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Dalamud.Bootstrap/SqexArg/EncodedArgument.cs

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+using System;
+using System.Buffers;
+using System.Buffers.Text;
+using System.Runtime.InteropServices;
+using System.Text;
+using Dalamud.Bootstrap.Crypto;
+
+namespace Dalamud.Bootstrap.SqexArg
+{
+    internal sealed class EncodedArgument
+    {
+        private static char[] ChecksumTable = new char[]
+        {
+            'f', 'X', '1', 'p', 'G', 't', 'd', 'S',
+            '5', 'C', 'A', 'P', '4', '_', 'V', 'L'
+        };
+
+        private readonly ReadOnlyMemory<byte> m_data;
+
+        private readonly uint m_key;
+
+        //private readonly Blowfish m_cachedBlowfish;
+
+        public EncodedArgument(ReadOnlyMemory<byte> data, uint key)
+        {
+            m_data = data;
+            m_key = key;
+        }
+
+        public static bool TryParse(string argument, out EncodedArgument? value)
+        {
+            if (argument.Length <= 17)
+            {
+                // does not contain: //**sqex0003 + payload + checksum + **//
+                value = null;
+                return false;
+            }
+
+            if (!argument.StartsWith("//**sqex0003") || !argument.EndsWith("**//"))
+            {
+                value = null;
+                return false;
+            }
+
+            var checksum = argument[^5];
+            var payload = DecodeUrlSafeBase64(argument.Substring(12, argument.Length - 1 - 12 - 4)); // //**sqex0003, checksum, **//
+
+            if (!FindPartialKey(checksum, out var partialKey))
+            {
+                value = null;
+                return false;
+            }
+
+            for (var i = 0u; i <= 0xFFF; i++)
+            {
+                var key = (i << 20) | partialKey;
+            }
+        }
+
+        private static bool FindPartialKey(char checksum, out uint recoveredKey)
+        {
+            var index = MemoryExtensions.IndexOf(ChecksumTable, checksum);
+            
+            if (index < 0)
+            {
+                recoveredKey = 0;
+                return false;
+            }
+
+            recoveredKey = (uint) (index << 16);
+            return true;
+        }
+        
+        /// <summary>
+        /// Converts url safe variant of base64 string to bytes.
+        /// </summary>
+        /// <param name="payload">A url-safe variant of base64 string.</param>
+        private static byte[] DecodeUrlSafeBase64(string payload)
+        {
+            var base64Str = payload
+                .Replace('-', '+')
+                .Replace('_', '/');
+            
+            return Convert.FromBase64String(base64Str);
+        }
+    }
+}