Implemented separate signature validator

LiteCharms.Features.MidrandBooks/LiteCharms.Features.MidrandBooks.csproj

@@ -148,6 +148,7 @@
 
   <!-- Shared Usings -->
   <ItemGroup>
+    <Using Include="Microsoft.AspNetCore.Http" />
     <Using Include="System.Net.Sockets" />
     <Using Include="System.Text.RegularExpressions" />
     <Using Include="System.Web" />

LiteCharms.Features.MidrandBooks/Payments/PayfastService.cs

@@ -3,6 +3,7 @@ using LiteCharms.Features.Api.Configuration;
 using LiteCharms.Features.Hasher;
 using LiteCharms.Features.MidrandBooks.Payments.Models;
 using LiteCharms.Features.MidrandBooks.Postgres;
+using Microsoft.AspNetCore.Http;
 
 namespace LiteCharms.Features.MidrandBooks.Payments;
 
@@ -48,6 +49,35 @@ public sealed partial class PayfastService(IDbContextFactory<MidrandBooksDbConte
         }
     }
 
+    public static bool VerifyIncomingSignature(HttpRequest request, string passphrase)
+    {
+        var formFields = request.Form.ToDictionary(x => x.Key, x => x.Value.ToString());
+
+        if (!formFields.TryGetValue("signature", out string? incomingSignature))
+            return false;
+
+        var stringBuilder = new StringBuilder();
+
+        foreach (var key in formFields.Keys)
+        {
+            if (key.Equals("signature", StringComparison.OrdinalIgnoreCase))
+                continue;
+
+            string encodedVal = HttpUtility.UrlEncode(formFields[key].Trim());
+            string cleanVal = PercentEncodingRegex.Replace(encodedVal,  m => m.Value.ToUpperInvariant());
+
+            stringBuilder.Append($"{key}={cleanVal}&");
+        }
+
+        string encodedPassphrase = HttpUtility.UrlEncode(passphrase.Trim());
+        string safePassphrase = PercentEncodingRegex.Replace(encodedPassphrase,  m => m.Value.ToUpperInvariant());
+
+        stringBuilder.Append($"passphrase={safePassphrase}");
+
+        string generatedSignature = HashService.ToMd5Hash(stringBuilder.ToString()).Value;
+        return incomingSignature.Equals(generatedSignature, StringComparison.OrdinalIgnoreCase);
+    }
+
     public async ValueTask<Result<bool>> ValidateReferrerIpAsync(string remoteIpAddress, bool allowLoopback = false, CancellationToken cancellationToken = default)
     {
         if(payfastOptions.Value?.ValidHosts?.Length == 0)