+    }
 }