Add support for zstd compressed depot chunks

Author: xPaw

SteamKit2/SteamKit2/Steam/CDN/DepotChunk.cs

@@ -55,7 +55,11 @@ public static int Process( DepotManifest.ChunkData info, ReadOnlySpan<byte> data
                 var written = aes.DecryptCbc( data[ iv.Length.. ], iv, buffer, PaddingMode.PKCS7 );
                 var decryptedStream = new MemoryStream( buffer, 0, written );
 
-                if ( buffer.Length > 1 && buffer[ 0 ] == 'V' && buffer[ 1 ] == 'Z' )
+                if ( buffer[ 0 ] == 'V' && buffer[ 1 ] == 'S' && buffer[ 2 ] == 'Z' && buffer[ 3 ] == 'a' ) // Zstd
+                {
+                    writtenDecompressed = VZstdUtil.Decompress( buffer.AsSpan( 0, written ), destination );
+                }
+                else if ( buffer.Length > 1 && buffer[ 0 ] == 'V' && buffer[ 1 ] == 'Z' )
                 {
                     writtenDecompressed = VZipUtil.Decompress( decryptedStream, destination, verifyChecksum: false );
                 }

SteamKit2/SteamKit2/SteamKit2.csproj

@@ -55,6 +55,7 @@
     </PackageReference>
     <PackageReference Include="protobuf-net" Version="3.2.46" />
     <PackageReference Include="System.IO.Hashing" Version="9.0.1" />
+    <PackageReference Include="ZstdSharp.Port" Version="0.8.4" />
   </ItemGroup>
 
 </Project>

SteamKit2/SteamKit2/Util/VZstdUtil.cs

@@ -0,0 +1,44 @@
+using System;
+using System.Buffers;
+using System.IO;
+using System.Runtime.InteropServices;
+
+namespace SteamKit2
+{
+    static class VZstdUtil
+    {
+        private const uint VZstdHeader = 0x56535A61;
+
+        public static int Decompress( ReadOnlySpan<byte> buffer, byte[] destination )
+        {
+            if ( MemoryMarshal.Read<uint>( buffer ) != VZstdHeader )
+            {
+                throw new InvalidDataException( "Expecting VZstdHeader at start of stream" );
+            }
+
+            var sizeCompressed = MemoryMarshal.Read<int>( buffer[ ^15.. ] ); // TODO: I am not convinced this is correct -- maybe its the frame size
+            var sizeDecompressed = MemoryMarshal.Read<int>( buffer[ ^11.. ] );
+
+            if ( buffer[ ^3 ] != 'z' || buffer[ ^2 ] != 's' || buffer[ ^1 ] != 'v' )
+            {
+                throw new InvalidDataException( "Expecting VZstdFooter at end of stream" );
+            }
+
+            if ( destination.Length < sizeDecompressed )
+            {
+                throw new ArgumentException( "The destination buffer is smaller than the decompressed data size.", nameof( destination ) );
+            }
+
+            using var zstdDecompressor = new ZstdSharp.Decompressor();
+
+            var input = buffer[ 4..^12 ];
+
+            if ( !zstdDecompressor.TryUnwrap( input, destination, out var sizeWritten ) || sizeDecompressed != sizeWritten )
+            {
+                throw new InvalidDataException( $"Failed to decompress Zstd (expected {sizeDecompressed} bytes, got {sizeWritten})." );
+            }
+
+            return sizeDecompressed;
+        }
+    }
+}