From fb64e46022e77f923ca21920250be0c33e7aac06 Mon Sep 17 00:00:00 2001
From: real-zony <zony@myzony.com>
Date: Mon, 31 May 2021 19:15:36 +0800
Subject: [PATCH] feat: working....ncm decryptor.

---
 .../MusicDecryption/NcmMusicDecryptor.cs      | 68 ++++++++++++++++++-
 1 file changed, 67 insertions(+), 1 deletion(-)

diff --git a/src/ZonyLrcTools.Cli/Infrastructure/MusicDecryption/NcmMusicDecryptor.cs b/src/ZonyLrcTools.Cli/Infrastructure/MusicDecryption/NcmMusicDecryptor.cs
index 9a2e27e..ab95522 100644
--- a/src/ZonyLrcTools.Cli/Infrastructure/MusicDecryption/NcmMusicDecryptor.cs
+++ b/src/ZonyLrcTools.Cli/Infrastructure/MusicDecryption/NcmMusicDecryptor.cs
@@ -1,13 +1,79 @@
-using System.Threading.Tasks;
+using System;
+using System.IO;
+using System.Linq;
+using System.Security.Cryptography;
+using System.Threading.Tasks;
 using ZonyLrcTools.Cli.Infrastructure.DependencyInject;
+using ZonyLrcTools.Cli.Infrastructure.Exceptions;
 
 namespace ZonyLrcTools.Cli.Infrastructure.MusicDecryption
 {
+    /// <summary>
+    /// NCM 音乐转换器，用于将 NCM 格式的音乐转换为可播放的格式。
+    /// </summary>
     public class NcmMusicDecryptor : IMusicDecryptor, ITransientDependency
     {
+        protected readonly byte[] AesCoreKey = {0x68, 0x7A, 0x48, 0x52, 0x41, 0x6D, 0x73, 0x6F, 0x35, 0x6B, 0x49, 0x6E, 0x62, 0x61, 0x78, 0x57};
+        protected readonly byte[] AesModifyKey = {0x23, 0x31, 0x34, 0x6C, 0x6A, 0x6B, 0x5F, 0x21, 0x5C, 0x5D, 0x26, 0x30, 0x55, 0x3C, 0x27, 0x28};
+
         public Task<byte[]> Convert(byte[] sourceBytes)
         {
+            var stream = new MemoryStream(sourceBytes);
+            var streamReader = new BinaryReader(stream);
+
+            var lengthBytes = new byte[4];
+            lengthBytes = streamReader.ReadBytes(4);
+            if (BitConverter.ToInt32(lengthBytes) != 0x4e455443)
+            {
+                throw new Exception();
+            }
+
+            lengthBytes = streamReader.ReadBytes(4);
+            if (BitConverter.ToInt32(lengthBytes) != 0x4d414446)
+            {
+                throw new Exception();
+            }
+
+            stream.Seek(2, SeekOrigin.Current);
+            stream.Read(lengthBytes);
+
+            var keyBytes = new byte[BitConverter.ToInt32(lengthBytes)];
+            stream.Read(keyBytes);
+
+            // 对已经加密的数据进行异或操作。
+            for (int i = 0; i < keyBytes.Length; i++)
+            {
+                keyBytes[i] ^= 0x64;
+            }
+
+            var deKeyDataBytes = GetBytesByOffset(DecryptAes128Ecb(AesCoreKey, keyBytes), 17);
+
+            var modifyDataBytes = new byte[streamReader.ReadInt32()];
+            stream.Read(modifyDataBytes);
+            for (int i = 0; i < modifyDataBytes.Length; i++)
+            {
+                modifyDataBytes[i] ^= 0x63;
+            }
+
             throw new System.NotImplementedException();
         }
+
+        private byte[] GetBytesByOffset(byte[] srcBytes, int offset = 0)
+        {
+            var resultBytes = new byte[srcBytes.Length - offset];
+            Array.Copy(srcBytes, offset, resultBytes, 0, srcBytes.Length - offset);
+            return resultBytes;
+        }
+
+        private byte[] DecryptAes128Ecb(byte[] keyBytes, byte[] data)
+        {
+            var aes = Aes.Create();
+            aes.Padding = PaddingMode.PKCS7;
+            aes.Mode = CipherMode.ECB;
+            using var decryptor = aes.CreateDecryptor(keyBytes, null);
+            var result = decryptor.TransformFinalBlock(data, 0, data.Length);
+
+            return result;
+        }
     }
 }
\ No newline at end of file