feat(OTP): [PM-18612] Change email OTP to six digits

* Change email OTP to 6 digits

* Added comment on base class

* Added tests

* Renamed tests.

* Fixed tests

* Renamed file to match class

src/Core/Auth/Identity/TokenProviders/EmailTokenProvider.cs

@@ -1,5 +1,6 @@
 using System.Text;
 using Bit.Core.Entities;
+using Bit.Core.Services;
 using Bit.Core.Utilities;
 using Microsoft.AspNetCore.Identity;
 using Microsoft.Extensions.Caching.Distributed;
@@ -7,6 +8,9 @@ using Microsoft.Extensions.DependencyInjection;
 
 namespace Bit.Core.Auth.Identity.TokenProviders;
 
+/// <summary>
+/// Generates and validates tokens for email OTPs.
+/// </summary>
 public class EmailTokenProvider : IUserTwoFactorTokenProvider<User>
 {
     private const string CacheKeyFormat = "EmailToken_{0}_{1}_{2}";
@@ -16,16 +20,25 @@ public class EmailTokenProvider : IUserTwoFactorTokenProvider<User>
 
     public EmailTokenProvider(
         [FromKeyedServices("persistent")]
-        IDistributedCache distributedCache)
+        IDistributedCache distributedCache,
+        IFeatureService featureService)
     {
         _distributedCache = distributedCache;
         _distributedCacheEntryOptions = new DistributedCacheEntryOptions
         {
             AbsoluteExpirationRelativeToNow = TimeSpan.FromMinutes(5)
         };
+        if (featureService.IsEnabled(FeatureFlagKeys.Otp6Digits))
+        {
+            TokenLength = 6;
+        }
+        else
+        {
+            TokenLength = 8;
+        }
     }
 
-    public int TokenLength { get; protected set; } = 8;
+    public int TokenLength { get; protected set; }
     public bool TokenAlpha { get; protected set; } = false;
     public bool TokenNumeric { get; protected set; } = true;
 

src/Core/Auth/Identity/TokenProviders/EmailTwoFactorTokenProvider.cs

@@ -4,19 +4,27 @@
 using Bit.Core.Auth.Enums;
 using Bit.Core.Auth.Models;
 using Bit.Core.Entities;
+using Bit.Core.Services;
 using Microsoft.AspNetCore.Identity;
 using Microsoft.Extensions.Caching.Distributed;
 using Microsoft.Extensions.DependencyInjection;
 
 namespace Bit.Core.Auth.Identity.TokenProviders;
 
+/// <summary>
+/// Generates tokens for email two-factor authentication.
+/// It inherits from the EmailTokenProvider class, which manages the persistence and validation of tokens, 
+/// and adds additional validation to ensure that 2FA is enabled for the user.
+/// </summary>
 public class EmailTwoFactorTokenProvider : EmailTokenProvider
 {
     public EmailTwoFactorTokenProvider(
         [FromKeyedServices("persistent")]
-        IDistributedCache distributedCache) :
-        base(distributedCache)
+        IDistributedCache distributedCache,
+        IFeatureService featureService) :
+        base(distributedCache, featureService)
     {
+        // This can be removed when the pm-18612-otp-6-digits feature flag is removed because the base implementation will match.
         TokenAlpha = false;
         TokenNumeric = true;
         TokenLength = 6;

src/Core/Constants.cs

@@ -124,6 +124,7 @@ public static class FeatureFlagKeys
     public const string SetInitialPasswordRefactor = "pm-16117-set-initial-password-refactor";
     public const string ChangeExistingPasswordRefactor = "pm-16117-change-existing-password-refactor";
     public const string RecoveryCodeLogin = "pm-17128-recovery-code-login";
+    public const string Otp6Digits = "pm-18612-otp-6-digits";
 
     /* Autofill Team */
     public const string IdpAutoSubmitLogin = "idp-auto-submit-login";